GB2197433A - Fluid control valves - Google Patents

Abstract

In a fluid pressure control valve 10 for controlling the flow of fluid between an actuator 21 and a reservoir, the valve comprising a bore 13 in which is slidably mounted a spool member 40 biassed against a seat 37 upstream of which (in one mode of operation) is the actuator, an area of the spool member 40 is exposed to the upstream pressure and the spool member is arranged so that on movement off its seat 37 the downstream fluid pressure acts on substantially equal and opposite areas thereof to provide a fluid balance. <IMAGE>

Description

SPECIFICATION Improvements relating to fluid control valves This invention relates to a fluid control valve and is for particularly concerned with a fluid control valve which is used to control the flow of fluid to and from an actuator and also to lock the actuator in position when the fluid flow is terminated. Such control valves are well known and may be of the kind for example as described in US Specification 4 346 733.

A problem with these valves has been in providing a satisfactory method of relieving pressure without damage to the equipment when the pressure in the actuator exceeds a predetermined value, as may for example happen when an excess load is applied to it. For example the actuator may be a jacking mechanism and in which case when the jack is in the raised position a further load could be deposited on it which would cause an excessive fluid pressure in the system which would, if not relieved, seriously damage the equipment and/or circuit.

It is an object of the present invention to provide a fluid control valve for controlling the flow of fluid between an actuator and a reservoir which will provide an improved pressure relieving function.

A fluid control valve according to the present invention for controlling the flow of fluid between an actuator and a reservoir comprises a bore in the valve in which is slidably mounted a spool valve biassed against a seat upstream of which is the actuator, an area of the spool valve being exposed to the upstream pressure and the spool valve being arranged so that on movement off its seat the downstream fluid pressure acts on substantially equal and opposite areas thereof to provide a fluid balance.

Preferably the area of the spool valve exposed to upstream pressure has an equivalent and opposite area exposed to atmosphere.

Thus there is no biassing effect generated by downsteam pressure within the valve spool.

The spool valve is preferably biased by means of a spring located in a chamber which is exposed to the downstream pressure and which provides oil damping thereof.

Preferably the spool valve is also operable by means of a pilot pressure and preferably means are provided for changing the pilot pressure ratio. The means for changing the pilot ratio conveniently comprise means for varying the area exposed to pilot pressure.

The pilot pressure conveniently acts on a pilot piston in the form of a sleeve surrounding the spool valve, and in this case the pilot pressure can be altered by selection of a sleeve of an appropriate diameter, a non-operative sleeve being used if necessary to occupy the remaining space.

The invention may be performed in various ways and two specific embodiments will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-section through a pilot assisted relief valve according to the present invention and incorporated in a circuit; and Figure 2 is a similar view to Figure 1 with a modified pilot piston arrangement.

In the Figures the valve is of the cartridge type indicated generally at 10 and adapted to fit into an opening indicated generally at 11 in a housing block 12.

The opening 11 comprises a bore 13 and a counterbore 14 joined by- a shoulder at 15 and leading from which are three ports indicated at 16, 17 and 18, the port 16 communicating with the operative side 20 of an actuator indicated at 21, the port 17 communicating with a control valve as more fully described below and the port 18 communicating with a pilot valve.

The port 16 communicates via a line 22 with the lower side of the actuator 20 whilst the annular side 23 of the actuator communicates via a line 24 with a control valve assembly 25 which in turn communicates via a pressure line 26 with a hydraulic pump 27 and a return line 28 to a reservoir.

The control valve communicates via a line 29 with the port 17 and the pilot port 18 communicates via a line 30 with the line 24.

The control valve 25 is a three position valve the top being the "lower" position, the intermediate being the "hold" position, and the bottom position being the "raise" position.

The cartridge valve comprises an outer flange 31 arranged to be secured by a screw thread 32 into the housing 12. The cartridge valve is provided with an annular body indicated generally at 33 the inner end 34 of which is spaced from the bore 13 and houses within it a non-return valve 35 biassed by means of a spring 36 against a seat 37 and movement of which permits substantially unimpeded fluid flow through the port 38 in the annular body 33.

Slidably mounted in the annular body 33 is a spool valve 40 and which is arranged in its closed position to seat against the non-return valve 35 in which in fact is formed the seat 37. It is to be noted at this stage that the diameter of the portion indicated at 42 and against which the piston valve seats is of a smaller diameter than the bore 41 in which the right hand end of the spool valve indicated at 43 slides. The spool valve 40 has at its end 43 a chamber 44 in it and which has a port 45 which communicates via port 46 with port 17.

In the arrangement shown in Figure 1 the pilot port 18 communicates via a restrictive orifice 47 with face 48 of a larger diameter portion 49 of the piston valve 40 which forms the pilot piston. The portion 49 forming the pilot piston slides in a bore 50. Within the portion 49 is mounted a spring 51 and which biases the spool valve against its seat, move ment of the larger diameter portion 49 being permitted to the left up to an end closure which communicates with atmosphere via a passage 53, a ring seal 54 being provided to prevent ingress of dirt but allowing free flow of air. The spool valve is provided with sliding seals 55, 56, 57 and 58.

