Know Your Air Brakes
Page 43
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Part 4—Twin-cylinder Compressors
FROM the single-cylinder compressor described last week we will now take a look at a heavier range of compressors capable of much greater capacity. Again, Clayton-Dewandre makes various models of twin-cylinder pumps, both airand watercooled, although with these heavier models it is common practice to mount the unloader valve on the air reservoir, or in-line between compressor and reservoir.
The design of the pump remains the same —disc valves and springs, automatic in operation—and these pumps can work in either direction. On the water-cooled models the cylinder heads themselves are water jacketed and fed from the vehicle cooling system. Again, lubrication is pressure-fed to the big-end bearings with splash feed to other working parts. With the unloader valve mounted away from the compressor, the feature of governing and pressure control is arranged differently. Instead of the compressor being permitted to freewheel, as it were, when the pressure is at maximum, the pump continues to operate normally and the unloader valve is designed to exhaust the surplus air to atmosphere. As this valve has special features it is worthy of separate examination in another article.
To return to compressors, the Westinghouse company makes a water-cooled twincylinder pump, the TU-Flo 500. This is capable of really high speeds—up to 3,000 r.p.m., in fact—with a swept volume of 28 Cu. ft./min. This represents quite a load, and at a working pressure of 100 lb., 5 h.p. is accounted for. Lubrication is similar to the Clayton pumps, with only the big ends pressure fed. On this pump, however, it is necessary that the cylinder bores as well as the cylinder heads are water cooled; therefore, the water jacket is very similar to that of the c.i. engine.
Pressure control is arranged through unloader mechanism in the pump. Operating against the inlet valve, a governor valve mounted on the pump and in circuit with the air reservoir, actuates the unloading pistons mounted below the inlet valves. These pistons lift and reseat the inlet valves, as the reservoir pressure rises and falls; thus the system is safeguarded and the pump can freewheel when the governed pressure is reached. An important feature of this pump, particularly in respect of its high capacity, is in connection with the air input side. The air supply can be drawn either from the main engine air cleaner, an anti-freeze unit, or the standard Westinghouse strainer, mounted directly on the pump. This strainer protects the pump, ensures long life to valves throughout the system and considerably reduces input noise.
The strainer is packed with curled hair, and at each vehicle service period the strainer should be stripped, all parts washed clean, including the curled hair. The hair should be soaked in engine oil, squeezed dry and repacked. Where the pump has its own lubrication system, this system should have the same oil 'change attention as the main engine.