GMC 1976 ZEO 6083 Maintenance Manual page 537

GENERAL INFORMATION
The housing and internal parts of the pump are
inside the reservoir so that the pump parts operate
submerged in oil. The reservoir is sealed against the
pump housing, leaving the housing face and the shaft
hub exposed. The reservoir has a filler neck fitted
with a cap. A shaft bushing and seal are pressed into
the housing from the front . The drive shaft is in-
serted through this seal and bushing. A large hole in
the rear of the housing contains the functional parts;
namely ring, rotor, vanes and plates . A smaller hole
contains the control valve assembly and spring .
The thrust plate (figure 2) is located by two
dowel pins on the inner face of the housing. This
plate has four central blind cavities for undervane oil
pressure . The two outer blind cavities direct dis-
charge oil through the two cross-over holes in the
pump ring (figure 3), through the pressure plate, and
into cavity 1 (figure 4) . The two outside indentations
in the thrust plate are for intake of the oil from the
suction part of the pump .
The pump ring (figure 3) is a plate having the
mating surfaces ground flat and parallel . The center
hole is a two lobed cam in which the rotor and vanes
operate. The ring is placed next to the thrust plate,
and located with the same dowel pins .
The pressure plate is fitted against the ring and
located with the same two dowel pins . This plate has
six through ports. The four central through ports
Figure 3-Pump Ring and Rotor
OPERATION
Slow Cornering (Figure 4)
STEERING
9- 5
connect from cavity 1 (figure 4) to supply undervane
oil pressure . The two outer ports pass oil under dis-
charge pressure to cavity 1 . The two indentations are
for oil intake from the suction part of the pump,
cavity 7 (figure 4) into the rotor.
The reservoir is for oil storage. It receives and
directs the return oil back to the make-up passage of
the pump .
The drive shaft is fitted with a pulley and is belt
driven from the crankshaft . The rotor is loosely
splined to the drive shaft and secured with a retain
ing ring . It is located centrally within the ring and
between the thrust and pressure plates . The ten vanes
are mounted in radial slots in the rotor (figure 3) .
The mode of operation of the power steering
pump is based upon the demand of the power steer-
ing gear . The various major modes of operation are:
slow cornering, moderate to high speed straight
ahead driving, and cornering against the wheel stop .
The pump is designed to recognize these conditions
as required by the steering gear valve and compen-
sates for them internally .
As the drive shaft turns the rotor, the vane tips
follow the inner cam surface of the pump ring, mov-
ing outward and inward twice during each revolu
tion . This results in a complete pumping cycle every
180 degrees of rotation (figure 3) . Oil is moved in the
spaces between the vanes. As the vane tips move
outward, oil is sucked into the intervane spaces
through four suction ports in the pressure and thrust
plates . The pressure of the oil is raised, and the oil
is discharged from the pump ring, as the vane tips
move inward . High pressure oil discharges into
cavity 1, (figure 4), through two open ports in the
pressure plate, and through two blind ports in the
thrust plate, which are connected to cavity 1 by the
cross-over holes in the ring . A portion of this oil is
circulated through the central port system in the
pressure plate, forcing the vanes to follow the cam
surface of the ring . The ring-rotor leakage oil (12) is
used for bushing lubrication and then bled to the
reservoir .
During slow cornering maneuvers, the oil pres-
sure required will usually not exceed 400 psi. The
RPM of the pump is not high enough to require
internal bypassing of oil, therefore, the pump bypass
port to (5) remains closed . The high pressure dis-
charge oil (7) is slightly lower in pressure than the
internal high pressure oil (1). The drop in pressure
occurs as oil flows through the flow control orifice
(2). This reduces the pressure at the bottom end of
the pump control valve (9) because the orifice (11) is