S O far we have dealt with two items at opposite
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ends of the air-brake system —the slack adjuster and the foot operating valve—and now it may be more sensible If we follow the pattern of the system item by item, from air input at the compressor to the point where the brake shoe meets the drum.
The standard commercial vehicle compressor is a reciprocating piston-type pump, made to suit the lighter vehicles, with an input of around 5 cult/min. of free air, and increasing in capacity with vehicle size up to the heavy vehicle and trailer calling for around 15 cult/min.
The normal operating speed is around 1,000 r.p.m., the top operating pressure around 105 lb. p.s.i., and the cylinder needs to be air or water cooled. Without cooling, lubrication would fail, causing rapid wear or seizure: therefore cooling is essential.
A flat-topped piston is used, fitted with two normal compression rings, with a scraper ring fitted at the bottom of the skirt. The moving parts are lubricated either from the main engine supply, or from a small sump and delivery pump inside the compressor; usually the big end bearing is pressure fed, other parts living off splash.
To pump air and store it under pressure calls for the use of valves, and these are in the simplest possible form—two flat discs per cylinder covering perfectly flat mated surfaces with light coil springs holding them on their seats, where they will hold inlet and delivery ports closed until the pump is revolved.
As the pump is going to take in air, it follows that this air can be cold, wet and dirty, and dangerous in all three forms. Therefore, this air must be filtered to keep out abrasive dirt, and in winter time the inrushing air must be clear of moisture that can freeze, either by passing the air through an anti-freeze device, or by turning the pump inlet to face the exhaust system where it can breathe warm air.
To describe the pump in action, the first downward movement of the piston creates a vacuum below the inlet valve disc, normal air pressure above at 14.7 lb. depresses this lightly loaded inlet valve, and air passes through the now open valve until the swept area of the cylinder is full and also at 14.7