Steering and Alignment

Steering and Alignment
Chapter 25 Steering and Alignment

Objectives (1 of 2) Identify the components of the steering system of a heavy-duty truck. Describe the procedure for inspecting front axle components for wear. Explain how toe, camber, caster, axle inclination, turning radius, and axle alignment affect tire wear, directional stability, and handling. Describe the components and operation of a worm and sector shaft and a recirculating ball-type steering gear. Explain how to check and adjust a manual steering gear preload and backlash.

Objectives (2 of 2) Identify the components of a power steering gear and pump and explain the operation of a power steering system. Describe the components and operation of a pneumatic steering system. Describe the components and operation of an electronically variable power steering system. Describe the components and operation of a load sensing power assist steering system.

The steering system in a heavy-duty truck is expected to:
Deliver precise directional control of the chassis at both gross and unloaded vehicle weight Be able to minimize driver effort while retaining some road feel Truck steering systems can be either manual or power-assisted. Some hydraulic power-assisted systems are available with electronic or load sensing controls.

Coupled to the steering column upper shaft by a pair of yokes and the U-joint assembly is the lower shaft assembly. The steering column assembly usually supports some other switch and control components. This helps keep the driver’s hands nearby. A multifunction stalk switch, which contains the turn signal switch at a minimum, is usually mounted on the left side of the steering column. If the vehicle is equipped with a trailer service brake control valve, it is clamped onto the right side of the steering column.

Pitman Arm A Pitman arm is a steel lever, splined to the sector (output) shaft of the steering gear. The end of the Pitman arm moves through an arc with the sector shaft center forming its center. The Pitman arm functions to change the rotary motion of the steering gear sector shaft into linear motion. The length of the Pitman arm affects leverage and therefore steering response. A longer Pitman arm will generate more steering motion at the front wheels for a given amount of steering wheel movement.

Drag Link A drag link is a forged rod that connects the Pitman arm to the steering control arm. The drag link can be a one- or two-piece component. The length of two-piece design is adjustable, which makes it easy to center the steering gear with the wheels straight ahead. The drag link is connected at each end by ball joints. These ball joints help isolate the steering gear and Pitman arm from axle motion.

Steering Column Arm The steering control arm connects the drag link to the steering knuckle on the driver side of the vehicle. A steering control arm is also known as a steering arm, a control arm, and a steering lever. When the drag link is moved in a linear direction, the steering control arm moves the steering knuckle, which changes the angle of the steering knuckle spindle.

Steering knuckles mount to the front axle by kingpins or knuckle pins.
Kingpins provide the ability to steer the vehicle. Spindle The steering knuckle incorporates the spindle onto which wheel bearings and wheel hubs are mounted, plus a flange to which the brake spider is bolted. A steering control arm is attached to the left side steering knuckle. Ackerman arms are attached to both steering knuckles. STEERING KNUCKLE Steering knuckles (Figure 25–4) mount to the rigid front axle beam by means of steel pins known as kingpins or knuckle pins. They provide the ability for the pivoting action required to steer the vehicle. The steering knuckle incorporates the spindle onto which wheel bearings and wheel hubs are mounted, plus a flange to which the brake spider is bolted. A steering control arm is attached to the upper portion of the left side steering knuckle and Ackerman (tie-rod) arms are attached to both left and right steering knuckles. A kingpin can be either tapered (Figure 25–5A) or straight (Figure 25– B). Tapered pins are drawn into the axle center and secured by tightening a nut at the upper pin end. Straight kingpins are secured to the axle with tapered draw keys that bear against flats on the pin. Tapered pins are usually sealed and may not require periodic lubricating. Straight pins have a cap on either end (top and bottom) to retain grease. Zerk-type grease fittings are used to lubricate steering knuckles. Good quality chassis grease is required to lubricate most steering knuckles, but you should check the OEM recommendation. On newly introduced unitized steer axles, the steering knuckle is lubed-for-life; no attempt should be made to lubricate these. The steering control arm is fastened to the upper knuckle of the steering knuckle with a key and nut. The Ackerman arm is fastened to the lower knuckle in the same way. Steering knuckles are required to have regular lube service. If the system is not connected to the auto-lube circuit, steering knuckles should be greased by raising the steering axle off the ground. This removes the weight from the wheels and helps properly distribute grease through the kingpin. A left side Ackerman arm is shown in Figure 25–5.

King Pins Tapered Straight Lubed for life
Drawn into the axle center and secured by tightening a nut at the upper pin end Usually sealed and may not require periodic lubricating Straight Secured to the axle with tapered draw keys that bear against flats on the pin Have a cap on either end (top and bottom) to retain grease. Zerk-type grease fittings are used to lubricate steering knuckles. Lubed for life On newly introduced unitized steer axles, the steering knuckle is lubed-for-life; no attempt should be made to lubricate these. STEERING KNUCKLE Steering knuckles (Figure 25–4) mount to the rigid front axle beam by means of steel pins known as kingpins or knuckle pins. They provide the ability for the pivoting action required to steer the vehicle. The steering knuckle incorporates the spindle onto which wheel bearings and wheel hubs are mounted, plus a flange to which the brake spider is bolted. A steering control arm is attached to the upper portion of the left side steering knuckle and Ackerman (tie-rod) arms are attached to both left and right steering knuckles. A kingpin can be either tapered (Figure 25–5A) or straight (Figure 25– B). Tapered pins are drawn into the axle center and secured by tightening a nut at the upper pin end. Straight kingpins are secured to the axle with tapered draw keys that bear against flats on the pin. Tapered pins are usually sealed and may not require periodic lubricating. Straight pins have a cap on either end (top and bottom) to retain grease. Zerk-type grease fittings are used to lubricate steering knuckles. Good quality chassis grease is required to lubricate most steering knuckles, but you should check the OEM recommendation. On newly introduced unitized steer axles, the steering knuckle is lubed-for-life; no attempt should be made to lubricate these. The steering control arm is fastened to the upper knuckle of the steering knuckle with a key and nut. The Ackerman arm is fastened to the lower knuckle in the same way. Steering knuckles are required to have regular lube service. If the system is not connected to the auto-lube circuit, steering knuckles should be greased by raising the steering axle off the ground. This removes the weight from the wheels and helps properly distribute grease through the kingpin. A left side Ackerman arm is shown in Figure 25–5.

Ackerman Arm An Ackerman or tie-rod arm is the means used to transfer and synchronize steering action on both steer wheels on a steering axle. Ackerman arms are drop-forged, tempered steel levers that are angled to define the steering geometry required on turns. One end of an Ackerman arm is keyed and bolted to the lower portion of the steering knuckle. The other end is taper bored to allow a tie-rod ball stud to be clamped to it.

Front-end Alignment (1 of 2)
A front end that is properly aligned will result in: Easier steering Longer tire life Directional stability Less wear on front-end components Better fuel economy Increased safety FRONT-END ALIGNMENT The components of a truck front axle, steering system, and suspension must be precisely aligned to ensure the vehicle steers and tracks true. When properly aligned, the wheels of both a loaded and unloaded vehicle should contact the road properly, allowing each wheel to rotate without scuffing, dragging, or slipping on the road. A front end that is properly aligned will result in: • Easier steering • Longer tire life • Directional stability • Less wear on front-end components • Better fuel economy • Increased safety The primary alignment angles are toe, caster, and camber. Kingpin inclination, turning angle, Ackerman geometry, and axle alignment will also have some effect on tire wear and steering characteristics. Front-end alignment should be checked at regular intervals, particularly after the front suspension has been subjected to severe impact loads. Before checking and adjusting alignment, components such as wheel bearings, tie-rods, steering gear, shock absorbers, and tire inflation should be inspected and corrected when necessary.

