US 3916765 A
Automatic return of a hydraulic press tool at the end of its working stroke when the pressure acting on its piston member reaches a predetermined value, and automatic resetting of the apparatus at the end of the return stroke is achieved by a main change over valve in interlocking relation with a secondary change over valve. Each of these valves is actuated by pressure acting on a small area of the valve element at a valve seat which opens access of the pressure to a larger piston area of the valve element against a spring. The main change-over valve is arranged when moved to its operative position by the rise in working pressure to divert the pressure supply from the working-cylinder and chamber to a return cylinder, until, at the end of the return stroke, the pressure rise in the return cylinder, acting via a line on the valve seat of the secondary change-over valve causing a land of the latter to establish a drain connection for the valve seat of the main c-o valve. The main change-over valve is then returned by its spring to its normal position.
Claims available in
Description (OCR text may contain errors)
United States Patent  Hilton Nov. 4, 1975 1 AUTOMATIC CONTROL OF RAM-TYPE FLUID-PRESSURE OPERATED ACTUATOR UNITS  Inventor: John Leslie Hilton, Ringwood,
England  Assignee: The Plessey Company Limited,
Essex, England  Filed: May 25, 1973  Appl. No.: 364,157
 Foreign Application Priority Data May 31, 1972 United Kingd m 25474/72 May 31, 1972 United Kingdom 25475/72  U.S. Cl. 91/307; 91/308; 91/318;
 Int. Cl. FOIL 25/06  Field of Search 91/307, 318, 306
 References Cited UNITED STATES PATENTS 1,952,690 3/1934 Strom, Sr 91/318 3,225,663 12/1965 Pelisson 91/318 FOREIGN PATENTS OR APPLICATIONS 939,238 2/1956 Germany 91/306 Primary Examiner-Paul E. Maslousky Attorney, Agent, or Firm-Scrivener Parker Scrivener & Clarke  ABSTRACT Automatic return of a hydraulic press tool at the end of its working stroke when the pressure acting on its piston member reaches a predetermined value, and automatic resetting of the apparatus at the end of the return stroke is achieved by a main change over valve in interlocking relation with a secondary change over valve. Each of these valves is actuated by pressure acting on a small area of the valve element at a valve seat which opens access of' the pressure to a larger piston area of the valve element against a spring. The main change-over valve is arranged when moved to its operative position by the rise in working pressure to divert the pressure supply from the working-cylinder and chamber to a return cylinder, until, at the end of the return stroke, the pressure rise in the return cylinder, acting via a line on the valve seat of the secondary change-over valve causing a land of the latter to establish a drain connection for the valve seat of the main c-o valve. The main change-over valve is then returned by its spring to its normal position.
A pressure switch by-passing the starting switch for the motor driving the pump may be provided which is closed against spring action by the pressure at the valve seat of the main change-over valve so as to stop the pressure supply after each completion of a cycle comprising a working stroke and return stroke. 1f the working piston is equipped with an approach-relief pressure utilization arrangement, the pressure released by this arrangement from the working chamber may be directed to the control chamber of the main change-over valve to cause immediate commencement of a return stroke of the working piston in the case of an obstruction having struck during the approach portion of the working stroke of the press tool.
1 Claim, 4 Drawing Figures US. Patent Nov. 4, 1975 Sl 1eet1 of3 3,916,765
12 32a 321; 3a. 2 19a 33 7&6
U.S. Patent Nov. 4, 1975 Sheet 2 0f3 3,916,765
U.S. Patent Nov. 4, 1975 Sheet 3 of3 3,916,765
AUTOMATIC CONTROL OF RAM-TYPE FLUID-PRESSURE OPERATED ACTUATOR UNITS This invention relates to ram-type fluid-pressureoperated actuator units and has for an object to provide improved apparatus including such units, which when a stroke is terminated by the rams striking an end stop operative at the end of a normal working stroke or an obstruction met by the ram of a press tool during a socalled approach stroke preceding the beginning of the working stroke proper, or some other obstacle, will automatically produce a control function causing the unit to automatically initiate a return stroke.
