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Page 58
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New Steering Gear
A NY component for a commercial vehicle which possesses the merit of simplicity attracts much attention on that account alone. When it has the added advantages of easy and cheap manufacture it is all the more striking. Such is the state of affairs appertaining to the Douglas steering set made by the B.M.D. Gear Co., Lansbury, The Avenue, Leagrave; Luton, Bedfordshire. Reference to the sectional drawing which we reproduce shows that the steering column carries a screw, the two being welded together. The screw works inside a split nut, which forms the lower bearing for the column ; the upper bearing—of the four-point, ball type, takes end thrust and permits instant tightening. The nut is split to permit adjustment by closing more tightly the nut around the screw. This is effected by the removal of a plug in the casing, the insertion of a special key and rotation of the self-locking screw.
-This nut, which has a large bearing surface in the case, slides some 21 ins. from full lock 'one way to the opposite end of its travel. In the side of the nut is a hardened bush, into which fits a hardened ball on the end of the stamped lever on the drop-arm shaft.
It will be realized that the nut has a partially rotary motion, in addition to its sliding in the casing. In the cen
tral part of the travel, with the front wheels straight ahead, the normal speed of action of the screw is transmitted to the drop arm, but on right lock the movement is accelerated, whilst on left Jock it is decelerated, in order to suit Ackerman requirements. If a variation of this characteristic be desired it can easily be arranged in the course of manufacture by locating the rocker shaft to the right or left of the centre line.
Sliding friction takes place between the flanks of the threads on the screw and nut ; this gives the required damping action. Indentation shocks are taken by line contact around the rocker-arm ball ; as these do not occur • always in the same area, added to the fact that the ball makes but one-fifth of a revolution in its full travel, giving, therefore, practically stationary contact at the moment of shock, the life of the gear should be long.
Care is taken to see that the screw and nut are of equal length; this prevents unequal wear and tight or loose places are avoided. There is no backlash at any portion of the rocker-arm travel. As the inner column always remains concentric with the outer column or casing, the installation of central controls is facilitated.
As a result of the generous bearing areas provided wear has been found to be negligible. After one special test, designed to simulate 20,000 miles' work in bad circumstances, there was fin. added backlash at the rim of a 17-in. wheel.