and to the discharge port. The rear pump flow pass-
Power steering pump and flow sensor
es through radial holes (3) in the control spool to
the center of the spool where it joins with the flow
The Tyrone power steering unit is very similar in
from the front pump and goes to the discharge port.
construction to the power sensor unit. It is made
up of two piggyback pumps with a common suction,
As pump RPM increases, pump flow increases. As
a n d a valve bolted to the pumps so that it re-
pump flow increases oil flows through the hole (4)
c e i v e s the flow from both pumps. The valve is
at the right end of the control spool and builds pres-
speed sensitive and controls pump flow according
sure in the cavity (5). When pump volume is suffi-
to the RPM at which the pumps rotate.
cient to create more pressure in the cavity (5) than
there is spring tension, the spool will shift to the
The purpose of this unit IS to provide a more uni-
left.
form steering time from stop to stop, regardless
of engine RPM's, From low idle to approximately
As the spool moves to the left, it begins to bypass
1600 RPM. both sections of the pump supply the
some oil from the front pump back to suction (See
steering system. From 1600 RPM to maximum RPM,
position 2). The amount of oil bypassed from the
the control valve unloads one pump by bypassing
front pump is determined by the pressure differen-
its flow back to the suction side of the pumps - the
tial on the spool caused by the amount of flow from
other pump continues to supply the steering sys-
the rear pump. When the spool is completely shifted
tem
to the left (See position 3), all the oil from the
front pump is bypassed to suction.
Operation of Unit (See page 195)
As the speed of the pump decreases, the flow
When the RPM of the pumps is below a prede-
t e r m i n e d value, both pumps deliver flow to the
from the rear pump decreases, the pressure dif-
discharge port of the control valve. The flow from
f e r e n t i a l acting on the spool decreases and the
the front pump passes around the control spool and
spool moves to the right, thus bypassing less oil
through the holes at (1), past the check valve (2),
to the suction side.
[42]