Front-end Alignment (2 of 2)
The primary alignment angles are: Toe Caster Camber Kingpin inclination Turning angle FRONT-END ALIGNMENT The components of a truck front axle, steering system, and suspension must be precisely aligned to ensure the vehicle steers and tracks true. When properly aligned, the wheels of both a loaded and unloaded vehicle should contact the road properly, allowing each wheel to rotate without scuffing, dragging, or slipping on the road. A front end that is properly aligned will result in: • Easier steering • Longer tire life • Directional stability • Less wear on front-end components • Better fuel economy • Increased safety The primary alignment angles are toe, caster, and camber. Kingpin inclination, turning angle, Ackerman geometry, and axle alignment will also have some effect on tire wear and steering characteristics. Front-end alignment should be checked at regular intervals, particularly after the front suspension has been subjected to severe impact loads. Before checking and adjusting alignment, components such as wheel bearings, tie-rods, steering gear, shock absorbers, and tire inflation should be inspected and corrected when necessary.

Toe (1 of 2) Toe is the tracking angle of the tires from a true straight-ahead track. If a line were to be drawn through the centerline of each tire and compared to the centerline of the vehicle (the true straight- head position), the amount of deviation between the two lines indicates the toe angle of the tire. When the tire centerline is exactly parallel with the vehicle centerline, the toe angle is zero. When the front end of the tire points inward toward the vehicle, the tire has toe-in. When the front of the tire points outward from the vehicle, the tire has toe-out. You can see toe-in and toe-out by looking at Figure 25–6, in which you are observing the tires from an overhead viewpoint. The ideal toe angle when a vehicle is running loaded down a highway is zero. We set toe angles statically. The objective of setting toe at a specified angle when aligning the front end is to have zero toe at highway speeds. Incorrect toe angles not only accelerate tire wear but also can have an adverse effect on directional stability of the vehicle. In fact, incorrect toe angles have the potential to cause more front tire wear than any other incorrect alignment angle. Too much toe-in produces a scuffing, or a featheredge, along the inner edges of the tires. Excessive toe-out produces a similar wear pattern along the outer edge of the tires. In extreme cases of toe-in or toe-out, feathered edges develop on the tread across the entire width of the tread face of both radial and bias-ply tires. Because the objective is to have a zero toe angle under running conditions, most truck specifications require a zero or a slight toe-in setting. When a fully loaded vehicle is moving at highway speeds, there is a slight tendency of steering tires to toe-out. Any looseness in the steering linkage and tie-rod assembly also will contribute to the toe-out tendency. Today, most radial steering tires are set with zero toe angle and bias-ply tires are set with a fractional toe-in. The ideal toe angle when a vehicle is running loaded down a highway is zero. We set toe angles statically. The objective of setting toe at a specified angle when aligning the front end is to have zero toe at highway speeds.

Toe (2 of 2) Incorrect toe angles not only accelerate tire wear but also can have an adverse effect on directional stability of the vehicle. Incorrect toe angles have the potential to cause more front tire wear than any other incorrect alignment angle. Too much toe-in produces a scuffing, or a featheredge, along the inner edges of the tires. Excessive toe-out produces a similar wear pattern along the outer edge of the tires. When a fully loaded vehicle is moving at highway speeds, there is a slight tendency of steering tires to toe-out. Any looseness in the steering linkage and tie-rod assembly also will contribute to the toe-out tendency.

Camber Steering tires also are designed to use a positive camber angle setting. We will take a closer look at camber later but, because it influences the toe setting, we will define it here. Camber is a measure of the angle a wheel leans away or toward the frame. Positive camber means that the tires lean away from the truck frame at the top. A positive camber setting is used to help compensate for that slight tendency of steering tires to toe-out when the vehicle is moving.

Measuring Toe (1 of 2) First check kingpin inclination, camber, and caster. Correct, if necessary. You should not make an adjustment to toe angle until the other factors of front-wheel alignment are known to be within specifications. Adjustment of toe angle or dimension requires lengthening or shortening the tie-rod dimension. This is achieved by loosening the tie-rod end clamp bolts and then rotating the cross tube. Measuring Toe Toe is specified in either degrees or fractions of inches depending on the type of alignment instruments you are using. When a toe-in setting is required, settings of 1/16 inch 6 1/32 inch are usually specified. The toe specification usually depends on whether radial or bias-ply tires are used. Adjustment of toe angle or dimension requires lengthening or shortening the tie-rod dimension. This is achieved by loosening the tie-rod end clamp bolts and then rotating the cross tube. To determine the causes of excessive tire wear that you suspect to be related to toe angle, you should first check kingpin inclination, camber, and caster. Correct, if necessary, in this order. These are covered later in this chapter. You should not make an adjustment to toe angle until the other factors of front-wheel alignment are known to be within specifications. When measuring toe angle, the front suspension should be neutralized. To neutralize the suspension, roll the vehicle back and forth about a half vehicle length. This relaxes the front suspension and steering linkages. Neutralizing the front suspension is important before making front-end adjustments, especially if the vehicle has been jacked up on either side to scribe the tires. This operation causes the front wheels to angle as each is returned to the floor. When possible, use a scissor jack that cradles both sides of the front axle when working on front ends. If you began by neutralizing the front end, both front wheels should at this point be in the exact straight-ahead position. Toe-in measurements should be taken across the hub centerline, front and rear on each tire (Figure 25–7). Make sure that the wheels are on the ground and fully supporting the vehicle weight. Measure and record the measurements. The difference between the front and rear measurements is the toe specification.

Measuring Toe (2 of 2) Neutralize the suspension first.
When measuring toe angle, the front suspension should be neutralized: To neutralize the suspension, roll the vehicle back and forth about a half vehicle length. This relaxes the front suspension and steering linkages. Neutralizing the front suspension is important before making front-end adjustments, especially if the vehicle has been jacked up on either side to scribe the tires. This operation causes the front wheels to angle as each is returned to the floor. Make sure that the wheels are on the ground and fully supporting the vehicle weight. Measure and record the measurements.

Shop Talk When tie-rod ends are replaced, they must be threaded into the tie-rod cross tube sufficiently so that clamp pressure is applied directly over the threads under the clamp. The threaded end should extend past the slot in the cross tube. CAUTION: Recheck the toe setting after any change in caster or camber angle.

Deep Drop Tie-Rod Ends Toe-in tends to increase slightly on some steer axles when loaded and stationary. It is therefore important that vehicles with set-back axles and deep drop tie-rod ends are given some special attention. Because of the turning geometry that results from set-back and deep drop tie-rod end axles, the tie-rod end clamps if facing forward can interfere with the axle I-beam on a full lock turn. This interference will not necessarily impede steering control, but it can reduce the effective turn angle of the vehicle.

Caster (1 of 2) CASTER Caster is the forward or rearward tilt of the kingpin centerline when viewed from the side of the vehicle. Zero caster occurs when the centerline of the kingpin is exactly vertical. Positive caster indicates the kingpin is tilted rearward as shown in Figure 25–9. Negative caster indicates that the kingpin is tilted forward. Caster is a directional stability angle only. Incorrect caster by itself will not affect tire wear. Most heavy duty trucks are designed with some degree of positive caster. Positive caster creates a trailing force in the front wheels, which tends to keep them tracking straight ahead. Positive caster tends to make the steering axle wheels want to return to a straight ahead position after a turn has been made. The principle is exactly the same as that used in tilting the front fork of a bicycle, which makes it possible to ride the bicycle without holding the handlebars. It is the opposite of the wobble-wheel effect of a supermarket pushcart when mobile. Positive caster also means that when the front wheels of the truck are turned, one side of the vehicle raises slightly when the other side is lowered. When the steering wheel is released, the weight of the vehicle forces the lifted side downward, resulting in the wheels returning to a straight-ahead steering track. In other words, positive caster produces a condition in which the right wheel resists a right turn and the left wheel resists a left turn. The further into a turn, the more this resistance increases. Caster settings generally affect steering performance in the following ways: Too little caster can cause wheel instability, wandering, and poor wheel recovery. Too much caster can result in hard steering, darting, oversteer, and low speed shimmy.

