US3592107A

US3592107A - Hydraulic control system

Info

Publication number: US3592107A
Application number: US829655A
Authority: US
Inventors: Gosta Ahlenius
Original assignee: LUNDBERGS MEK BRDR
Current assignee: BRODERNA LUNDBERGS MEKANISKA VERKSTAD AB; LUNDBERGS MEK BRDR
Priority date: 1968-06-06
Filing date: 1969-06-02
Publication date: 1971-07-13
Anticipated expiration: 1988-07-13
Also published as: SE322469B; GB1275636A

Abstract

Hydraulic control system producing parallel linkage of a toolholder located on control arms actuated by means of a hydraulic cylinder mechanically coupled between the toolholder and the control arms, the control units for said arms being allotted a hydraulic valve containing a supervisory unit detecting the movements of the arms and in dependence on the direction of movement of said arms, permitting the passage of fluid to a hydraulic unit allotted to said valve, said unit assuming a first position for the passage of fluid to one side of the cylinder on movement of the control arms in one direction under the influence of said fluid, and a second position for passage of fluid to the other side of the cylinder on movement of the control arms in the opposite direction.

Description

United States Patent Gallo Ahlenlus i Oppange, Enlarger. Sweden [2|] AppLNo 829.655

[72] Inventor [54] HYDRAULIC CONTROL SYSTEM 13 Claims, 3 Drawing Figs.

[52] U.S.Cl n 9l/l70R. 60/S2HE.2l4/l40SL [5|] lnt.Cl .7 Ftllh U00, B66f 9/00. F02f 3/62 [50] FleldolSe-rdr 2l4/I40.

[56] References Cited UNITED STATES PATENTS 2,8ll,265 l0/l957 Wagner........................ 2l4/l40SL 2.914.202 ll/l959 Wagner Primary Examiner- Edgar W. Geoghegan Arromey-Sughrue, Rothwell, Mion, Zinn 8t MacPcak ABSTRACT: Hydraulic control system producing parallel linkage of a toolholder located on control arms actuated by means of a hydraulic cylinder mechanically coupled betvveen the toolholder and the control arms, the control units for said arms being allotted a hydraulic valve containing a supervisory unit detecting the movements of the arms and in dependence on the direction of movement of said arms, permitting the passage of fluid to a hydraulic unit allotted to said valve, said unit assuming a first position for the passage of fluid to one side of the cylinder on movement of the control arms in one direction under the influence of said fluid, and a second position for passage of fluid to the other side of the cylinder on movement of the control arms in the opposite direction HYDRAULIC CONTROL SYSTEM This invention relates to a hydraulic control system with angle transmitter to a hydraulic control valve, the transmitter working in dependence on the position of the tool holder. especially a control system for producing parallel linkage of toolholders for cranes, loaders or the like.

On loading and excavating operations and the like with, for example, constructional machinery it is desirable that the tool of the constructional machine can be made to assume a con stant angle with the underlying surface during raising or lower ing of the booms supporting the tool in order that material car ried by the tool shall not fall out. For this purpose simultane ous control of the vertical positions of the booms and of the angular position of the tool in relation to the booms can be effected manually from the driver's cabin of the constructional machine.

It is usual, however, that a lifting movement must be made simultaneously with a rotary movement around an essentially vertical shaft or simultaneously with maneuvering of the machine. This involves difficulty in simultaneous manual supervision of the angular position of the tool in relation to booms, and so in relation to the ground surface. For this reason machines of these kinds are equipped with devices for automatic parallel linkage of toolholders, and so of tools, during the lifting movement of the booms. These devices usually consist of mechanical rods, slewing brackets and the like. But as the too] must also be usable for dumping and bending up in any position whatsoever of the booms, for example for unloading of the contents of the tool, such devices for parallel linkage of the tool are relatively complicated. They are also subjected to severe strains, so that rods, links, bearings etc. must be robustly dimensioned. The costs of such devices are therefore relatively high, and as they are also subjected to damage owing to their exposed positions, expensive stoppages occur.

As the devices for parallel linkage of the toolholder must be robustly dimensioned, and must be so designed moreover that the tool can be used for dumping and bending up irrespective of the positions of the booms, it is difficult to avoid that the parallel linkage devices involve some limitation on the means for operation of the tool. This means that many operations take a longer time than if the maneuverability is unlimited. This has an unfavorable effect on the operating costs.

