Peterbilt Essentials Module5 Axles Suspensions 1b1p60
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p e te rb ilt e sse nt ia ls MO D ULE 5
• Front and Rear Axles • Steering • Front and Rear Suspensions • Drivelines
peterbilt NEW essentials – Module 5 Introduction
How To Use New Peterbilt Essentials
This module will cover axle configurations, front and rear suspension, steering operation, hubs/drums, drivelines and power take-offs
1. Print the module and study the information. To print, click the printer icon on your browser. Highlight material that is new to you, or complex.
The design and placement of axles under a truck frame vary. This module describes common configurations for front and rear axles, how they operate and the rationale for axle placement.
2. When you are ready to take the online test, click the "Begin" button in the "Test" column for the desired module. When the test is completed, it will automatically be scored and the results will be entered in the Peterbilt training records database.
Succeeding modules present more detailed information about major truck operating systems.
3. Upon successful completion of all modules, you will receive a personalized certificate.
It is recommended that you complete these training modules in sequence since each succeeding module builds on the previous module.
Published by Peterbilt Motors Company. This material is intended for Peterbilt training purposes and may not be sold, given, loaned or reproduced, in whole or in part, including photocopying, without the express written consent of Peterbilt Motors Company.
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Front Axle
Axle Configurations
One of the primary functions of a truck’s front axle assembly is to help the weight of the vehicle. The front axle typically carries between 25 percent and 37 percent of the gross vehicle weight (GVW). The amount of weight the front axle carries depends primarily on the axle’s position (set-forward or set-back) and the fifth wheel setting. However, to maximize Federal Bridge Law legal limits, the front axle will carry about 15 percent of the combined weight of the tractor, trailer and cargo. The front axle also serves as a foundation for the truck’s steering components and transmits braking torque to the front suspension springs.
You will often see the designations 4x4, 4x2, 6x4 and 6x6 in connection with truck and tractor configurations. These are indicators of the total number of axles compared to the number of driving axles. The first number indicates the number of “axle ends” (each axle has two ends); the second indicates how many of them are live (driven by the engine). For example, “6x4” means that the truck has three axles (six axle ends) and that two of the axles (four axle ends) are being driven by the engine. A “6x6” designation indicates that all three axles, including the front axle, are being driven by the engine.
Weight Distribution and Axle Position Since weight and the distribution of weight have so much to do with the function of the front axle, it is useful to review some common concepts associated with weight, beginning with gross vehicle weight and gross combination weight. • Gross Vehicle Weight (GVW) is the total weight of a fully equipped truck and payload. •
Gross Combination Weight (GCW) is the total weight of a fully-equipped truck or tractor, trailer or trailers and payload.
As indicated earlier, the front axle carries a significant portion of the gross weight of a tractor-trailer combination or straight truck. How much it actually carries depends in part upon where the axle is positioned relative to the payload. At this point the concepts of the set-forward front axle (SFFA) and the set-back front axle (SBFA) need to be defined. Both are measurements of the distance from the bumper to the centerline of the front axle. • The SFFA on a Peterbilt is about 29- to 31-inches.
The axle center is known as the beam, and is most commonly a forged steel I-beam. Round tubular stock of tempered seamless steel can also be specified for the front axle beam; this is frequently used in applications that involve extremely heavy loads.
•
There are in general two types of front axles: driving or non-driving. A driving axle is one that is receiving power from the engine to provide vehicle motion. Except for some off-road or extremely heavy-duty applications, most front axles are non-driving.
The SBFA on a Peterbilt ranges from 47- to 51-inches.
Since the location of the front axle affects the length of the wheelbase, it also affects the percentage of weight distributed to the axle. With a set-forward axle, less weight will be transferred to the front axle. With a set-back axle, more weight will be transferred to the front axle. A set-back axle usually helps maximize loads, because more weight will be distributed to the front axle. On the other hand, a set-forward axle helps distribute loads when the possibility of overloading the front axle exists.
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peterbilt NEW essentials – Module 5 Axle Ratings
Front Drive Axles
Front axles are rated according to the weight they can carry. The axle rating a truck owner selects will depend on GVW or GCW. Peterbilt offers non-driving axles rated from 10,000 to 22,000 pounds. Drive axles rated from 12,000 to 23,000 pounds are also available. It’s important to , however, that the load capacity of an installed front axle is determined by the Gross Axle Weight Rating (GAWR) rather than by the rating of the axle alone. Gross Axle Weight Rating is the rated capacity of the suspension system and front axle components, which include the springs, tires, steering gear, and wheels in addition to the axle. The lowest-rated of these weight-sensitive components will determine the GAWR.
Steerable drive axles are typically used on heavyduty on/off-road vehicles that require greater traction and maneuverability than can be obtained from a 6x4 configuration. A transfer case remotely mounted to the transmission is commonly used to drive a front drive axle. The transfer case is designed with front and rear drivelines that provide torque to the front and rear axles respectively.
The following components are considered in determining Gross Axle Weight Rating.
Component
Weight Rating
Axle
12,000 pounds
Suspension
12,000 pounds
Wheels
12,080 pounds
Tires
10,860 pounds
Steering Gear
12,000 pounds
Front Drive Axle
As another choice on trucks with steerable drive axles, Peterbilt can also install an all-wheel drive system instead of a transfer case. The all-wheel drive system is a power divider that is often referred to as a “spaghetti drive” because of the number and configuration of the drivelines. The system’s design calls for a driveline from the transmission to the rear axle and then an additional driveline from the rear axle to the front axle.
Since the GAWR cannot exceed the capacity of the lowest-rated component, it corresponds in this case to the rating of the tires: GAWR = 10,860 pounds.
“Spaghetti Drive”: the Dana Spicer All-Wheel Drive System
Whether a transfer case or a spaghetti drive is used, torque is distributed to the front wheels in the same proportion as to the rear wheels by means of a differential. Like a rear drive axle, a front drive axle must provide a sufficient range for all operating speeds; this means that the front drive axle must have the same gear reduction capabilities as the rear axle. 4
New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Front Axle Steering Since the front axle is essential to the steering of the vehicle, design and maintenance of the front axle and the steering components attached to it are major considerations in determining steering ease and tire wear. Peterbilt vehicles are standard with power steering, which means that steering is assisted by means of a hydraulic pump that circulates fluid from a reservoir through the steering gear and back. An engine-mounted two-quart fluid reservoir is standard, but to ensure optimum performance by maintaining proper fluid temperatures, the size of the reservoir is designed to increase as the front axle GAWR of the vehicle increases. For GAWRs from 16,000 to 20,000 pounds, Peterbilt offers a twelve-quart reservoir.
•
The rack piston’s movement, in mesh with gears on the sector (steering gear output) shaft, causes the sector shaft to rotate left or right.
•
The steering gear output shaft or sector shaft, working through the pitman arm and drag link, pushes or pulls the steering arm, which rotates the wheel assembly on the knuckle pin (or “king pin”).
•
The Ackermann arm transmits torque to the tie-rod assembly, which transfers the torque to the opposite Ackermann arm, which then turns the opposite wheel.
STEERING KNUCKLE
Steering Gear Operation Here is how the steering system works: • When the steering wheel is turned, force is transmitted through the intermediate shafts to the steering gear input shaft. Resistance causes the steering gear’s internal torsion bar to be twisted by the input shaft.
Ackermann arm
Axle Beam with Steering Components (Left Side of Vehicle)
Steering Geometry (Alignment) The alignment that keeps a truck rolling efficiently both straight ahead and through curves is achieved through a balance of several factors: • Caster
Typical Steering Linkage
•
The twisting of the torsion bar actuates a control valve, which allows pressurized fluid to enter one end of the rack piston cylinder. The end of the cylinder that the fluid enters is determined by the direction the wheel was turned; the control valve also permits open flow from the opposite end to aid in the cooling of the fluid. The pressure of the fluid against the rack piston acts to reduce the amount of steering effort required to move the rack piston along the input shaft.
•
Camber
•
Ackermann geometry
•
Toe-in/Toe-out
The following is a brief discussion of each of these properties and how they are related to the operation of a Peterbilt truck.
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peterbilt NEW essentials – Module 5 Caster positive Camber
Caster is the fore or aft tilt of the top of the steering knuckle pin as it might be viewed from the side of the vehicle. “Positive” caster is the tilt of the top of the knuckle pin toward the rear of the vehicle; negative caster is the tilt of the pin toward the front of the vehicle. Peterbilt uses
Negative Camber
Vertical C/L
Illustration of Camber
nonadjustable setting. Camber can be adjusted only by bending the axle beam, which is not recommended.
Ackermann Geometry Illustration of Caster
While a truck is moving straight ahead, the front wheels should be tracking parallel to each other. However, when the vehicle encounters a curve in the road, parallel operation of the wheels would cause one tire to side-slip because the wheels would be forced to rotate around circles of different diameters. To ensure that the inner
a caster of +4 degrees to maintain steering stability and steering return to center. Caster angle is determined by the installed position of the steer axle; it can be adjusted by inserting wedge-shaped shims between the front suspension springs and the front axle beam (or the spacer block if the truck is equipped with one). Although incorrect caster adjustment has a negligible effect on tire wear, it may affect steering effort and stability. A greater positive caster angle than is specified may result in excessive steering effort. A smaller caster angle than is specified may result in vehicle wander or poor steering return to center.
ackermann arm
Camber The vertical tilt of the wheel as it might be seen from the front of the truck is called camber. Positive camber is an outward tilt of the wheel at the top; negative camber is an inward tilt of the wheel at the top. A front axle will deflect slightly under a load. To offset this deflection and bring the axis of the knuckle pin closer to the point of the road and the center line of the tire, a small amount of positive camber is designed into the axle.
tie rod
Axle, Ackermann Arms and Tie Rod
Excessive positive camber can result in wander, steering difficulty and abnormal wear on the outer area of the tire. Excessive negative camber can cause inside tire wear.
wheel will always turn through a shorter circumference than the outer wheel, each Ackermann arm (or “tie-rod arm”) is positioned at an angle to the tie rod. This angle is
Camber is a condition that is machined into the axle by Peterbilt and is generally considered a permanent, 6
New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Wheel Cut and Turning Radius
determined by the tire width and the wheelbase. During turning, the geometry of the Ackermann arms causes the angle of the inner wheel to become greater and the angle of the outer wheel to become smaller. Because Peterbilt offers a large number of wheelbase choices, Ackermann arm options are selected to minimize tire side-slip.
