9
General Operation
What Happens During a Steering Maneuver
When the driver turns the steering wheel, he transmits force from the
steering wheel to the steering gear input shaft. A torsion bar, pinned
at its one end to the input shaft and at its other end to the worm shaft,
turns with the input shaft and exerts a rotational force on the worm
shaft. In response to this rotational force, the worm shaft, acting
through the recirculating ball mechanism, tries to move the rack piston
axially through the gear housing cylinder bore.
The rack piston's axial movement is resisted by its engagement to the
sector shaft, which is connected by linkage to the steered wheels.
Because of this resistance, the torsion bar is twisted by the input shaft,
thereby actuating the control valve. Pressurized fluid, directed by the
control valve, assists in moving the rack piston axially through the
cylinder bore. The rack piston then turns the sector shaft to steer the
vehicle.
Shock Loads to the Gear
If the steered wheels receive a shock load, the shock forces are trans-
mitted through the sector shaft to the rack piston, and on to the worm
shaft. The internal geometry of the steering gear causes the control
valve to send high-pressure fluid to the correct cylinder cavity to resist
the shock forces. By absorbing the shock forces hydraulically, the
steering gear prevents objectionable kickback at the steering wheel.
Unloading (Poppet) Valves
Most TAS gears are equipped with two unloading valves, one at each
end of the rack piston. One valve or the other, depending on the direc-
tion of turn, will trip as the steered wheels approach the axle stops
(which must be set according to manufacturer's specification). The
tripped valve reduces pressure in the gear and helps to reduce heat
generated by the pump. At the same time, the valves also reduce
forces on the steering linkage. These valves are automatically set to
axle stops after installation in vehicle at first full right and left turn.
Relief Valve
Some TAS gears, (with or without poppets), are supplied with a relief
valve. The relief valve limits maximum supply pressure to protect the
power steering gear, but it does not reduce pressure as the steered
wheels approach the axle stops.
Bleed Systems
Some TAS gears which are mounted with the output shaft above the
rack piston bore are equipped with either an automatic bleed system or
a manual bleed screw.
The procedures for Flushing and Air Bleeding the System are found in
the Steering Maintenance Guideline packet (TRW Document #800).
Replacement of damaged automatic bleed plugs, and manual bleed
screws is described on page 52.