The present invention is intended to eliminate these draw backs and to provide a cheap and reliable device which by simple means solves the problem of parallel linkage of toolholders. The invention consists of a hydraulic control system for parallel linkage oftool holders arranged on control arms actuated, for example, by means of hydraulic control cylinders, preferentially on cranes, loaders or the like, comprising a hydraulic cylinder mechanically coupled between the toolholder and the control arms and characterized chiefly in that the control units for the control arms are allotted a hydraulic valve, comprising a supervisory unit which detects the movements of the control arms, the supervisory unit being arranged so as to permit the passage of fluid to a regulating unit allotted to the hydraulic valve in dependence on the direction of movement of the control arms, and the regulating unit is arranged to assume a first position for the passage of fluid on movement of the control arms in one direction under the influence ofthe fluid to one side of the hydraulic cylinder and to assume a second position for passage of fluid to the other side of the hydraulic cylinder on movement of the control arms in the opposite direction.

On hydraulically operated cranes, loaders and the like it is advisable, with the arrangement according to the invention, to use the fluid in the control cylinders for control of the regulating unit and for regulation of the hydraulic cylinder of the toolholder. According to a further development of the invention it is proposed that the regulating unit and channels in the hydraulic valve are so arranged that the hydraulic cylinder of the tool holder is supplied with pressure fluid when the tool is to be rotated counter to a force acting upon it but with return fluid when the tool is to be rotated in the opposite direction. This is especially advisable, for example, for tractors carrying a bucket on the booms. When the booms are to be raised, this takes place counter to the force of gravity due to the dead weight, tool and load, if any, in the tool, while the tool is being rotated in the same direction as the forces of gravity. The capacity of the hydraulic pump of the device can thereby be fully utilized for the lifting movement.

The conditions are reversed when the booms are to be lowered, which takes place under the influence of the forces of gravity, while the tool in such case is being rotated counter to the forces of gravity, the which rational movement at this stage is accordingly accomplished by means of pressure fluid.

For transmission of the relative positions of the toolholder and booms to the supervisory unit it is suggested according to a further development of the invention that a Bowden cable or the like be arranged in such a way that one end of the Bowden cable wire is coiled up on a pulley rigidly connected to the toolholder, while the other end of the wire is taken to the supervisory unit via a pulley located at the pivoting axis of the booms on the machine. On raising of the booms, for example, at a constant angle between them and the toolholder, the wire of the Bowden cable is coiled up on the pulley at the pivoting axis of the booms, whereby the wire, preferably against the ac tion of a spring, moves the supervisory unit a certain distance in the hydraulic valve. In this way, as described in greater detail below, communications are opened between different channels in the hydraulic valve in such a way that the regulating unit is brought into action, with the result that the hydraulic cylinder of the toolholder is supplied with fluid for rotation of the toolholder, and so the tool, to the desired position. During this rotation of the tool the wire is uncoiled from the pulley on the toolholder, so that the supervisory unit is returned to neutral after the tool has assumed the desired position under spring action. The actuation of the toolholder in the opposite direction when the booms are lowered takes place in the reverse manner.

With reference to the attached drawings one embodiment of the invention will now be described and other characteristics and advantages of a device according to the invention will be demonstrated.

FIG. 1 shows a tractor with bucket attachment equipped with a device according to the invention,

FIG. 2 shows a hydraulic valve according to the invention,

FIG. 3 is a schematic diagram of the hydraulic connections between the hollow screws 2832 of the hydraulic valve [2 of FIG. 2 and the hydraulic cylinders 5 and I4 and control valve [0.

FIG. 1 shows a tractor equipped with a bucket attachment comprising booms 2 and a toolholder 3 on which is a bucket 4. The booms are raised and lowered by means of hydraulic cylinders 5, the booms pivoting around a shaft 6. The control cylinders 5 are actuated from a control valve 10 by means of levers It. The bucket attachment is shown in two positions, a lowered position 2, 3, 4, S and raised position 2 3 4 5 With the tool 4 at a given angle to the ground, accordingly, the booms 2, (2') and the tool holder 3, (3' assume different angles to one another depending on the positions of the booms.

