outer rotor. The inner rotor is off center from the outer
As the oil pump shaft turns, the inner rotor causes
the outer rotor to spin. The eccentric action of the two
rotors forms pockets that change size. A large pocket is
formed on the inlet side of the pump. As the rotors turn,
the oil-filled pocket becomes smaller, as it nears the
outlet of the pump. This action squeezes the oil and
makes it spurt out under pressure. As the pump spins,
this action is repeated over and over to produce a
relatively smooth flow of oil.
The GEAR pump (fig. 6-22) consists of two pump
gears mounted within a close-fitting housing. A shaft,
usually turned by the distributor, crankshaft, or
accessory shaft, rotates one of the pump gears. The gear
turns the other pump gear that is supported on a short
shaft inside the pump housing.
As a safety factor to assure sufficient oil delivery
under extreme operating conditions, the oil pump (gear
or rotary) is designed to supply a greater amount of oil
than is normally required for adequate lubrication. This
requires that an oil pressure relief valve be incorporated
in the pump to limit maximum oil pressure.
The pressure relief valve is a spring-loaded bypass
valve in the oil pump, engine block, or oil filter housing.
The valve consists of a small piston, spring, and
cylinder. Under normal pressure conditions, the spring
holds the relief valve closed. All the oil from the oil
pump flows into the oil galleries and to the bearings.
However, under abnormally high oil pressure
conditions (cold, thick oil, for example), the pressure
relief valve opens. Oil pressure pushes the small piston
back in its cylinder by overcoming spring tension. This
allows some oil to bypass the main oil galleries and pour
back into the oil pan. Most of the oil still flows to the
bearings and a preset pressure is maintained. Some
pressure relief valves are adjustable. By turning a bolt
or screw or by changing spring shim thickness, the
Oil on the inlet side of the pump is caught in the gear
teeth and carried around the outer wall inside the pump
housing. When oil reaches the outlet side of the pump,
the gear teeth mesh and seal. Oil caught in each gear
tooth is forced into the pocket at the pump outlet and
pressure is formed. Oil squirts out of the pump and to
the engine bearings.
pressure setting can be altered.
Figure 6-22.Gear-type oil pump.