EP0019597A2

EP0019597A2 - Safety device for hydraulic circuits

Info

Publication number: EP0019597A2
Application number: EP80830027A
Authority: EP
Inventors: Giuseppe Cinotto
Original assignee: CHS GIUSEPPE CINOTTO HYDRAULIC SYSTEM SpA
Current assignee: CHS GIUSEPPE CINOTTO HYDRAULIC SYSTEM SpA
Priority date: 1979-05-18
Filing date: 1980-05-06
Publication date: 1980-11-26
Also published as: EP0019597A3; IT7968061A0; IT1118648B

Abstract

A safety device (24) is provided for use, for example, in conjunction with a double-acting hydraulic actuator (18) operating a lifting arm. The safety device (24) is interposed between the actuator (18) and the fluid line (20) along which fluid is supplied to raise the arm. In the event of accidental rupturing of this fluid line (20), the safety device (24) operates to prevent uncontrolled lowering of the arm. The device (24) has a working section which comprises a non-return valve (36) inserted in said line (20) such as to normally only allow fluid into the actuator (18), and control means selectively operable to inhibit the non-return valve (36) and permit fluid to exit from the actuator (18). The control means is itself controlled from a controlling section of the device (24) in such a manner that the control means only operates in the presence both of a control pressure signal (Ps) indicating that the arm is to be lowered, and of normal operating pressures in said line (20).

Description

The present invention relates to a safety device for hydraulic circuits comprising a source of fluid under pressure, a double-acting hydraulic actuator having a cylinder and a piston dividing the internal volume of the actuator cylinder into first and second working chambers with the piston rod extending through the second working chamber, a distributor arranged to receive fluid under pressure from said source, first and second fluid lines intercommunicating said distributor with said first and second working chambers resDectively, and an hydraulic servo-control arranged to control said distributor by the application of a control pressure thereto.
In particular the invention relates to a safety device for use with a double-acting actuator controlling the lifting arm of an excavator, power-lift or similar machine, the device being arranged to prevent the uncontrolled lowering of a load carried by the lifting arm of the.machine in the event of the accidental rupture of the fluid supply line which supplies fluid to the actuator during raising of the load.
Safety devices for preventing such uncontrolled operation of an actuator are known but traditionally employ two pressure take-offs, one'from each of the two fluid lines supplying the actuator. These known devices operate by monitoring the pressure difference between the two fluid lines.
The object of the present invention is to provide a safety device which is particularly reliable, and has a greater operating safety and is more convenient to use than known safety devices.
With a view to achieving this object, the present invention provides a safety device for hydraulic circuits, particularly for excavators, power-lifts or similar machines, characterised in that it comprises a valve unit placed in said first fluid line and having an automatic controlling section and a working valve section controlled by the controlling section: the controlling section comprising a body having an input connected to the said hydraulic servo-control and-an output connected to the working section, and a valve body slidable mounted in said body and displaceable between an open position in which said input and output are placed in communication with each other, and a closed position in which communication between said input and output is prevented; the working section comprising a body connected to the output of the controlling section and having an input connected to said first fluid line and an output connected to said hydraulic actuator, said input and output being also connected to the body of the controlling section; a non-return valve being placed within the body of the working section, said non-return valve being arranged for connection between said input and output of the work ing section, when the flow of fluid in the working section is directed from the input to the output, uniquely in the presence of an input pressure in the working section; control means being provided to maintain said non-return valve in its open position, when the flow of fluid in the working section is directed from the output to the input, uniquely in the presence of an output pressure in the control ling section.
The valve body of the controlling section of the safety device is preferably formed with a first surface arranged to be subjected to the input pressure of the working section and with a second surface arranged to be subjected to the output pressure of the working section, the valve body being, in addition, subjected to the action of adjustable bias means tending to maintain the said valve body in its open position; the sum of the forces acting on the said valve body being such that in the presence of normal working pressures the valve body is in its open position, whilst in the absence of the input pressure in said working section the valve body is displaced by the output pressure of the working section, against the action of the said adjustable bias means, in its closed position.
A safety device embodying the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 is a diagram of an hydraulic circuit incorporating the safety device;
Figure 2 is a simplified functional representation of the safety device;
Figure 3 is a longitudinal section of a working valve section of the safety device;
Figure 4 is a longitudinal section of a controlling section of the safety device; and
Figure 5 is a longitudinal section of a modified form of the working valve section shown in Figure 3.