Above the control valve 10 there is a hydraulic valve I2 according to the invention, from which fluid pressure lines 13 run to a hydraulic cylinder [4 between the boon is Z'and the toolholder 3. Furthermore, for transmission of the movements of the booms 2 and of the angle at every instant between the booms 2 and the toolholder 3 to the hydraulic valve 12 there is a Bowden cable 9 between the toolholder 3 and the hydraulic valve 12. The wire l5 of the Bowden cable 9 is coiled up at one end on a pulley 8 fixed to the toolholder 3 and at its other end, via a pulley 7 located on the shaft 6 of the booms 2, is fixed to a supervisory unit in the hydraulic valve 12. The wire 15 may, if necessary, be taken over an additional pulley in order to obtain a suitabae direction in relation to the supervisory unit. For raising of the bucket attachment, for example from the position marked 2, 3, 4, to the position marked 2, 3', 4', 5', the wire ofthe Bowden cable 9 is coiled up round the pulley 7. If the angle between the booms 2 and the toolholder 3 remains constant during this movement, the position of the wire 15 at the toolholder is fixed, so that the wire 15 actuates the supervisory unit in such a way that the latter is pulled in the direction out from the hydraulic valve. In the reverse manner the wire 15 is uncoiled from the pulley 7 when the booms 2 are lowered, whereby the wire tension diminishes and falls to zero, as a result of which the supervisory unit is moved in the direction into the hydraulic valve 12 under spring action. In this way impulses are obtained in the hydraulic valve 12, which, in the manner recounted below, result in the fact that the hydraulic cylinder 14 of the toolholder 3 is caused to function in such a way that thc toolholder 3, and so the tool 4, is carried upwards or downwards, during the movements ofthe booms, at a constant angle with the ground.

FIG. 2 shows the various parts of the hydraulic valve [2 in detail. The valve contains, as mentioned, a supervisory unit having a control rod 16 passing through most ofthe valve and, on the control rod, slotted clamping sleeves l8, 18', which are at least partly tapered. The supervisory unit also comprises a control plunger 17 which, by means of tapered shoulders 52, 52', is arranged to engage with the tapered surfaces of the clamping sleeves l8, [8, The clamping sleeves l8, [8' are kept apart by the action of a compression spring 19. At one end of the control rod 16 there is a supporting washer 23 against which rests one end of a return spring 22, the other end of which rests against a stop washer bearing on a shoulder in the valve housing. At the other end of the control rod [6 is a second stop washer 20'. The end of the control rod [6 projecting from the valve housing is designed to be connected to the wire 15 of the Bowden cable. Furthermore the idle position of the control rod 16 relative to the valve housing is adjustable with a trimming screw 24.

For removal ofany fluid leakage around the control plunger 17 there are two

chambers

36 and 4| around the control plunger 17 in the valve housing which communicate with channels for return fluid in the valve housing.

The hydraulic valve also comprises a regulating unit consist ing of a regulating plunger 25 having a hole passing through it axially. At one end of the plunger 25 is a throttling disc 26 and at its other end a spring 27.

The hydraulic valve has also a number of fluid channels and five

hollow screws

28, 29, 30, 31, 32 passing through the valve housing for connection both to the control valve 10 of the control cylinders 5 and to leads running to the control cylin ders 5 and the hydraulic cylinder 14.

The hydraulic valve functions as follows.

