Power-assisted steering On highway vehicles is required to function with full mechanical redundancy in the event of a loss of power-assist system Full power steering system Found on some off highway equipment. This equipment often uses hydrostatic steering: in the event of loss of hydraulic power, this will result in no asteering capability Steering System Components steering wheel, steering column, steering gear, and sterring linkages that actually move the steering tires Typical size of a heavy-duty trucks 22 to 24 inches U-joints should be lubed in the same manner as those in the drive shaft assembly. It should come out all four trunnion journals. Many steering column related problems are caused by U-joints failures Steering Gear A gearbox that both multiplies steering input torque and changes its direction Three general categories of heavy-duty steering gears Worm and sector shaft; Recirculating ball and Rack and pinion Pitman Arm Is a steel lever, spline attached 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. Drag link Is a forged rod that connects the Pitman arm to the steering control arm The drag link is connected at each end by ball and socket joints. Ball and socket joints help isolate the steering gear and Pitman arm from axle motion 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 Control Arm Connects the drag link to the steering knuckle on the driver side of the vehicle Kingpins The linkage pin used in a steering knuckle that permits the assembly to pivot 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 or straight. Tapered pins are drawn into the axle center and secured by tightening a nut at the upper pin end. A straight kingpin is secured to the axle with tapered draw keys that bear against flats on the pin Zerk-type grease fittings are used to lubricate steering knuckles 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. A ball joint and socket joint are manufactured using forged steel ball with a threaded stud attached to it. Ball and socket joints are used in the steering linkage to accomodate motion between the axle and the frame rails that results when the front axle springs flex and do so without affecting steering. Ackerman Arm Or tie-rod arm is a means used to transfer and synchronize steering action on both steer wheels on a steering axle. 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 Tie-rod Transfers steering motion from the steering gear side of the chassis to the opposite side by means of an adjustable rod, forcing the steering knuckles to act in unison A tie-rod assembly consists of a cross tube and two tie-rod ends. The tie-rod ends are ball sockets that connect to the Ackerman arms on each steering knuckle Tie-rod ends are ball sockets that connect to the Ackerman arms on each steering knuckle.Each tie-rod end is threaded onto eiher end of the cross tube to connect tit to tapered sockets in the Ackerman arms. One side of the tie-rod has left-hand threads, and its oppostie side has right-hand threads The length of a tie-rod assembly defines the toe setting. Its length is determined by the postion of the tie-rod ends 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 Toe The tracking angle of the tires from a true straight-ahead track Toe-in When the fronbt end of the tire points inward toward the vehicle Toe-out When the front of the tire points outward from the vehicle In extreme cases of toe-in and toe-out, feathered edges develop on the tread across the entire width of the tread face of both radial and bias-ply tires Too much toe-in produces scuffing or a featheredge along the inside of the tires. Excessive toe-out produces a similar wear pattern along the outside of the tires Most radial steering tires are set with aero toe angle and bias-ply tires are set with a fractional toe-in Steering tires are designed to use a positive camber angle setting Camber is a measure of the angle a wheel leans away or toward the frame A positive camber setting is used to help compensate for the slight tendency of steering tires to toe-out when vehicle is moving Toe-in setting is 1/16 inch+ – 1/32 inch (1.6 =- 0.8mm). it depends on whether radial or bias-ply tires Adjustment of toe angle or dimension requires lengthening or shortening the tie-rod dimensions. Loosen the tie-rod end clamp bolts and then rotating the cross tube 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 Caster is the forward or rearward tilt of the kingpin centerline when viewed from/ the side of the vehicle Positive caster indicaTES THAT THE KINGPIN IS TILTED REARWARD NEGATIVE CASTER INDICATES THAT THE KINGPIN IS TILTED FORWARD Caster settings generally affect steering performance: 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 Recommended Caster Settings: Tandem Drive axle: 1/2 to 1 1/2 degrees positive Single drive: 1 1/2 to 2 1/2 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 then on the right Kingpin inclination (KPI) is the inclination of the steering axis to a verical polane that is the equivalent of steering axis inclination (SAI) in automobiles front ends Ackerman Geometry is the means used to steer a vehicle so that the tires track reely during tires oversteer is an overresponse to steering input, which resutls in vehicle yaw understeer is the underreponse to steering input. most often calsuing stgeering slip at high speeds Dog-tracking a misalignment condition in which the rear wheels of a truck run on a different track linethat the front steering wheels, creating a thrustline irregularity
