Function and Linkage of a Steering System
The function of a steering system is to convert the rotary movement of the steering wheel in driver’s hand into the angular turn of the front wheels on road. Additionally, the steering system should provide mechanical advantage over front wheel steering knuckles, offering driver an easy turning of front wheels with minimum effort in any desired direction. The main causes of stiff steering include (i) insufficient lubrication of the king-pins or steering linkage, (it) tyre pressure too low, (Hi) wheels out of track, i.e. toe-in not correct, and (iv) stiffness in the steering column itself, caused by lack of lubricant or over tightening.
The steering system is designed to enable the driver to control and continuously adjust the steered path of the vehicle. Also it provides a positive response to whatever direction the driver
Fig.27.42. Relationship of steer angle speed and vehicle speed for various steering conditions.
may makes on the steering wheel. To achieve these objectives, a suitable mechanical linkage is incorporated between the front steered road-wheels and the driver’s steering-wheel. This
Fig. 27.43. Schematice view of steering linkage.
Fig. 27.44. Steering layout of light truck.
Fig. 27.45. Steering layout for car.
mechanism operates effectively under all normal conditions without interfering with the wheel traction or with the suspension movement.
The steering linkage shown in Fig. 27.43 (schematic view) performs the above functions. When driver turns the steering wheel, motion is transmitted down through the steering tube to the steering gear. The steering tube revolves inside the steering column. The steering gear changes the direction of motion and increases the turning force applied by driver at the steering wheel in accordance with the gear ratio. The gear rotates the steering arm (Pitman arm), which transfers the motion to the steering knuckles through the steering gear connecting rod, tie-rod, and knuckle arms. This type of linkage is called the relay steering linkage.
The layout of any steering linkage depends largely on the type of vehicle to which it is fitted. A commercial vehicle uses a rigid axle beam front suspension steering system (Fig. 27.44). A car generally relies on independent front suspension steering system (Fig. 27.44).
Fig. 27.46. A typical axle beam steering linkage layout.
Axle-beam Suspension Steering System
This steering system (Fig. 27.46) incorporates a steering-wheel to impart motion to the steering-box which transfers the steering effort through the drop-arm and drag-link directly to one of the two stub-axles pivoting at the ends of the axle-beam. Both the stub-axles are joined together by a track-rod. Figure 27.47 illustrates the axle beam steering layout in one of its views and the functions of the components are as follows :
Steering Box. The steering box uses a reduction gear which provides a much larger force to the steering linkage with only a small effort. Simultaneously, the degree of stub-axle movement is decreased for a given angular movement of the steering wheel so that the oversensitivity of the steering with respect to driver’s touch on the wheel is reduced.
This forged lever-arm is bolted on to a tapered steering-box output rocker-shaft and it hangs or drops downwards. It imparts a circular-arc movement to the drag-link through its swing action.
This tubular rod converts the circular movement of the drop-arm into a linear push or pull motion of the drag-link arm, attached rigidly to one of the stub-axles. A ball-joint is fitted at each end of the rod so that a relative movement is provided in planes. Figure 27.48 shows an alternative transverse drag link layout suitable for cross country applications.
Fig. 27.48. Axle-beam steering linkage with transverse located drag link.
This arm joints the drag-link to one of the stub-axles and provides sufficient leverage to convert the linear movement of the drag-link to an angular movement about the stub-axle king-pin.
Drag-link Suspension and Steering Interaction.
The axle-beam pivots about the fixed front shackle-pins and moves up and down in a circular arc. Also the drag-link pivots about the drop-arm ball-joint during any vertical movement of the axle. When the effective arc radius of the axle movement and the drag-link arm end are approximately equal, the suspension axle movement relative to the chassis is independent of the vehicle’s steered path. If a slight difference exists during deflection of the suspension, then the drag-link proportionally increases or decreases the relative angular position of the stub-axle about the kind-pin. This causes the steering to continuously twitch or jerk when encountering rough surface conditions.
The stub-axle is a short axle-shaft to which one steered road-wheel is mounted. It uses two extended horizontal prongs that fit over the ends of the axle-beam. The king-pin, a short circular bar, passes vertically through both prongs and the eye of the axle-beam to form the hinge pivot. The stub-axle acts as the wheel axle as well as the pivot support member in the horizontal plane.
Each stub-axle uses a forged track-rod arm bolted approximately at right angles to the wheel axis in the horizontal plane. This arm provides the leverage to rotate the stub-axle about the king-pin. This rotary movement is transferred to the other stub-axle through the track-rod.
A tubular track-rod spans the wheel track and pivots together the two stub-axles. The ends of this rod carry ball-joints, which in turn are bolted to the track-rod arms of each stub axle. These ball-joints are allowed to move only in the horizontal plane. The drag-link movement is either a pull or a push action and rotates one of the stub-axles. This motion is transferred to the other stub-axle through the track-rod.
27.6.3. Independent Suspension Steering System
In the rigid-beam suspension, the stub-axle is pivoted at each end of the axle-beam. Consequently the relative movement is permitted only in the horizontal plane due to which effective track-rod length is not affected by any vertical suspension deflection.
Independent-suspension steering, on the other hand, copes with up a down movement of each stub-axle independent of the other due to which the distance between track-rod-arm ball-joint centres varies continually. Therefore, if a single track-rod joins the two stub-axles together, the slightest bump or rebound tends to pull both stub-axle arms at once and thus interferes with the steering-track toe-in or toe-out. To overcome the problem of the changing distance between track-rod-arm ball-joint centres, a three-piece track-rod is used. The centre portion of the track rod may be a relay-rod suspended between the steering-box drop-arm and an idler arm fixed to the body structure (Fig. 27.49). Also the centre portion may from the track shaft of a rack-and-pinion steering-box (Fig. 27.50). In both the cases, this part moves only in the horizontal plane. Movement in the vertical plane is provided by the two outer connecting rods, known as tie-rods. The tie-rods swing about the ball-joints placed at the end of the middle track rod member. In earlier designs, independent suspension steering incorporated stub-axles and king-pin pivots similar to those used with the axle-beam. But current systems use ball-swivel joints for the stub axle pivot and are also spaced further apart.
Large cars normally use the system shown in Fig. 27.49. When the steering-wheel is acted, the drop-arm conveys movement to the relav-rod, which inturn transmits this motion to both
Fig. 27.49. Split track-rod with relay-rod and idler steering linkage layout.
tie-rods and stub-axles.The drop-arm and idler-arm relay joints provide movement only in the horizontal plane. The tie-rod joints provide movement in both the horizontal and vertical planes.
The most popular steering system used for small and medium cars is shown in Fig. 27.50. This type of steering box has a rack-and-pinion housing bolted along the body cross-member. The angular movement of steering wheel is converted to a linear to-and-fro movement of the rack. Each end of the rack shaft is attached to a tie-rod by means of a ball-and-socket joint. The outer tie-rod ends also use ball-joints, which are bolted to the stub-axle track-rod arms. The rack
Fig. 27.50. Rack-and-pinion steering linkage layout.
shaft thus provides the transverse steering thrust and the tie-rod ball joints allow pivoting in two planes.