On raising of the booms 2 pressure fluid from the control valve 10 passes through the hollow screw 29 to the control cylinders 5 and return fluid from the latter returns via the hollow screw 28 to the control valve 10. The screw 28, however, is not hollow throughout, but return fluid is forced into a channel 34 down into a chamber 53 in the hydraulic valve 12, thence over to another chamber 54 and via a channel 33 back into the hollow screw and on to the control valve 10. As soon as this fluid flow for raising of the booms 2 has started, the control rod 16 in the hydraulic valve [2 is drawn up, as described above, by the wire 15 owing to the movements of the booms and the coiling of the wire around pulley 7. A chamber 40 around the control plunger 16 in the hydraulic valve 12 is thereby placed in open communication via a slot 43 in the control plunger with another chamber 39 around the control plunger 17. Through a channel 35 leading from chamber 53 around the regulating plunger 25 to chamber 39 some of the return fluid is thereby led via chambers 39 and 40 and a channel to a chamber 56 at the other end of the regulating plunger 25. The fluid is thereby passed through the throttling disc 26 under a certain pressure, the throttling producing a pressure dif ference between the two sides of the throttling disc. The regulating plunger 25 is thereby moved against the action of spring 27. The communication between chambers 53 and S4 is thus closed, while a communication is opened between chamber 53 and another chamber 55. Return fluid from the control cylindcrs 5 can then freely flow into the hollow screw 31 and thereafter through one of leads 13 to the hydraulic cylinder 14. Return fluid from the hydraulic cylinder 14 can either be led via the hollow screw 32 and through channels not shown in the drawing to the hollow screw 30 for return fluid or via a differential piston valve, for example of the type described in our Swedish Pat. and substitute therefor application No. 6614/65 which corresponds to US, Pat. No. 3,438,307, back to the pressure side of the hydraulic cylinder 14. The control fluid entering the regulating plunger through the throttling disc 26 is led off through holes 60 in the regulating plunger 25 into channel 59 leading to a chamber 48 and to the hollow screw 30 for removal of return fluidv As long as the hydraulic cylinder 14 is kept in working position in this way, the angle between the tool holder 3 and the booms 2 changes and the wire 15 is continuously uncoiled from pulley 8. This uncoiling is compensated by a continuous coiling up of the wire on pulley 7. If the angle between the toolholder 3 and the boom 2 becomes too large, the wire slackens, the result being that the supervisory unit is forced into the valve housing under the action of the return spring 22. The communication between chambers 39 and 40 is then throttled so that the regulating plunger 25 returns to neutral position under the action of spring 27. On lowering of the booms 2 pressure fluid flows in the reverse manner through the hollow screw 28 and return fluid returns through hollow screw 29. At the end ofthe hollow screw 29 facing the control valve 10 there is a throttling disc not shown in the drawing, which is displaccable under the action of the prevailing fluid pressure. When the fluid flows in the direction from the hydraulic valve 12 to the control valve 10, this throttling disc assumes throttling position. Some of the return fluid is thereby caused to flow out into channel 49, which leads to a third chamber 38 around the control plunger 17. As soon as the booms 2 commence their downward movement, wire 15 is uncoiled from pulley 7, so that the control rod 16, and therefore the control plunger 17, are forced into the valve housing under spring action. Chamber 38 is thereby placed in communication with a fourth chamber 37 around the control plunger via a channel 42 in the control plunger. The return fluid then flows through channel 39, chamber 38, channel 42, chamber 37, a channel 44 running from chamber 37 to chamber 40, and through channel 45 to chamber 56. Through the fact that throttling takes place at one end of the hollow screw 29, the return fluid has a rather higher pressure on lowering than on raising of the booms 2. The pressure difference between the two sides of the throttling disc 26 thereby produces a movement of the regulating plunger 25 against the action of spring 27, which is sufficient to open a communication between chamber 54 and another chamber 58 around the regulating plunger in the valve housing. The distance between chambers 54 and 58 is rather larger than between chambers 53 and 54. in the inner position of the regulating plunger, accordingly, a communication is open between chambers 54 and 58, whereas the communciations between chambers 53 and 54 and between chambers 55 and 53 closed. The pressure fluid through the hollow screw 28 can then not flow into chamber 53 but is led entirely via chamber 58 out into a channel 50 leading from the latter to hollow screw 32. From hollow screw 32 the pressure fluid then flows through one ofleads 13 to the hydraulic cylinder 14, whereby the toolholder is brought into parallel linkage during the lowering of booms 2. The lowering of the booms thus takes place under the influence of forces of gravity, while the entire capacity of the fluid pump is utilized for upward rotation of the tool in relation to the booms.

Since, in this way no pressure fluid is supplied to the pressure side of the control cylinders, a nonrcturn valve 51 is provided in the valve housing chamber 48 which communicates with hollow screws 30 for return of hydraulic fluid to the control valve 10 and channel 35. As soon as a negative pressure arises in the side of each of the control cylinders 5 which is to be filled with fluid during the lowering of booms 2, the nonreturn valve 51 opens and return fluid flows out chamber 48 and control valve via channel 35, chamber 53 and channel 34 to control cylinders 5. For tilting ofthe tool 4 the wire of the Bowden cable 9 is uncoiled from pulley 8, with the result that the wire is slackened and that the control rod l6 is forced to its innermost position in the valve housing under spring ac tion. The control plunger [7 is then in a position such that chamber 37 communicates with chamber 38 and therefore with channel 49. On a downward movement of the booms 2 the tool is immediately bent up, the bending-up movement continuing until the wire 15 has been coiled round pulley 8 so far that the wire is tensioned sufficiently to pull the control rod 16 slightly out of the valve housing. By means of the trimming screw 24 the innermost position of the control rod 16 can be regulated, which affects also the length of wire which, after tilting of tool 4, must be coiled around pulley 8 before the wire is tensioned. In this way it is possible to regulate the angular position to which the tool 4 is returned after tilting.