In my co-pending patent application Ser. No. 196,052 on which US. Pat. No. 3,735,632 has issued a fluid-pressure actuation system for press tools has been described which, during an approach portion of the working stroke of the press tool, limits the actuating pressure to a predetermined or adjustable maximum while permitting pressure to rise well above this maximum during the remaining part of the working stroke. It will be readily appreciated that in the case of press tools a relatively long initial part of the working stroke of the press piston, hereinafter called approach stroke, is often required in order to move the press tool over a region required to facilitate the insertion and/or removal of the workpieces, while a high pressure force is only required for a short working stroke proper. In the case, for example, of rivetting tools, or of so-called crimping tools for use in the establishment of electrical connections, there is a risk that the working tool may meet, during the approach stroke prior to the working stroke proper, some object which obstructs the approach movement, and which might be liable either to be damaged itself or to cause damage to other objects if the approach stroke would proceed after striking the obstruction, and in my said co-pending patent application it has been proposed to reduce this risk by the provision of a relief passage which via a pressure limiting valve leads to a lower pressure, and a port, controlled by the piston movement, which is open during the approach stroke and is blanked-off by the piston movement at the end of the approach stroke, and according to one form of the present invention, the relief passage of such system may be arranged to lead to an actuating chamber associated with a change-over valve so as to operate this valve to reverse the action of the fluid pressure on the ram actuator unit so as to automatically return that unit to its starting position when, during the approach-stroke, the ram meets an obstruction leading to a pressure rise in the working chamber sufficient to i open the pressure-limiting valve.
It is also an object of the invention to provide an improved ram-type fluid-pressure-operated apparatus which, when the pressure in a working cylinder of the ram unit, for example in the working cylinder of a fluidpressure-operated press tool, reaches a predetermined value, will automatically switch-over the supply of fluid pressure from a fluid-pressure supply line so as to produce return movement and which, at the end of such return movement, will automatically restore itself to a position ready for a further operating stroke of the working cylinder.
Such apparatus according to the invention may include two fluid-pressure-actuated change-over valves, hereinafter respectively called main change-over valve and secondary change-over valve, each of which in- 2 cludes a loading spring normally holding the valve element in one predetermined position, and each of which is movable to a second position by fluid pressure acting, in opposition to the action of this spring, on a small piston area of the valve element to admit, when the action of this pressure is sufficient to overcome the loading of the spring, such fluid to act on a larger area of the valve element, thereby increasing the force moving the valve element to the said second position and holding it there until the pressure drops to a value very much lower than that of the pressure which initiated the operation of the valve. The main change-over valve is arranged to be acted-upon, via a restrictor, by the pressure in the said working chamber during the working stroke of the unit and to establish, when in its normal position, connection from the fluid-pressure supply line to the said working chamber while venting the return cylinder, and in its second position to vent the working chamber to a low pressure and isolate it from the fluid-pressure supply, and establish connection between the fluid pressure supply line and the return cylinder and isolating the latter from the low pressure, and the secondary change-over valve is arranged to be acted-upon by the pressure in the return cylinder and to control the connection between the downstream side of said restrictor and such low pressure so that when the secondary change-over valve is in its normal position, it isolates the said downstream side from the said low pressure, while when said secondary change-over valve is in its said second position, it establishes connection between said downstream side and said low pressure. If desired, the downstream side of the restrictor may be additionally connected to a holding cylinder which, when, and as long as pressure at said downstream side exceeds a predetermined minimum maintains, against the action of a spring, a switching device in a position permitting continued supply of operating fluid for the apparatus, but which automatically cuts off, until manually reset, such supply when the pressure at said downstream side drops below this predetermined value. The apparatus according to the invention may furthermore, and irrespective of whether or not such cut-off means are incorporated, be provided with a connection communicating with the working chamber and including a retaining valve opening at an approach-stroke pressure substantially lower than the maximum working-stroke pressure to ensure automatically the return of the working piston to its normal position without completion of its working stroke if, during the approach portion of the working stroke, an obstacle is encountered which causes the operating pressure in the working cylinder to rise beyond the said approach-stroke pressure.
