MAINTENANCE OF FORKLIFTS - OD1660 - LESSON l/TASK 1
pressure from the main hydraulic pump while the
rear
steering
operates off pressure from the rear steering pump.
(2) The power steering control valves generally use the hydraulic
pressure in the steering system to urge the valve spool to its
neutral position.
This is called hydraulic reaction.
As the
pressure in the hydraulic cylinder builds up, the hydraulic centering
force increases.
Thus, the operator's effort on the steering wheel
in turning a corner is greater than in a straight ahead road
correction condition.
Because the hydraulic pressure is low in the
straight ahead driving condition, it is necessary to add the
centering springs to assist, the hydraulic reaction in giving the
system road feel.
The feel and steering effort created by the
centering springs is constant for any steering condition and is
generally tailored to suit the vehicle application by changing the
spring load to give the degree of steering effort desired.
(3) When the valve is in the center position, the oil pressure at
its two cylinder ports is low and equal and produces ineffective
forces in the cylinder.
This results in no movement of the piston
and no circulation of oil in the lines to the cylinder; however, oil
is circulating from the pump through the control valve to the
reservoir with only sufficient pressure to overcome friction of the
lines and the fittings.
(4) Whenever the operator's efforts at the steering wheel overcome
the centering effect of the springs and/or the hydraulic reaction,
the valve spool is moved axially, restricting one of the return
passages to the outlet port, thus causing an immediate increase in
pressure at one of the cylinder ports and in one end of the cylinder.
At the same time, the other passage is enlarged, allowing the oil
from the discharging end of the cylinder free passage to the outlet
port and return to the reservoir. The immediate effect is increased
pressure in one end of the cylinder to actuate the piston which
applies hydraulic power directly to the steering gear pitman arm or
linkage part to which the cylinder is attached.
Full pressure is
obtained with a spool travel of about sixty-five one-thousandths of
an inch. The slightest movement results in a pressure differential.
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