DE2340663A1 - SERVO-CONTROLLED, HYDRAULIC POWER TRANSFER DEVICE - Google Patents

Description

Servo-controlled hydraulic power transmission device

The invention relates to hydraulic power transmission devices, for example, axial piston pumps or motors, and is particularly concerned with a pump or a motor of this type, which is characterized by a variable fluid displacement and its displacement adjustable by a directly operated or remote controlled hydraulic pressure control system is. In addition, the device according to the invention have a remotely controlled, self-balancing, hydrostatic drive control device.

The term "pump" used in the following is intended to be simultaneous also include engine-type hydraulic power transmission devices. Axial piston pumps and motors

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Patent attorneys Dipl.-Ing. Martin Licht, Dipl.-Wirtsch.-Ing. Axel Hansmann, Dipl.-Phys. Sebastian Herrmann

which are equipped with various types of control devices are already known and in the following US patents described:

2 749 B44, 2 835 268, 2 915 985, 3 o17 864, 3 o89 426,

3 25o 277, 3 286 6o1, 3 3o2 585, 3 381 624, 3 4o5 646, 3 422 767.3 429 225, 3 461 277, 3 51o 231, 3 554 671, 3,588,286. These references show the general prior art known in this field.

According to the invention, a force or fluid transmission device is now intended of the axial piston type, in which the fluid displacement elements and the controls that work with them form an integral unit that is relatively simple is constructed and easy to install, adjust and maintain. In addition, in this axial piston force transmission device the fluid displacement continuously from a maximum displacement setting in one direction over the displacement value zero to a maximum displacement setting be adjustable in the other direction. Furthermore the device according to the invention is to be built in this way be that the fluid displacement control system is through an improved hydrostatic control device can be operated remotely, so that the device can be used and operated more universally.

For this purpose, according to the invention, a servo-controlled Transmission device of the axial piston type created, at dBr in a body a rotatable piston barrel is located, in which a swash plate is also located within a curve bearing seat located in the body forming tilting disk is pivotably mounted, and finally several pistons in the piston barrel are axially displaceable and abut on the drum disc,

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The pistons are designed so that sis when rotating the barrel and with respect to the swash plate perform a back and forth motion. Two drives are hinged to the tilting disk opposite to this disc from a position in which it causes maximum fluid displacement of the pistons, in a first direction of flow via a position in which the pistons have minimal fluid displacement cause up to a position in which a maximum fluid displacement in a second and opposite flow direction takes place to pivot. The drives are connected in such a way that they are acted upon with pressure medium via a servo selector valve. This valve has a externally operated valve control device, with dBr the fluid pressure either one or can be fed to the other drive. Furthermore, this valve is equipped with a valve cam follower which is mechanically articulated to the tilting disc and is designed in such a way that it flows through the fluid can shut off by the control valve as soon as the tilting plate has been pivoted into a position is in which it is in a linear relationship to a corresponding position of the control valve device stands. The valve control device can be controlled directly via a hydrostatic or an electrical control system operate or operate remotely. The geometry of the mechanical linkage through which the valve control device is hinged to the tilting disk is chosen so that the effect of the tilting disk on the control valve device is essentially linear.

The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing. In the drawing show:

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Figure 1 is a side view of an axial piston pump,

Figure 2 is a partially cut-away, rear end view of the pump of Figure 1,

Figure 3 is a partial axial sectional view of the pump longitudinally the line 3-3 in Figure 2,

Figure 4 is a sectional view of the pump along the line 4-4 in Figure 3,

FIG. 5 is a partial sectional view of the pump control along the line 5-5 in Figure 2,

FIG. 6 is an outline view of the control linkage of FIG Pump,

FIG. 7 shows a sectional view of the pump control mechanism along the line 7-7 in FIG. 2, in connection with a schematic representation of a hydrostatic drive for the control mechanism connected at some distance;

FIG. B is an enlarged partial sectional view of the control device of Figure 7, which shows this control device in the switching state in which it causes a change in the displacement of the pump, and

FIG. 9 is a partial sectional view of the control device of Figure 7, which shows this control device in the switching state in which it is an opposite Changes the displacement of the pump causes

In Figures 1-4, a hydraulic power transmission pump or a related motor 9oodes Axial-

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piston type shown, which has a drive shaft 11 which extends through an opening block 12 into a rotatable piston drum 1o. which in turn is rotatably supported in a pump body 13 by a bearing arrangement 14. Several pump pistons 15 are arranged in cylinders 16 which form the running drum 1o. On every piston a piston shoe 17 is rotatably mounted, which by a shoe holder arrangement 19 on a sliding washer 18 is held. This sliding plate is also called a swash plate in the following. The swash plate and the mounting arrangement 19 are mounted on the tilting cam 21, which in turn is seated in a bearing bracket 22 which is formed by an end cap 23. The end cap rests against a spacer ring 24 that holds the bearing assembly 14 and carries the drum Io into the correct operating position as soon as the end cap 23 with the body 13 is assembled in the manner shown. The end cap 23 does not protrude from the body 13 Connection elements shown (for example head screws} in connection. A bearing 25 is located between, the complementary, cylindrical surfaces the tilting curve 21 and the bearing block 22, so that a fairly slight rotational movement of the tilting curve in the Storage chair is made possible. The bearing 25 can best be seen in FIGS.