Caster (2 of 2) Caster is the forward or rearward tilt of the kingpin centerline when viewed from the side of the vehicle. Zero caster occurs when the centerline of the kingpin is exactly vertical. Positive caster indicates the kingpin is tilted rearward. Negative caster indicates that the kingpin is tilted forward. Caster is a directional stability angle only. Incorrect caster by itself will not affect tire wear. Most heavy-duty trucks are designed with some degree of positive caster. CASTER Caster is the forward or rearward tilt of the kingpin centerline when viewed from the side of the vehicle. Zero caster occurs when the centerline of the kingpin is exactly vertical. Positive caster indicates the kingpin is tilted rearward as shown in Figure 25–9. Negative caster indicates that the kingpin is tilted forward. Caster is a directional stability angle only. Incorrect caster by itself will not affect tire wear. Most heavy duty trucks are designed with some degree of positive caster. Positive caster creates a trailing force in the front wheels, which tends to keep them tracking straight ahead. Positive caster tends to make the steering axle wheels want to return to a straight ahead position after a turn has been made. The principle is exactly the same as that used in tilting the front fork of a bicycle, which makes it possible to ride the bicycle without holding the handlebars. It is the opposite of the wobble-wheel effect of a supermarket pushcart when mobile. Positive caster also means that when the front wheels of the truck are turned, one side of the vehicle raises slightly when the other side is lowered. When the steering wheel is released, the weight of the vehicle forces the lifted side downward, resulting in the wheels returning to a straight-ahead steering track. In other words, positive caster produces a condition in which the right wheel resists a right turn and the left wheel resists a left turn. The further into a turn, the more this resistance increases. Caster settings generally affect steering performance in the following ways: Too little caster can cause wheel instability, wandering, and poor wheel recovery. Too much caster can result in hard steering, darting, oversteer, and low speed shimmy.

Positive Caster Positive caster creates a force in the front wheels, which tends to keep them tracking straight ahead. Positive caster tends to make the steering axle wheels want to return to a straight ahead position. Positive caster also means that when the front wheels of the truck are turned, one side of the vehicle raises slightly and the other side is lowered. When the steering wheel is released, the weight of the vehicle forces the lifted side downward, resulting in the wheels returning to a straight-ahead position. Caster settings generally affect steering performance in the following ways: Too little caster can cause wheel instability, wandering, and poor wheel recovery. Too much caster can result in hard steering, darting, oversteer, and low speed shimmy.

Caster Recommendations
Examples are measured weight down on a level floor: Tandem drive axle: ½–1½ degrees positive Single drive: 1½–2 ½ degrees positive No more than 1/2 degree difference between the left and right wheels Positive caster on the left wheel should not be greater than on the right. Caster specifications are based on vehicle design load which will usually result in a level frame. If the frame is not level when alignment checks are made, this must be factored in the caster measurement. Recommended Caster Settings Vehicle manufacturers specify varying caster settings, depending on the truck model, what type of load it is to carry, whether it is a tandem or single drive axle, and whether it has manual or power steering. The objective is to provide just enough positive caster to ensure wheel stability and good wheel recovery. The following settings are some examples used for vehicles with manual steering gear, measured weight down on a level floor: • Tandem drive axle: 1/2–11/2 degrees positive • Single drive: 11/2–21/2 degrees positive • No more than 1/2 degree difference between the left and right wheels • Positive caster on the left wheel should not be greater than on the right. Caster specifications are based on vehicle design load (no payload), which will usually result in a level frame. If the frame is not level when alignment checks are made, this must be factored in the caster measurement. With the vehicle on a smooth, level surface, measure the frame angle with a protractor or inclinometer placed on a frame rail, as shown in Figure 25–10. Positive frame angle is defined as forward tilt (front end down) and negative angle as tilt to rear (front end high).

Twisted Axles In a few cases, steering problems can be traced to a twist in the front axle. A twisted steer axle can be caused by an actual structural twist, or one induced by improper use of caster shims. A twisted axle should be suspected whenever: The difference in caster angle exceeds 1/2 degree from side to side The caster shims in place differ by 1 degree or more A low speed shimmy exists, and there is no evidence of looseness elsewhere in the steering system A leading or darting condition persists Twisted Axles In a few cases, steering problems can be traced to a twist in the front axle. A twisted steer axle can be caused by an actual structural twist, or one induced by improper use of caster shims. A twisted axle should be suspected whenever: • The difference in caster angle exceeds 1/2 degree from side to side • The caster shims in place differ by 1 degree or more • A low speed shimmy exists, and there is no evidence of looseness elsewhere in the steering system • A leading or darting condition persists

Camber Excessive positive camber causes the tire to wear on its outside shoulder. Excessive negative camber causes the tire to wear on its inside shoulder. Unequal camber in the front wheels also can cause the steering to lead to the right or left. The truck will lead to the side that has the most positive camber. CAMBER Camber is the inward or outward tilt of the top of the wheels when viewed from the front of the vehicle, as shown in Figure 25–17. If the centerline of the tire is exactly vertical, the tire has a camber setting of zero degrees. If the top of the tire tilts outward, it has positive camber, and if the top of the tire tilts inward, it has negative camber. Positive and negative camber is illustrated in Figure 25–18. Camber settings of 11/4 degree for the curb side and 13/4 degree for the driver side wheels are typical. The amount of camber used depends on the kingpin inclination (KPI) setting. Kingpin inclination is the inclination of the steering axis to a vertical plane that is the equivalent of steering axis inclination (SAI) in automotive front ends. Incorrect camber angle results in abnormal wear on the side of a tire tread. Excessive positive camber causes the tire to wear on its outside shoulder. Likewise, excessive negative camber causes the tire to wear on its inside shoulder. Unequal camber in the front wheels also can cause the steering to lead to the right or left. The truck will lead to the side that has the most positive camber. Camber adjustments are not often made on heavy duty truck axles. Some service facilities with specialized equipment adjust camber by cold bending axle beams, but, generally, axle manufacturers do not approve of this practice. Bending an axle can damage the structural integrity of the beam, increase the risk of metal fatigue, and lead to the axle breaking. Heating and bending the axle should never be attempted as it removes the heat tempering.

Kingpin Inclination (KPI)
The amount that the top of the kingpin inclines away from vertical as viewed from the front of the truck In conjunction with camber angle, places the approximate center of the tire tread footprint in contact with the road Reduces steering effort and improves the directional stability Cannot be adjusted in trucks Once set, KPI should not change unless the front axle has been bent. Corrections or changes accomplished by replacement of broken, bent, or worn parts

Turning Angle or Radius
Turning angle or radius is the degree of movement from straight-ahead to either an extreme right or left position. Two factors limit the turning angle. Tire interference with the chassis and steering gear travel To avoid tire interference or bottoming of the steering gear, there are adjustable stop screws on the steering knuckles Turning radius or angle should be checked using the radius gauge.

Shop Talk The power steering gear pressure relief valve should open just before the steering stop screw contacts the axle stop. You may have to adjust the power steering gear so that power-assist stops approximately 1 degree before the steering stops contact. Failure to do this will result in slamming of the steering stops on full lock turns.

