c. Lubricating System. The crankcase rotating and reciprocating parts are lubricated by a positive
displacement rotary gear type oil pump. Refer to FIGURE 1-4. Oil is drawn up from the crankcase oil sump
through an oil strainer to the oil pump. The pump forces oil, under pressure, through the crankshaft and
connecting rods to lubricate the crankpin journals, the main journals, the wrist pin bearings, and the cylinder
(1) Oil Pump. The oil pump is an integral gear type pump with an adjustable pressure relief valve.
Refer to FIGURE 1-5. It is flange-mounted, piloted to the oil pump housing, and direct driven by the
(2) Oil Pressure Gauge. Normal oil pressure is between 18 and 20 psig.
d. Drive Power. A motor pulley diameter is selected to obtain the pressure and air delivery without
overloading the motor or operating the compressor beyond or below the designed speed range. The motor
pulley is coupled to the compressor sheave using vee belts. Refer to FIGURE 1-6.
e. Hydraulic Unloader Valve. The hydraulic unloader valve permits the driving unit and compressor to
attain full rated speed, and assures that oil pressure has been established before the compression of air begins.
The hydraulic unloader valve is considered an integral part of the air compressor. Refer to FIGURE 1-7.
f. Mode of Control. Receiver or plant air system pressure is controlled within limits by automatically
stopping and starting the compressor as the air pressure reaches a maximum preset pressure and then drops to
a minimum preset pressure.