Wheel cut determines a vehicle’s turning radius. Since larger wheel cut angles imply improved maneuverability, wheel cut figures are often quoted in promotional materials. “Inside” wheel cut angles (which are numerically greater) often are the ones quoted, but it is actually the outside tire that turns the vehicle. The significant wheel cut angle, then, is the one for the outside wheel in each turn direction. Factors influencing wheel cut angles include the sizes of the tires and the wheels, and the width (or track) of the front axle. All of these factors affect how far the tire can be turned before it touches a chassis component, which is often the limiting factor.
Wheel Toe-in/Toe-out “Toe” is the relationship of the distance between the front of the front tires and the rear of the front tires. When the distance at the front is smaller than the distance in the rear, the wheels are said to be “toed-in”. When the distance in the rear is smaller, the wheels are “toed-out.” “Zero toe” is the condition of the wheels when they are parallel. Because of the forces acting on them, a truck’s front tires tend to toe-out as the vehicle travels down the road. If the toe setting of the wheels were zero, excess rolling resistance, tire wear and vehicle wander would likely result. Therefore, wheels should be toed-in about one-eighth of an inch. front view
toe-in
Turning Radius: The Significant Wheel Cut is the One for the Outside Wheel
front view
With some exceptions, a Peterbilt with identical specs to a vehicle made by another OEM will have a greater wheel cut, in part because of the Peterbilt’s steering component design. The position of the steering gear is a good example of this. On a Peterbilt, the steering gear is located forward of the axle, where it won’t obstruct the path of the wheel. This design increases the angle that the wheel can be turned.
toe-out
Toe-in and Toe-out
Turning radius is the arc described by the center of the track of the outside front wheel in the tightest turn a truck can make. The tightest turn is described by the angle of the outside wheel when the inside front wheel reaches maximum wheel cut. There are 2 turning radius measurements: • Curb-to-Curb – the arc described by the front wheel. •
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Wall-to-Wall – the arc described by the outside edge of the bumper or fender. New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Front Suspensions
Peterbilt Split Progressive Springs
Peterbilt offers several front leaf suspension systems in various capacities; all are designed to achieve optimum ride characteristics under varying operating loads and applications. The 12,000-pound suspensions are offered with a choice of springs: •
Peterbilt taper leaf
•
Peterbilt split progressive leaf
Peterbilt split progressive springs are rated at 23,000 pounds. This suspension provides a comfortable ride with minimal maintenance for a range of heavy loads. The unique design minimizes friction by using only the number of springs required to carry the load at a given time. The lower leaves act as “helper” springs, becoming active only as the spring pack flattens under the weight of a load. This design provides a smooth ride when the vehicle is lightly loaded and allows a gradual stiffening of the ride as the load is increased.
Peterbilt Taper Leaf Springs Tightly clamped front leaf spring assemblies used on many trucks typically have substantial friction between the leaves. This tends to cause stiffness of ride quality, especially without a load. Since the friction between spring leaves is a major contributor to a stiff ride, each Peterbilt taper leaf suspension uses only two or three contoured leaves: fewer leaves – less friction. The thickness of each leaf is tapered so that it can absorb stresses uniformly from its thin ends to its thick center area. This creates a smooth ride under light loads and also heavy loads. The unique two-leaf design of the Peterbilt suspension also reduces weight, which can translate into greater payload potential.
Peterbilt Split Progressive Springs
Peterbilt Taper Leaf Springs
Peterbilt offers taper leaf springs rated at (10,000 only on medium duty), 12,000, 14,600, 16,000, 18,000, 20,000 and 23,000 pounds. All taper leaf springs include tubular shock absorbers for positive rebound control.
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peterbilt NEW essentials – Module 5 Hubs/Drums
Oil Seals
A hub-piloted wheel mounting system (PHP-10) is standard on Peterbilt trucks. Hub-piloted wheels are centered around the hub with close tolerances between the wheel center and machined surfaces on the hub. This hub-pilot mounting system is more effective at centering the wheel, resulting in a smoother ride and better tire wear. There are also fewer parts in a hub-piloted mounting system.
Peterbilt installs oil seals and sight glasses on all non-driving axles – a feature that makes for ease of maintenance and contributes to extended wheel bearing life.
Benefits of Peterbilt’s PHP-10 •
The elimination of inner capnuts, which simplifies installation and saves weight.
•
Improved wheel centering for a smoother ride and even tread wear.
•
All right-hand thread wheel studs for simplified maintenance and parts stocking.
Oil Seal Sight Glass
Hub-Piloted Wheel Mounting System
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peterbilt NEW essentials – Module 5 Drivelines The driveline is an extremely important link in the drivetrain. Its purpose is to transfer power and speed variations from the transmission to the drive axles. The shaft must be strong enough to handle the maximum low gear torque from the engine as it comes from the transmission. It should also be light in weight and dynamically balanced to prevent excessive noise and vibration. To meet these requirements, a tubular steel shaft with universal crossts and a slip t are used. If the driveline were solidly connected at both ends, the length would remain constant. This is not possible, however, because the shaft must become longer or shorter to accommodate the up-and-down motion of the vehicle’s suspension across uneven road surfaces. This is accomplished with a splined slip t at the front end of the driveshaft where it is attached to the output shaft of the transmission. The front end of the driveshaft has internal splines that match the external splines on the driveshaft body.
Midship Bearing
transmission or axles. The bearing caps allow the yokes to rotate around the cross assembly and also permit the cross assembly to pivot inside the yokes. Driveshaft construction varies according to function and location on the vehicle. A driveshaft may transmit torque to a driving axle directly, or to an auxiliary transmission. Dual driving rear axles are connected by an interaxle driveline. On long-wheelbase vehicles, it may be necessary to use two or more driveshafts ed by an intermediate bearing, more commonly called a midship bearing (or carrier bearing). A midship bearing allows free rotation while maintaining the driveline’s position relative to the transmission and axle. Peterbilt uses drivelines manufactured from high-quality steel tubing.
Since the driveshaft’s front end is solidly mounted to the transmission, the driveshaft, when properly lubricated, will slide in and out of the driveshaft end along the splines as the suspension moves up and down. The ends of the driveshaft must also be able to adjust for the forces acting upon them. They must accommodate power transfer, frame and axle twist, as well as nonparallel t angles and the changes in t angles caused by uneven road surfaces. The universal ts (U-ts) allow the driveshaft ends to adjust for these changing forces by enabling the driveshaft to operate at different angles. A U-t consists of a cross assembly with precision bearing caps at each end and two yokes, one on the driveshaft and the other on the
Slip t at Front of Driveshaft
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peterbilt NEW essentials – Module 5 Rear Axles Rear axles are manufactured in multiple designs for both single and tandem configurations. Peterbilt offers rear axles to fit any application ranging from 21,000 to 46,000 pounds GAWR. Regardless of the axle specified, the basic functions of a rear axle are the same: • the load. •
Retain and position the wheels.
•
Act as a mounting surface for the suspension.
•
Provide for the braking system.
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Encase the differential gearing and axle shafts.
•
Provide power transfer to the ground.
•
Allow a differential action to each side during a turn. 5. The axle shaft side gears are in mesh with the four differential pinion gears.
Differential
6. Since the ring gear is bolted to the differential case, any time the ring gear rotates, the differential case and the spider, along with the four differential pinion gears, must also rotate.
A differential is used in all rear driving axles to vary wheel speed in turns. When a vehicle is driven in a straight line, the wheels rotate at the same speed. When the vehicle negotiates a curve, the outer drive wheel must travel faster to cover a greater distance than that of the inner drive wheel. To allow one wheel to go faster or slower than the other, a differential is required. If no differential were used, the wheels would skid in turns.
7. The two side gears, splined to the axle shafts (one on each side), are free to turn within the differential casing.
Differential Operation A differential works in the following manner: 1. Power from the driveline is transferred to the input yoke of the differential. The input yoke is splined and bolted to a pinion gear. 2. The pinion gear is in constant mesh with the differential ring gear. The ring is bolted to the differential casing. 3. Power is transferred through the differential casing to internal side gears to which the axle shafts are splined; this completes the flow of power to the wheels. 4. A “spider” shaft s four differential pinion gears. Each pinion is free to rotate around the spider shaft on which it is mounted.
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peterbilt NEW essentials – Module 5 Rear Axle Operation
Let’s examine what happens when a vehicle makes a turn a right turn, in this example. During turning, the pinion gears start to rotate freely around their spider shafts. The right axle side gear begins to slow due to tire rolling resistance encountered from turning. The spider pinion gears begin to “walk around” the right side gear, causing the left side gear and axle shaft to speed up. This allows one wheel to turn faster than the other during the turn. The action during a left-hand turn is similar, but starts from the left axle side gear.