Ackerman Geometry (1 of 2)
Ackerman geometry is the means used to steer a vehicle so that the tires track freely during a turn. During a turn, the inboard wheel on a steer axle has to track a tighter circle than the outer wheel. In other words, the outer tire has to travel just a little further because of the wider arc it has to follow. Ackerman geometry is also known as toe-out during turns. It allows the inner and outer wheel to turn at different angles so that both wheels can negotiate the turn without scrubbing. This is shown in Figure 25–21. To achieve the required Ackerman angle, the steering linkage must be arranged so the axes of the steering wheels intersect on the axis of the rear axle (Figure 25–21). On single drive axles, the centerline of the drive axle is the rear axis. On tandem drive axles, the intersecting axis is centered between the two drive axles (Figure 25–22). The Ackerman angle built into the steering geometry actually provides perfect rolling angles for both front wheels at only one turning angle. Anything other than this turning angle introduces an error, the size of which depends on the length and inclination of the tie-rod arms. As a result, the lengths of the cross tube and tie-rod arms have to be selected so as to minimize the error throughout the full range of steering angles possible on a particular front end. In trucks, the Ackerman steering error has to be smallest at low turning angles where the most steering corrections are made and at the highest road speeds. Toe-out on turns is accomplished by having the ends of lower steering arms (those that connect to the tie-rods) closer together than the kingpins, as shown in Figure 25–23. Actual toe-out during a turn depends on the length and angle of the steering control arms and the length of the cross tube. Even if the toe-in setting with the wheels in a straight-ahead position is correctly adjusted, a bent steering arm can cause the toe-out on a turn to be incorrect, causing tire scuffing. Ackerman geometry is the means used to steer a vehicle so that the tires track freely during a turn. During a turn, the inboard wheel on a steer axle has to track a tighter circle than the outer wheel. Ackerman geometry is also known as toe-out during turns. It allows the inner and outer wheel to turn at different angles so that both wheels can negotiate the turn without scrubbing.

Ackerman Geometry (2 of 2)
Toe-out on turns is accomplished by having the ends of lower steering arms (those that connect to the tie-rods) closer together than the kingpins. Actual toe-out during a turn depends on the length and angle of the steering control arms and the length of the cross tube. Even if the toe-in setting with the wheels in a straight-ahead position is correctly adjusted, a bent steering arm can cause the toe-out on a turn to be incorrect, causing tire scuffing. ACKERMAN GEOMETRY Ackerman geometry is the means used to steer a vehicle so that the tires track freely during a turn. During a turn, the inboard wheel on a steer axle has to track a tighter circle than the outer wheel. In other words, the outer tire has to travel just a little further because of the wider arc it has to follow. Ackerman geometry is also known as toe-out during turns. It allows the inner and outer wheel to turn at different angles so that both wheels can negotiate the turn without scrubbing. This is shown in Figure 25–21. To achieve the required Ackerman angle, the steering linkage must be arranged so the axes of the steering wheels intersect on the axis of the rear axle (Figure 25–21). On single drive axles, the centerline of the drive axle is the rear axis. On tandem drive axles, the intersecting axis is centered between the two drive axles (Figure 25–22). The Ackerman angle built into the steering geometry actually provides perfect rolling angles for both front wheels at only one turning angle. Anything other than this turning angle introduces an error, the size of which depends on the length and inclination of the tie-rod arms. As a result, the lengths of the cross tube and tie-rod arms have to be selected so as to minimize the error throughout the full range of steering angles possible on a particular front end. In trucks, the Ackerman steering error has to be smallest at low turning angles where the most steering corrections are made and at the highest road speeds. Toe-out on turns is accomplished by having the ends of lower steering arms (those that connect to the tie-rods) closer together than the kingpins, as shown in Figure 25–23. Actual toe-out during a turn depends on the length and angle of the steering control arms and the length of the cross tube. Even if the toe-in setting with the wheels in a straight-ahead position is correctly adjusted, a bent steering arm can cause the toe-out on a turn to be incorrect, causing tire scuffing.

Axle Alignment (1 of 2) All of the axles should be perpendicular to the vehicle’s centerline. The thrustline thus created is parallel to the vehicle centerline. If they are not positioned perpendicular to the vehicle centerline, the rear wheels will not track directly behind the front wheels, and the thrustline of the rear wheels deviates from the centerline of the vehicle. The steering fights the vehicle thrustline, resulting in an un-centered steering wheel and accelerated front tire wear. AXLE ALIGNMENT When a vehicle is running on the highway, all of the axles should track perpendicular to the vehicle centerline; that is, the rear wheels should track directly behind the front wheels when the vehicle is moving straight ahead. When this happens, the thrustline created by the rear wheels is parallel to the vehicle centerline, as shown in Figure 25–24A. However, if the axles are not running perpendicular to the vehicle centerline, the rear wheels will not track directly behind the front wheels, and the thrustline of the rear wheels deviates from the centerline of the vehicle, as shown in Figure 25–24B. The steering controls fight the vehicle thrustline, resulting in an uncensored steering wheel and accelerated front tire wear, and which causes the vehicle to oversteer when turning in one direction and understeer when turning in the other direction. Oversteer is an over response to

Axle Alignment (2 of 2) On a single-axle vehicle, the rear-axle thrustline can be off if the entire axle is offset or if only one wheel has an improper toe angle. On a tandem axle, there are a number of different combinations that can cause incorrect tracking. One method of checking a single axle for misalignment is to clamp a straightedge across the frame so that it is square with the frame rails. Measure from the center of the hub to the straightedge. The distances on each side should be within 1/8 inch of each other. When the steering controls fight the vehicle thrustline there will be an uncentered steering wheel and accelerated front tire wear, and which causes the vehicle to oversteer when turning in one direction and understeer when turning in the other direction. Oversteer is an over response to steering input, which results in vehicle yaw or lateral tracking off the intended turning radius. Understeer is an under response to steering input, most often causing steering tire slip at high speeds. Incorrect directional tracking can occur on single-axle vehicles, tandem-axle vehicles, and trailers. On a single-axle vehicle, the rear-axle thrustline can be off if the entire axle is offset or if only one wheel has an improper toe angle. On a tandem axle, there are a number of different combinations that can cause incorrect tracking. Figure 25–25 illustrates some of those combinations. One method of checking a single axle for misalignment is to clamp a straightedge across the frame so that it is square with the frame rails on each side. Then measure from the straightedge to the center of the hub. The distances on each side should be within 1/8 inch of each other. If not, the axle must be aligned.

Trailer Tracking It is also possible for the trailer axles to be out of alignment and cause a tracking problem. Depending on the severity of the trailer misalignment, it might be possible to see the effects of the misalignment as the trailer travels down the road. Usually, the trailer will travel at an angle to the tractor. Misalignment also makes it very hard to back up the trailer. This is commonly called dog- tracking.

Axle Offset Another problem is an axle that is not centered with the centerline of the vehicle. When an axle is offset and the vehicle is driven straight down a highway, the steering wheel should be centered and the vehicle will not dog-track. However, as soon as it is cornered, it will oversteer in one direction and understeer in the other.