The hydraulic circuit shown in Figure 1 is arranged to control the movement of an arm 10 which, for example, forms part of a power-lift not illustrated. The hydraulic circuit comprises a pump 12 which supplies fluid under pressure to a three-position hydraulic distributor 14 of a type known per se, controlled by means of a manually operable servo control 16, also of known type. The hydraulic distributor 14 is connected by first and second lines 20 and 22 to the cylinder of a double-acting hydraulic actuator 18 the piston rod of which is pivoted to the arm 10. The piston of the actuator 18 divides the internal volume of the actuator cylinder into first and second working chambers with the piston rod extending through the second working chamber. The lines 20 and 22 communicate with the first and second working chambers respectively. The safety device 24 embodying the invention is disposed in the first line 20 and is provided with means (not shown) enabling its secural to the cylinder of the actuator 18. The safety device 24 is functionally represented by the diagram of Figure 2 and is arranged to allow uninhibited operation of the actuator ₁8 in the presence of normal operating conditions of the hydraulic circuit while preventing sudden exit of fluid from the first working chamber of the actuator cylinder (and thus sudden lowering of the arm 10) should the line 20 become ruptured.
The safety device 24 is constituted by a unit comprising a working valve section 26 (see Figure 3), and a controlling section 28(Figure 4).
The working section 26 includes a cylinder 30 having an input duct 32 connected to the first-line 20, and an output duct 34 which lies offset with respect to the duct 32, and communicates with the first working chamber of the hydraulic actuator 18. A selectively-operable non-return valve 36 is disposed within the cylinder 30 and comprises a valve body 38 in the form of a cylindrical sleeve the axis of which is directed perpendicularly to the ducts 32 and 34. One end opening of the sleeve 38 communicates with the duct 32 and can be closed off by the cooperation of a spherical valve closure member 42 slidably mounted in the interior of the sleeve 38 with a valve seat formed internally around the said one end opening. At its opposite end the sleeve 38 has an end plate 43 which faces a plate 54 of the cylinder 30. A helical spring 51 reacts between the plates 43 and 54. The interior of the sleeve 38 communicates with the duct 34 through a circular array of calibrated radial holes 44 formed in the cylindrical wall of the sleeve 38 and, in addition, through at least one further calibrated radial hole 46 axially spaced from the holes 44.
An element 48 in the form of a cup is slidably mounted in the interior of the body 38 and sealingly engages the cylindrical wall of the sleeve 38. The bottom of the cup element 48 bears against the spherical valve closure member 42 and is provided with inclined discharge holes 50. The cup element 48 is urged towards the valve closure member 42 by the action of an helical spring 52 reacting against the inner face of the end plate 43 of the sleeve 38. The spring 52 therefore tends to maintain the valve closure member 42 against the valve seat 40, thereby closing the non-return valve 36 and preventing communication between the ducts 32 and 34. In the closed state of the valve 36, the valve closure member. ₄₂ also serves to close off the holes 44 while leaving open the hole 46.
The non-return valve 36 is associated with a control device 56 controlled by the controlling section 26. The control device 56 comprises a shaft 58 slidably mounted in the cylinder 30 coaxially with the sleeve 38. One end of the shaft 58 bears against the valve closure member 42 on the side thereof opposite the cup element 48. The other end of the shaft 58 has a hollow portion 60 which lies in a cavity 62 of the cylinder 30 and bears against the bottom of a cup-shaped body 64 slidably mounted in the cavity 62. The body 64 is urged in a direction away from the shaft 58 by a helical spring 67. The region of the cavity 62 lying on the opposite side of the bottom of the cup-shaped body 64 to the non-return valve 36 communicates, through a duct 66, with an output 68 of the controlling section 28 of the safety device 24. The duct 66 is formed in a threaded plug which is engaged in the cylinder 30 and serves to close off the cavity 62. This plug is formed with a cylindrical cavity 72 which opens coaxially into the cavity 62. An auxiliary piston 70 is slidably mounted in the cavity 72 and sealingly engages the walls of the cavity 72. One end of the auxiliary piston 70 projects from the cavity 72 into the cavity 62 where it bears against the bottom of the cup-shaped body 64.
The input duct 32 and the output duct 34 respectively communicate via a duct 74 and a duct 76 formed in the cylinder 30, with an input 78 and an input 80 of the controlling section 28. In addition, the duct 76 communicates via a duct 82 with the cavity 72 containing the auxiliary piston 70. The output duct 34 also communicates with a fluid- collection reservoir 84 (see Figures 1 and 2) by means of a pair of ducts 86, 88 in which there are respectively disposed a manually operable bypass valve 90 and an anti-surge valve 92 of type known per se.
The controlling section 28 of the safety device 24 (see Figure 4) comprises a cylinder 94 formed with an input duct 96 which communicates with the output of the servo control 16. The duct 96 is also selectively connectable, by means of a slide valve or spool valve 98, with the output 68 which, as previously mentioned, is connected to the duct 66 of the working section 26. The valve 98 is constituted by a cylindrical body sealingly slidable in a cavity 100 formed in the cylinder 94 and into which opens the input duct.96'. This cylindrical body has a cylindrical head end portion 102, a central portion 104 of reduced diameter, and a hollow end portion 106. The interior of the hollow end portion 106 communicates with the cavity 100 via an axial passage 108 formed in the central portion 104 and a series of radial holes 110 provided in the lateral wall of the valve body in the zone intermediate the cylindrical head portion 102 and the central portion 104. The cavity ₁00 is formed with an annular groove 101 which communicates with the collection reservoir 84 through a duct 112 formed in the cylinder 94. Axial displacement of the body of the valve 98, provides for the selective connection of the output duct 68 to the input duct 96 or to the duct 112 this axial displacement being brought about in a manner to be described hereinafter.