In order that the invention may be more readily understood, a number of embodiments will now be described with reference to the accompanying drawings, in which:
FIG. 1 is a somewhat diagrammatic layout of one automatically reversing flu id-pressure operated press tool the operating cylinder and the change-over valves of which are shown generally in axial section,
FIG. 2 similarly shows a modified embodiment incorporating means for automatically arresting operation after completion of a working stroke and return stroke of the working-cylinder piston,
FIG. 3 shows a third embodiment incorporating means for prematurely effecting the change-over from a forward stroke to a return stroke if, during the approach part of the working stroke, the piston strikes an 3 obstacle, and
FIG. 4 shows a fourth embodiment which incorporates both the means for automatically arresting operation and the means for prematurely effecting the change-over to return-stroke movement respectively shown in FIGS. 2 and 3.
Referring now first to FIG. 1, the illustrated embodiment comprises a pump 1, shown as a gear pump, which serves to supply liquid under pressure for the operation of a hydraulic working unit 5, and which when operating draws hydraulic fluid from a reservoir 2 through a suction line 1a and supplies it under pressure to a delivery line lb which leads into the cylindrical bore of the housing 31 of a main change-over valve 3.
branch of the delivery line leads, via a restrictor 11d, to a small-diameter valve seat 8 at one end of the j housing 31. A valve spool 32 equipped with three longitudinally spaced lands hereinafter referred-to as the left-hand land 32a, the centre land 32b and the right hand land 320, is longitudinally slidable in the bore of the housing 31 and is urged by a loading spring 33, which is adjustable by a screw-threaded end plug 6, into contact of one of its ends with the valve seat 8. Movement of the valve spool in the opposite direction is limited by shim discs 7 inserted between the inner end of the plug 6 and the end of the loading spring 33, if desired or necessary, with the addition of annular shims 7a in order to permit independent selection of the spring tension for a given end position of the stroke of the spool 32. In the illustrated normal position of the valve spool 32, the port at which the pump-delivery line lb enters the bore of the valve housing 31, lies between the centre land 32b and the right-hand land 32c of the valve spool 32, so as to communicate with a port 3a which is connected to the working chamber 9 of the hydraulic working unit 5. This unit contains a piston member 11 operating in the working cylinder 10 of, for example, a hydraulically operated press tool, and since the port 3a leading to its working chamber 9 is in this position of the valve spool 32 also located between the said two lands 32b and 32c of the spool, the working chamber 9 is supplied with hydraulic fluid under pressure delivered to the valve 3 by the pump 1 via the delivery line 1b. The part of the bore of the main changeover valve 3 beyond the right-hand land 32c communicates permanently, by a drain line 40, with the reservoir 2. A further line 19a connects the end portion of the working cylinder 10 at the opposite side of the working piston, which is hereinafter referred-to as the return cylinder 19, with a port in the bore of the valve housing 31 which, when the spool 32 is in its illustrated normal position, is also located beyond the right-hand land 320 so that the return cylinder 19 is drained to the reservoir 2. Another drain connection 41, preferably provided with a restrictor 41a, communicates with a part of the bore of the valve housing 31 which, throughout the movement of the valve spool 32, is located between the left-hand land 32a and the centre land 32b. A secondary change-over valve 4, which is a spool valve, whose spool has a left-hand land 14a and a right-hand land 14b, is normally held by a loading spring 16 in its illustrated position, in which it isolates the valve seat 8 of the main change-over valve 3 from a drain line 22, whereas when actuated against its loading spring, the secondary change-over valve will drain the branch line 1c which leads from the pump-delivery line lb via the restrictor 1d to the valve-seat 8, by connecting it via a port connection 13 and the drain line 22 to the reservoir 2. As long as the secondary change-over valve 4 remains in its illustrated normal position, the valve seat 8 facing the end of the spool 32 of the main changeover valve 3 is under pump-delivery pressure due to its connection to the delivery line lb via the branch line 10. The action of this pressure upon the valve spool 32 in the area of the valve seat 8 is opposed only by the force of the loading spring 33, which is so chosen as to prevent movement of the spool under the action of the pump delivery pressure during the working stroke of the piston member 11 but to be overcome when, at the end of the working stroke, that pressure rises due to the fact that further movement of the working piston is prevented by abutment means. As a result, the valve spool 32 is then moved by the pressure action in