Know Your Air Brakes
Page 47
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Part 3—Single-cylinder Compressors
lb. p.s.i. At this point the spring returns the valve disc to its closed position and as the piston starts to rise, the air in the cylinder is compressed under the delivery valve, the pressure building up until the delivery valve spring is overcome and the valve opens to let the compressed air through.
This is the two-cycle operation complete, as far as the pump is concerned, but there is another necessary feature worked into the operation. This is the unloader valve, the purpose of which is to govern the pressure in order not to overload the pump or air-storage tank. In addition to the unloader valve it is the normal practice to introduce a safety or blow-off valve, which will open at around 150 lb. and so safeguard the systems in the event of the unloader valve sticking.
The operation of this valve—the unloader —is simple, and again the feature of balance I mentioned earlier under operating valves is used. The unloader valve is in two operat
ing parts the governor valve and the unloader. The governor valve is a ball and seating and the entry to this ball valve is coupled to the air tank. Thus, as top pressure is reached in the tank, the ball is moved off its seat and air then passes to the unloader chamber. This depresses the unloader plunger, which moves into contact with the normal inlet valve in the cylinder head, depressing this valve and thus stopping further air being compressed.
The compressor will then run under no-load conditions until the vehicle brake is operated and the air pressure in the tank falls. As the pressure falls the ball valve returns to its seat, cutting off the air from the tank. The plunger is then able to return under its own spring tension, and the inlet valve, in closing, brings the pump into operation again.
This is the single-cylinder compressor in its simplest form, a Clayton-Dewandre with, in this instance, the unloader, governor and safety valves handily mounted for easy servicing.