Electric Pressure-reducing Valve

Abstract

An electric pressure-reducing valve is provided for regulating a hydraulic utilization pressure by means of a low electric control current and comprises two stages, i.e., a primary stage constituted by a magnetic force motor and a stop-valve and jet-nozzle system, and a secondary stage constituted by a hydraulic slide valve servo-controlled by the pressure regulated in the primary stage. The primary stage is wholly constructed along a single axis of revolution: the power developed by the magnetic force motor is directed along this axis, the movement of the stop-valve is a movement of translation along this axis, the jet-nozzle is mounted on this axis; the primary stage is mounted on the body of the electric pressure-reducing valve so as to form an independent unit which is readily removable along said axis without any risk of upsetting the adjustments.

Description

The present invention has for its object an apparatus known as an electric pressure-reducing valve, the function of which is to permit the control of a hydraulic pressure by means of a small electric control current.

For the same purpose, it is generally known to utilize servo distributors. This kind of apparatus usually gives satisfactory results, but it has the disadvantage of a high production cost.

The electric pressure-reducing valve according to the invention presents the advantage of having a much simpler construction and, consequently, being substantially less costly than its predecessors.

The electric pressure-reducing valve according to the invention comprises two stages, a primary stage constituted by a magnetic force motor and by a stop-valve and nozzle system, and a secondary stage constituted by a hydraulic slide valve servo controlled by the pressure regulated by the primary stage.

The electric pressure-reducing valve in accordance with the invention is especially characterized in that:

A. The primary stage is wholly constructed along an axis of revolution: the power developed by the force-motor is directed along this axis, the movement of the stop-valve is a translation along this axis, the nozzle or jet is placed on this axis; the primary stage is mounted on the body of the electric pressure-reducing valve as an independent unit which is readily removable by dismantling along the said axis, without risk of interfering with the adjustment;

B. For a constant supply pressure, and in the absence of hydraulic flow, the law of operation which associates the output pressure of the electric pressure-reducing valve with its control current is substantially linear;

C. In addition, if desired but not necessarily, the slope of the said law of operation is independent of the supply pressure; in other words, the same variation of intensity of the control current always produces the same variation of the utilization pressure, and these variations are proportional, over the range between the maximum supply pressure and zero pressure, which are physical limits of the utilization pressure.

In a more detailed manner, the law specified in (b) will be defined by referring to the curves shown in FIG. 1, given by way of nonlimitative example, in which the intensity I of the electric control current is plotted on the abscissa and the utilization pressure PU delivered by the secondary stage is plotted on the ordinate. With the electric pressure-reducing valve according to the invention, the law of current-pressure is linear (the straight line LO in FIG. 1) for the value of the supply pressure PAO at which the primary stage has been adjusted. On the other hand, when the supply pressure falls to a value PA1, less than PA0, the law of current-pressure has a level portion (the broken line L1 of FIG. 1). The utilization pressure PU first remains substantially constant and equal to PA1 as long as the control current has not reached a value IS, which value increases as the supply pressure PA1 becomes lower.

This characteristic with a flat-topped portion is obviously not troublesome in applications in which the supply pressure PA is approximately constant, and it still remains permissible in other applications. However, there exist applications in which the supply pressure is variable and in which furthermore the said flat portion may represent a disadvantage.

The invention enables this disadvantage to be eliminated when so desired. According to a further arrangement of the invention, the electric pressure-reducing valve can, according to paragraph (c) above, provide a law of current-pressure which is always linear and of constant slope, in spite of variations of supply pressure. This law according to (c) will be defined by referring to the curves given in FIG. 2 by way of nonlimitative example, these curves also having the intensity I of the electric control current plotted on the abscissa and the utilization pressure PU on the ordinate.

With this other arrangement according to the invention, for different values of PA, for example PA1, PA2, PA3, there is obtained a system of parallel straight lines, all characterized by a constant ratio between the variation of the control current intensity I and the variation of the utilization pressure PU. It will be observed that the level pressure zones such as that shown in FIG. 1 have been eliminated.

