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 manufacturers
maintenance manual for the correct type of oil and the
proper procedure for checking and topping off.
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.