The hollow end portion 106 of the slide valve 98 is guided in a slidable manner between the walls of the cavity 100 and the enlarged head 114 of a shaft 116. The end of the shaft 116 remote from the head 114 is threadedly engaged in a block 118 which is located in a flanged bush 120 mounted in the cylinder 98. The shaft 116 carries a stack of dished plate springs 122 which react between the block 118 and a washer 124 located in the cavity 100 and bearing against the end of the hollow end portion 106 of the valve 98. On its side away from the springs 122, the block 118 is engaged by an adjustment screw ₁₂₆ accessible from the exterior of the cylinder 94.
The interior of the flanged bush 120 communicates via a duct 128 with the input 78 which, as previously mentioned, communicates with the input duct 32 of the working section 26.. A.
A piston 130 is slidably mounted in a seal-tight manner inside the cavity 132 of a cylindrical bush 134 which is mounted in the cylinder 94 coaxially with the cavity 100. The piston 130 bears against the end face of the valve end portion 102. The cavity 132 communicates, through an axial passage 136;with a duct 140 connected to the input 80 which, as previously mentioned, communicates with the output duct 34 of the working section 26. A needle valve 138 controls fluid flow through the passage 136. A piston 142 bears against one end of the needle valve 138 to control its operation. The piston 142 is slidably mounted in a seal-tight manner in a cavity 144 of an end cap 145 of the cylinder 94. The piston 142 is provided with a stem 146 which mounts a stack of dished plate springs 148 serving to urge the piston 142 into engagement with the needle valve 138. The portion of the cavity 144 lying on the opposite side of the piston 142 to the stem 146 communicates with the input duct 96 through a duct 150 formed in the cylinder 94.
The operation of the safety device 24 will now be described.
The operator by manually acting on the servo control 16, selectively sets the state of the distributor 14 such as to cause the delivery of pressurised fluid from the pump 12 either to the first line 20 to bring about a raising of the arm ₁₀, or to the second line 22 to cause lowering of the arm 10. A change in the selected state of the distributor is initiated by the appropriate application to the distributor 16 of a control fluid pressure P of a value of seven bars by the servo control 16 (in contrast, the pressure of fluid delivered by the pump 12 can be as much as 350 bars).
In the presence of normal operating conditions in the hydraulic circuit, the safety device 24 is arranged to allow the uninhibited flow of fluid therethrough; however, should abnormal conditions arise, such as rupture of the line 20, which could lead to uncontrolled lowering of the arm 10, the safety device 24 effectively operates to prevent a sudden exit of fluid from the cylinder of the actuator 18. To this end the non-return valve 36 of the working section 26 is operatively inserted in the line 20 except when the distributor 14 is set for' lowering of the arm 10 and the controlling section 28 simultaneously detects the presence of normal operating conditions of the hydraulic circuit; in these latter conditions the non-return valve 36 is rendered inoperative by the control device 56.
Considering first the situation in which the distributor is set to deliver fluid under pressure to the duct 20, it will be seen from Figure 1 that in this case no control pressure P_s is supplied to the safety device 24. As will be more fully described hereinafter, in the absence of the pressure P₅ the shaft 58 of the control device 56 is arranged not to interfere with the operation of the valve 36 as a simple non-return valve.
Fluid from the line 20 enters the input duct 32 of the safety device 24 at an input pressure P_d and causes the axial displacement of the sleeve 38 against the action of the spring 51. As a result, the non-return valve 36 is opened to allow fluid to pass through the output duct 34 (at pressure P ) and into the interior of the hydraulic actuator 18. In this operating state, removal of the input pressure P_d (for example, due to the accidental rupture of the line 20) will result in the spring 52 closing the non-return valve 36 and thereby preventing uncontrolled lowering of the arm 10. The provision of the anti-surge valve 92 allows the absorption of the pressure peaks occurring upon rupture of the line 20, whilst the by-pass valve 90 allows a subsequent slow and gradual manual lowering of the arm 10 to be achieved.
When the distributor 14 is set in a state corresponding to lowering of the arm 10, fluid under pressure coming from the actuator 18 reaches the non-return valve 36 through the output duct 34. In this operating state, the controlling section 28 of the safety valve 24 acts to supply or cut off the control fluid pressure P_s to the working section 26 in dependence on whether normal or abnormal operating conditions are present (the latter conditions corresponding, for example, to rupture of the line ₂₀). If the control pressure signal P is fed to the working section 26, the non-return valve 36 is rendered inoperative and is opened to allow the passage of fluid through the holes 44 and the consequent lowering of the arm 10. If, on the other hand, the pressure signal P_s is not supplied to the working section 26, the non-return valve 36 is allowed to close to prevent the lowering of the arm 10.
