Design and Operation
Instructions on Design, Operation, Maintenance and Inspection
3
The steering valve consists of the valve rotor (C) which is carried in a needle bearing in the worm
a
nd is provided with six control grooves on the circumference, and of the valve sleeve (D) on the
worm.
A torsion bar (E) pinned to the valve rotor (C) and the worm (D) keeps the steering valve in
mid-position as long as no effort is exerted on the steering wheel.
A pressure relief valve (G) limiting the maximum pressure within the steering system may be
integrated in the steering gear housing.
In addition, a replenishment valve (H) sucking oil from the return oil line when a steering motion
without hydraulic assistance takes place can be fitted to the housing or to the steering valve.
In comparison with constant ratio steering gears, variable ratio steering gears are more direct
around centre than outside the mid-position area, which has a favourable effect on the steering
performance during straight ahead driving as minor steering corrections only, if any, are required.
At the same time, in the static parking range requiring a wider steering wheel turning angle a
higher hydraulic torque is available at the sector shaft owing to the more indirect steering ratio.
In the event of a failure of the hydraulic assistance the steering efforts at the steering wheel rim
are lower in this range than they would be for a constant ratio steering gear.
The 3 functional drawings to Figs. 1 to 3 give a simplified representation of the steering valve and
the oil flow. In addition, these figures give a cross-sectional view of the steering valve in order to
schematically represent the connections from the steering valve to the cylinder chambers and the
mode of operation of the valve.
2 Operation
When a torque is transmitted from the steering input shaft to the worm or vice-versa, the torsion
b
ar is subjected to a deformation in the elastic area of its length, causing a torsion to occur be-
tween the valve rotor and the valve sleeve and, thus, to move the control grooves of the valve
rotor away from the mid-position as compared with the position of the valve sleeve control
grooves.
When the steering wheel is released, the action of the torsion bar will make the steering valve
return to the neutral (mid) position.
Through the bore in the housing, the oil flows into the annular groove of the valve sleeve and is
fed to the arcuate control grooves of the inner valve rotor through three symmetrically-arranged
radial bores.
The position of the control grooves in the valve rotor and the valve sleeve is such that, in the
mid-position of the steering valve, the oil can flow through the inlet slots (J and K) to the axial
grooves (N and O) of the valve sleeve, which are also arcuate. From there, the oil can freely flow
through radial bores to the cylinder chambers.
As long as the steering valve is in the mid-position, the oil can flow to both cylinder chambers,
and via the three return grooves (P) in the valve rotor it can also flow off to the oil tank.
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