TM 5-3895-346-14
POWER STEERING
OPERATION
OPERATION
Pump Operation
Pumps are composed principally of a pressure plate, ring, rotor, vanes, and wear plate. (Refer to Figs. 1, 2, 3,
and 4. ) The rotor is driven within the cartridge by a drive shaft, coupled to a power source. As the rotor speed
increases, centrifugal action causes the vanes to follow the cam-shaped contour of the pump ring. (Fig.
5.)System pressure fed behind the vanes assures sealing contact of the vanes on the ring cam contour during
normal operation.
The ring is shaped so that two opposing pumping chambers are formed, thus canceling any hydraulic loads on
the bearings. Radial movement of the vanes, and rotation of the rotor, causes the chamber area between
vanes to increase in size at the inlet (large diameter) section of the ring. This results in a low pressure, or
vacuum in the chamber. This pressure differential causes oil to flow into the inlet, where it is trapped between
the rotating vanes and is forced, through porting in the pressure plate to discharge into the system as the
chamber size decreases at the pressure quadrant (small diameter) of the ring.
Flow Control and Relief Valve
Maximum pump delivery and maximum system pressure are determined by the integral flow control and relief
valve in a special outlet cover used on pumps in this series. This feature is illustrated schematically in Fig. 6.
An orifice in the cover limits maximum flow. A pilot-operated-type relief valve shifts to divert excess fluid
delivery to tank, thus limiting the system pressure to a prescribed maximum.
Fig. 6 shows the condition when the total pump delivery can be passed through the orifice. This condition
usually occurs only at low drive speeds. The large spring chamber is connected to the pressure port through an
orifice. Pressure-in this chamber equalizes pressure at the other end of the relief valve spool and the light
spring holds the spool closed. Pump delivery is blocked from the tank port by the spool land.
When pump delivery is more than the flow rate determined by the orifice plug, a pressure buildup forces the
spool open against the light spring. Excess fluid is throttled past the spool to the tank port as shown in Fig. 6.
If pressure in the system builds up to the relief valve setting (Fig. 6), the pilot poppet is forced off its seat. Fluid
in the large spring chamber flows through the spool and out to tank. This flow causes a pressure differential on
the spool, shifting it against the light spring. All pump delivery is thus permitted to flow to tank.
Operating Instructions
Normally, these pumps require no manual priming. However, it is essential that, after starting, a minimum
drive speed of 600 RPM be held until the pump picks up its prime and pressure is built up in the system.
Failure to observe the above precaution can result in scoring and possible seizure of the pump due to a lack of
oil for lubrication.
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