the valve seat 8, so that the working pressure is now admitted to the portion 12 of the bore of the valve housing 31, where it will act on the left-hand land 32a of the valve spool, whose area is much greater than that of the valve seat 8. The valve spool 32 of the main change-over valve will therefore move rapidly to its opposite end position, in which the port 3a leading to the working chamber 9 of the working cylinder 10 is separated by the centre land 32b of the valve spool 32 from. the pump-delivery line 1b and placed into communication, via the space between the centre and left-hand lands 32a, 32b, with the restricted drain line 41, which permits liquid from the port 3a to return to the tank 2. At the same time the right-hand land 320 of the valve spool 32 will isolate the retum-cylinder line 19a from the drain line 40 and connect it, via the chamber between the centre and right-hand lands 32b, 320 of the valve spool 32, to the pump-delivery line lb, thus causing the working piston 11 to carry out a return stroke.
The spool 14 of the secondary change-over valve 4 is urged, by the spring 16, into its illustrated position, in which its valve-shaped end closes a valve seat 18 which, similarly to the valve seat 8 in the main change-over valve 3, leads to an end portion 15 of the bore of the valve housing so that the pressure admitted to this portion 15 through the valve seat 18, once the action of the spring 16 is overcome, will act on the surface of the left-hand land 14a of this spool 14, and the area at the end of this land is much larger than the opening area of the valve-seat 18. This ensures completion of the stroke of the valve spool 14 and retention of the valve spool in its resulting position until the pressure in portion 15 of the valve-housing bore drops to a value very much lower than that which initiated the operation of the valve. The right-hand land 14b of the valve spool 14 is so axially spaced from the land 14a that in the illustrated normal position of the spool 14 this land 14b blanks-off the port 13 leading to the branch line 1c and thus isolates this branch line from the drain line 22. The valve seat 18 is connected by a line 21 to the return cylinder 19 of the working unit 5, and the loading spring 16 of the secondary change-over valve 4 is so dimensioned as to permit movement of the spool 14 from its illustrated position when the pressure in the return cylinder 19 reaches a value slightly higher than that required to overcome the normal resistance of the piston member 11 in the working cylinder 10 during its retum-stroke movement. As a result when, at the end of the return stroke of the working piston, the pressure in the return cylinder 19 reaches this higher value, the valve spool 14 of the secondary change-over valve will be switched over from its illustrated position to its other end position. When the valve spool 14 of the secondary change-over valve 4 is thus displaced, both the port connection 13 from the branch line 1c and the port of the drain line 22 will be between the two lands 14a and 14b of the secondary change-over valve 4 thus causing the valve seat 8 of the main change-over valve 3 to be drained to the tank 2 via the drain line 22. The restrictor 1d in the line 10 nevertheless permits the operating pressure in the pump-delivery line lb, and thus the pressure in the line 21, to be maintained, for the time being, at a level sufficient to hold the secondary change-over valve 4 in its operative position. The connection of the valve seat 8 of the main change-over valve 3 to the drain line 22 via the secondary changeover valve 4 also releases the pressure from the chamber 12 which acts on the left-hand land 32a of the spool 32 of the main change-over valve, so that the loading spring 33 can now move the spool 32 of the main change-over valve 3 back to its illustrated position, in which it will once more close the valve seat 8. Soon after the commencement of this return movement of the spool 32, the right-hand land 320 of this spool will blank-off the port of return cylinder line 19a, thus cutting-off the supply of pressure liquid from the pumpdelivery line 1b to the return cylinder 19, but since at this stage no outlet is available for the release of liquid from the system comprising the return cylinder 19, the lines 21 and 19a, and the chamber 15 of the secondary change-over valve 4, the loading spring 16 of that valve is still prevented from returning the secondary changeover valve 4 to its illustrated position until the return movement of the spool 32 of the main change-over valve 3 to its illustrated position is nearly completed and its right-hand land 32c has thus once more opened the drain connection from return-cylinder line 19a to the drain line 40. Only then the spring 16 is able to commence returning the spool 14 of the secondary change-over valve 4, so that the spool 32 of the main change-over valve 3 will have reached its illlustrated position before the land 14b of the secondary changeover valve reaches a position blanking off the port connection 13 from the valve seat 8 of the main changeover valve 3 and thus isolating this valve seat from the drain line 22.
