912 Screw and nut steering gear mechanism

To introduce the principles of the steering gearbox, the screw and nut type mechanism will be examined as this is the foundation for all the other types of steering box gear reduction mechanisms.

A screw is made by cutting an external spiral groove around and along a cylindrical shaft, whereas a nut is produced by cutting a similar spiral groove on the internal surface of a hole made in a solid block.

The thread profile produced by the external and internal spiral grooves may take the form of a vee, trapezoidal, square or semicircle, depending upon the actual application.

A nut and screw combination (Fig. 9.2) is a mechanism which increases both the force and

Fig. 9.2 Screw and nut friction steering gear mechanism

movement ratios. A small input effort applied to the end of a perpendicular lever fixed to the screw is capable of moving a much larger load axially along the screw provided that the nut is prevented from rotating.

If the screw is prevented from moving longitudinally and it revolves once within its nut, the nut advances or retracts a distance equal to the axial length of one complete spiral groove loop. This distance is known as the thread pitch or lead (p).

The inclination of the spiral thread to the perpendicular of the screw axis is known as the helix angle (a). The smaller the helix angle the greater the load the nut is able to displace in an axial direction. This is contrasted by the reduced distance the nut moves forwards or backwards for one complete revolution of the screw.

The engaged or meshing external and internal spiral threads may be considered as a pair of infinitely long inclined planes (Fig. 9.3(a and b)). When the nut is prevented from turning and the screw is rotated, the inclined plane of the screw slides relative to that of the nut. Consequently, a continuous wedge action takes place between the two members in contact which compels the nut to move along the screw.

Because of the comparatively large surface areas in contact between the male and female threads and the difficulty of maintaining an adequate supply of lubricant between the rubbing faces, friction in this mechanism is relatively high with the result that mechanical efficiency is low and the rate of wear is very high.

A major improvement in reducing the friction force generated between the rubbing faces of the threads has been to introduce a series of balls (Fig. 9.4) which roll between the inclined planes as the screw is rotated relatively to the nut.

The overall gear ratio is achieved in a screw and nut steering gearbox in two stages. The first stage occurs by the nut moving a pitch length for every one complete revolution of the steering wheel. The second stage takes place by converting the linear movement of the nut back to an angular one via an integral rocker lever and shaft. Motion is imparted to the rocker lever and shaft by a stud attached to the end of the rocker lever. This stud acts as a pivot and engages the nut by means of a slot formed at right angles to the nut axis.

Square Cut Screw From Steering Gear Box
Fig. 9.4 Screw and nut recirculating ball low friction gear mechanism
Fig. 9.3 (a and b) Principle of screw and nut steering gear

Forward and reverse efficiency The forward efficiency of a steering gearbox may be defined as the ratio of the output work produced at the drop arm to move a given load to that of the input work done at the steering wheel to achieve this movement.

i.e. Forward efficiency =

Output work at drop arm

Input work at steering wheel

Conversely the reverse efficiency of a steering gearbox is defined as the ratio of the output work produced at the steering wheel rim causing it to rotate against a resisting force to that of the input work done on the drop arm to produce this movement.

i.e. Reverse efficiency =

Output work at steering wheel

Input work at drop arm

A high forward efficiency means that very little energy is wasted within the steering gearbox in overcoming friction so that for a minimum input effort at the steering wheel rim a maximum output torque at the drop arm shaft will be obtained.

A small amount of irreversibility is advantageous in that it reduces the magnitude of any road wheel oscillations which are transmitted back to the steering mechanism. Therefore the vibrations which do get through to the steering wheel are severely damped.

However, a very low reverse efficiency is undesirable because it will prevent the self-righting action of the kingpin inclination and castor angle straightening out the front wheels after steering the vehicle round a bend.

helix angle (wedge angle) is much too small when the nut is made to move the screw. Thus when the mechanism is operated in the reverse direction the efficiency (reverse) is considerably lower than when the screw is moving the nut. Only if the inclined plane angle was to be increased beyond 40° would the nut be easily able to rotate the screw.

The efficiency of a screw and nut mechanism will vary with the helix angle (Fig. 9.5). It will be at a maximum in the region of 40-50° for both forward and reverse directions and fall to zero at the two extremes of 0 and 90° (helix angle). If both forward and reverse efficiency curves for a screw and nut device were plotted together they would both look similar but would appear to be out of phase by an amount known as the friction factor.

Selecting a helix angle that gives the maximum forward efficiency position (A) produces a very high reverse efficiency (A') and therefore would feed back to the driver every twitch of the road wheels caused by any irregularities on the road surface. Consequently it is better to choose a smaller helix angle which produces only a slight reduction in the forward efficiency (B) but a relatively much larger reduced reverse efficiency (B-B'). As a result this will absorb and damp the majority of very small vibrations generated by the tyres rolling over the road contour as they are transmitted through the steering linkage to the steering gearbox.

A typical value for the helix angle is about 30° which produces forward and reverse efficiencies of about 55% and 30% without balls respectively. By incorporating recirculating balls between the screw and nut (Fig. 9.4) the forward and reverse efficiencies will rise to approximately 80% and 60% respectively.

Relationship between the forward and reverse efficiency and the helix angle (Figs 9.3, 9.4 and 9.5) The forward efficiency of a screw and nut mechanism may be best illustrated by considering the inclined plane (Fig. 9.3(a)). Here the inclined plane forms part of the thread spiral of the screw and the block represents the small portion of the nut. When the inclined plane (wedge) is rotated anticlockwise (moves downwards) the block (nut) is easily pushed against whatever load is imposed on it. When the screw moves the nut the condition is known as the forward efficiency.

In the second diagram (Fig. 9.3(b)) the block (nut) is being pressed towards the right which in turn forces the inclined plane to rotate clockwise (move upward), but this is difficult because the

S 80

S 80

Maximum forward

A'

r— Maximum reverse

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40

50 60

70

Fig. 9.5 Efficiency curves for a screw and nut recirculating ball steering gear

Helix angle (cleg]

Fig. 9.5 Efficiency curves for a screw and nut recirculating ball steering gear

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