Briefly, following an arrangement in accordance with the invention, in the electric pressure-reducing valve comprising a primary stage and a secondary stage, the primary stage comprising a magnetic force motor, a moving stop-valve displaced by the said motor, a nozzle placed in front of the said stop-valve, and an orifice plate mounted upstream of the said nozzle and supplied under pressure by a hydraulic source, the secondary stage comprising a hydraulic slide valve controlled by the control pressure established by the primary stage between the said orifice plate and the said nozzle and regulating the said utilization pressure, with zero hydraulic flow, following a law as a function of the electric control current which is substantially linear for a constant supply pressure, the said primary stage is wholly constructed along an axis of revolution Y-Y, the power developed by the said force-motor being directed along the said axis Y-Y, the movement of the said stop-valve is in translation along the said axis Y-Y, the said nozzle is a body of revolution about the said axis Y-Y, the whole of the primary stage being capable of removal as a unit along the said axis Y-Y, and the said hydraulic slide valve is acted upon in opposite directions, over two equal sections, on the one hand by the said control pressure and on the other hand by the said utilization pressure, the said hydraulic slide valve having a neutral position in which it does not modify the said utilization pressure, on one side of which it is displaced if the utilization pressure becomes less than the prescribed value and in which it connects the said utilization pressure to exhaust.

Briefly, following another arrangement according to the invention, in the electric pressure-reducing valve comprising a primary stage and a secondary stage, the primary stage comprising a magnetic force-motor, a movable stop-valve displaced by the said motor, a nozzle placed in front of the said stop-valve, and an orifice plate mounted upstream of the said nozzle and supplied under pressure by a hydraulic source, the secondary stage comprising a hydraulic slide valve controlled by the control pressure established by the primary stage between the said orifice plate and the said nozzle and regulating the said utilization pressure, at zero hydraulic flow, following a law as a function of the electric control current which is substantially linear for a constant supply pressure and has a substantially constant slope for a variable supply pressure, the said primary stage is wholly constructed along an axis of revolution Y-Y, the power developed by the said force-motor is directed along the said axis Y-Y, the movement of the said stop-valve is in translation along the said axis Y-Y, the said nozzle is a body of revolution about the said axis Y-Y, the whole of the primary motor can be removed as a unit along the said axis Y-Y, and the said hydraulic slide valve is acted upon in opposite directions, on the one hand over a first section by the said control pressure and over a second section by the said utilization pressure, and on the other hand over a third section equal to the said second section by the said supply pressure, the said hydraulic slide valve having a neutral position in which it does not modify the said utilization pressure, on one side of which it is displaced if the utilization pressure becomes less than the prescribed value and at which it supplies the said utilization pressure from the said supply pressure, and on the other side of which it is displaced if the utilization pressure becomes greater than the prescribed value and at which it connects the said utilization pressure to exhaust.

The invention, and two embodiments of the electric pressure-reducing valve according to the invention, will now be described with reference to the accompanying drawings, given by way of nonlimitative examples. In these drawings:

FIG. 3 is a diagrammatic view in axial section of an electric pressure-reducing valve according to the invention, providing the law of current-pressure shown in FIG. 1;

FIG. 4 is the view in elevation corresponding to FIG. 3;

FIG. 5 is a view in axial section taken along the line V-V of FIG. 4, but to twice the scale;

FIG. 6 is a view in cross section taken along the line VI-VI of FIG. 5;

FIG. 7 is a view in cross section along the line VII-VII of FIG. 4;

FIG. 8 is a view in plan looking from above, of FIG. 4;

FIG. 9 is a view in half-longitudinal section of the secondary stage;

FIG. 10 shows a further electric pressure-reducing valve according to the invention, in cross section passing through the axis X-X of the slide valve of the secondary stage and normal to the axis Y-Y of the primary stage;

FIG. 11 is a cross section passing through the axis Y-Y of the primary stage and normal to the axis X-X of the slide valve of the secondary stage, taken along the line XI-XI of FIG. 10;

FIG. 12 is a detail, in cross section through the axis Y-Y of the primary stage, along the line XII-XII of FIG. 10, showing the electrical connections of the operating coil.

With reference to FIGS. 3 and 5, there will be described below the structure of the primary stage of this electric pressure-reducing valve according to the invention.

The magnetic motor, of the polarized type, comprises a permanent magnet 1 with radial magnetization, a coil 2, a moving core 3 and a magnetic circuit consisting of the parts 4, 5 and 6. The airgap 7 is determined by the space comprised between the core 3 and the part 4. The airgap 8 corresponds to the space comprised between the said core 3 and the part 5, and the airgap 9 to the space comprised between the said core 3 and the part 6.