It will be readily appreciated that, in the arrangement illustrated in FIG. 1, a fresh cycle of operations identical to the one above described will commence as soon as the main change-over valve has again reached its illustrated normal position, so that the apparatus will perform, as long as the pump 1 is driven and not rendered inoperative by any hydraulic switch means, a continuous sequence of operating cycles each comprising a working stroke of the piston 11 which, when a predetermined hydraulic pressure has been reached in the chamber 9 of the working cylinder 10 is terminated and followed by a return stroke, at the end of which, when the piston member 1 1 strikes an end stop, for example the bottom of the working cylinder 10, the next working stroke immediately commences.
FIG. 2 illustrates a modification of this apparatus, in which operation is arranged to stop each time a working stroke followed by a return stroke has been completed, and wherein operation of a switch, either manually or by some automatic means, is then required for the commencement of the next working stroke. In the apparatus illustrated in FIG. 2 all the elements described with reference to FIG. 1 are likewise present, and the same references have been used for indicating identical parts. It will therefore suffice for the under- 6 standing of the apparatus of FIG. 2 to refer to the description of FIG. 1 as far as the operation of these parts is concerned, and only the additional features will now be described in detail.
A pressure-switch mechanism 34 is provided in order to arrest the operation of the apparatus each time a working stroke followed by a return stroke has been completed. This mechanism comprises a piston 25 which operates in a hydraulic cylinder 24 and is urged by a spring 26 to the illustrated position. In this position a pressure chamber 23 at the opposite side of the piston 25 is at its minimum volume, and the pressure switch is arranged when in this position to cut-off the pressure supply to the pump-delivery line 1b. In the illustrated embodiment this is effected by the use of an electric switch 27 which, when the piston 25 is at the other end of its stroke, establishes connection, in parallel to a starting switch ST, between a mains supply line P and a feed line Q for an electric motor M which is arranged to drive the gear pump 1.. The starting switch ST is biassed by a spring STl to its open position, and a hydraulic line 23a connects the pressure chamber 23 of the switch mechanism 34 with the valve seat 8 of the main change-over valve 3 so that when, after operation of the starting switch ST, the pump 1 has commenced delivering liquid through delivery line lb to the working cylinder 5, the resulting establishment of working pressure in the line 1b will cause liquid under pressure to be delivered via the branch line 1c to move the piston 25 of the pressure switch 34 from its illustrated OFF position to its opposite or ON position, in which the switch 27 is closed, so that operation of the motor M and pump 1 will now continue even when the starter switch ST is allowed to open. The loading of the pressureswitch spring 26 is so chosen that the piston 25 will remain in this position, thus ensuring continuation of the operation of the pump not only during the working stroke but also during the return stroke of the working unit 5, as long as the pressure in return cylinder 19, and thus in the return-cylinder line 21, remains high enough for the normal execution of a return stroke of the piston member 11. When however, after completion of the return movement of the piston 11, the valve seat 8 of the main change-over valve 3 is vented via a lowresistance vent line 22 by operation of the secondary change-over valve 4, the spring 26 will return the piston 25 of the pressure-switch device 34 to its illustrated position, thereby opening the switch 27-and thus cuttingoff the connection between the motor-feed line Q and the mains line P. The motor M will therefore now stop and the supply of liquid to the working cylinder 5 will be discontinued until a fresh cycle of operations is initiated by closing once more the starter switch ST.