The supply of the control pressure P_s to the working section 26 depends on the position of the valve 98 which is determined by the relative magnitudes of the various pressure and mechanical forces acting on the valve. The equation of equilibrium of the valve 98 is as follows:
in which X_m is the force exerted by the spring 122, A is the surface area of the valve 98 subject to the pressure P_d, B is the surface area of the head 114 of the shaft 116 subject to the pressure P_s, and C is the surface area of the piston 130 subject to the pressure P .
In the presence of normal operating conditions of the hydraulic circuit, the following condition of unbalance exists in the controlling section 28:
so that the head portion 102 of the slider of the valve 98 bears against the bush 134 as is illustrated in Figure.4. In this position, the valve 98 communicates the input duct 96 with the output 68 and therefore with the duct 66 of the working section ₂6. As a result, the control pressure P_s is fed to the control device 56 of the working section 26 which as a consequence moves the shaft 58 and valve closure member 42 to the right as viewed in Figure 3 to maintain the return valve 36 open. In fact, in the presence of normal operating conditions in the hydraulic circuit, the following condition of unbalance exists in the working section 26:
where D is the area of the auxiliary piston 70 subject to the pressure P_c, E is the area of the cup-shaped body 64 subject to the pressure P_s, _F is the area of the valve closure member 42 subject to the pressure P_c, X_p is the force exerted by the spring 67, and X₁ is the force exerted on the spring 52.
Should the duct 20 become ruptured so that the pressure P_d is removed, the following condition of unbalance exists in the controlling section 28:
In this case, the valve 98 takes up a position in which it interrupts communication between the input duct 96 and the output 68 but establishes communication between the output 68 and the duct 112 connected to the reservoir 84. As a result, the control pressure P_s is no longer fed to the control device 56 of the working section 26 and as a consequence movement of the shaft 58 to the left as viewed in Figure 3 becomes possible so as to allow closure of the non-return valve 36. In fact, in the absence of the control pressure P_s, the following condition of unbalance exists in the working section 26:
where the term R represents the passive resistance of the working section 26.
The non-return valve 36 will therefore move to its closed position to prevent lowering of the arm 10. The pressure P acts on the area F of the closure member 42 through the hole 46.
Because the non-return valve 36 must always close in the event of accidental rupture of the line 20 but not when the actuator 18 is being unloaded under normal working conditions, the values of the parameters X_m (adjustable) and C must be chosen in accordance with the previously indicated equation of equilibrium for the valve 98, assuming for P a range of values lying between 50 and 350 bars with a corresponding range of values for P lying between 4.5 and 7 bars.
Moreover, in order to ensure that the valve 36 does not interfere with the operation of the hydraulic circuit under normal operating conditions, it is necessary that this valve is completely open when P_s = 7 bars, this being the value of the control pressure which, as previously mentioned, is used to cause a change of state in the distributor 14. This condition determines the values of the areas . E,F,D and of the cross-sectional area of the shaft 58 of the working section 26.
It is clear from the foregoing that operation of the described safety device is dependent on the difference between the pressures P and P_d being used to control the valve 98. Since during lowering of the arm 10 this pressure difference is a function of the rate of flow of fluid through the valve 36 (which, in turn, is a function of the speed of lowering effected by the hydraulic actuator 18) then unless suitable compensatory measures are taken, the point of operation of the safety device to close off the duct 34 from the duct 32 will be dependent on the speed of operation of the actuator 18. To render the operation of the valve 98 independent of the rate of fluid flow, the controlling section 28 is so designed that the pressure force P . B exerted on the valve 98 increases in correspondence with the rate of fluid flow through the valve 36 in order to counteract the increase in the difference between the pressures P_cand P_d.
Variations in the pressure force P . B is achieved by varying the fluid flow between the ducts ₉₆ and 112 in dependence on the position of the valve body 98, the valve position being in turn dependent on the difference between the pressures P_c and P_d.
When the distributor 14 is set into its third state in which neither line 20, 22 is connected to the pump 12, the pressure P_d in the input duct 32 will generally gradually fall due to blow-by. Unless otherwise prevented, this would result in the valve 98 eventually closing with the result that upon the distributor 14 being subsequently set to lower the arm ₁0, the valve 98 will remain closed preventing this lowering because the pressure P_d is at a level corresponding to rupture of the line 20. In order to avoid this situation the needle valve 138 is arranged to prevent the pressure P from acting on the area C of the piston 130 of the controlling section 28 whenever the control pressure P_s is absent. The equation of equilibrium of the needle valve 138 is as follows:
where X_b is the force exerted by the springs 1₄8, _G is the area of the end of the needle valve 138 subject to the pressure P , and I is the surface area of the needle valve 138 subject to the pressure P_c.
It can be seen that for the needle valve 138 to close with P_s = O, then:
This condition must be met even with values of

