by the air as it is being compressed. The lubricating oil is force fed to the required lubricating points by a means called a pressure differential system. Figure 8-19 shows the operation of this lubrication system, trace it as you follow the text. As the unit is started, air begins the compression cycle leaving the compressor and entering the air receiver. A factory-set minimum pressure valve, located on the air receiver, remains closed to allow rapid buildup of air pressure. The high pressure air in the air receiver is the force that moves the oil through the oil lines to the working parts of the compressor. An oil filter is placed in the system to remove impurities. After leaving the filter, a thermostatic control valve directs heated oil through an oil cooler to keep the oil temperature between 130°F and 180°F. Oil already cool bypasses this step. The oil is then directed to the intake side of the compressor where it is injected into the cylinder (vane type) or dual-bore cylinders (screw type) for sealing purposes and to cool the air as it is being compressed. Oil is also directed into the air intake control assembly and all bearings and other moving parts at the same time. The air-oil mix exits the compressor at the discharge end and re-enters the air receiver. The oil is removed from the air by means of an air-oil-labyrinth-separator which returns it to the sump where it starts the cycle again.

Some vane and screw type of air compressors use a mechanical type of oil pump in the lubrication system. You should check the level of the compressor oil daily, before operation. Refer to the manufacturer's maintenance manual for the correct type of oil and the proper procedure for checking and topping off.

CAUTION

Because the system is under high pressure, the vane and screw types of air compressors must be shut down and unloaded before oil is added to the system.

Preventive maintenance procedures for all three types of air compressors are outlined in current manuals for the unit you are working on or operating. USE THEM!!! Oil should be changed according to these manuals, in most cases, at 500 hour intervals. The compressor oil filter and air separator should not be overlooked and the air filter, taking into account operating conditions, should be inspected daily. When you operate air compressors at any time, do not leave the unit unattended while it is running.

Figure 8-19.-Rotary vane air compressor lubrication system.