When the working cylinder 5 is that of a press tool or other apparatus that is equipped with a safety device which operates to arrest the working stroke during an approach portion of the working stroke when the element moved by the piston member 11 encounters an obstacle, it may be desired to ensure that in this case the working piston immediately returns to its illustrated initial position without requiring the intervention of the operator, and FIG. 3 illustrates a modified form of apparatus in which this is achieved by the combination of the apparatus illustrated in FIG. 1 with a connection communication with the working chamber and including a retaining valve opening at an approachstroke pressure substantially lower than the maximum working-stroke pressure.
As in the case of FIG. 2, substantially all the apparatus described with reference to FIG. 1 has been retained unaltered and indicated by the same reference numbers in the embodiment of FIG. 3, so that for the understanding of the modified form illustrated, it will suffice to describe the modifications that have been effected. These consist basically in that the working-cylinder unit 5 of FIG. 1 has been replaced by a workingcylinder unit 5a, which has an additional outlet which is controlled by a valve 42 arranged in the working piston 11a and is connected to a line 29b, and the change-over valve 3A, which is identical in all other respects to the change-over valve 3 of FIG. 1, has in its cylindrical valve housing 31a, in addition to the ports provided in the valve housing 31 of FIG. 1, an additional port 12a which leads from the line 29b into the chamber 12 that extends from the valve seat 8 to the left-hand land 32a of the valve spool 32.
The valve 42 is a ball valve, which is urged by a spring 43 on to an annular shoulder 44 in an axial bore 45 of the working piston 11a so as to normally prevent flow from the working chamber 9 through the bore 45 and a cross-bore 46. This cross-bore opens into a sleeve-like chamber 47 which is so arranged in a cylinder-head member 48 as to face the cross-bore 46 throughout a so-called approach portion of the stroke of the working piston, and to which the line 29b is connected. The spring 43 is so preset by means of a screw plug 49 on which the shoulder 44 is formed, as to allow the valve 42 to open when, during the approach portion of the working stroke of the working-cylinder unit 5a, the piston member 11a of this unit meets an obstacle which causes the pressure in its working chamber 9 to rise above a relatively low maximum preset to be just sufficient to overcome normal friction and like resistances during the approach movement. A path is thus opened which permitting liquid under pressure from the line 3a which interconnects the working chamber 9 of the working cylinder a with a port in the cylinder 31a of the main change-over valve 3A, to flow through the line 29b into the chamber 12 between the valve seat 8 and the left-hand land 32a of the valve 3A. The pressure in the working chamber 9 of the working cylinder unit 5a will thus act on the land 32a of the valve spool 32 to overcome the pressure of the loading spring 33 and move the valve spool 32 of the main change-over valve 3A to its operative position, in which valve 3A causes pressure fluid from the pump-delivery line 1b to be redirected to reach the return cylinder 19 instead of the working chamber 9 of the unit 5a exactly as it does when reversal of the position of the change-over valve is effected by a rise of the pressure in the branch line 1c at the end of a normal working stroke of the piston unit 11a. FIG. 4differs from FIG. 3 in exactly the same way in which FIG. 2 differs from FIG. 1, the means for automatically arresting operation at the end of a return stroke of the working cylinder piston being connected to the line 13 leading to the valve seat 8 of the main change-over valve 3a of FIG. 4 and to power-supply lines for the motor of the pump 1 in exactly the same manner in'which these parts are connected to the line 13 of pump 1 of FIG.