P = 350 bars. c

Whatever the value of P , upon the distributor 14 being set to lower the arm 10 the needle valve 1.38 must be arranged to open after the opening of the non-return valve 36 under the action of the control pressure P_s on the control device 56; in this manner, provided the line 20 is not ruptured, pressure will be re-established in the duct 32 to ensure proper operation of the controlling section 28. To achieve the delayed opening of the needle valve 138, the non-return valve 36 is arranged to open with values of P_s = 1.66 bars when P_c = 50 bars and with P_s = 3.44 bars when P_c = 350 bars. By suitable choice of the values of the parameters X_b, G, H and I it is possible to arrange for the needle valve 138 to open with values of P_s ⁼ 7 when P_c = 50 and P_s = 4.5 when P_c = 350..
A modified form of the working section 26 of the safety device 24 is shown in Figure 5. The working section 26 shown in Figure 5 differs from that illustrated in Figure 3 only in that the valve closure member of the non-return valve 36 is constituted by a cup-shaped body 1.60 rather than by a spherical member 42. The cup-shaped body 160 is slidably mounted in the interior of the sleeve 38 and is formed with a frusto-conical end 162 arranged to cooperate with the valve seat 40. The spring 52 reacts directly against the cup-shaped body 160 in such a way as to urge the frusto-conical end 162 towards the valve seat 40. The interior of the cup-shaped body 160 communicates with the interior of the sleeve 38 by means of a series of calibrated radial holes 164 formed in the lateral wall of the body 160. The radial holes 164 serve the same function as the hole 46 described with reference to Figure 3. The operation of the Figure 5 form of the working section 26 is the same as for the Figure 3 form.
The described safety device is particularly suitable for use with double-acting hydraulic actuators incorporated in power-lifts, excavators and the like.