The needs of different applications have led to the design of many different rear axle configurations. A vehicle may be equipped with a single rear axle or with a tandem, in which two rear axle assemblies are used. Each type functions in the same basic manner. In this section we will cover the most widely used axles: • Single-reduction axle
When Turning a Tandem Axle
•
Double-reduction axle
•
Two-speed axle
Although they are of a different type than the gears in transmissions, rear axle gears do the same type of work when it comes to gear reduction and torque multiplication. The rear axle has to absorb the torque multiplication sent back from the transmission, multiply it again, change the direction 90 degrees and send it out to the wheels.
All Wheels Rotate at Different Speeds
Axle Differential Compensates for speed variations between each set of axle wheels
Single-Reduction Axle In a standard rear axle, the input or driving gear is called a pinion. The driven gear, or output, is called a ring gear. A single-speed, single-reduction rear axle has one ring and pinion gear set. The axle ratio is determined by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. For example, if a ring gear has thirtynine teeth and the pinion gear has nine teeth, the ratio is 4.33-to-1 (39 divided by 9 = 4.33). Multiple gear sets are available in a variety of ratios that can be tailored to the needs of a particular operation. There can be over fifteen ratios available for a given axle model.
Inter-Axle Differential Compensates for variance of speed between axles
Differential Operation During a Right Turn
Double-Reduction Axle A single-speed, double-reduction axle has two sets of gears in which reduction takes place twice (once through each set of gears). Axles such as this are usually found in heavier-duty applications in which additional strength and torque are required, such as in dump, mixer or off-highway work and also where speed is not needed. There are two methods of obtaining double reduction: a planetary gear arrangement or a hypoid-helical design. With the planetary double-reduction method, the first reduction occurs in the ring and pinion. The ring gear has an additional set of teeth that drive a planetary gear set, where the second reduction takes place. In the hypoid-helical design, the first reduction takes place in the ring and pinion, but the ring gear is mounted on a
Planetary Double-Reduction Gearing
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peterbilt NEW essentials – Module 5 shaft that has a helical drive gear cut into it. This powers the helical-driven gear for the second reduction. Both types also have a variety of ratios available. The axle ratio for a double-reduction axle is determined by multiplying the ratio of the first reduction times the ratio of the second reduction.
Two-Speed Axle Two-speed rear axles are almost identical to singlespeed, double-reduction axles, but the two-speed type incorporates a powered shifting mechanism. This shifting arrangement locks or releases the planetary gear set to provide high and low ranges. In the low range, the planetary gear set works just like the double-reduction, single-speed gear set that it actually is. In the high range, the planetary set is locked up, eliminating the doublereduction feature. The axle becomes a single-speed, single-reduction unit. With a two-speed axle air shift system, the driver controls the range with a shift button located on the top of the gear shift lever. The rear axle can be shifted into low, then high, each time the transmission is shifted, providing two gear ratios for each transmission gear ratio. This allows added flexibility along with increased gear reduction when required.
Inter-axle Differential
valve should be placed in the lock position any time that the vehicle encounters ice, snow, wet surfaces, mud or loose terrain. However, the lockout selector should not be engaged when a wheel is already slipping or spinning. Another purpose of an inter-axle differential is to distribute power to both axles. The inter-axle differential is part of the forward rear axle.
Tandem Axles Tandem rear axle assemblies combine two single axle units with a power divider; they are connected by an inter-axle driveline. Options include single-reduction tandems, twospeed tandems and double-reduction tandems.
Interaxle Differential
Inter-axle Differential The inter-axle differential is a mechanism that allows faster or slower rotation of one axle in relation to the other. This is necessary because tandem axles will rotate at different speeds when the vehicle turns a corner or negotiates uneven road surfaces, or when different or mismatched tire sizes are used. When extra traction is required under adverse road conditions, the inter-axle differential can be locked out through a dash-mounted air-operated control switch. With lockout engaged, the dash-mounted valve is in the “lock” position and the inter-axle differential acts as a solid shaft and does not compensate for differences in axle speed. When the lockout selector is placed in the “unlock” position, the lockout is disengaged and the interaxle differential operates normally. The lockout selector
Power from Driveline
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peterbilt NEW essentials – Module 5 Auxiliary Axles
The power is transferred through the inter-axle differential unit to both the forward and rear axles. This transfer through the power divider unit distributes power more or less equally from the transmission to both axle assemblies. The differential in each drive axle will provide individual differential action to each axle’s left and right wheels.
Auxiliary axles are often specified in addition to the vehicle’s steer axle and rear drive axle(s). All auxiliary axles are non-driving axles; that is, they do not receive power from the engine and therefore do not help move the vehicle down the road. The sole purpose of an auxiliary axle is to increase a vehicle’s legal load capacity. Auxiliary axles may be self-steering and liftable, fixed and liftable, or fixed and non-liftable. Although there are many variations and GAWR’s, an auxiliary axle is generally classified as either a pusher axle or a tag axle.
Earlier we mentioned that rear axles are available in a variety of ratios so it is possible to spec an axle that is best suited to a particular operation. Below is a list of typical rear axle ratios.
Typical Rear Axle Ratios 2.64 2.85 2.93 3.08 3.25
3.36 3.55 3.70 3.90 4.11
4.33 4.56 4.63 4.88 5.29
5.43 5.57 6.17 6.50 7.17
So far, we have examined individual components and individual gear ratios. They work in combination so the performance of each item in the powertrain is compounded. This compounding of the ratios of the individual components results in total gear reduction.
Liftable Pusher Axle
A pusher axle is an auxiliary non-driving axle located in front of the drive axle(s). It can be located close to the rear driving axles or placed some distance ahead. The pusher’s location depends on the customer’s weight distribution needs. A tag axle is an auxiliary axle located behind the driving axle. A tag axle’s spacing is also determined by the customer’s particular operation but is more limited. In most instances, pusher axles are preferred.
Total Gear Reduction Total gear reduction is the maximum reduction attainable from all the components in the powertrain. It is determined by multiplying each ratio by the other. The table below illustrates the process. Transmission Ratio
Axle Ratio
Total Gear Reduction
4.46 x
4.56 =
20.34 to 1
14.78 x
4.56 =
67.40 to 1
With the 4.46/4.56 example, the total reduction is 20.34 to 1. If the engine produces 500 pound-feet of torque with this powertrain, it would be multiplied 20.34 times, which totals 10,170 pound-feet. If the same engine is used in the 14.78/4.56 example, the 500 pound-feet of torque is multiplied 67.40 times, for a total of 33,700 pound-feet. Powertrains that combine high horsepower, high torquerise engines with deep reduction ratios in the transmission and rear axle can create a real “stump puller”.
Liftable Tag Axle
Occasionally a customer will specify a single drive axle with a tag or pusher mounted to a tandem suspension to save weight, or reduce the maintenance and operating costs associated with a tandem drive axle.
, the higher the numerical ratio, the slower the vehicle’s top speed. The lower the numerical ratio, the faster it will run, given the same engine rpm, powertrain and tire size. 14
New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Rear Suspensions Selecting the correct rear suspension for a given application has a direct bearing on the performance, economy and reliability of the vehicle. The rear suspension must perform a variety of functions, including: • the payload of the vehicle. •
Transmit acceleration and braking forces to the frame.
•
Articulate tandem axles for all road conditions.
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Provide an acceptable ride both loaded and unloaded.
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Track properly to provide safe steering.
•
Be easily serviced and maintained. Slipper Type Spring End
Several types of rear suspensions are found on heavyduty vehicles. Each is specifically designed to handle the load requirements of a specific maximum vehicle rating. Overloading – and thus exceeding the maximum suspension load rating – results in damage to the suspension system itself and to other components. Most suspensions systems can be easily identified by their design characteristics, and they can be classified into one of three types: • Spring •
Walking-beam
•
Air
Spring-Type Suspensions Taper Leaf Rear Suspension Spring (Peterbilt Quadraflex)
Suspension springs generally refer to multi-leaf or taper leaf designs. They are commonly attached to the axle housing with U-bolts, nuts and lock washers. Peterbilt rear spring suspensions are mounted to the frame with “slipper” type ends. The spring ends are allowed to slide, or “slip,” in the frame bracket as conditions change. When a load is placed on the vehicle, the spring will deflect and lengthen. This forces the point of spring with the frame mounting bracket to move toward the center of the spring, which stiffens the spring’s rate of deflection. This also allows for compression and rebound of the springs when the road wheels are displaced by varying road surface conditions.
Peterbilt Quadraflex Taper Leaf Suspension The Quadraflex Taper Leaf features a 38,000-pound capacity and unique two-leaf taper leaf springs. High grade spring steel and a specialized manufacturing process allow Peterbilt to use springs that achieve a heavy load capacity with only two spring leaves instead of three. The two contoured leaves of the Quadraflex Taper Leaf rear springs are tapered in thickness so stresses are absorbed uniformly from the thin ends to the thick center areas.
Peterbilt offers a proprietary spring suspension: the Peterbilt Quadraflex Taper Leaf.
Peterbilt Quadraflex Taper Leaf Suspension
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peterbilt NEW essentials – Module 5 Walking-Beam Type Suspensions
Since friction contributes to a “stiff” ride, and since there is less inter-leaf friction in a two-leaf spring than in the conventional three-leaf spring configuration, the two-leaf design provides a more comfortable ride. The design is also light in weight.
Peterbilt offers walking-beam (sometimes referred to as equalizing beam) suspensions rated from 36,000 to over 65,000 pounds. These suspensions lower the center of gravity of the axle loads since the beam is actually below the axle center line. The equalizing beams utilize the lever principle to absorb road shock and bumps; torque rods add to the suspension’s effectiveness by controlling axle torque.