The core 3 is rigidly fixed to a rod 11, the two extremities of which are respectively suspended from two fine metallic diaphragms 12 and 13. The core 3 can thus move without frictional contact on any other member. Two springs 14 and 15 ensure its return to its mean position. The support 6 on which the spring 14 is indirectly supported, comprises a screw 17 which, by acting on the cup 18 of this spring enables the position of the core 3 to be adjusted.

Magnetic shunts 19 arranged in holes formed in a spacing member 21 of the magnetic circuit 4 permit the adjustment of the gain of the force-current law. A closure member or stop-valve 23, fixed to the extremity of the rod 11 opposite to the screw 17 is mounted facing a jet-nozzle 24 with lateral outlets 25a, the whole being arranged, as already stated, along the general axis Y-Y. This primary stage finally comprises an orifice plate 25 protected by a filter 26.

All the elements of this primary stage are contained in a body 27 and thus constitute an independent unit, which can be extracted from the remainder of the apparatus without risk of putting it out of adjustment. This body 27 is extended by an endpiece 28 fixed on the body 27 by a nut 29 (see FIG. 5). This endpiece 18 is in turn closed by the base of the fluidtight electric socket 30.

The secondary stage having an axis X-X (see FIGS. 3, 7, 8 and 9), comprises a slide valve 31 moving inside a casing 32 and acted upon by two restoring springs 34 and 35. The chamber 36 (FIGS. 3 and 9), located at one of the extremities of the slide valve, communicates continuously with the chamber 37 comprised between the nozzle jet 24 and the orifice plate 25 (FIGS. 3 and 5), in which the primary stage regulates the control pressure PC. At the opposite extremity of the slide valve 31, the chamber 38 communicates with the utilization orifice (pressure PU). The elements of this secondary stage are mounted in a body 39 on which the primary stage is fixed in a removable manner.

The casing 32 is held in position by the endpiece 41, which comprises a screw 42 by means of which the law of current-pressure can be adjusted. An endpiece 43 closes the chamber 36. Finally, on the body 39 are provided the three hydraulic connections for supply (pressure PA), utilization (pressure PU) and return R (FIGS. 3, 4, 7 and 8), together with the fixing lugs (not shown).

The operation of this electric pressure-reducing valve is effected as follows:

According to the shape of the current-pressure characteristic which it is desired to obtain, the utilization pressure PU is a substantially linear function, increasing or decreasing, of the current I passing through the coil 2, on the assumption that the supply pressure PA is constant.

There is considered for example the case in which the pressure PU is a decreasing function of I. When the current I is zero, the stop-valve or closure member 23 occupies a position very close to the jet nozzle 24, at a distance which can be adjusted by means of the screw 17 in order to obtain the desired maximum pressure of PU. In fact, this distance determines the pressure in the chamber 37. As long as the pressure PU is less than the pressure PC existing at 37, the slide valve 31 moves towards the left (FIG. 5) and it delivers from PA towards PU. In consequence, the pressure PU progressively increases; the slide valve 31 closes its passages when an equilibrium of the pressures is reached.

When the current I is not zero, the magnetic motor applies to the moving core 3 a force which, acting in opposition to that applied by the springs 14 and 15, moves the closure member 23 away from the jet nozzle 24, which has the effect of reducing the pressure in the chamber 37. The result is that the slide valve 31 is displaced towards the right and puts the utilization PU in communication with the return R, up to the moment when an equilibrium of the pressures is again obtained, but at a lower value.

It is clear that the force applied on the core by the magnetic motor has for its origin the combination in the airgaps 7 and 9, of the flux generated by the coil 2, with the flux created by the permanent magnet of the groove 64 of the casing, a groove 74 which can come opposite passages 64 of the latter flux being distributed between the airgaps 7 and 9, after having passed across the airgap 8.

It will be understood that the particular constructional features of the electric pressure-reducing valve described above are not of any limitative nature.

Another electric pressure-reducing valve according to the invention, which ensures the current-pressure law shown in FIG. 2, will now be described with reference to FIGS. 10 to 12.

Referring to FIG. 10, the secondary stage of this embodiment comprises, housed in a body 51 in a fluidtight manner, as shown, a cylindrical casing 52 having an axis X-X mounted between two sockets 53 and 54 forming abutments and two fluidtight plugs 55 and 56 held by threaded nuts 57 and 58 screwed on the body 51. A cylindrical slide valve 59 having an axis X-X slides freely between the socket-abutments 53 and 54.

