95 Variableratio rack and pinion

Variable-ratio rack and pinion can be made to improve both manual and power assisted steering operating characteristics. For a manual rack and pinion steering system it is desirable to have a moderately high steering ratio to provide an almost direct steering response while the steering wheel is in the normally 'central position' for straight ahead driving and for very small steering wheel angular correction movement. Conversely for parking manoeuvres requiring a greater force to turn the steering wheel on either lock, a more indirect lower steering ratio is called for to reduce the steering wheel turning effort. However, with power assisted steering the situation is different; the steering wheel response in the straight ahead driving position still needs to be very slightly indirect with a relatively high steering ratio, but with the power assistance provided the off-centre steering response for manoeuvring the vehicle can be made more direct compared with a manual steering with a slightly higher steering ratio. The use of a more direct low steering ratio when the road wheels are being turned on either lock is made possible by the servo action of the hydraulic operated power cylinder and piston which can easily overcome the extra tyre scrub and swivel-pin inclination resisting force. The variable-ratio rack is achieved by having tooth profiles of different inclination along the length of the rack, accordingly the pitch of the teeth will also vary over the tooth span.

With racks designed for manual steering the centre region of the rack has wide pitched teeth with a 40° flank inclination, whereas the teeth on either side of the centre region of the rack have a closer pitch with a 20° flank inclination. Conversely, power assisted steering with variable-ratio rack and pinion (see Fig. 9.37(c)) has narrow pitch teeth with 20° flank inclination in the central region; the tooth profile then changes to a wider pitch with 40° flank inclination away from the central region of the rack for both steering locks.

Pressure angle

Pressure angle

(a) Central rack teeth

Pressure angle - 40°

(b) Off-centre rack teeth Narrow pitch (p) Wide pitch (P)

(a) Central rack teeth

Wide pitch (P)

(b) Off-centre rack teeth Narrow pitch (p) Wide pitch (P)

Wide pitch (P)

(c) Variable-ratio tooth rack
Small p.c.d.

Transition

480 180 120 60 30 0 30 60 120 180 480

Turning steering wheel to left Turning steering wheel to right

Steering wheel and pinion rotation (deg) (d) Rack and pinion movement ratio from lock to lock of the steering wheel Fig. 9.37(a-d) Variable ratio rack and pinion steering suitable for power assisted steering

With variable-ratio rack and pinion involute teeth the rack has straight sided teeth. The sides of the teeth are normal to the line of action, therefore, they are inclined to the vertical at the pressure angle. If the rack has narrow pitch 'p' 20° pressure-angle teeth, the pitch circle diameter (2R) of the pinion will be small, that is, the point of contact of the meshing teeth will be close to the tip of the rack teeth (Fig. 9.37(a)), whereas with wide pitched 'P' 40° pressure-angle tooth contact between teeth will be near the root of the rack teeth (Fig. 9.37(b)) so its pitch circle diameter (2R) will be larger.

The ratio of steering wheel radius to pinion pitch circle radius (tooth contact radius) determines the movement ratio. Thus the smaller the pitch circle radius of the pinion for a given steering wheel size, the greater will be the movement ratio (see Fig. 9.37(d)), that is, a smaller input effort will be needed to steer the vehicle, but inversely, greater will be the steering wheel movement relative to the vehicle road wheel steer angle.

This design of rack and pinion tooth profile can provide a movement-ratio variation of up to 35% with the number of steering wheel turns limited to 2.8 from lock to lock.

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