Claims

1. A safety device for hydraulic circuits, particularly for excavators, power-lifts or similar machines, comprising a source of fluid under pressure, a double-acting hydraulic actuator having a cylinder and a piston dividing the internal volume of the actuator cylinder into first and second working chambers with the piston rod extending through the second working chamber, a distributor arranged to receive fluid under pressure from said source, first and second fluid lines intercommunicating said distributor with said first and second working chambers respectively, and an hydraulic servo-control arranged to control said distributor by the application of a control pressure thereto, characterised in that said safety device comprises a valve unit (24) placed in said first fluid line (20) and having an automatic controlling section (28) and a working valve section (26) controlled by the controlling section (28); the controlling section (28) comprising a body (94) having an input (96) connected to the said hydraulic servo-control (16) and an output (68) connected to the working section (26), and a valve body (98) slidable mounted in said body (94) and displaceable between an open position inwhich said input (96) and output (68) are placed in communication with each other, and a closed position in which communication between said input (96) and output (68) is prevented; the working section (26) comprising a body (30) connected to the output (68) of the controlling section (28) and having an input (32) connected to said first fluid line (20) and an output (34) connected to said hydraulic actuator (18), said input (32) and output (34) being also connected to the body (94) of the controlling section (28); a non-return valve (36) being placed within the body (30) of the working section (26), said non-return valve (36) being arranged for connection between said input (32) and output (34) of the working section (26), when the flow of fluid in the working section (26) is directed from the input (32) to the output (34), uniquely in presence of an input pressure (P_d) in the working section (26); control means (56) being provided to maintain said non-return valve (36) in its open position, when the flow of fluid in the working section (26) is directed from the output (34) to the input (32), uniquely in presence of an output pressure (P_s) in the controlling section (28).

2. A safety device according to claim 1, characterised in that the said valve body (98) of the controlling section (28) is formed with a first surface (A) arranged to be subjected to the input pressure (P_d) of the working section (26) and with a second surface (C) arranged to be subjected to the out put pressure (P_c) of the working section (26), the valve body (98) being, in addition, subjected to the action of adjustable bias means (122) tending to maintain the said valve body (98) in its open position; the sum of the forces acting on the said valve body (98) being such that in the presence of normal working pressure the valve body (98) is in its open position, whilst in the absence of the input pressure (P_d) in said working section (26) the valve body (98) is displaced by the output pressure (P ) of the working section (26) against the action of the said adjustable bias means (122), into its closed position.

3. A safety device according to claim 1 and 2, characterised in that the controlling section (28) further comprises a shut-off valve (138) which is responsive to the presence or absence of said input pressure (P_s) of the controlling section (28) to respectively take up an open position in which the controlling section (28) and the output (34) of the working section (26) are placed in communication with each other, and a closed position in which said communication is prevented.

4. A safety device according to claim 3, characterised in that the said shut-off valve includes a valve needle (138) cooperating with a valve seat (136) formed in the body (94) of the controlling section (28), the said valve needle (138) being arranged to be urged towards the open position of the shut-off valve under the action of said output pressure (P ) of the working section (26), and-being urged towards the closed position by resilient means (148) the action of which is opposed by the input pressure (P_s) of the controlling section (28).