The casing 52 is provided with a plurality of external grooves, each communicating with radial passages, namely, going from left to right in FIG. 10: a groove 61 which is connected (in a manner not shown), to the return to the tank and therefore under pressure; a groove 62 connected to the utilization conduit and therefore under the utilization pressure PU; a groove 63 coupled to the supply conduit and therefore under the supply pressure PA; a groove 64 which is also under the utilization pressure PU; and a groove 65 connected to the output of the primary stage and therefore under the control pressure PC.

The slide valve 59 is provided externally with a plurality of external grooves, separated from each other by three shoulders, namely, going from left to right in FIG. 10: an endpiece 71 of reduced diameter, a groove 72 which can come opposite passages of the groove 61 of the casing and which faces passages of the groove 62 of the casing, a groove 73 which is facing the groove 63 of the casing and which can come opposite passages 64 of the casing, and an endpiece 75 of reduced diameter.

An axial and then radial passage 76 provides a continuous communication between the chamber 77 on the left-hand side of the slide valve and the groove 73, and a radial then axial passage 78 forms a continuous communication between the groove 74 and the chamber 79 on the right-hand side of the slide valve.

To sum-up, to the left of a central position, the slide valve puts its groove 72 into communication with the grooves 61 and 62 of the casing 52 and thus connects the utilization conduit to the return to the tank, which reduces the utilization pressure PU, and to the right of this central position, the groove 73 of the slide valve 59 puts into communication the grooves 63 and 64 of the casing 52, and therefore puts the supply conduit into communication with the utilization conduit, which increases the utilization pressure PU.

Now, the slide valve 59 slides freely in the casing 62 under the action of the hydraulic pressures which are applied to it, and which are: on the one hand in the left-hand chamber 77, the supply pressure PA arriving through the groove 63, the groove 73 and the axial passage 76, and acting towards the right on the annular section S1 of the endpiece 71, on the other hand, in the right-hand chamber 79, the utilization pressure PU coming in through the groove 64, the groove 74 and the axial passage 78, and acting towards the left on the annular section of the endpiece 75, which has the same annular section S1 as the endpiece 71, and finally the control pressure PC, coming in through the groove 65 and acting towards the left on the annular section S2. In consequence, the slide valve 59 will be in equilibrium if:

1. (PA)(S1)=(PU)(S1)+(PC)(S2)

therefore:

1. (PU)=(PA)-(PC).sup.. (S2)/(S1)

With reference to FIG. 11, the magnetic motor of the polarized type comprises a permanent magnet 81 with radial magnetization, a coil 82, a moving core 83 and a magnetic circuit composed of the members 84, 85 and 86. The working airgaps 87 and 88 correspond respectively to the space comprised between the core 83 and the member 84, and to the space comprised between the said core and the member 86. The core 83 is rigidly fixed to a rod 89, the two extremities of which are suspended respectively from two thin metallic diaphragms 91 and 92; the core 83 is thus able to move without rubbing contact with any other part. The elastic return of the moving system 83--89 is ensured by the two diaphragms 91 and 92 which are notched and tend to lift this moving system, and by a helicoidal spring 93 which tends to pull it down. This spring 93 works in compression and its supporting point 94 can be displaced by an adjustment screw 95, accessible through a threaded plug 96.

The above members of the primary stage are housed in a cylindrical chimney 97, having an axis Y-Y, of the body 51, closed by an endplate 98 and a nut 99.

The two connection wires 101 of the coil 82 (see FIG. 12) pass into a recess 102 formed in the body 51 and are connected to an electric socket 103 which is removable by means of screws 104 (see FIG. 10).

In the body 51 and again along the axis Y-Y is formed a chamber 111, in which a closure member 112, fixed to the lower extremity of the rod 89 and therefore movable in translation parallel to the axis Y-Y, moves in front of a jet nozzle 113 mounted in a fixed position along the axis Y-Y and with fluidtight sealing in the body 51. This jet nozzle 113 is fed through the intermediary of an orifice plate 114 and a cylindrical filter 115 from the supply conduit 116. The jet nozzle 113 and its associated parts are held by a plug 117 and a screw 118 and are thus easily accessible.

