Power assisted steering system

DETAILED DESCRIPTION OF DRAWINGS The power assisted steering system shown schematically in FIG.
1 has a reservoir 1 of hydraulic fluid which supplies a constant volume output pump 2 which will usually be driven by the engine of a vehicle in which the system is installed.
The output from the pump 2 is delivered by way of a fluid pressure inlet line 3 to a conventional open centre power assistance control valve 4 which is adjustable, again in conventional manner, by a steering torque applied to an input 5 to control fluid flow to a power assistance servo motor such as a double operating piston and cylinder device 6 which provides assistance to displacement of steerable vehicle wheels as appropriate.
Fluid from the control valve 4 passes by way of an outlet or return line 7 to the reservoir 1.
Usually the control valve 4 will be both open centre and open return whereby when that valve is in its neutral condition (which would be consistent with zero steering torque) both the fluid pressure line 3 and both pressure chambers of the piston and cylinder device 6 communicate with the return line 7.
Communicating between the pressure inlet line 3 and the return line 7 is a by-pass valve 8 having a valve member 9 which is displaceable between a fully open condition in which it permits maximum fluid to flow therethrough from the pressure line 3 to the return line 7 and a closed condition in which it permits minimum or no fluid to flow therethrough from the pressure line 3 to the return line 7.
The valve member 9 is biased by a spring 10 so that the by-pass valve is normally in its fully open condition.
The displacement of the valve member 9 determines the proportion of fluid which flows from the pressure line 3 to the return line 7 by way of a control port 11 in the by-pass valve so this valve may be regarded as a variable orifice or metering valve.
Furthermore, the by-pass valve 8, or more particularly the displaceable valve member 9, reacts in response to the development of a predetermined fluid pressure differential between pressure in the line 3 and that in the return line 7 so that when this differential exceeds the force of the biasing spring 10, the valve member 9 can be displaced to progressively close the port 11 and thereby reduce the volume of fluid flow which can pass through the valve 8 (and thereby make a progressively increasing proportion of fluid flow available to the power assistance control valve 4)