Peterbilt rear spring suspensions feature a simple, lowmaintenance design: • The steel wear pads that isolate frame brackets from spring wear are replaceable and easy to maintain. •
The load equalizer evenly distributes the load between the forward and rear drive axles and prevents the overloading of either axle during road fluctuations. The load equalizer pivots on shear rubber bushings and thus requires no lubrication.
•
Four radius rods transmit axle acceleration and braking forces to the frame. This relieves the springs from these forces and increases the life of the springs.
Walking-Beam Type Suspension [Haul Maxx]
Regardless of road or load conditions, the axles are always parallel, which results in better tire wear, stability and freedom from maintenance. All suspension models of this type use four-point frame mounting to eliminate concentrations of stresses at any one point on the vehicle frame.
Radius Rod (lower center, below spring leaves)
The two beams, one on each side of the vehicle, are connected to the forward and rear drive axles, which helps lateral stability and maintains a constant parallel relationship of the axles. Torque rods are connected from each rear axle to a frame crossmember to prevent axle rotation caused by driving and braking forces. Most walking-beam suspensions employ rubber center bushings at the beam center pivot point to provide weight capacity, and axle-insulating end bushings and connections. Torque rods also are used. None of these bushings require lubrication. Overall, walking-beam suspensions are reliable and perform well in a wide range of heavy-duty on/off highway applications.
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Air Type Suspensions
Standard axle spacing (dimension between the center of the two rear axles) on the Low Air Leaf suspension is 52-inches; axle spacing of 54-inches is available as an option. Optional axle spacing provides customers with the opportunity to maximize payload potential in accordance with local regulations.
Nothing compares to the feel of a truck that is equipped with an air suspension. Customers appreciate the superior ride and enhanced handling from an air suspension. Peterbilt offers a full line of air suspensions, including the Low Air Leaf, the Flex Air, the Air Leaf with Tracking Rods and the Air Trac.
The Low Air Leaf is designed specifically for use with “high cube” general freight trailers or for auto carriers or flat beds, which require a low frame or fifth wheel height. It features a ride height of 19.5-inches, which is approximately 2.5-inches lower than standard. A reduction of about 4-inches can be achieved with low profile tires. An additional 2-inches is available by ordering this suspension as a “low-low” air leaf suspension. This is one of the lowest frame heights in the industry – an advantage that translates into improved ride stability and less concern about over interference.
Low Air Leaf Peterbilt’s Low Air Leaf suspension is available as a dual tandem suspension application on Class 8 and single and dual Class 7 vehicle. It is a low-maintenance suspension that requires no lubrication. It has a rated capacity of 40,000 pounds and a maximum GCW rating of 143,000 pounds. The smooth, controlled ride of this suspension is a result of these features: • A spring eye at the leading end of the leaf spring assembly transmits axle acceleration and braking forces through the leaf springs to the frame. •
A tapered spring leaf distributes bending stresses evenly throughout the leaf, minimizing friction and weight.
•
Air bags mounted to the leaf springs on the side of each axle carry most of the load.
•
Transverse tracking rods are included to control lateral (side-to-side) axle movement and help maintain correct axle alignment.
Flex-Air The Peterbilt Flex-Air suspension is a low-maintenance design that also requires no lubrication. It has a rated capacity of 38,000-pounds and a GVW rating of 143,000-pounds The Flex-Air is the lightest of the industry’s air suspensions. Flex-Air’s design features these components: • Underslung axle drive beams bolt to the axle and provide mounting for the air bag and front spring. This provides the low ride height. •
Transverse tracking rods and parallel radius rods keep the axle positioned for controlled handling and easily maintained axle alignment.
•
The leading semi-elliptical taper leaf spring and air spring enhance ride characteristics and all-around stability.
•
Fewer parts and non-welded axle seats ease service and extend axle life.
•
Flex-Air is designed to provide an excellent ride and superior handling for the driver.
•
Flex-Air is perfect for on-highway customers who need the stability of lower ride height for high cube van trailers.
Features of the Low Air Leaf Suspension
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Air Leaf with Tracking Rods
Air Trac
The Peterbilt Air Leaf with Tracking Rods is another lowmaintenance suspension with a superior ride. The tapered spring leaves are attached to the frame brackets with a spring eye that is bolted to the leaf. Wide air spring brackets (sometimes called “air bag paddles”) the air bags and effectively distribute the axle’s loads. Transverse tracking rods provide additional vehicle stability by helping to maintain axle alignment and by controlling lateral movement of the drive axles. This increases performance and reduces wear on the suspension.
The Air Trac suspension is a rugged and versatile air suspension designed for heavy-duty on- or off-highway applications. The Air Trac provides a superior combination of ride, stability and durability. Equipped with two taper leaf springs, sturdy air bag mounting brackets and transverse tracking rods, the Air Trac is similar to the
This suspension is very effective for all on-highway freight services. The overall durability of its design also allows it to be used in some on/off-highway applications, such as semi end dump tractors. The suspension’s durable design also allows for extensive use of aluminum in the mounting brackets for the leaf springs and air bags. As a result, the suspension is comparable in weight to most four-spring suspensions without sacrificing strength. This provides the customer the benefit of air suspension without the usual weight penalty. 52-inch axle spacing is standard on the Peterbilt Air Leaf with Tracking Rods suspension; optional 54-inch axle spacings are available if required. The suspension has a rated capacity of 38,000 pounds and a maximum GCW rating of 143,000 pounds.
Peterbilt Air Trac Suspension
Air Leaf with Tracking Rods suspension. However, the Air Trac is also designed with longitudinal radius rods to transmit acceleration and braking forces to the frame. The springs use a slipper style front bracket that includes a replaceable wear pad. The slipper spring improves ride characteristics and exerts less stress on the spring. The wear pads can reduce maintenance costs and downtime. This added durability allows the Air Trac to be used for all highway freight applications, as well as for several severe off-highway applications such as logging, refuse dump and low-bed operations. The Air Trac is available in both single and tandem drive axle configurations. Single drive application is available in 20,000- and 23,000-pound capacities, and tandem drive applications in 40,000-, 44,000- and 46,000-pound capacities. Standard axle spacing for the dual drive Air Trac is 52-inches. Optional axle spacings of 54, 60, 65 and 72-inches are also available. The Air Trac has a maximum GCW rating of 125,000 pounds for single axles and 180,000 pounds for tandems.
Peterbilt Air Leaf with Tracking Rods Suspension
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Air Springs
Height Control Valve
The air spring on a Peterbilt air suspension is a rubberized fabric tube. On the Air Leaf with Tracking Rods and the Air Trac, each air spring is equipped with an internal bump block, which protects the air spring and the frame rail in the event of total air loss. The Low Air Leaf uses a framemounted axle stop so the entire inside of the air spring can be used to hold air. This increases the volume in the air spring and improves the ride.
The height control valve (sometimes called the leveling valve) keeps the frame at a constant height in changing load conditions by varying the pressure in the air springs. When a load is applied to the vehicle, the frame lowers; this activates the control valve through a linkage. The control valve allows more air to enter the spring, which raises the frame. When the load is removed, the control valve exhausts air from the air spring, lowering the frame. When load conditions remain constant, the height control lever arm stays in the neutral position. This means the valve is closed and no air can flow into or out of the air spring. The design of the height control valve allows it to maintain correct frame height as well as the required air volume for proper operation of the suspension.
Air Spring Bump Block Piston
Air Spring Cutaway
Height Control Valve
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Power Take-offs A power take-off (PTO) is a mechanical device driven from either the transmission or the engine. It is used to transmit power to another mechanical device, such as a compressor or hydraulic pump. We will discuss a transmission-driven PTO as it would be used with a hydraulic pump system. There are many variations in applications that use PTOs.
Hydraulic Pump (Foreground) Connected to Side-mounted PTO
An inlet and return fluid line run from the pump to the frame-mounted reservoir. A valve on the return line allows hydraulic pressure from the pump to flow to a cylinder, or ram, which operates the various hydraulically driven machinery. The PTO application familiar to most people is one that provides power to a dump body or dump trailer. Other applications include wreckers or tow trucks, aerial bucket applications, fire trucks, refuse packers, liquid and dry bulk delivery tankers and fertilizer or salt spreaders. Transit mixers typically have PTO-driven hydraulic motors powering the mixer drum. The list is nearly endless, and all of these devices have specific requirements for PTO’s and PTO-driven equipment.
Power Tower
A PTO system consists of the PTO unit, a hydraulic pump, a fluid reservoir, valves and the connecting hardware, which includes fluid lines, shift cables, fittings and the driveline. Most heavy-duty transmissions can be used with a PTO. The transmission cover plates, when removed, expose the drive gear of the transmission that will be used to power the PTO unit. Side-mount and bottom-mount locations on the transmission are commonly used. Some customers prefer a top-mount (“power tower”) PTO, but this configuration is rare and is usually used with an auxiliary transmission. The mounting location will primarily determine the pitch line velocity (measurement of PTO application capability) because of different gear tooth and leverage from the driving gear. The PTO controls can be mounted on the dash or on the floor, and can be either electrically or air-operated.
For heavy-duty applications like mixers, PTO’s can be front crankshaft or rear engine driven. Other applications include very heavy off-road mining or oilfield equipment. When the PTO is driven from the front of the crankshaft and the pump or mechanical device is mounted to the front frame/ bumper extension, it is commonly referred to as a frontengine power take-off (FEPTO). When the PTO is driven from the rear of the crankshaft, it is called a rear-engine power take-off (REPTO). Selection factors for a PTO include the following: • Type of equipment.
A pump is connected to the PTO. Some configurations call for an integral pump, which means it is part of the PTO unit itself. Other configurations call for the pump to be mounted on a frame or bumper extension; in these cases a driveline connects the PTO unit and the pump.
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Power required, torque and shaft speed.
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Type of service – continuous or intermittent.
