General Design
Description of the TRW Commercial Steering
Division Power Steering Pump
The PS power steering pump is a balanced, positive
displacement, sliding vane type, two line pump with an
internal pilot operated flow control and relief valve. The
components of this description are broken down and
explained below:
Balanced
The pumping element has two pumping pockets opposed
180° from each other which balance the internal forces
due to the pressure generated by the pumping action.
Positive Displacement
The pump will output a fixed volume for each revolution
of the input shaft. This fixed volume is determined by the
internal contour of the cam ring.
Sliding Vane Type
This describes the type of pumping element. The PS
pumping element consists of three components.
1 The rotor that holds the vanes and is driven by
the engine with the pump input shaft.
2 The vanes that slide back and forth in slots in the
rotor while following the internal contour of the
cam ring as the rotor is being turned, thus a
sliding vane.
3 The cam ring that contains the internal contour
that defines the amount of fluid that is output
with each revolution of the rotor.
Two Line
The PS pump requires an inlet line to supply oil to the
pump and an outlet line to take the oil supplied by the
pump to the steering gear. All excess, (bypass), oil is
diverted internally in the pump housing back to the inlet
of the pumping element. Other systems may have a third
line which takes this excess oil back to the reservoir.
Internal Flow Control Valve
The pump has a pilot operated valve built into the pump
housing that will control the amount of oil that is output to
the steering gear. This allows the output flow to remain
within specification for almost any input speed variation.
Relief Valve
The pump has a pilot operated relief valve built into the
flow control valve spool which will limit the maximum
pressure the pump can produce. When the pressure limit
has been reached, the relief section will cause the flow
control to bypass more oil internal to the pump, limiting
the outlet pressure.
General Operation
What is a Power Steering Pump?
The pump is the heart of the hydraulic steering system. It
converts the rotational energy supplied by the engine into
hydraulic energy, flow and pressure, for use by the
steering gear.
Theory of Operation
All pumps function by creating a partial vacuum at the
inlet, which causes atmospheric pressure to force fluid
into the pump from the reservoir. The pump then pushes
this fluid into the system for use. The fluid is used to
power the steering gear. Pump output flow relates to
steering gear speed and pump output pressure relates to
steering gear force (work).
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