When the action of the magnetic motor brings the closure member 112 closer to the jet nozzle 113, the pressure in the chamber 119 upstream of the jet nozzle increases. This pressure is the control pressure PC and is sent via lateral openings 113a in nozzle 113 into the groove 65 of the secondary stage through a conduit system (not shown).

It will be noted that, with the arrangements described above, on the one hand the hydraulic portion of the primary stage is readily accessible, for example in order to clean the filter or the jet nozzle, and on the other hand, its electrical portion is easily accessible. In addition, these parts are interchangeable by means of the adjustment alone of the spring 93 by its screw 95. These advantages are obviously associated with the systematically axial structure (with an axis Y-Y) given to the two hydraulic and electrical portions of the primary stage of the electrodistributor.

On the other hand, the direction of the current I in its coil 82 is such that it produces a control pressure PC at the output of the primary stage which is an increasing function of the current:

3. (PC)=kI

in which k is a positive constant. A combination of the equations (2) and (3) shows that:

4. (PU)=(PA)-aI

in which a is a constant equal to (S2/S1).sup.. k. In other words, the slope a of the straight line representing the current-pressure law is independent of the supply pressure PA. This is what is shown in FIG. 2.

From the technological point of view, the construction of the second stage shown in FIG. 10 should be noted. The two endpieces 71 and 75 are machined on the slide valve 59 in a concentric manner with respect to the three bearing surfaces which slide in the bore of the casing. The two sockets 53 and 54 are centered in the bore of the casing 52, these sockets each comprising a bore in which is allowed to slide one of the endpieces 71 and 75. It can thus be seen that as a result of the centering of the sockets 53 and 54 in the casing 52, there are only two concentricities to be observed: that of the end pieces 71 and 75 with the three bearing surfaces of the slide valve, and that of the bore and the external machining of the sockets 53 and 54. These two concentricities can furthermore be obtained with a high degree of accuracy by a single machining operation. In fact, by means of an appropriate jig, the sockets 53 and 54 can be held on the endpieces 71 and 75, and in a single operation it is possible to grind the three bearing surfaces of the slide valve and the external diameters of the sockets 53 and 54.

In this way it is possible to obtain almost perfect concentricities and in consequence to provide a frictionless sliding movement of the slide valve, at the same time having very small clearances (between the slide valve 59 and the casing 52, between the endpieces 71 and 75 and between the sockets 53 and 54 and finally between the sockets 53 and 54 and the casing). These small clearances enable very small friction to be obtained at the level of the second stage.

In addition, it will be observed that the two passages 76 and 78 in the interior of the slide valve 59, which on the one hand bring the pressure PA into the chamber 77 and on the other hand lead the pressure PU into the chamber 79 permit the machining of the body 51 to be simplified.

Claims

What I claim is:

1. An electric pressure-reducing valve for regulating a hydraulic utilization pressure as a function of a low electric control current, comprising a primary stage and a secondary stage, the primary stage comprising a magnetic force motor, a movable closure member coupled to and displaced by said motor, a jet-nozzle placed in front of said closure member, and an orifice plate mounted upstream of said jet-nozzle and supplied under pressure by a hydraulic source, the secondary stage comprising a hydraulic slide valve controlled by the control pressure established by the primary stage between said orifice plate and said jet-nozzle and regulating said utilization pressure with zero hydraulic flow as a function of the electric control current which is substantially linear for a constant supply pressure, said primary stage being wholly constructed along an axis of revolution Y-Y, the power developed by said force-motor being directed along said axis Y-Y, the movement of said closure member by said motor being in translation along said axis Y-Y, said jet-nozzle being a body of revolution about said axis Y-Y, means supporting the whole of the primary stage as a removable unit along said axis Y-Y, means for supplying said hydraulic slide valve in opposite directions, on two equal sections, on the one hand with said control pressure and on the other with said utilization pressure, and means providing said hydraulic slide valve with a neutral position in which it does not modify said utilization pressure, on one side of which it is displaced if the utilization pressure becomes less than the prescribed value and in which it supplies said utilization pressure from said supply pressure, and on the other side of which it is displaced if said utilization pressure becomes higher than the prescribed value and in which it causes said utilization pressure to escape.