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Output shaft rotation direction.
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Speeds.
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Reversibility.
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System dimensions and clearance.
New Peterbilt Essentials 8/2008
W
p e te rb ilt e sse nt ia ls MO D ULE 5
• Front and Rear Axles • Steering • Front and Rear Suspensions • Drivelines
peterbilt NEW essentials – Module 5 Introduction
How To Use New Peterbilt Essentials
This module will cover axle configurations, front and rear suspension, steering operation, hubs/drums, drivelines and power take-offs
1. Print the module and study the information. To print, click the printer icon on your browser. Highlight material that is new to you, or complex.
The design and placement of axles under a truck frame vary. This module describes common configurations for front and rear axles, how they operate and the rationale for axle placement.
2. When you are ready to take the online test, click the "Begin" button in the "Test" column for the desired module. When the test is completed, it will automatically be scored and the results will be entered in the Peterbilt training records database.
Succeeding modules present more detailed information about major truck operating systems.
3. Upon successful completion of all modules, you will receive a personalized certificate.
It is recommended that you complete these training modules in sequence since each succeeding module builds on the previous module.
Published by Peterbilt Motors Company. This material is intended for Peterbilt training purposes and may not be sold, given, loaned or reproduced, in whole or in part, including photocopying, without the express written consent of Peterbilt Motors Company.
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Front Axle
Axle Configurations
One of the primary functions of a truck’s front axle assembly is to help the weight of the vehicle. The front axle typically carries between 25 percent and 37 percent of the gross vehicle weight (GVW). The amount of weight the front axle carries depends primarily on the axle’s position (set-forward or set-back) and the fifth wheel setting. However, to maximize Federal Bridge Law legal limits, the front axle will carry about 15 percent of the combined weight of the tractor, trailer and cargo. The front axle also serves as a foundation for the truck’s steering components and transmits braking torque to the front suspension springs.
You will often see the designations 4x4, 4x2, 6x4 and 6x6 in connection with truck and tractor configurations. These are indicators of the total number of axles compared to the number of driving axles. The first number indicates the number of “axle ends” (each axle has two ends); the second indicates how many of them are live (driven by the engine). For example, “6x4” means that the truck has three axles (six axle ends) and that two of the axles (four axle ends) are being driven by the engine. A “6x6” designation indicates that all three axles, including the front axle, are being driven by the engine.
Weight Distribution and Axle Position Since weight and the distribution of weight have so much to do with the function of the front axle, it is useful to review some common concepts associated with weight, beginning with gross vehicle weight and gross combination weight. • Gross Vehicle Weight (GVW) is the total weight of a fully equipped truck and payload. •
Gross Combination Weight (GCW) is the total weight of a fully-equipped truck or tractor, trailer or trailers and payload.
As indicated earlier, the front axle carries a significant portion of the gross weight of a tractor-trailer combination or straight truck. How much it actually carries depends in part upon where the axle is positioned relative to the payload. At this point the concepts of the set-forward front axle (SFFA) and the set-back front axle (SBFA) need to be defined. Both are measurements of the distance from the bumper to the centerline of the front axle. • The SFFA on a Peterbilt is about 29- to 31-inches.
The axle center is known as the beam, and is most commonly a forged steel I-beam. Round tubular stock of tempered seamless steel can also be specified for the front axle beam; this is frequently used in applications that involve extremely heavy loads.
•
There are in general two types of front axles: driving or non-driving. A driving axle is one that is receiving power from the engine to provide vehicle motion. Except for some off-road or extremely heavy-duty applications, most front axles are non-driving.
The SBFA on a Peterbilt ranges from 47- to 51-inches.
Since the location of the front axle affects the length of the wheelbase, it also affects the percentage of weight distributed to the axle. With a set-forward axle, less weight will be transferred to the front axle. With a set-back axle, more weight will be transferred to the front axle. A set-back axle usually helps maximize loads, because more weight will be distributed to the front axle. On the other hand, a set-forward axle helps distribute loads when the possibility of overloading the front axle exists.
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Axle Ratings
Front Drive Axles
Front axles are rated according to the weight they can carry. The axle rating a truck owner selects will depend on GVW or GCW. Peterbilt offers non-driving axles rated from 10,000 to 22,000 pounds. Drive axles rated from 12,000 to 23,000 pounds are also available. It’s important to , however, that the load capacity of an installed front axle is determined by the Gross Axle Weight Rating (GAWR) rather than by the rating of the axle alone. Gross Axle Weight Rating is the rated capacity of the suspension system and front axle components, which include the springs, tires, steering gear, and wheels in addition to the axle. The lowest-rated of these weight-sensitive components will determine the GAWR.
Steerable drive axles are typically used on heavyduty on/off-road vehicles that require greater traction and maneuverability than can be obtained from a 6x4 configuration. A transfer case remotely mounted to the transmission is commonly used to drive a front drive axle. The transfer case is designed with front and rear drivelines that provide torque to the front and rear axles respectively.
The following components are considered in determining Gross Axle Weight Rating.
Component
Weight Rating
Axle
12,000 pounds
Suspension
12,000 pounds
Wheels
12,080 pounds
Tires
10,860 pounds
Steering Gear
12,000 pounds
Front Drive Axle
As another choice on trucks with steerable drive axles, Peterbilt can also install an all-wheel drive system instead of a transfer case. The all-wheel drive system is a power divider that is often referred to as a “spaghetti drive” because of the number and configuration of the drivelines. The system’s design calls for a driveline from the transmission to the rear axle and then an additional driveline from the rear axle to the front axle.
Since the GAWR cannot exceed the capacity of the lowest-rated component, it corresponds in this case to the rating of the tires: GAWR = 10,860 pounds.
“Spaghetti Drive”: the Dana Spicer All-Wheel Drive System
Whether a transfer case or a spaghetti drive is used, torque is distributed to the front wheels in the same proportion as to the rear wheels by means of a differential. Like a rear drive axle, a front drive axle must provide a sufficient range for all operating speeds; this means that the front drive axle must have the same gear reduction capabilities as the rear axle. 4
New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Front Axle Steering Since the front axle is essential to the steering of the vehicle, design and maintenance of the front axle and the steering components attached to it are major considerations in determining steering ease and tire wear. Peterbilt vehicles are standard with power steering, which means that steering is assisted by means of a hydraulic pump that circulates fluid from a reservoir through the steering gear and back. An engine-mounted two-quart fluid reservoir is standard, but to ensure optimum performance by maintaining proper fluid temperatures, the size of the reservoir is designed to increase as the front axle GAWR of the vehicle increases. For GAWRs from 16,000 to 20,000 pounds, Peterbilt offers a twelve-quart reservoir.
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The rack piston’s movement, in mesh with gears on the sector (steering gear output) shaft, causes the sector shaft to rotate left or right.
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The steering gear output shaft or sector shaft, working through the pitman arm and drag link, pushes or pulls the steering arm, which rotates the wheel assembly on the knuckle pin (or “king pin”).
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The Ackermann arm transmits torque to the tie-rod assembly, which transfers the torque to the opposite Ackermann arm, which then turns the opposite wheel.
STEERING KNUCKLE
Steering Gear Operation Here is how the steering system works: • When the steering wheel is turned, force is transmitted through the intermediate shafts to the steering gear input shaft. Resistance causes the steering gear’s internal torsion bar to be twisted by the input shaft.
Ackermann arm
Axle Beam with Steering Components (Left Side of Vehicle)
Steering Geometry (Alignment) The alignment that keeps a truck rolling efficiently both straight ahead and through curves is achieved through a balance of several factors: • Caster
Typical Steering Linkage
•
The twisting of the torsion bar actuates a control valve, which allows pressurized fluid to enter one end of the rack piston cylinder. The end of the cylinder that the fluid enters is determined by the direction the wheel was turned; the control valve also permits open flow from the opposite end to aid in the cooling of the fluid. The pressure of the fluid against the rack piston acts to reduce the amount of steering effort required to move the rack piston along the input shaft.
•
Camber
•
Ackermann geometry
•
Toe-in/Toe-out
The following is a brief discussion of each of these properties and how they are related to the operation of a Peterbilt truck.
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Caster positive Camber
Caster is the fore or aft tilt of the top of the steering knuckle pin as it might be viewed from the side of the vehicle. “Positive” caster is the tilt of the top of the knuckle pin toward the rear of the vehicle; negative caster is the tilt of the pin toward the front of the vehicle. Peterbilt uses
Negative Camber
Vertical C/L
Illustration of Camber
nonadjustable setting. Camber can be adjusted only by bending the axle beam, which is not recommended.
Ackermann Geometry Illustration of Caster
While a truck is moving straight ahead, the front wheels should be tracking parallel to each other. However, when the vehicle encounters a curve in the road, parallel operation of the wheels would cause one tire to side-slip because the wheels would be forced to rotate around circles of different diameters. To ensure that the inner
a caster of +4 degrees to maintain steering stability and steering return to center. Caster angle is determined by the installed position of the steer axle; it can be adjusted by inserting wedge-shaped shims between the front suspension springs and the front axle beam (or the spacer block if the truck is equipped with one). Although incorrect caster adjustment has a negligible effect on tire wear, it may affect steering effort and stability. A greater positive caster angle than is specified may result in excessive steering effort. A smaller caster angle than is specified may result in vehicle wander or poor steering return to center.
ackermann arm
Camber The vertical tilt of the wheel as it might be seen from the front of the truck is called camber. Positive camber is an outward tilt of the wheel at the top; negative camber is an inward tilt of the wheel at the top. A front axle will deflect slightly under a load. To offset this deflection and bring the axis of the knuckle pin closer to the point of the road and the center line of the tire, a small amount of positive camber is designed into the axle.
tie rod
Axle, Ackermann Arms and Tie Rod
Excessive positive camber can result in wander, steering difficulty and abnormal wear on the outer area of the tire. Excessive negative camber can cause inside tire wear.
wheel will always turn through a shorter circumference than the outer wheel, each Ackermann arm (or “tie-rod arm”) is positioned at an angle to the tie rod. This angle is
Camber is a condition that is machined into the axle by Peterbilt and is generally considered a permanent, 6
New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Wheel Cut and Turning Radius
determined by the tire width and the wheelbase. During turning, the geometry of the Ackermann arms causes the angle of the inner wheel to become greater and the angle of the outer wheel to become smaller. Because Peterbilt offers a large number of wheelbase choices, Ackermann arm options are selected to minimize tire side-slip.