Electronic Alignment Equipment
Wheel/axle alignment measurements can be taken in a number of ways. A number of electronic alignment systems exist. Electronic alignment systems use computers to analyze and display alignment conditions. The electronic methods include: Light beam alignment Laser alignment

Beam Alignment Systems
Laser beam equipment projects light beams onto charts, scales, and sensors to measure toe, caster, and camber. The wheel-mounted light projectors can be focused to any length wheelbase and to check alignment angles. Two- and four-wheel models are available. Mirrors redirect the light beams from the projector back onto scales mounted in the rear toe boxes of the projectors. BEAM ALIGNMENT SYSTEMS Light beam or laser alignment systems use wheel mounted instruments to project light beams onto charts, scales, and sensors to measure toe, caster, and camber and to note the results of alignment adjustments. The wheel-mounted light projectors can be focused to handle any length wheelbase. The system checks all alignment angles. Two- and four-wheel models are available. Four- heel models reference all measurements to the rear wheel thrustline to ensure precise alignment. They can check rear camber and toe without moving the vehicle or switching the instruments. This is done by redirecting the light beam through the use of mirrors and precisely positioned scales built into the projectors and the rear wheel instruments. The light projectors are mounted on the front steering axle, and the rear wheel instruments are mounted on the rear axle of the tandem drive axle. The function of the projectors is to project light in two directions: forward onto the wheel alignment chart positioned in front of the vehicle and rearward onto scales built into the rear wheel instruments. This setup enables you to measure and record the camber of the rear axle of the tandem drive axle. To set the thrust angle of the rearward tandem axle, the front steering wheels are positioned straight ahead so that the light beam is equal on the rear scales of the rear wheel instruments. The steering wheel is then locked in place with a special tool, and the toe of the front wheel is adjusted to specification. Front wheel toe readings are taken off the chart positioned in front of the vehicle. To adjust the rear axle of the tandem drive axle, the rear scales of the rear wheel instruments are lifted to expose mirrors. These mirrors redirect the light beam from the projector back onto scales mounted in the rear toe boxes of the projectors. The rearward tandem axle can then be aligned, based on the readings shown on the rear toe box scales. To align the front steering axle, the front wheels are set straight ahead and camber and caster measurements are taken off the chart positioned in front of the vehicle. Adjustments are made as needed. With the wheels still in the straight-ahead position, toe can be measured and adjusted as needed. To align the forward drive axle of a tandem drive truck so that the rear scales of the rear wheel instruments are lifted to expose the mirrors. The beams from the projectors are now directed back into the rear toe boxes of the projectors. Adjustments are then made based on the reading on the rear toe box scales. Figure 25–29 shows a technician fitting a self centering wheel adaptor system as used with the Hunter 811 T alignment system.

Sensor/Computer Alignment (1 of 4)
This shows an initial analysis made by Hunter WinAlign software as displayed by a Windows driven PC. WinAlign automatically calculates the correction for the technician. As the adjustment is made, the arrow moves across the bar graph target guiding the technician. A typical display screen is shown in Figure 25–32. This shows an initial analysis made by Hunter WinAlign software as displayed by a Windows driven PC. WinAlign automatically calculates the correction for the technician. As the adjustment is made, the arrow moves across the bar graph target guiding the technician. When the adjustment comes within spec, the bar graph changes color from red to green, as shown in Figure 25–33. A typical system provides for four-wheel alignment with four sensors (two axles simultaneously), or for two-wheel alignment with two sensors (one axle at a time). All wheels are aligned to a common centerline for precise alignment. By moving instruments, the system can also check both rear axles of a tandem drive axle, as well as the front steering axle. This means all axles can be aligned relative to a common centerline. Using software such as Hunter’s WinAlign makes alignment adjustments easy. After the truck specifications and VIN are keyed or downloaded into the system computer, the alignment can begin. By pressing the appropriate key, the thrust angle of the rear drive axle is computed and displayed on the display monitor. Adjustments to this angle can then be made until it is within specifications. The caster and toe is then measured and adjusted on the front steering axle. Once again, a few simple keystrokes compute the data and track alignment progress. After the front axle is aligned, the front sensors are moved to the forward axle of the tandem pair. The thrust angle of this axle is then measured by the sensors and adjusted as needed. Like the light beam system explained earlier, this computer-controlled system can be adapted to align trailer axles by using a gauge bar attached to the trailer kingpin, offering a much greater degree of precision than equivalent bazooka alignment equipment.

Sensors are mounted at each wheel for fast, precise alignment. Alignment readings, specifications, and step-by-step instructions are displayed on a display monitor. Keyboard-entered specifications are automatically compared against the actual angles of the vehicle, with the results displayed on the display screen. Specifications can be retained in computer memory for future use or on CD. As adjustments are made on the truck, these are automatically displayed on the monitor, enabling a high degree of precision. SENSOR AND COMPUTER ALIGNMENT SYSTEMS The most advanced alignment systems in use today are controlled by microprocessors. They use sensors mounted at each wheel for fast, precise alignment. A computer-controlled system is shown in Figures 25–28 and 25–31. Alignment readings, specifications, and step-by-step instructions are displayed on a display monitor (cathode ray tube or CRT). Keyboard entered specifications are automatically compared against the actual angles of the vehicle, with the results displayed on the display screen. Specifications can be retained in computer memory for future use and correlated with the OEM specifications held in memory or on CD. Most computer-controlled alignment equipment is user-friendly and provides step-by-step instructions and graphics on the display monitor. As adjustments are made on the truck, these are automatically displayed on the monitor, enabling a high degree of precision. A typical display screen is shown in Figure 25–32. This shows an initial analysis made by Hunter WinAlign software as displayed by a Windows driven PC. WinAlign automatically calculates the correction for the technician. As the adjustment is made, the arrow moves across the bar graph target guiding the technician. When the adjustment comes within spec, the bar graph changes color from red to green, as shown in Figure 25–33. A typical system provides for four-wheel alignment with four sensors (two axles simultaneously), or for two-wheel alignment with two sensors (one axle at a time). All wheels are aligned to a common centerline for precise alignment. By moving instruments, the system can also check both rear axles of a tandem drive axle, as well as the front steering axle. This means all axles can be aligned relative to a common centerline. Using software such as Hunter’s WinAlign makes alignment adjustments easy. After the truck specifications and VIN are keyed or downloaded into the system computer, the alignment can begin. By pressing the appropriate key, the thrust angle of the rear drive axle is computed and displayed on the display monitor. Adjustments to this angle can then be made until it is within specifications. The caster and toe is then measured and adjusted on the front steering axle. Once again, a few simple keystrokes compute the data and track alignment progress. After the front axle is aligned, the front sensors are moved to the forward axle of the tandem pair. The thrust angle of this axle is then measured by the sensors and adjusted as needed. Like the light beam system explained earlier, this computer-controlled system can be adapted to align trailer axles by using a gauge bar attached to the trailer kingpin, offering a much greater degree of precision than equivalent bazooka alignment equipment.

When the adjustment is within spec, the bar graph changes from red to green. A typical system provides for: Four-wheel alignment with four sensors Two-wheel alignment with two sensors All wheels are aligned to a common centerline for precise alignment. By moving instruments, the system can also check both rear axles of a tandem drive axle, as well as the front steering axle. When the adjustment comes within spec, the bar graph changes color from red to green, as shown in Figure 25–33. A typical system provides for four-wheel alignment with four sensors (two axles simultaneously), or for two-wheel alignment with two sensors (one axle at a time). All wheels are aligned to a common centerline for precise alignment. By moving instruments, the system can also check both rear axles of a tandem drive axle, as well as the front steering axle. This means all axles can be aligned relative to a common centerline. Using software such as Hunter’s WinAlign makes alignment adjustments easy. After the truck specifications and VIN are keyed or downloaded into the system computer, the alignment can begin. By pressing the appropriate key, the thrust angle of the rear drive axle is computed and displayed on the display monitor. Adjustments to this angle can then be made until it is within specifications. The caster and toe is then measured and adjusted on the front steering axle. Once again, a few simple keystrokes compute the data and track alignment progress. After the front axle is aligned, the front sensors are moved to the forward axle of the tandem pair. The thrust angle of this axle is then measured by the sensors and adjusted as needed. Like the light beam system explained earlier, this computer-controlled system can be adapted to align trailer axles by using a gauge bar attached to the trailer kingpin, offering a much greater degree of precision than equivalent bazooka alignment equipment.

Like the light beam system, computer-controlled systems can be adapted to align trailer axles by using a gauge bar attached to the trailer kingpin, offering a much greater degree of precision. Computerized alignment systems make truck alignments an exact science. They will also measure and display frame offset angles. This allows technicians to true truck and trailer chassis and suspensions.

