Illustration of the principles of the ZF recirculating ball steering in the neutral position (vehicle travelling in a straight line).
The steering valve, the working piston and the mechanical gear sit in a common housing. The two valve pistons of the steering valve have
been turned out of their operating plane to make the diagram easier to see. The individual parts are:
1 gear housing 9/10 valve piston 2 piston with steering nut 11/12 inlet groove
3 steering spindle connection 13/14 radial groove
4 steering shaft with toothed segment 15/16 return groove
5 steering worm roller with valve body 17 fluid reservoir
6 balls 18 torsion bar
7 recirculation tube 19 hydraulic pump
8 fluid flow limitation valve 20 pressure-limiting valve
In a situation where there is no torque, for example during straight running, the oil flows direct from the steering valve 6 back to the pump 1 via the return line 4.
The method of operation of the steering valve is shown in Fig. 9.1-17, using the example of recirculating-ball steering. In a similar way to rack and pinion steering, it is integrated into the input shaft of the steering gear. As is the case with most hydraulic power steering systems, the measurement of the steering-wheel torque is undertaken with the use of a torsion bar 18. The torsion bar connects the valve housing 5 (part of the steering screw) to the valve pistons 9/10 in a torsionally elastic way. Steering- wheel torque generates torsion of the torsion bar. These valve pistons then move and open radial groove 13 or 14, depending on the direction of rotation. This leads to a difference in pressure between pressure chambers D1 and D2. The resultant axial force on the working piston 2
is calculated using Equation 9.1-2. Because phyd,2 also
operates in the interior space of the piston behind the
steering screw 5, the surface areas are the same on both
sides:
pD 2
9.1.4.2 Electro-hydraulic power
steering systems
With electro-hydraulic power steering systems, the power-steering pump driven by the engine of the vehicle via V-belts is replaced by an electrically operated pump.
Fig. 9.1-18 shows the arrangement of the system in an Opel Astra (1997). The electrically operated power pack supplies the hydraulic, torsion-bar controlled steering valve with oil. The pump is electronically controlled - when
servo boost is not required, the oil supply is reduced.
The supply of energy by electricity cable allows greater flexibility with regard to the position of the power pack. In the example shown, it is located in the immediate vicinity of the steering gear. Compared with the purely hydraulic system, the lines can be made con- siderably shorter and there is no cooling circuit. The steering gear, power pack and lines are installed as a ready-assembled and tested unit.
To sum up, electro-hydraulic power steering systems
offer the following advantages:
2 The pressure supply unit (Fig. 9.1-19) can be accom-
FPi ¼ phyd;1 or 2 APi
¼ phyd;1 or 2 4Pi
(9.1.2)
modated in an appropriate location (in relation to
space and crash safety considerations).
đang được dịch, vui lòng đợi..