In operation when it is desired to raise the load the control valve 25 is put into the "raise" position and in this position fluid flows from line 26 through line 29, port 17, port 46, port 45 and causes the non-return valve 35 to open and then flows through port 38, line 16, line 22, and into the actuator chamber 20 of the actuator 21.

When the load has reached the desired position the control valve is placed in the hold position and in this position the annular side 23 of the actuator communicates via line 24 to the reservoir, the pump being isolated. In this position also the pilot valve is connected to the reservoir via restrictive passage 47, port 18 and line 30. The non-return valve 35 is also held on its seat by the fluid pressure in line 22 which has free access to its seat 37.

The load induced pressure in line 22 is applied to the annulus defined by the diameter of the portion 42 and the larger diameter of the bore 41 in which slides the right hand end of the spool valve 43. This does not have any effect under normal conditions as the pressure is well short of the opposing force applied by the spring 51.

When it is desired to retract the actuator the "lower" position is selected and this applies pressure to the annular side 23 of the actuator and also to the pilot piston via face 48 opposing the pressure of the spring and has the effect of intensifying the load induced pressure. These two pressures combined move the spool valve against the spring allowing communication via port 38, chamber 44, ports 45, 46, 17 to the reservoir since the piston is pushed to the left.

The annular area defined by the valve seat at 37 and the larger diameter at 41 is exposed to variations in the load induced fluid pressure from the actuator 20. When the fluid pressure acting on the annular area exerts a bias which exceeds the spring load generated by spring 51 the spool valve 40 will move away from its seat 37 permitting fluid to relieve through chamber 44 and ports 45 and 46 to reservoir via the control valve 25.

The diameter at 58, i.e. the diameter of the bore in which the spring 51 is mounted in the spool valve is substantially equal to that of the valve seat at 37 so that the relieved fluid pressure or "downstream" pressure and which flows freely from chamber 44 to chamber 60 in which the spring is mounted via passage 59 is reacting on equal areas at both ends and does not exert any additional bias through the spool valve. The diameter at 58 is slightly less than that at 41 and provides an annulus of substantially equal area to that formed between the valve seat at 37 and the bore at 41.This annular area is exposed to atmospheric pressure via passage 53 so that the pressure sensitive valve annulus in the relief valve function is biased only by the spring and an annulus subjected to atmospheric pressure so that there is no biassing effect generated by downstreain pressure within the valve spool.

Additional opening bias is provided when fluid pressure is directed to the pilot annulus as will be required when the valve is used to control the lowering of a hydraulic actuator supporting an external load. The seal diameter at 58 prevents downstream fluid pressure communicating with the non-pressurized side of the annular pilot piston which would otherwise increase the closing bias of the valve.

In the arrangement shown in Figure 1 it should be noted that the pilot piston indicated at 49 has a stepped area indicated at 62 so that the pilot piston has a relatively large area exposed to pilot pressure that is to say the whole of the area between diameter 50 and diameter 41, the shoulder 62 being acted upon by the pilot piston. This arrangement can give a large pilot ratio.

In the arrangement shown in Figure 2 instead of the pilot piston 49 a static sliding sleeve 64 is provided and in this case the pilot pressure only acts on the area between the inner diameter of the static sliding sleeve.

That is the diameter at seal 57 and the outer diameter of the spool valve as indicated at 41. This gives a relatively low pilot pressure ratio.

The pilot pressure ratio can be varied by the use of two sleeves that is a static sliding sleeve such as 64 inside which is a pilot piston as at 49, the diameters being variable to give the pilot ratio required.

The invention provides a valve which is not substantially biassed by relieved or downstream pressure but which houses the pressure controlling spring within a fluid filled chamber which additionally provides a damping chamber by discharging and inducing fluid through the orifice 59 during valve movement which must take place when operating in the overload relief or pilotted load lowering modes.

Claims (7)

1. A fluid pressure control valve for controlling the flow of fluid between an actuator and a reservoir comprising a bore in the valve in which is slidably mounted a spool valve biassed against a seat upstream of which is the actuator, an area of the spool valve being ex posed to the upstream pressure and the spool valve being arranged so that on movement off its seat the downstream fluid pressure acts on substantially equal and opposite areas thereof to provide a fluid balance.

2. A valve as claimed in claim 1 in which the area of the spool valve exposed to the upstream pressure has anequivalent and opposite area exposed to atmosphere.

3. A valve as claimed in claim 1 or claim 2 in which the spool valve is biassed by means of a spring located in a chamber which is exposed to the downstream pressure to provide oil damping thereof.

4. A valve as claimed in claim 3 in which the spool valve is biassed by means of a pilot pressure.

5. A valve as claimed in claim 4 in which the pilot ratio is variable.

6. A valve as claimed in claim 5 in which the means for varying the pilot ratio comprise means for varying the area exposed to pilot pressure.

7. A valve as claimed in claim 6 in which the pilot piston is in the form of a sleeve surrounding the spool valve, selection of an appropriate diameter sleeve providing the required pilot ratio, a non-operative sleeve being used to occupy the remaining area.