Wheel cut determines a vehicle’s turning radius. Since larger wheel cut angles imply improved maneuverability, wheel cut figures are often quoted in promotional materials. “Inside” wheel cut angles (which are numerically greater) often are the ones quoted, but it is actually the outside tire that turns the vehicle. The significant wheel cut angle, then, is the one for the outside wheel in each turn direction. Factors influencing wheel cut angles include the sizes of the tires and the wheels, and the width (or track) of the front axle. All of these factors affect how far the tire can be turned before it touches a chassis component, which is often the limiting factor.
Wheel Toe-in/Toe-out “Toe” is the relationship of the distance between the front of the front tires and the rear of the front tires. When the distance at the front is smaller than the distance in the rear, the wheels are said to be “toed-in”. When the distance in the rear is smaller, the wheels are “toed-out.” “Zero toe” is the condition of the wheels when they are parallel. Because of the forces acting on them, a truck’s front tires tend to toe-out as the vehicle travels down the road. If the toe setting of the wheels were zero, excess rolling resistance, tire wear and vehicle wander would likely result. Therefore, wheels should be toed-in about one-eighth of an inch. front view
toe-in
Turning Radius: The Significant Wheel Cut is the One for the Outside Wheel
front view
With some exceptions, a Peterbilt with identical specs to a vehicle made by another OEM will have a greater wheel cut, in part because of the Peterbilt’s steering component design. The position of the steering gear is a good example of this. On a Peterbilt, the steering gear is located forward of the axle, where it won’t obstruct the path of the wheel. This design increases the angle that the wheel can be turned.
toe-out
Toe-in and Toe-out
Turning radius is the arc described by the center of the track of the outside front wheel in the tightest turn a truck can make. The tightest turn is described by the angle of the outside wheel when the inside front wheel reaches maximum wheel cut. There are 2 turning radius measurements: • Curb-to-Curb – the arc described by the front wheel. •
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Wall-to-Wall – the arc described by the outside edge of the bumper or fender. New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Front Suspensions
Peterbilt Split Progressive Springs
Peterbilt offers several front leaf suspension systems in various capacities; all are designed to achieve optimum ride characteristics under varying operating loads and applications. The 12,000-pound suspensions are offered with a choice of springs: •
Peterbilt taper leaf
•
Peterbilt split progressive leaf
Peterbilt split progressive springs are rated at 23,000 pounds. This suspension provides a comfortable ride with minimal maintenance for a range of heavy loads. The unique design minimizes friction by using only the number of springs required to carry the load at a given time. The lower leaves act as “helper” springs, becoming active only as the spring pack flattens under the weight of a load. This design provides a smooth ride when the vehicle is lightly loaded and allows a gradual stiffening of the ride as the load is increased.
Peterbilt Taper Leaf Springs Tightly clamped front leaf spring assemblies used on many trucks typically have substantial friction between the leaves. This tends to cause stiffness of ride quality, especially without a load. Since the friction between spring leaves is a major contributor to a stiff ride, each Peterbilt taper leaf suspension uses only two or three contoured leaves: fewer leaves – less friction. The thickness of each leaf is tapered so that it can absorb stresses uniformly from its thin ends to its thick center area. This creates a smooth ride under light loads and also heavy loads. The unique two-leaf design of the Peterbilt suspension also reduces weight, which can translate into greater payload potential.
Peterbilt Split Progressive Springs
Peterbilt Taper Leaf Springs
Peterbilt offers taper leaf springs rated at (10,000 only on medium duty), 12,000, 14,600, 16,000, 18,000, 20,000 and 23,000 pounds. All taper leaf springs include tubular shock absorbers for positive rebound control.
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Hubs/Drums
Oil Seals
A hub-piloted wheel mounting system (PHP-10) is standard on Peterbilt trucks. Hub-piloted wheels are centered around the hub with close tolerances between the wheel center and machined surfaces on the hub. This hub-pilot mounting system is more effective at centering the wheel, resulting in a smoother ride and better tire wear. There are also fewer parts in a hub-piloted mounting system.
Peterbilt installs oil seals and sight glasses on all non-driving axles – a feature that makes for ease of maintenance and contributes to extended wheel bearing life.
Benefits of Peterbilt’s PHP-10 •
The elimination of inner capnuts, which simplifies installation and saves weight.
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Improved wheel centering for a smoother ride and even tread wear.
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All right-hand thread wheel studs for simplified maintenance and parts stocking.
Oil Seal Sight Glass
Hub-Piloted Wheel Mounting System
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Drivelines The driveline is an extremely important link in the drivetrain. Its purpose is to transfer power and speed variations from the transmission to the drive axles. The shaft must be strong enough to handle the maximum low gear torque from the engine as it comes from the transmission. It should also be light in weight and dynamically balanced to prevent excessive noise and vibration. To meet these requirements, a tubular steel shaft with universal crossts and a slip t are used. If the driveline were solidly connected at both ends, the length would remain constant. This is not possible, however, because the shaft must become longer or shorter to accommodate the up-and-down motion of the vehicle’s suspension across uneven road surfaces. This is accomplished with a splined slip t at the front end of the driveshaft where it is attached to the output shaft of the transmission. The front end of the driveshaft has internal splines that match the external splines on the driveshaft body.
Midship Bearing
transmission or axles. The bearing caps allow the yokes to rotate around the cross assembly and also permit the cross assembly to pivot inside the yokes. Driveshaft construction varies according to function and location on the vehicle. A driveshaft may transmit torque to a driving axle directly, or to an auxiliary transmission. Dual driving rear axles are connected by an interaxle driveline. On long-wheelbase vehicles, it may be necessary to use two or more driveshafts ed by an intermediate bearing, more commonly called a midship bearing (or carrier bearing). A midship bearing allows free rotation while maintaining the driveline’s position relative to the transmission and axle. Peterbilt uses drivelines manufactured from high-quality steel tubing.
Since the driveshaft’s front end is solidly mounted to the transmission, the driveshaft, when properly lubricated, will slide in and out of the driveshaft end along the splines as the suspension moves up and down. The ends of the driveshaft must also be able to adjust for the forces acting upon them. They must accommodate power transfer, frame and axle twist, as well as nonparallel t angles and the changes in t angles caused by uneven road surfaces. The universal ts (U-ts) allow the driveshaft ends to adjust for these changing forces by enabling the driveshaft to operate at different angles. A U-t consists of a cross assembly with precision bearing caps at each end and two yokes, one on the driveshaft and the other on the
Slip t at Front of Driveshaft
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Rear Axles Rear axles are manufactured in multiple designs for both single and tandem configurations. Peterbilt offers rear axles to fit any application ranging from 21,000 to 46,000 pounds GAWR. Regardless of the axle specified, the basic functions of a rear axle are the same: • the load. •
Retain and position the wheels.
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Act as a mounting surface for the suspension.
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Provide for the braking system.
•
Encase the differential gearing and axle shafts.
•
Provide power transfer to the ground.
•
Allow a differential action to each side during a turn. 5. The axle shaft side gears are in mesh with the four differential pinion gears.
Differential
6. Since the ring gear is bolted to the differential case, any time the ring gear rotates, the differential case and the spider, along with the four differential pinion gears, must also rotate.
A differential is used in all rear driving axles to vary wheel speed in turns. When a vehicle is driven in a straight line, the wheels rotate at the same speed. When the vehicle negotiates a curve, the outer drive wheel must travel faster to cover a greater distance than that of the inner drive wheel. To allow one wheel to go faster or slower than the other, a differential is required. If no differential were used, the wheels would skid in turns.
7. The two side gears, splined to the axle shafts (one on each side), are free to turn within the differential casing.
Differential Operation A differential works in the following manner: 1. Power from the driveline is transferred to the input yoke of the differential. The input yoke is splined and bolted to a pinion gear. 2. The pinion gear is in constant mesh with the differential ring gear. The ring is bolted to the differential casing. 3. Power is transferred through the differential casing to internal side gears to which the axle shafts are splined; this completes the flow of power to the wheels. 4. A “spider” shaft s four differential pinion gears. Each pinion is free to rotate around the spider shaft on which it is mounted.
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Rear Axle Operation
Let’s examine what happens when a vehicle makes a turn a right turn, in this example. During turning, the pinion gears start to rotate freely around their spider shafts. The right axle side gear begins to slow due to tire rolling resistance encountered from turning. The spider pinion gears begin to “walk around” the right side gear, causing the left side gear and axle shaft to speed up. This allows one wheel to turn faster than the other during the turn. The action during a left-hand turn is similar, but starts from the left axle side gear.
The needs of different applications have led to the design of many different rear axle configurations. A vehicle may be equipped with a single rear axle or with a tandem, in which two rear axle assemblies are used. Each type functions in the same basic manner. In this section we will cover the most widely used axles: • Single-reduction axle
When Turning a Tandem Axle
•
Double-reduction axle
•
Two-speed axle
Although they are of a different type than the gears in transmissions, rear axle gears do the same type of work when it comes to gear reduction and torque multiplication. The rear axle has to absorb the torque multiplication sent back from the transmission, multiply it again, change the direction 90 degrees and send it out to the wheels.