Warning All steering mechanisms are critical safety items.
A vehicle should be deadlined (OOS report) when a defect is reported. It is essential that instructions in the service literature are adhered to. Failure to observe these procedures may result in loss of steering with life-threatening results.

Warning When a vehicle is operated at temperatures below 30°F with SAE 90 weight oil in a manual steering gear, it can turn to a grease-like consistency, resulting in stiff, sluggish steering. This can compromise accident avoidance maneuvers because of slow steering response. When operating in temperatures continuously below 30°F, install a lighter oil in manual steering gear, such as SAE 75 weight oil.

Shop Talk Before performing the remaining checks, apply the parking brakes and chock the rear tires. Raise the vehicle until the front tires leave the pavement and then place safety stands under the frame rails. Be sure that the stands will support the weight of the vehicle.

Caution Before performing any servicing procedure on the front suspension, set the parking brake and block the drive wheels to prevent the vehicle from moving. After jacking the truck up until the steering axle wheels are raised off the ground, support the chassis with safety stands. Never work under a vehicle supported only by a jack.

Warning Do not drive the vehicle with too much lash in the steering gear. Excessive lash is a sign of an improperly adjusted steering gear or worn or otherwise damaged steering gear components. Driving the vehicle in this condition could result in a loss of steering control.

Manual Steering Gears A steering gear is often referred to as a steering box. It is an assembly of gears contained within a housing. Manual steering gears have lubricant within the housing. Two basic types of manual steering gears are used in heavy-duty trucks. Worm and sector shaft type Recirculating ball and worm type

Recirculating Ball Gears
The input shaft of this type of steering gear is also connected to a worm gear, but the worm gear used is straight (see Figure 25–54) in a recirculating ball steering gear. Mounted on the worm gear is a ball nut. The ball nut has internal spiral grooves that mate with the threads of the worm gear. The ball nut also has exterior gear teeth on one side. These teeth mesh with teeth on a sector gear and shaft. In the grooves between the ball nut and the worm gear are ball bearings. The ball bearings allow the worm gear and ball nut to mesh and move with little friction. When the steering wheel is turned, the input shaft rotates the worm gear. The ball bearings transmit the turning force from the worm gear to the ball nut, causing the ball nut to move up and down the worm gear. Ball return guides are connected to each end of the ball nut grooves. These allow the ball bearings to circulate in a continuous loop. As the ball nut moves up or down on the worm gear, it causes the sector gear to rotate, which, in turn, causes the Pitman arm to swivel back and forth. This motion is transferred to the steering arm and knuckle to turn the wheel. Mounted on the worm gear is a ball nut with internal spiral grooves containing recirculating ball bearings. The ball bearings transmit the turning force from the worm gear to the ball nut, causing the ball nut to move up and down the worm gear. Ball return guides are connected to each end of the ball nut grooves. These allow the ball bearings to circulate in a continuous loop. The ball nut also has exterior gear teeth that mesh with the sector gear. As the ball nut moves up or down on the worm gear, it causes the sector gear to rotate, which, in turn, causes the Pitman arm to swivel back and forth.

Power-assist Steering Systems
A power steering system must default to manual operation in the event of power-assist circuit failure. Truck hydraulic power-assisted steering systems contain a dedicated pump that delivers hydraulic fluid under pressure to a power-assist steering gear. In a gear such as this, the hydraulic control circuit is located within the steering gear so it is known as an integral system. POWER-ASSIST STEERING SYSTEMS Truck power-assisted steering systems are not so different from manual steering systems. As we said in the introduction to this chapter, any power steering system used in a highway vehicle must default to manual operation in the event of power-assist circuit failure. Power-assist steering systems use the same steering axle and steering linkage as manual systems and both have similar steering column assemblies. The real difference is in the addition of a hydraulic assist circuit. Most trucks today use hydraulic power-assist. There are some older air-assist systems still around but these tend to be after-market units installed to trucks with a conventional manual steering gear system. Truck hydraulic power-assisted steering systems contain a dedicated pump that delivers hydraulic fluid under pressure to a power-assist steering gear. In a gear such as this, the hydraulic control circuit is located within the steering gear so it is known as an integral system. There are also some nonintegral systems, but these can be regarded as obsolete.

Power Steering Pump The power steering pump is used to produce the hydraulic circuit flow required for the power-assist to the steering gear. They can be either gear- or belt-driven, depending on the engine application, but the majority are gear-driven by an engine accessory drive. Power steering pumps are usually mounted near the front of the engine. Most power steering pumps are similar in construction.

Pump Operation Centrifugal force and hydraulic pressure loads the vanes outward against an oval cam ring. As the rotor turns, the volume between vanes changes. When they move past the inlet port, the volume increases; oil is drawn in. As they approach the discharge port, volume is reduced, creating pressure rise and oil is discharged. Pump Operation The pump shaft is driven by the engine and turns the pump rotor. Vanes or rollers turn with the rotor and a combination of centrifugal force and hydraulic pressure loads them outward against a cam ring or liner. The inside of the cam ring is oval. When the rotor turns in the liner, the volume of the space changes between the rotor, the cam ring chamber, and two of the vanes. When two vanes move past the inlet side of the cam ring, the volume between rollers increases, creating a low-pressure area. As the rotor turns, the volume between the vanes and the seal they create in the cam ring is reduced, creating pressure rise, after which the hydraulic oil is discharged through the outlet. A typical pump has two inlet and two outlet ports. This helps smooth pump operation and increase pump displacement by providing two delivery pulses per revolution. Hydraulic fluid used in the pump is stored in a reservoir that might be remotely mounted or might be an integral part of the pump assembly. Fluid is routed between the pump and the reservoir and between the pump and the steering gear via hoses and lines. Peak pressure is controlled by a relief valve. The fluid used in the hydraulic circuit can be engine oil, power-steering fluid, or hydraulic fluid, dependent on OEM specification and user preference.

Flow Control and Pressure Relief Valves
The flow control valve regulates pressure and flow output from the pump to provide for consistent power-assist during variations in engine rpm. When the engine speed increases, the pump can deliver more flow than is required to meet system requirements. When outlet pressure reaches a preset level, the pressure relief ball is forced off its seat, creating a greater pressure differential at the two ends of the flow control valve. This allows the flow control valve to open wider, permitting more pump pressure to flow back to the pump inlet, and pressure is held at a safe level. Flow Control and Pressure Relief Valves The flow control valve in the pump regulates pressure and flow output from the pump (Figure 25–64). This valve is necessary because of variations in engine rpm and the requirement for consistent power-assist when the engine is operated at low speeds. It is positioned in a chamber subject to pump outlet pressure at one end and supply hose pressure at the other. A spring is used at the supply pressure end to help maintain a balance. As the fluid is discharged from the pump rotor, it passes the end of the flow control valve and is forced through an orifice that causes a slight drop in pressure. This reduced pressure, aided by the springs, holds the flow control valve in the closed position. All pump flow is directed to the steering gear. When the engine speed increases, the pump can deliver more flow than is required to meet system requirements. Because the outlet orifice restricts the amount of fluid leaving the pump, the difference in pressure at the two ends of the valve becomes greater until pump outlet pressure overcomes the combined force of supply line pressure and spring force. The valve is pushed down against the spring, opening a passage that returns the excess flow back to the inlet side of the pump. A spring and ball contained inside the flow control valve form a pressure relief valve and are used to relieve pump outlet pressure. This is done to protect the system from damage due to excessive pressure when the steering wheel is full-lock held against the stops. Because flow in the system is severely restricted in this position, the pump would continue to build pressure until a hose ruptured or the pump destroyed itself. When outlet pressure reaches a preset level, the pressure relief ball is forced off its seat, creating a greater pressure differential at the two ends of the flow control valve. This allows the flow control valve to open wider, permitting more pump pressure to flow back to the pump inlet, and pressure is held at a safe level.

