CONSTRUCTION AND FUNCTION ZF SINGLE CIRCUIT POWER STEERING
GEAR, TYPE 7421
The ZF Single Circuit Power Steering Gear comprises a completely mechanical steering gear unit,
where the power is transmitted from the steering spindle to the steering nut and from there to the
sector shaft. In a single circuit power steering gear there is one hydraulic control valve before
steering nut, centrally in relation to steering spindle and organically attached to steering case.
The valve in front of the steering nut controls steering circuit I with either a drive dependent pump or
an engine-driven pump and an additional drive-dependent pump (3-pump steering system). If one
circuit fails - for example circuit II because the engine has stopped - hydraulic assistance is
maintained by the additionally connected emergency steering pump which is driven by the forward
movement of the vehicle. The valve housings of each steering circuit accommodate a replenishing
valve, through which oil can be in-taken from the return line when one circuit is without hydraulic
assistance, and a pressure-relief valve which protects the lines if an obstacle is encountered or
limits operating pressure in steering systems without steering-lock limitation.
Steering systems with mechanical steering-lock limitation:
Adjustment is by means of adjuster rings on the steering spindle on the steering gear.
Outside the steering gear each circuit requires:
One or two pressure oil pumps per steering circuit. The flow volume must be designed in such a
manner that a steering speed of approximately 1.5 steering wheel turns per second is possible. On
engine-dependent pump, this steering speed should be available within entire speed range of
engine as required for normal driving. In some cases, this steering speed can be reduced in lower
speed range, but should not drop below one steering wheel turn per second at idle speed. For the
drive-dependent pump the transmission ratio from drive to pump should be selected in such a
manner that the above steering speed is attained at a vehicle speed of approx. 10 km/h.
One or more power cylinders attached to the steered axle or axles of the vehicle and connected to
the steering system valve by high-pressure hoses or pressure pipes. A ball joint or universal joint
connects the piston rod of the power cylinder to the steering arm or track arm, or in some cases to
the track rod.
To obtain hydraulic support via steering, which is initiated by turning the steering wheel, the worm
and the two valve sleeves (on single circuit power steering gears one valve sleeve) must be
displaced from their neutral position. The worm is held in this neutral position by means of cup
springs. For this reason, forces must be provided to overcome the preload of the spring. The
steering nut, positively connected with sector shaft and with the steered wheels, will resist the rotary
movement. When the steering system is operated, the worm is therefore axially loaded via steering
nut and ball chain and the resistance of the spring is overcome. As a result of the connected
movement of the valve sleeves from their neutral position the valve control grooves will be adjusted.