TM 5-3810-307-24-1-2
3.
FUNCTION AND ADJUSTMENT OF HYDRAULIC STEERING LIMIT (STEERING
LOCK)
NOTE
Find numbers in ( ) refer to the illustrations in Section 12.
3.1
ZF Single Circuit Power Steering Gear, Type 7421
3.1.1
Function
The housing cover (21) holds the control piston (224) which is axially moved by the teeth at face
end on sector shaft. On one side of the control piston are two steering limit valves (231), their balls
are moved by cones on control piston via pins (229). When the sector shaft is rotated, the pistons
will remain in rest position until a cone of the control piston encounters and thereby raises a pin. As
a result, the hydraulic steering limit can be separately controlled for each steering circuit. The two
steering limit valves are connected with remote control valve by means of a common control line.
The steering limit valves are simultaneously connected with the power cylinders by way of check
valves. When the control piston is moved to the right when the steering wheel is turned, steering
limit valve (b2) will open after a given pitman arm deflection. The opening point of the steering limit
valve can be changed by screwing in or out.
The result is that the oil pressure in the control line to remote control valve drops and the oil will flow
through the opened check valves in the power cylinder sides connected to the return flow line. The
connections of the power cylinders are selected in such a manner that with the steering limit valve
opened no pressure rests against check valve or that the respective power cylinder side is
connected with the return flow line. The remote control valve opens so that the oil flow generated by
the pumps flows into the return line. The steering limit valve (a2) remains closed during this
procedure.
If the control piston is moved to the left when the steering wheel is turned, the steering limit valve
(a2) will open after a given pitman arm deflection, so that the pressure of the oil in the control line
will drop and the remote control valve will let the oil flow from the pump into the return line.
With the steering limit valves opened the steering can be turned up to stop in steering or in power
cylinder at increased manual force and considerably reduced hydraulic support (e.g. with drag link
disconnected).
3.1.2
Adjustment
Screw in pressure gauge (pressure range up to 250 bar) into pressure line between pump and
pressure connection 1 of steering control valve (refer to Fig. 3) and jack steering axle(s). Position
vehicle jack at axle. If the vehicle has no rigid steering axle(s), the steered wheels for adjusting the
hydraulic steering limit must be placed on rotary supports; the steering axle must be definitely
loaded for the approximate compensation of deflection faults which may occur while measuring. On
vehicles with hinged frame steering no axle need be jacked up. Turn steering with engine running
without major manual force up to wheel lock. Have another mechanic hold a spacing sheet 2 - 3
mm thick between the wheel lock components so that a small reserve is still available.