In the supervisory unit there are also stop washers 20, 20' and a strain-relieving sleeve 21. The function of these is to permit trimming with the trimming screw 24 and to render the valve inoperative in the event that the control rod is subjected to too great a movement owing to a fault. If, for example, the control rod is pulled violently in the direction out of the valve housing, the strain-relieving sleeve 21 is brought up against the stop washer 20' and thus brings the slotted clamping sleeve 18 out of engagement with the shoulder 52. The control rod is thus rendered freely movable in the control plunger 17. Correspondingly the clamping sleeve [8' can be taken out of engagement with the shoulder 52' via the stop washer 20 in the event, for example, that the trimming screw 24 is unscrewed too far from the valve housing.

Through the fact that the control rod 16, and so the control plunger I7, is actuated by wire 15 of the Bowden cable 9, the stroke of the control plunger increases during raising or lowering of the booms 2 in the event that the angle between toolholder 3 and booms 2 should remain constant or change too slowly owing, for example, to some inertia in the regulating plunger 25. A passage is thus opened still further between chambers 37 and 38 or 39 and 40 and additional control fluid is supplied to the regulating plunger. An absence of reaction thus increases the action of the valve, so ensuring reliable functioning of the hydraulic valve. Furthermore manual con' trol of tool 4 is possible regardless of the position of booms 2 since the hollow screws 31 and 32 are in communication with control valve 10 of the lifting attachment.

Although the invention has been described with reference to one of its embodiments, it can be arbitrarily varied within the scope ofthe following claims.

What I claim is:

l. Hydraulic control system for producing parallel linkage of a toolholder located, for example, on control arms actuated by means of hydraulic control cylinders, preferentially for cranes, loaders or the like, comprising a hydraulic cylinder mechanically coupled between the toolholder and the control arms, characterized in that the control units for the control arms are allotted a hydraulic valve containing a supervisory unit detecting the movements of the control arms and, in dependence on the direction of movement of the control arms, permitting the passage of fluid to a regulating unit allotted to the hydraulic valve, the regulating unit assuming a first position for the passage of fluid to one side of the hydraulic cylinder on movement of the control arms in one direction under the influence of said fluid and a second position for passage of fluid to the other side of the hydraulic cylinder on movement of the control arms in the opposite direction.

2, Hydraulic control system according to claim 1, characterized in that the fluid let through by the supervisory unit to the regulating unit consists of at least a part of the fluid of the control cylinders, preferentially the return fluid.

3. Hydraulic control system according to claim 1, characterized in that the fluid let through by the regulating unit to the hydraulic cylinder consists of at least a part of the fluid of the control cylinders.

4. Hydraulic control system according to claim 3, charac terized in that the fluid let through by the regulating unit to the hydraulic cylinder on actuation of the toolholder in one direction against a force acting upon the toolholder carrying the tool consists of the pressure fluid of the control cylinders and, on actuation of the toolholder in the opposite direction, consists ofthe return fluid of the control cylinders.

5. Hydraulic control system according to claim 1, wherein a shaft is operatively mounted on each of said control arms and said toolholder, said control system characterized in that the supervisory unit is designed to be actuated by movements of the control arms via a Bowdcn cable or the like, the wire of which passes over pulleys located on the shaft of the control arms and on the shaft of the toolholder.

6. Hydraulic control system according to claim 5, characterized in that the control unit contains a control plunger detachably located on and concentric with a control rod and preferably spring-loaded.

7. Hydraulic control system according to claim 6, further comprising a detachable fitting connected to said control plunger of said control rod, characterized in that the detachable fitting of the control plunger to the control rod is comprised of slotted clamping sleeves on the control rod which have outer tapered shoulders for engagement with inner tapered shoulders in the control plunger.

