A Servo Steering Mechanism
Page 35
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With Rotary Control
New A.E.C. Dcsign Employing Pressure Fluid and Devised to Function Under Only Heavy Torque
INapplying the servo principle to the steering gear of heavy road vehicles several considerations arise which distinguish this specific application from the embodyment of the scheme in other mechanisms, such as brale systems. As an example, may be named the desirability of incorporating a means for rendering the servo device operative only when a' muscular effort above a pre-determined magnitude is .exerted on tie steering wheel. Another instance is the importance of ensuring a quick reversal of the 'action of the servo motor.
A design, in which attention has been paid to these points, has just been evolved by the Associated Equipment Co., Ltd., Southall, Middlesex; it is also characterized by the fact that the control apparatus comprises a rotary device built into the hub of the steering wheel so that it can be fitted' to a standard steering column.
It is explained that a servo steering system in which all movements of the wheel energize the servo motor is extravagant of pressure fluid—compressed air or liquid—and, Moreover, that power is unnecessary except when the driver requires to lock over the front wheels quickly and to a considerable extent.
In the A.E.C. system a double-acting cylinder operates a suitable member of 'the steering linkage, and is connected to the valve (1) under the steering wheel by two pipes from the two ends of the cylinder. One of these connections is indicated at 2 in the sketch above.
A spindle (3) actuates the valve, receiving rotary motion through an arm (4) from a collar (5). The lastnamed is free to move axially on the steering column, with which it turns.
A bell-crank (6) causes the collar to rise and fall, its upper arm engaging a slot (7) in the hub of the wheel itself. Thus, angular mdvement of the wheel relative to the column imparts rotation to the valve spindle (3).
Under light effort, the steering wheel does not move relatively to the column, because the torque is trans.
mated direct, via the medium of a spring-loaded plunger (8) engaging a
recess in a flange secured to the column. Under greater effort, however, the plunger is displaced and the
valve is accordingly operated. In these circumstances torque is transmitted from the wheel to the steeringz column flange through spring abutments (9).
Turning now to the actual elements of the valve, we find that part 10 is fixed and part II movable. Pressure
fluid enters at 12. By turning valve7 element 11 in the appropriate direction, fluid is admitted by slot 13 to port 14.
and thence, via passage 2 to the power cylinder. At the same timE slot 15 puts the other cylinder pipe in connection' with an exhaust port (16). When the valve is in its inoperative position, slot 15 connects ports 17 and 18, so that the pressures on the two sides of the piston of the power-cylinder are balanced.
As a result of this arrangement, cylinder pressure does not normally fall to atmospheric pressure, with the result that the servo motor operates without the delay of preliminary building up of pressure.
• The apparatus is .described in full detail in recently published patent specification No. 549,354, which also bears the name Cl S. R. Stock, and from which the data for the accompanying drawings have been obtained.