All Wheels Rotate at Different Speeds
Axle Differential Compensates for speed variations between each set of axle wheels
Single-Reduction Axle In a standard rear axle, the input or driving gear is called a pinion. The driven gear, or output, is called a ring gear. A single-speed, single-reduction rear axle has one ring and pinion gear set. The axle ratio is determined by dividing the number of teeth on the ring gear by the number of teeth on the pinion gear. For example, if a ring gear has thirtynine teeth and the pinion gear has nine teeth, the ratio is 4.33-to-1 (39 divided by 9 = 4.33). Multiple gear sets are available in a variety of ratios that can be tailored to the needs of a particular operation. There can be over fifteen ratios available for a given axle model.
Inter-Axle Differential Compensates for variance of speed between axles
Differential Operation During a Right Turn
Double-Reduction Axle A single-speed, double-reduction axle has two sets of gears in which reduction takes place twice (once through each set of gears). Axles such as this are usually found in heavier-duty applications in which additional strength and torque are required, such as in dump, mixer or off-highway work and also where speed is not needed. There are two methods of obtaining double reduction: a planetary gear arrangement or a hypoid-helical design. With the planetary double-reduction method, the first reduction occurs in the ring and pinion. The ring gear has an additional set of teeth that drive a planetary gear set, where the second reduction takes place. In the hypoid-helical design, the first reduction takes place in the ring and pinion, but the ring gear is mounted on a
Planetary Double-Reduction Gearing
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 shaft that has a helical drive gear cut into it. This powers the helical-driven gear for the second reduction. Both types also have a variety of ratios available. The axle ratio for a double-reduction axle is determined by multiplying the ratio of the first reduction times the ratio of the second reduction.
Two-Speed Axle Two-speed rear axles are almost identical to singlespeed, double-reduction axles, but the two-speed type incorporates a powered shifting mechanism. This shifting arrangement locks or releases the planetary gear set to provide high and low ranges. In the low range, the planetary gear set works just like the double-reduction, single-speed gear set that it actually is. In the high range, the planetary set is locked up, eliminating the doublereduction feature. The axle becomes a single-speed, single-reduction unit. With a two-speed axle air shift system, the driver controls the range with a shift button located on the top of the gear shift lever. The rear axle can be shifted into low, then high, each time the transmission is shifted, providing two gear ratios for each transmission gear ratio. This allows added flexibility along with increased gear reduction when required.
Inter-axle Differential
valve should be placed in the lock position any time that the vehicle encounters ice, snow, wet surfaces, mud or loose terrain. However, the lockout selector should not be engaged when a wheel is already slipping or spinning. Another purpose of an inter-axle differential is to distribute power to both axles. The inter-axle differential is part of the forward rear axle.
Tandem Axles Tandem rear axle assemblies combine two single axle units with a power divider; they are connected by an inter-axle driveline. Options include single-reduction tandems, twospeed tandems and double-reduction tandems.
Interaxle Differential
Inter-axle Differential The inter-axle differential is a mechanism that allows faster or slower rotation of one axle in relation to the other. This is necessary because tandem axles will rotate at different speeds when the vehicle turns a corner or negotiates uneven road surfaces, or when different or mismatched tire sizes are used. When extra traction is required under adverse road conditions, the inter-axle differential can be locked out through a dash-mounted air-operated control switch. With lockout engaged, the dash-mounted valve is in the “lock” position and the inter-axle differential acts as a solid shaft and does not compensate for differences in axle speed. When the lockout selector is placed in the “unlock” position, the lockout is disengaged and the interaxle differential operates normally. The lockout selector
Power from Driveline
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Auxiliary Axles
The power is transferred through the inter-axle differential unit to both the forward and rear axles. This transfer through the power divider unit distributes power more or less equally from the transmission to both axle assemblies. The differential in each drive axle will provide individual differential action to each axle’s left and right wheels.
Auxiliary axles are often specified in addition to the vehicle’s steer axle and rear drive axle(s). All auxiliary axles are non-driving axles; that is, they do not receive power from the engine and therefore do not help move the vehicle down the road. The sole purpose of an auxiliary axle is to increase a vehicle’s legal load capacity. Auxiliary axles may be self-steering and liftable, fixed and liftable, or fixed and non-liftable. Although there are many variations and GAWR’s, an auxiliary axle is generally classified as either a pusher axle or a tag axle.
Earlier we mentioned that rear axles are available in a variety of ratios so it is possible to spec an axle that is best suited to a particular operation. Below is a list of typical rear axle ratios.
Typical Rear Axle Ratios 2.64 2.85 2.93 3.08 3.25
3.36 3.55 3.70 3.90 4.11
4.33 4.56 4.63 4.88 5.29
5.43 5.57 6.17 6.50 7.17
So far, we have examined individual components and individual gear ratios. They work in combination so the performance of each item in the powertrain is compounded. This compounding of the ratios of the individual components results in total gear reduction.
Liftable Pusher Axle
A pusher axle is an auxiliary non-driving axle located in front of the drive axle(s). It can be located close to the rear driving axles or placed some distance ahead. The pusher’s location depends on the customer’s weight distribution needs. A tag axle is an auxiliary axle located behind the driving axle. A tag axle’s spacing is also determined by the customer’s particular operation but is more limited. In most instances, pusher axles are preferred.
Total Gear Reduction Total gear reduction is the maximum reduction attainable from all the components in the powertrain. It is determined by multiplying each ratio by the other. The table below illustrates the process. Transmission Ratio
Axle Ratio
Total Gear Reduction
4.46 x
4.56 =
20.34 to 1
14.78 x
4.56 =
67.40 to 1
With the 4.46/4.56 example, the total reduction is 20.34 to 1. If the engine produces 500 pound-feet of torque with this powertrain, it would be multiplied 20.34 times, which totals 10,170 pound-feet. If the same engine is used in the 14.78/4.56 example, the 500 pound-feet of torque is multiplied 67.40 times, for a total of 33,700 pound-feet. Powertrains that combine high horsepower, high torquerise engines with deep reduction ratios in the transmission and rear axle can create a real “stump puller”.
Liftable Tag Axle
Occasionally a customer will specify a single drive axle with a tag or pusher mounted to a tandem suspension to save weight, or reduce the maintenance and operating costs associated with a tandem drive axle.
, the higher the numerical ratio, the slower the vehicle’s top speed. The lower the numerical ratio, the faster it will run, given the same engine rpm, powertrain and tire size. 14
New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Rear Suspensions Selecting the correct rear suspension for a given application has a direct bearing on the performance, economy and reliability of the vehicle. The rear suspension must perform a variety of functions, including: • the payload of the vehicle. •
Transmit acceleration and braking forces to the frame.
•
Articulate tandem axles for all road conditions.
•
Provide an acceptable ride both loaded and unloaded.
•
Track properly to provide safe steering.
•
Be easily serviced and maintained. Slipper Type Spring End
Several types of rear suspensions are found on heavyduty vehicles. Each is specifically designed to handle the load requirements of a specific maximum vehicle rating. Overloading – and thus exceeding the maximum suspension load rating – results in damage to the suspension system itself and to other components. Most suspensions systems can be easily identified by their design characteristics, and they can be classified into one of three types: • Spring •
Walking-beam
•
Air
Spring-Type Suspensions Taper Leaf Rear Suspension Spring (Peterbilt Quadraflex)
Suspension springs generally refer to multi-leaf or taper leaf designs. They are commonly attached to the axle housing with U-bolts, nuts and lock washers. Peterbilt rear spring suspensions are mounted to the frame with “slipper” type ends. The spring ends are allowed to slide, or “slip,” in the frame bracket as conditions change. When a load is placed on the vehicle, the spring will deflect and lengthen. This forces the point of spring with the frame mounting bracket to move toward the center of the spring, which stiffens the spring’s rate of deflection. This also allows for compression and rebound of the springs when the road wheels are displaced by varying road surface conditions.
Peterbilt Quadraflex Taper Leaf Suspension The Quadraflex Taper Leaf features a 38,000-pound capacity and unique two-leaf taper leaf springs. High grade spring steel and a specialized manufacturing process allow Peterbilt to use springs that achieve a heavy load capacity with only two spring leaves instead of three. The two contoured leaves of the Quadraflex Taper Leaf rear springs are tapered in thickness so stresses are absorbed uniformly from the thin ends to the thick center areas.
Peterbilt offers a proprietary spring suspension: the Peterbilt Quadraflex Taper Leaf.
Peterbilt Quadraflex Taper Leaf Suspension
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Walking-Beam Type Suspensions
Since friction contributes to a “stiff” ride, and since there is less inter-leaf friction in a two-leaf spring than in the conventional three-leaf spring configuration, the two-leaf design provides a more comfortable ride. The design is also light in weight.
Peterbilt offers walking-beam (sometimes referred to as equalizing beam) suspensions rated from 36,000 to over 65,000 pounds. These suspensions lower the center of gravity of the axle loads since the beam is actually below the axle center line. The equalizing beams utilize the lever principle to absorb road shock and bumps; torque rods add to the suspension’s effectiveness by controlling axle torque.
Peterbilt rear spring suspensions feature a simple, lowmaintenance design: • The steel wear pads that isolate frame brackets from spring wear are replaceable and easy to maintain. •
The load equalizer evenly distributes the load between the forward and rear drive axles and prevents the overloading of either axle during road fluctuations. The load equalizer pivots on shear rubber bushings and thus requires no lubrication.
•
Four radius rods transmit axle acceleration and braking forces to the frame. This relieves the springs from these forces and increases the life of the springs.