8. Hydraulic control system according to claim 7, charac terized in that the slotted clamping sleeves are designed, directly and/or via strain-relieving sleeves located on the control rod in predetermined limit positions of the supervisory unit, to engage with stop washers in the hydraulic valve for release of the engagement between the tapered shoulders on the clamping sleeves and the tapered shoulders in the control plunger, and so to release the engagement between the control rod and the control plunger in the said limit positions.

9. Hydraulic control system according to claim 1, characterized in that the supervisory unit has a trimming screw for adjustment of one limit position of the control unit, thereby permitting, within certain limits, optional parallel linkage angles to which the toolholder is returned after tilting of the tool.

10. Hydraulic control system according to claim 1, characterized in that the regulating unit consists of a plunger, preferably acted upon by a spring, having at one end a hole passing axially through it and throttled by a disc.

11. Hydraulic control system according to claim 1 characterized in that the hydraulic valve has hollow screws passing through it for pressure fluid and return fluid to and from the control cylinders of the control arms and the hydraulic cylinder of the toolholder to permit manual a justment of the positions of control arms as well as toolholder, said hollow screws within the hydraulic valve having bushings leading to channels in the hydraulic valve.

12. Hydraulic control system according to claim ll characterized in that, between channels for return fluid in the hydraulic valve and channels leading to one side ofthe control cylinders, there is a nonreturn valve for the passing of fluid from the return channels to the control cylinders on movement of the control arms in a direction essentially coinciding with the direction ofa force acting upon the tool.

13. Hydraulic control system according to claim 12, characterized in that the hydraulic cylinder of the toolholder is allotted to a differential piston valve for transmission of fluid from the suction side to the pressure side of the hydraulic cylinder during movement of the tool in a direction essentially coinciding with the direction of a force acting upon the tool.

Claims

1. Hydraulic control system for producing parallel linkage of a toolholder located, for example, on control arms actuated by means of hydraulic control cylinders, preferentially for cranes, loaders or the like, comprising a hydraulic cylinder mechanically coupled between the toolholder and the control arms, characterized in that the control units for the control arms are allotted a hydraulic valve containing a supervisory unit detecting the movements of the control arms and, in dependence on the direction of movement of the control arms, permitting the passage of fluid to a regulating unit allotted to the hydraulic valve, the regulating unit assuming a first position for the passage of fluid to one side of the hydraulic cylinder on movement of the control arms in one direction under the influence of said fluid and a second position for passage of fluid to the other side of the hydraulic cylinder on movement of the control arms in the opposite direction.

2. Hydraulic control system according to claim 1, characterized in that the fluid let through by the supervisory unit to the regulating unit consists of at least a part of the fluid of the control cylinders, preferentially the return fluid.

4. Hydraulic control system according to claim 3, characterized in that the fluid let through by the regulating unit to the hydraulic cylinder on actuation of the toolholder in one direction against a force acting upon the toolholder carrying the tool consists of the pressure fluid of the control cylinders and, on actuation of the toolholder in the opposite direction, consists of the return fluid of the control cylinders.

5. Hydraulic control system according to claim 1, wherein a shaft is operatively mounted on each of said control arms and said toolholder, said control system characterized in that the supervisory unit is designed to be actuated by movements of the control arms via a Bowden cable or the like, the wire of which passes over pulleys located on the shaft of the control arms and on the shaft of the toolholder.

8. Hydraulic control system according to claim 7, characterized in that the slotted clamping sleeves are designed, directly and/or via strain-relieving sleeves located on the control rod in predetermined limit positions of the supervisory unit, to engage with stop washers in the hydraulic valve for release of the engagement between the tapered shoulders on the clamping sleeves and the tapered shoulders in the control plunger, and so to release the engagement between the control rod and the control plunger in the said limit positions.

11. Hydraulic control system according to claim 1 characterized in that the hydraulic valve has hollow screws passing through it for pressure fluid and return fluid to and from the control cylinders of the control arms and the hydraulic cylinder of the toolholder to permit manual adjustment of the positions of control arms as well as toolholder, said hollow screws within the hydraulic valve having bushings leading to channels in the hydraulic valve.

12. Hydraulic control system according to claim 11 characterized in that, between channels for return fluid in the hydraulic valve and channels leading to one side of the control cylinders, there is a nonreturn valve for the passing of fluid from the return channels to the control cylinders on movement of the control arms in a direction essentially coinciding with the direction of a force acting upon the tool.