Power Steering Gears Power steering gears are similar to a manual recirculating ball steering gear with the addition of a hydraulic assist control mechanism. The power steering gearbox is charged with hydraulic fluid under pressure and uses a rotary control valve to control the flow of fluid. The movement of the ball nut is assisted by hydraulic pressure. The integral-type power steering has the rotary control valve and a power piston integrated with the gearbox. The rotary valve directs the oil pressure to the left or right chamber to steer the vehicle. The spool valve is actuated by a lever or a small torsion bar located in the worm gear. Power Steering Gears A couple of different types of power-assist, steering gearboxes are used in most medium- and heavy-duty trucks. One type is similar to a manual recirculating ball steering gear with the addition of a hydraulic assist control mechanism. A typical power steering gearbox is charged with hydraulic fluid under pressure and uses a rotary control valve to control the flow of fluid into, through, and out of the gear. Some gears are also equipped with a pressure relief valve. In a power steering gear, the movement of the ball nut, also called a piston or rack gear, is assisted by hydraulic pressure. When the steering wheel is turned, the rotary valve changes hydraulic flow to create a pressure differential on either side of the piston. The unequal pressure causes the piston to move toward the lower pressure, reducing the effort required to turn the wheels. The integral-type power steering has the rotary control valve and a power piston integrated with the gearbox. The rotary valve directs the oil pressure to the left or right chamber to steer the vehicle. The spool valve is actuated by a lever or a small torsion bar located in the worm gear (Figure 25–65).

Hydraulic Operation HYDRAULIC OPERATION The driven end of the worm gear (called a ball screw in power steering gears) rotates on a ball bearing contained in the valve body. Hydraulic oil under pressure enters and exits the power steering gear by means of hydraulic lines connected to threaded ports in the valve body. A pressure relief valve contained in the valve body prevents over-pressurization of the power steering gear. Hydraulic pressure in excess of the setting of the relief valve causes the valve to open a channel to the reservoir return side of the gear. The ball screw assembly is held in the valve housing by a valve nut, which forms the outermost element of the rotary control valve. The valve nut contains circular channels and radial passages, which serve to direct hydraulic oil into and out of the rotary control valve. The ball screw assembly forms the rotary control and consists of three parts: the input shaft, torsion bar, and ball screw (Figures 25–65 and 25–66). One end of the steering gear input shaft is finely splined for connection to the steering column, and the other end has a coarse spline that fits more loosely with a similar spline inside the worm screw. The coarse splines form mechanical stops that limit the amount or relative rotation between the ball screw and input shaft. A torsion bar connects the input shaft to the ball screw. Six evenly distributed longitudinal grooves are machined into the outer surface of the input shaft and correspond to six grooves machined into the bore of the ball screw. Holes extend from the outside surface of the ball screw into the six grooves in the bore. These holes allow pressurized oil to enter and exit the two inner elements of the rotary control valve. The six grooves in the bore of the ball screw are connected alternately to each side of the piston through three pairs of the drilled holes. The other three holes admit pressurized oil directly to three of the six grooves in the input shaft. The other three grooves in the output shaft carry oil to the return line connection. The length of the six pairs of grooves cut into the ball screw and input shaft allows large pressure changes to be achieved with a small rotational displacement of the valve elements. The rotary control valve is an open-center type that allows a continuous flow of oil (through the longitudinal grooves in the input shaft and bore of the ball screw) when held in the neutral position by the torsion bar (Figure 25–66). When steering effort is applied, the input shaft and ball screw tend to turn in unison; however, the spring action of the torsion bar results in the input shaft rotating slightly in advance of the ball screw. The six pairs of grooves that form the rotary control valve are displaced from their neutral flow position. As steering effort increases, so does the amount of displacement. Depending on the direction steered, the groove displacement of the input shaft directs hydraulic oil through the appropriate drilled passages in the ball screw to one side or the other of the piston. Hydraulic pressure acting on the piston surface eliminates much of the piston’s resistance to movement. Spring force exerted by the torsion bar causes the ball screw to rotate as piston resistance is removed. As the ball screw rotates, the relative groove displacement is eliminated, and the rotary valve returns to a neutral position. Moderate effort at the steering wheel produces smaller valve displacements and lower power assist, and in this way provides good steering feel. Good steering feel is a critical element in any power-assist system. At increased displacements, the pressure rises more rapidly, giving increased power assistance and faster response. Maximum pressure is developed after approximately 3 degrees displacement, providing a direct feel to the steering. Groove displacement is limited by the free-play of the stop spline mesh between the input shaft and ball screw. The splines take up the steering movement while allowing the torsion bar to hold the groove displacement. The torsion bar and stop splines form two parallel means of transmitting the steering torque. When no steering torque is applied, the torsion bar returns the valve grooves to a neutral position, allowing the pressurized oil to flow to the return line. Power-assisted movement of the valve nut within its bore is limited by poppet valves installed in both piston faces. When the piston approaches its extreme travel in either direction, the stem of the limiting poppet valve makes contact at the end of the piston bore. As piston travel continues, the limiting poppet is unseated and some hydraulic power-assist is removed as pressurized oil is diverted to the return line. As more and more power-assist is removed by the action of the limiting poppet valves, steering effort increases. The piston can travel to the extreme ends of its bore; however, the maximum steering assistance available is reduced to protect the steering components in the axle. The bypass valve is located in the valve body and permits oil to flow from one side of the piston to the other when it is necessary to steer the vehicle without the hydraulic pump in operation. Oil displaced from one side of the piston is essentially transferred to the other side, which prevents reservoir flooding and cavitation in the pressure line. The pressure relief valve is located in the valve body and limits internal hydraulic pressure to a preset maximum. The pressure relief valve can be set to various pressures; however, its setting is typically 150 psi lower than the power steering pump relief valve setting. The rotary control valve is an open-center type that allows a continuous flow of oil when held in the neutral position by the torsion bar. When steering effort is applied, the spring action of the torsion bar results in the input shaft rotating slightly in advance of the ball screw. The six pairs of grooves that form the rotary control valve are displaced from their neutral flow position. As steering effort increases, so does the amount of displacement. Depending on the direction steered, the groove displacement of the input shaft directs hydraulic oil through the appropriate drilled passages in the ball screw to one side or the other of the piston.

Troubleshooting Safety
Always block vehicle wheels. Stop the engine when working under a vehicle. Keep hands away from pinch points. Never connect or disconnect a hose or line containing pressure. Never remove a component or pipe plug unless all system pressure has been depleted. The pressures used in truck hydraulic power-assist steering systems are massive and can peak at 1,500 psi. Never exceed recommended pressure and always wear safety glasses. Never attempt to disassemble a component until after reading and understanding recommended procedures. Use only OEM-suggested replacement components. Only components, devices, and mounting and attaching hardware specifically designed for use in hydraulic systems should be used. Replacement hardware, such as hoses and fittings, should be of equivalent size, type, and strength as the original equipment. Devices with stripped threads or obvious physical damage should be replaced. Repairs requiring machining should not be attempted.

Caution As with manual steering gear, the Pitman arm should NEVER be removed from the sector shaft to center the steering. In some instances, this could require disassembly of the steering gear.

Shop Talk A power steering analyzer is the preferred method of assessing the performance of a hydraulic power-assist steering circuit. The power steering analyzer consists of hoses, quick release couplers, a flowmeter, pressure gauge, and flow control valve.