Walking-Beam Type Suspension [Haul Maxx]
Regardless of road or load conditions, the axles are always parallel, which results in better tire wear, stability and freedom from maintenance. All suspension models of this type use four-point frame mounting to eliminate concentrations of stresses at any one point on the vehicle frame.
Radius Rod (lower center, below spring leaves)
The two beams, one on each side of the vehicle, are connected to the forward and rear drive axles, which helps lateral stability and maintains a constant parallel relationship of the axles. Torque rods are connected from each rear axle to a frame crossmember to prevent axle rotation caused by driving and braking forces. Most walking-beam suspensions employ rubber center bushings at the beam center pivot point to provide weight capacity, and axle-insulating end bushings and connections. Torque rods also are used. None of these bushings require lubrication. Overall, walking-beam suspensions are reliable and perform well in a wide range of heavy-duty on/off highway applications.
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Air Type Suspensions
Standard axle spacing (dimension between the center of the two rear axles) on the Low Air Leaf suspension is 52-inches; axle spacing of 54-inches is available as an option. Optional axle spacing provides customers with the opportunity to maximize payload potential in accordance with local regulations.
Nothing compares to the feel of a truck that is equipped with an air suspension. Customers appreciate the superior ride and enhanced handling from an air suspension. Peterbilt offers a full line of air suspensions, including the Low Air Leaf, the Flex Air, the Air Leaf with Tracking Rods and the Air Trac.
The Low Air Leaf is designed specifically for use with “high cube” general freight trailers or for auto carriers or flat beds, which require a low frame or fifth wheel height. It features a ride height of 19.5-inches, which is approximately 2.5-inches lower than standard. A reduction of about 4-inches can be achieved with low profile tires. An additional 2-inches is available by ordering this suspension as a “low-low” air leaf suspension. This is one of the lowest frame heights in the industry – an advantage that translates into improved ride stability and less concern about over interference.
Low Air Leaf Peterbilt’s Low Air Leaf suspension is available as a dual tandem suspension application on Class 8 and single and dual Class 7 vehicle. It is a low-maintenance suspension that requires no lubrication. It has a rated capacity of 40,000 pounds and a maximum GCW rating of 143,000 pounds. The smooth, controlled ride of this suspension is a result of these features: • A spring eye at the leading end of the leaf spring assembly transmits axle acceleration and braking forces through the leaf springs to the frame. •
A tapered spring leaf distributes bending stresses evenly throughout the leaf, minimizing friction and weight.
•
Air bags mounted to the leaf springs on the side of each axle carry most of the load.
•
Transverse tracking rods are included to control lateral (side-to-side) axle movement and help maintain correct axle alignment.
Flex-Air The Peterbilt Flex-Air suspension is a low-maintenance design that also requires no lubrication. It has a rated capacity of 38,000-pounds and a GVW rating of 143,000-pounds The Flex-Air is the lightest of the industry’s air suspensions. Flex-Air’s design features these components: • Underslung axle drive beams bolt to the axle and provide mounting for the air bag and front spring. This provides the low ride height. •
Transverse tracking rods and parallel radius rods keep the axle positioned for controlled handling and easily maintained axle alignment.
•
The leading semi-elliptical taper leaf spring and air spring enhance ride characteristics and all-around stability.
•
Fewer parts and non-welded axle seats ease service and extend axle life.
•
Flex-Air is designed to provide an excellent ride and superior handling for the driver.
•
Flex-Air is perfect for on-highway customers who need the stability of lower ride height for high cube van trailers.
Features of the Low Air Leaf Suspension
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Air Leaf with Tracking Rods
Air Trac
The Peterbilt Air Leaf with Tracking Rods is another lowmaintenance suspension with a superior ride. The tapered spring leaves are attached to the frame brackets with a spring eye that is bolted to the leaf. Wide air spring brackets (sometimes called “air bag paddles”) the air bags and effectively distribute the axle’s loads. Transverse tracking rods provide additional vehicle stability by helping to maintain axle alignment and by controlling lateral movement of the drive axles. This increases performance and reduces wear on the suspension.
The Air Trac suspension is a rugged and versatile air suspension designed for heavy-duty on- or off-highway applications. The Air Trac provides a superior combination of ride, stability and durability. Equipped with two taper leaf springs, sturdy air bag mounting brackets and transverse tracking rods, the Air Trac is similar to the
This suspension is very effective for all on-highway freight services. The overall durability of its design also allows it to be used in some on/off-highway applications, such as semi end dump tractors. The suspension’s durable design also allows for extensive use of aluminum in the mounting brackets for the leaf springs and air bags. As a result, the suspension is comparable in weight to most four-spring suspensions without sacrificing strength. This provides the customer the benefit of air suspension without the usual weight penalty. 52-inch axle spacing is standard on the Peterbilt Air Leaf with Tracking Rods suspension; optional 54-inch axle spacings are available if required. The suspension has a rated capacity of 38,000 pounds and a maximum GCW rating of 143,000 pounds.
Peterbilt Air Trac Suspension
Air Leaf with Tracking Rods suspension. However, the Air Trac is also designed with longitudinal radius rods to transmit acceleration and braking forces to the frame. The springs use a slipper style front bracket that includes a replaceable wear pad. The slipper spring improves ride characteristics and exerts less stress on the spring. The wear pads can reduce maintenance costs and downtime. This added durability allows the Air Trac to be used for all highway freight applications, as well as for several severe off-highway applications such as logging, refuse dump and low-bed operations. The Air Trac is available in both single and tandem drive axle configurations. Single drive application is available in 20,000- and 23,000-pound capacities, and tandem drive applications in 40,000-, 44,000- and 46,000-pound capacities. Standard axle spacing for the dual drive Air Trac is 52-inches. Optional axle spacings of 54, 60, 65 and 72-inches are also available. The Air Trac has a maximum GCW rating of 125,000 pounds for single axles and 180,000 pounds for tandems.
Peterbilt Air Leaf with Tracking Rods Suspension
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Air Springs
Height Control Valve
The air spring on a Peterbilt air suspension is a rubberized fabric tube. On the Air Leaf with Tracking Rods and the Air Trac, each air spring is equipped with an internal bump block, which protects the air spring and the frame rail in the event of total air loss. The Low Air Leaf uses a framemounted axle stop so the entire inside of the air spring can be used to hold air. This increases the volume in the air spring and improves the ride.
The height control valve (sometimes called the leveling valve) keeps the frame at a constant height in changing load conditions by varying the pressure in the air springs. When a load is applied to the vehicle, the frame lowers; this activates the control valve through a linkage. The control valve allows more air to enter the spring, which raises the frame. When the load is removed, the control valve exhausts air from the air spring, lowering the frame. When load conditions remain constant, the height control lever arm stays in the neutral position. This means the valve is closed and no air can flow into or out of the air spring. The design of the height control valve allows it to maintain correct frame height as well as the required air volume for proper operation of the suspension.
Air Spring Bump Block Piston
Air Spring Cutaway
Height Control Valve
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New Peterbilt Essentials 8/2008
peterbilt NEW essentials – Module 5 Power Take-offs A power take-off (PTO) is a mechanical device driven from either the transmission or the engine. It is used to transmit power to another mechanical device, such as a compressor or hydraulic pump. We will discuss a transmission-driven PTO as it would be used with a hydraulic pump system. There are many variations in applications that use PTOs.
Hydraulic Pump (Foreground) Connected to Side-mounted PTO
An inlet and return fluid line run from the pump to the frame-mounted reservoir. A valve on the return line allows hydraulic pressure from the pump to flow to a cylinder, or ram, which operates the various hydraulically driven machinery. The PTO application familiar to most people is one that provides power to a dump body or dump trailer. Other applications include wreckers or tow trucks, aerial bucket applications, fire trucks, refuse packers, liquid and dry bulk delivery tankers and fertilizer or salt spreaders. Transit mixers typically have PTO-driven hydraulic motors powering the mixer drum. The list is nearly endless, and all of these devices have specific requirements for PTO’s and PTO-driven equipment.
Power Tower
A PTO system consists of the PTO unit, a hydraulic pump, a fluid reservoir, valves and the connecting hardware, which includes fluid lines, shift cables, fittings and the driveline. Most heavy-duty transmissions can be used with a PTO. The transmission cover plates, when removed, expose the drive gear of the transmission that will be used to power the PTO unit. Side-mount and bottom-mount locations on the transmission are commonly used. Some customers prefer a top-mount (“power tower”) PTO, but this configuration is rare and is usually used with an auxiliary transmission. The mounting location will primarily determine the pitch line velocity (measurement of PTO application capability) because of different gear tooth and leverage from the driving gear. The PTO controls can be mounted on the dash or on the floor, and can be either electrically or air-operated.
For heavy-duty applications like mixers, PTO’s can be front crankshaft or rear engine driven. Other applications include very heavy off-road mining or oilfield equipment. When the PTO is driven from the front of the crankshaft and the pump or mechanical device is mounted to the front frame/ bumper extension, it is commonly referred to as a frontengine power take-off (FEPTO). When the PTO is driven from the rear of the crankshaft, it is called a rear-engine power take-off (REPTO). Selection factors for a PTO include the following: • Type of equipment.
A pump is connected to the PTO. Some configurations call for an integral pump, which means it is part of the PTO unit itself. Other configurations call for the pump to be mounted on a frame or bumper extension; in these cases a driveline connects the PTO unit and the pump.
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•
Power required, torque and shaft speed.
•
Type of service – continuous or intermittent.
•
Output shaft rotation direction.
•
Speeds.
•
Reversibility.
•
System dimensions and clearance.
New Peterbilt Essentials 8/2008
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