maintain proper oil temperature by varying the
OIL PRESSURE RELIEF VALVE.-- An oil
pressure relief valve limits the maximum pressure
the oil cooler. A cutaway view of a thermostatic
within the system. The relief valve is preset to
bypass valve is shown in figure 5-9. This valve
relieve pressure and return the oil to the inlet side
consists of a valve body (having two inlet
of the lube pump. This valve is important if the
ports and one outlet port) and a spring-loaded
system has an oil cooler, since the cooler's thin
thermostatic element valve.
walls rupture easily.
The valve is spring-loaded so the valve will
open (bypassing the cooler) if the pressure through
CHECK VALVES.-- Check valves installed
the oil cooler drops too much because of denting
in the oil supply lines or filter housings pre-
or clogging of the cooler tubing.
vent oil in the reservoir from seeping (by
gravity) into the engine after shutdown. Check
valves prevent accumulations of undue amounts
of oil in the accessory gearbox, rear of the
The filters are an important part of the
lubrication system, since they remove most foreign
Such accumulations could cause excessive loading
particles in the oil. Without some type of filter
of accessory drive gears during starts, contamina-
in the oil system, dirt or metal particles suspended
in the oil could damage bearings, clog passages,
fires, and hot starts.
The check valves are usually of the spring-
and cause engine failure.
The filter bypass valve allows oil to flow
loaded ball-and-socket type, constructed for free
flow of pressure oil. The pressure required to open
around the filter element if it becomes clogged.
The bypass valve opens whenever a certain
these valves will vary. Most valves require from
pressure differential is reached because the filter
2 to 5 psi to permit oil to flow to the bearings.
became clogged. When this occurs, the filtering
THERMOSTATIC BYPASS VALVES.--
action is lost, allowing unfiltered oil to be pumped
Thermostatic bypass valves are included in oil
to the bearings. This is a dangerous situation;
however, unfiltered oil is better than no oil.
systems using an oil cooler. Their purpose is to
Figure 5-9.-Thermostatic bypass valve.