Westinghouse Introduces a Pressure Servo Brake
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DOWER-ASSISTED braking, in which compressed air 1 provides the motive force, is, we believe, a new development in the commercial-vehicle field. Several systems employing vacuum are, of course, in general use, whilst the same applies to pneumatic brakes incorporating no servo gear, First-class examples of both types are among the well-known products of the Westinghouse Brake and Signal Co., Ltd., 82, York Road, King's Cross, London, NJ, which concern has now evolved a pressure servo system.
In its simplest form the new Westinghouse apparatus transmits braking force solely by mechanical means. It can, however, be interconnected with the Lockheed hydraulic transmission system, whilst provision is made for connecting the main air cylinder, by means of piping, with separate cylinders on the front axles.
Reference to the accompanying diagrammatic sectional drawing will enable this description of the working of the device to be followed. Movement of the brake-pedal rod in a left-hand direction, as drawn, first takes up slack in the whole system of rods and levers. Then point B becomes a fulcrum and lever A swings through a few degrees, moving the control-valve rod to the right and compressing spring F.
This causes motion of the control piston, against the reltaSe spring. It travels to the right; and, in doing so, first clOses the exhaust valve, and then opens the inlet valve. The main compressed-air reservoir is in direct communication with the inlet-valve chamber, consequently, so soon as the valve opens, air flows, by port D, into the brake cylinder, thus applying the brake through lever C.
There is also a flow of air through the small passage E, which causes pressure to build up behind the control piston until enough force is exerted upon it further to compress spring F, with the result that the piston travels to he left and allows the inlet valve to close.
The brake is, therefore, held in this position—applied with a certain degree of pressure—until the driver either increases or diminishes the effort with which he is depressing the pedal. If he applies additional force, the control piston is moved again to the right, more air admitted, and a state of equilibrium reached by the same sequence
of events as before. .
If he releases the pedal, the piston travels farther to the left and allows the exhaust valve to open and air to escape from the cylinder. Thus it is clear that force applied to the pedal produces a directly proportional force on the rod to the brake.
The adjustable and fixed stops serve two purposes. Should no air pressure be available, the former abuts against the latter, providing a fulcrum at the base of lever A, so that the brake can be applied purely by muscular effort, whilst the adjustment affords an easy means for setting the maximum braking force obtainable irrespective of the pressure of the air in the reservoir. This is an important point enabling one model to be used for various powers. Access can be gained to the stop by removing a Small bottom cover.
The main advantage of the compressed air system is that greater power is available with less bulky apparatus, and that a greater reserve can be maintained. The Westinghouse apparatus is compact, and embodies well-tried features, the valve being similar to that in the company's air-pressure system. .