1 in the embodiment of FIG. 2. All the reference numbers of FIG. 3 have been retained and the reference numbers of the added parts are identical with those of the same added parts of FIG. 2, so that the construction illustrated in FIG. 4 will be readily understood vn'thout further description, and it will be readily appreciated that the main change-over valve 3a will be moved to cause a change from forward movement to reverse movement of the piston member 11a both when, at the end of a working stroke, the pressure supplied through lines 1b and 3a to the forward working chamber 9 of the working cylinder 5a reaches a value high enough to lift, by acting on the cross-section on the seat, the valve 3a off its seat 8 and when, due to an obstruction during the approach stroke, pressure from the working chamber 9 is allowed to obtain access via line 29b to the larger-area surface of the land 32a of the valve element. In both these cases the pressure in line 13 will, as long as, due to the changed position of valve element 32 pressure from pump line 1b is fed via line 19a to the returned-stroke side of the piston 1 la for the execution of its return stroke, the pressure in chamber 23 of the cylinder 24 of the pressure-switch mechanism 34 is maintained sufficiently high to hold the electric switch 27 closed against the action of spring 26, so that the motor M will continue driving the pump 1, and in both cases at the end of the return stroke the resulting momentary increase in pressure in line 19a is communicated via line 21 to the secondary change-over valve 4, causing the latter to vent line 13 to the tank 2, with the result that the pressure switch mechanism 34 will be moved by its spring 26 to its illustrated position, in which the electric switch 27 is opened and causes the motor M to stop until the motor is restarted by operation of the starter switch ST.
What we claim is:
1. Fluid-pressure-operated apparatus which com- .prises: an actuator unit having a working chamber and an output member movable, by pressure in said working chamber, to carry out a forward stroke in which the volume of said working chamber is increased, and which extends from a first position to a second position, and return means having a return chamber and operable by pressure in said return chamber to produce return movement of said output member towards said first position; change-over means including a main change-over valve and a secondary change-over valve, each constituted by a valve housing formed with a cylindrical bore having an access aperture at one end and with a plurality of ports spaced along said bore, by a valve spool longitudinally movable along said main bore between a first control position and a second control position and formed with a plurality of lands spaced longitudinally of the valve spool and including one land which throughout the movement of the spool isolates said access aperture from all said ports, and by a spring urging said valve spool to said first control position; the cylindrical bore of each change-over valve housing terminating at an end face that is perforated to provide said access aperture, the latter being of smaller diameter than and coaxial with said main bore and forming a valve seat at said end face, and the valve spool of each change-over valve having a reduceddiameter extension at one end constituting a valve element that closes said seat when said valve spool is in said first position, the ports of the main change-over valve including a supply port for connection to a supply of fluid under pressure, a first and a second service port respectively connected with said working chamber and with said return chamber, and low-pressure vent port means, said ports and port means being so arranged in relation to said lands that, when said valve spool is in said first control position, they establish communication between said first service port and said supply port while isolating the said first service port from the vent port means, and also establish communication between isolating said second service'port from said supply port, and that conversely, when the valve spool is in its second control position, they establish communication of said second service port with said supply port and isolate the second service port from the vent-port means, while also establishing communication of said first service pdrt with the vent port means and isolating the first service port from the supply port, the ports of the secondary change-over valve including a communication in communication with each other when the spool of the secondary change-over valve is in its second control position and are isolated from each other by one of the lands of the valve spool when said spool is in its first control position; passage means connecting the communication port of the secondary change-over valve with the valve seat in the end face of the bore of the housing of the main change-over valve and communicating through a restrictor with the supply port of the first change-over valve, and passage means connecting the access aperture of the secondary change-over valve port and a vent port which are arranged at such posiwith the return chamber of the actuator unit.
tions spaced along the cylindrical housing bore as to be