2. An electric pressure-reducing valve as claimed in claim 1, in which said motor of said primary stage comprises a permanent magnet with an axis Y-Y, with radial magnetization, a coil on the axis Y-Y, a movable core on the axis Y-Y, a magnetic circuit comprising two end plates, a yoke with its axis Y-Y with a spacing member and adjustable magnetic shunt means in said spacing member, a rod with its axis Y-Y rigidly fixed to said movable core, two thin metal diaphragms supporting said rod, an adjustable spring with its axis Y-Y at one extremity of said rod, said closure member with its axis Y-Y being at the other extremity of said rod, said means supporting the primary stage as a removable unit comprising a body on the axis Y-Y housing all said members of the primary stage.

3. An electric pressure-reducing valve as claimed in claim 1 wherein said means providing the slide valve with its neutral position comprises a casing slidably receiving said hydraulic slide valve therein, and two opposing springs, one of which includes adjustable means, said springs acting on said slide valve for restoring said slide valve to its neutral position, said means for supplying the hydraulic slide valve with the utilization and control pressures comprising at one extremity of said casing an endpiece closing a first chamber connected to and subjected to said utilization pressure, at the other extremity of said casing, an endpiece closing a second chamber connected to and subjected to said control pressure, said slide valve being provided with a blind axial bore and grooves positioned to cooperate with grooves in said casing for supplying said utilization pressure from said supply pressure when the slide valve is displaced towards the first said chamber, and in causing said utilization pressure to escape when said slide valve is displaced towards said second chamber.

4. An electric pressure-reducing valve for regulating a hydraulic utilization pressure as a function of a low electric control current, comprising a primary stage and a secondary stage, said primary stage comprising a magnetic force motor, a movable closure member coupled to and displaced by said motor, a jet-nozzle placed in front of said closure member, and an orifice plate mounted upstream of said jet-nozzle and supplied under pressure by a hydraulic source, said secondary stage comprising a hydraulic slide valve controlled by the control pressure established by the primary stage between said orifice plate and said jet-nozzle and regulating said utilization pressure, at zero hydraulic flow as a function of the electric control current which is substantially linear for a constant supply pressure and substantially of constant slope for a variable supply pressure, said primary stage being wholly constructed following an axis of revolution, the power developed by said force-motor being directed along said axis, the movement of said closure member by said motor being in translation along said axis, said jet-nozzle being a body of revolution about said axis, means supporting the whole of the primary motor as a removable unit along said axis, means for supplying said hydraulic slide valve in opposite directions, on the one hand on a first section by said control pressure and on a second section by said utilization pressure and on the other hand on a third section equal to said second section by said supply pressure, and means providing said hydraulic slide valve with a neutral position in which it does not modify said utilization pressure, on one side of which it is displaced if the utilization pressure becomes less than the prescribed value and in which it supplies said utilization pressure from said supply pressure, and on the other side of which it is displaced if the utilization pressure becomes greater than the value prescribed and in which it causes said utilization pressure to escape.

5. An electric pressure-reducing valve as claimed in claim 4, in which said motor of said primary stage comprises a permanent magnet having an axis Y-Y, with radial magnetization, a coil on the axis Y-Y, a movable core on the axis Y-Y, and a magnetic circuit comprising two end plates and a yoke having the axis Y-Y, a rod on axis Y-Y rigidly fixed to said core, two thin metal diaphragms supporting said rod, an adjustable spring means on the axis Y-Y, at one extremity of said rod, said closure member being on the axis Y-Y, at the other extremity of said rod, said means supporting the primary stage as a removable unit comprising a cylindrical chimney with the axis Y-Y housing all said members of the primary stage.

6. An electric pressure-reducing valve as claimed in claim 4 further comprising a casing slidably receiving said hydraulic slide valve, said means for supplying the hydraulic slide valve with the utilization and control pressures comprising at one extremity of said casing a socket-abutment and a plug defining a first chamber having a determined section on which is applied said supply pressure, at the other extremity of said casing, a socket-abutment and a plug defining a second chamber having the same section as said first chamber, on which is applied said utilization pressure, an intermediate shoulder of said slide valve forming therein an annular surface constituting said first section on which said control pressure is applied in the direction which pushes the slide valve towards said first chamber, said slide valve being provided with two blind axial bores and with grooves positioned to cooperate with grooves of said casing in supplying said utilization pressure from the supply pressure when the slide valve is displaced towards said second chamber and in causing said utilization pressure to escape with said slide valve is displaced towards said first chamber.