5. A safety device according to any one of claims 2 to 4, characterised in that the said first surface (A) of the valve body (98) is constituted by the end face of one end portion (106) of said valve body (98), said one end portion (106) being formed with an axial cavity communicating with the said pressure input (96) of the controlling section (28), and the valve (98) of said controlling section (28) further comprising a fixed shaft (116) provided at one end with a head (114) which slidably engages in seal-tight manner in the cavity of the said one end portion (106) of the valve body (98), and axial adjustment means (126) operable from externally of the controlling section body (94) to effect axial adjustment of the position of said fixed shaft (116).

6. A safety device according to any one of claims 2 to 5, characterised in that the valve body (98) of said controlling section (28) is formed with a third surface (B) arranged to be subjected to said input pressure (P_s) of the controlling section (28) in such a manner as to render the equilibrium of the valve of the controlling section (28) independent of the rate of flow of fluid through the non-return valve (36).

7. A safety device according to claim 6 when dependent on claim-5, characterised in that the said third surface (B) is constituted by the end face of the said shaft head (114).

8. A safety device according to claim 5, characterised in that the said adjustable bias means acting on the valve body (98) of the controlling section (28) is constituted by dished plate springs (122) carried by the said fixed shaft (116) and reacting against the said first surface (A) of the valve body (98).

9. A safety device according to any one of claims 3 to 7, characterised in that the non-return valve (36) of the working section (26)comprises

- a cylindrical sleeve (38) slidably mounted in the working section body (26) with its interior in communication with the input (32) of the working section (26) through an axial end opening of the sleeve (38), the lateral wall of the sleeve (38) being formed with at least one restricted aperture (44) through which the sleeve interior communicates with the output (34) of said working section (26),

- a valve closure member (42, 160) slidably mounted inside said sleeve (38), and

- resilient means (52) urging the said valve closure member (42, 160) against a valve seat (40) formed inside said sleeve (38) whereby to interrupt communication between said axial end opening and said restricted aperture (44); the said non-return valve (36) being formed with at least one further aperture (46, 164) which, with the non-return valve (36) held open by said control means (56) to permit fluid flow from the output (34) to the input (32) of the working section (26), serves to apply the output pressure (P_c) of the working section (26) to a surface (F) of the said valve closure member (42, 160) such as to urge the valve closure member (42, 160) towards the said valve seat (40).

10. A safety device according to claim 9, characterised in that the said control means (56) comprises a shaft (58) one end of which extends through said axial end opening of the non-return valve sleeve (38) to bear against the said valve closure member (42, 160) on its side opposite the said resilient means (52), the opposite end (60) of the shaft (58) being formed with a surface (D) arranged to be subject to the output pressure (P ) of the working section (26), and a surface (E) arranged to be subject to said output pressure (P_s) of the con- trolling section (28), the control means (56) further comprising resilient means (67) arranged to urge the shaft (58) in a direction away from said valve closure member (42, 160); the arrangement being such that the sum of the forces acting on the said shaft (58) in the presence of normal working pressures and of the output pressure (P ) of the controlling section (28) is sufficient to cause the shaft (58) to take up a position in which the valve closure member (42, 160) is held in its open position, whilst in the absence of the output pressure (P ) of the controlling section (28) the valve closure member (42, 160) is displaced into its closed position.

11. A safety device according to claim 9 or claim 10,- characterised in that the valve closure member (42) is spherical, the said further aperture (46) of the non-return valve (36) being formed in the lateral wall of the said cylindrical sleeve (38).

12. A safety device according to claim 9 or claim 10, characterised in that the valve closure member (160) is constituted by a cup-shaped body provided with a frusto-conical end (162) arranged to cooperate with the said valve seat (40); the said further aperture (164) of the non-return valve (36) being formed in the lateral wall of the said cup-shaped body (160).

13. A safety device according to any one of the preceding claims, characterised in that the device (24) further comorises a manually operable by-pass valve (90) in communication with the side of the non-return valve (36) which in use of the device (24) is nearest said actuator (48).

14. A safety device according to any one of the preceding claims, characterised in that the device (24) further comprises an anti-surge valve (92) in communication with the side of the non-return valve (36) which in use of the device (24) is nearest said actuator (18).

15. A safety device according to any one of the preceding claims, characterised in that the safety device is provided with means for securing the device directly to the cylinder of the hydraulic actuator (18).