Electronically Variable Steering
Optimized power steering control has the following features: Improved steering wheel road feel Directional stability enhanced by a steering effort proportional to driving speed Reduced dry park effort at the steering wheel Reduced system power consumption To better understand the advantages offered by electronically variable steering (EVS), the limitations of conventional power steering must be understood. In a conventional power steering system, the demand for power assistance is opposite to its availability. Designers are forced to compromise power steering pump optimization pressures. This results in a parasitic power loss that can amount to 40 to 50 percent of the power consumed by the steering system. It also results in light steering effort at highway speeds because the flow control setting is often high, as dictated by dry park conditions. Optimized power steering control has the following features: • Improved steering wheel road feel • Directional stability enhanced by a steering effort proportional to driving speed. • Reduced dry park effort at the steering wheel. • Reduced system power consumption. Because conventional power-assisted steering systems do not sense or respond to these variables, the electronically controlled version can provide a correction toward the ideal. Ideally, such a system changes the driver road feel and effort as a function of road speed and lateral acceleration. The electronic system shown in Figure 25–75 offers single-switch, operator-selectable steering modes. The system consists of a modified, integral power steering gear that includes a revised high-gain rotary control valve; a pulse width modulated (PWM), solenoid-controlled, variable-flow control pump; a reluctance-type speed sensor (similar to that used for electronic speedometers); and an electronic control module for closed-loop solenoid current control. Variable assist is provided by changing the integral- pump, control flow supplied to the steering gear. The steering gear provides the driver a variable torque input based on the variable flow control. Steering gear features include free play control, a high-efficiency manual section, low mechanical ratio, and a direct rotary valve. Power-assist is controlled by adaptive features built into the steering gear, such as torsion bar spring-rate and valve “feel” characteristics. Because the system modulates pump flow control to the steering gear, the steering gear rotary control valve helps achieve appropriate effort or “feel” changes. The potential to bypass pump flow internally, combined with the low power steering gear total loop drop at reduced flow, is the source of power consumption improvements. From the driver’s perspective, it is a little like having manual steering on the highway when running at highway speeds and enhanced power assisted steering when it comes to parking maneuvers.

EVS Pump Design The EVS steering system uses a solenoid-controlled variable-flow control concept. The pump assembly includes a high-efficiency vane pump, flow control, and proportional solenoid-type throttle valve. The system is designed with fail-safe features. When the solenoid is not energized, flow is low (at around 60 percent of its normal capacity), similar to conventional power steering systems.

EVS Electronic Control Unit (ECU)
The purpose of the electronic controller is to change the PWM actuation signal to the pump solenoid with respect to the road-speed sensor signal. The controller incorporates switches that allow for three potential operating modes with an LED display to assist in troubleshooting. ECONOMY MODE: In economy mode, flow control is at maximum below 15 mph (24 km/h); at its minimum, above 50 mph (80 km/h); and it decreases proportionally from 15 to 50 mph. NORMAL MODE: The normal mode has high flow at low speeds and decreases as speed increases (similar to economy mode), except the minimum flow is set at 13.6 L/min and is reached at about 40 mph (64 km/h). In this mode, the steering system acts much like a conventional power steering system with 13.6 L/min flow control. STANDBY MODE: If the vehicle is stationary for longer than 10 minutes, the system switches into a standby mode. This mode reduces flow to minimum power-assist as long as the speedometer indicates zero. When road speed is detected, the system automatically switches to its original mode.

PWM Actuation The PWM actuation circuit operates the solenoid valve by a fixed-frequency, pulse-width-modulated signal with closed-loop duty cycle control that uses the solenoid current as feedback. This ensures that the solenoid positioning is relatively constant with changes in vehicle battery voltage. It also allows the controller to provide current to the solenoid directly from the battery without need for high-current voltage regulators.

Load-sensing Power Steering
HYDRAULICALLY CONTROLLED: This non-electronic hydraulically controlled load-sensing system has a demand-type flow control that uses steering load pressure to control its variable-flow power-assist. SPEED PROPORTIONING: This system provides for speed proportioning of flow to the steering gear from a flow-controlled pump. Instead, the system relies on a control strategy in which the pressure requirement for power steering is a function of vehicle speed. The load-sensing system is similar to a conventional hydraulic power-assist circuit, except that a modification has been made to the flow-control section of the pump.

Combination Flow/Relief/Load-sensing Valve
Compared to the electronic version, the load-sensing system provides increased flow only to the steering gear “on demand” in response to steering pressure increase designated as “load sensing.” The load-sensing pump components include a high-efficiency, balanced vane pumping cartridge and a combination flow/relief/load-sense valve. The valve construction and operation within the circuit are shown in Figure 25–78. The valve spring has two functions. The first is to restrict the movement of the variable-orifice spool valve according to the steering load. The second is to load the relief poppet valve at the relief pressure setting. The small variable-orifice valve is exposed to return pressure on the opposite end of the spool. As pump outlet pressure increases, a progressively larger opening is produced by the moving spool. This larger opening allows a greater volume of fluid to flow into the spring-chamber end of the large flow control valve. Because the fluid must pass through a fixed orifice to escape this spring chamber, more fluid flow produces a higher pressure. The higher pressure adds to the load imparted to the flow control spool by the spring and increases the restriction of flow to the return circuit. As a result, more flow is forced through the primary control orifice and delivered to the steering gear. A conventional pressure- compensated flow control valve maintains a constant flow rate regardless of pressure, whereas the pump used on this system increases flow as pressure is increased.

Summary (1 of 6) Steering systems used in trucks must deliver precise directional control of the vehicle and its load, in both loaded and unloaded conditions, and at highway and park/stall speeds. Truck steering systems are either manual or power-assisted. Power-assist systems are required to default to manual operation in the event of a loss in the power-assist circuit. Power-assist systems can use either hydraulic or air-assist circuits. Improper steering adjustments and front-end alignment can lead to suspension and tire wear problems.

Summary (2 of 6) A properly aligned front-end results in:
Easier steering Increased tire life Directional stability Less wear and maintenance on front-end components Better fuel economy Increased safety Ackerman geometry provides toe-out on turns, permitting tires to roll freely during turns when each travels through a different arc.

Summary (3 of 6) Axle alignment measurements can be taken in a number of ways. Tram gauges and measuring tapes may be used as can light or laser beam alignment systems with computerized sensors and analysis. The most accurate and easiest to use alignment systems in use today are computer-controlled and feature in-memory specifications, step-by-step instructions, and user-friendly displays. Keyboard-entered specifications are automatically correlated to the actual angles measured on a vehicle, with the results displayed on the monitor screen.

Summary (4 of 6) Steering axle components should be inspected and lubricated routinely on a preventive maintenance schedule. Two general types of manual steering gear are used in heavy-duty trucks. They are the worm and sector shaft type and the recirculating ball and worm type. Some worm shaft preload is necessary to prevent unwanted random movement within a steering system. Two types of steering gear preloads have to be checked. Worm bearing preload and total mesh preload

Summary (5 of 6) Insufficient preload allows some lost motion in the gearset, requiring the driver to turn the steering wheel farther to get a steering response from the front wheels. Excessive preloads result in hard steering, darting, and oversteer. An integral power-assist steering system contains a pump that delivers hydraulic fluid under pressure to a power steering gear. The steering gear has an integral hydraulic control valve that measures steering effort and responds to provide the appropriate amount of assist in turning the wheels.

Summary (6 of 6) A power-assist steering gear is fundamentally similar to a manual recirculating ball steering gearbox with the addition of a hydraulic assist: It must default to manual operation in the event of a hydraulic circuit failure. Air-assisted steering systems use the vehicle compressed air supply as a means of supplementing manual steering operation. This is an external system consisting of an air torque valve contained in a dedicated drag link assembly and a double-acting power cylinder.

Steering and Alignment

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Steering and Alignment

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Presentation on theme: "Steering and Alignment"— Presentation transcript:

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