WiB can be seen from Figure 4, are also Halerungsconsole 26 present, which hold back the tilting curve 21 in the bearing bracket when the pump 9oo does not generate any delivery pressure. The delivery pressure generated by the pump 900 brings the tilting curve 21 to the bearing in a manner known per se 25 to the fixed system as soon as the pump is in operation.

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As can be seen from Figure 3, the shoe holder arrangement 19 one shoe holder plate 27 on which the piston shoes 17 during operation of the pump 9oo the swash plate 1B. The mounting plate is from the tilting curve 21 by a combination of radial and thrust bearing 28 and a splined shaft 29 supported, which are in an existing in the tilt curve Counterbore 31 extends into it. The shaft 29 is in the tilt curve 21 by a provided with a shoulder Nut 32 is resiliently attached, which carries the spring 33, for example a plate spring, and is compressed in the State holds. It should be noted that the shoulder of the nut 32 in the counterbore 31 game has, so that the mounting plate 27 is very much in the case of the action of axial forces occurring along the shaft 29 can move easily. The purpose of this construction shown and described is to provide only a limited one To allow lifting of the piston shoes 17. Such lifting is known per se and in the case of the at the beginning mentioned publications described devices of this type realized.

As can also be seen from Figure 3, the inner end of the drive shaft 11 is splined in the Running drum 1o fastened with the help of a pressure snap ring 34, which the shaft in the inner axial position holds and is fastened by a fastening element, for example a cap screw 35, which against a spring 36, for example a plate spring, and a washer 37 is tightened, the washer 37 rests against the drum 1o, as from. can be seen in the drawing.

As can be seen from Figures 3 and 4, the counterbore 31 opens into a bore 38 which forms a socket,

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which receives the main body of a control arm 39. The Stsuerarm 39 is in the bore 38 by a Snap ring or snap ring 41 held. The front one The end of the control arm 39 forms a fork 42 which receives a control arm pin 43.

The end cap 23 forms opposed cylinders 44 and 44. 45, which have a two-ended piston 46 take up. The center of the piston 46 is cut away to receive the steering arm pin 43 and the fork 42 and pierced. Appropriate seals, such as Q-rings, are provided at each end of the piston 46. The ends of the cylinders 44 and 45 are closed by heads 47 and 48 which are attached to the end cap 23 through Fastening elements not shown, for example head screws, are fastened. With the construction shown the piston 46, the cylinders 44 and 45 and the heads 47 and 48 form chambers 49 and 41.

As shown partially and somewhat schematically in Figures 2 and 3, the end cap 23 and the heads delimit 47 and 48 fluid channels 52 and 53 which are in communication with channels 54 and 55 which are in a Servo control unit 56 are located. The continuation of the channels 54 and 55 can be seen in FIGS. 7-9.

The servo control unit 56 and its associated mechanical Linkage with the tilting curve 21 can best be seen from FIGS. 4-7, the linkage as such especially Figure 6 can be seen.

As already mentioned, the tilting cam 21 is held in the bearing bracket 22 by a mounting bracket 26. the Console 26 form curved slots (FIG. 4) through which dowel pins 2o and

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extend. Thus, these pins move on a curved path when the tilt curve is in the bearing block 22 turns.

As can be seen from Figures 4-6, the dowel pin is 3o hinged to a feedback lever 95 which is attached to a control shaft 92 by a key 96. The control shaft 92 is supported in a bearing 93 and extends into the body 57 of the control unit 56 into it. A feedback bracket 91 is attached to the front end of the shaft by a key 94. The shared Ends of the bracket 91 encompass the central portion of a driver valve sleeve 61 and fit into a circumferential groove, which is formed in the sleeve so that the sleeve within the control body 57 to an axial movement is caused as soon as the shaft 92 via the lever 95 by the curved movement of the dowel pin 3o in Rotation is offset.

It should be noted that the fluid displacements in and out of chambers 49 and 51 causes a corresponding curved movement of the tilt cam 21 out. The movement of the tilt curve 21 calls for its part a corresponding movement of the dowel pin 3o, which is transmitted to the shaft 92 via the feedback lever 95 By rotating the shaft 92, the bracket 91 is pivoted. The movement of the bracket 91 in turn causes a corresponding axial movement of the driver sleeve 61.

The entire mechanical feedback linkage system is best seen in FIG. It should be noted that that the geometry of the mechanical linkage should be such that the reaction of the swash plate in the Setting a specific fluid displacement

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mange on the setting of the control valve spindle 62 is substantially linear.

It can be seen from FIGS. 5 and 6 and corresponding to FIGS. 0 and 9 that the control unit 56 has a body 57 on ^WB on a side cover 58 and an upper end cover 59. The body 57 delimits a bore which receives the driver valve 61 in a reciprocating manner and in a fluid-tight manner. A hollow control valve spindle 62 is seated in the driver sleeve 61 and is mounted in it so that it can move back and forth and is fluid-tight.

As can be seen from Figure 7, the above-mentioned channels 54 and 55 are with the piston displacement chambers 59 and 51 of Figure 3 in conjunction. The body 57 also contains channels 82, 83, 84 and 85, as shown in FIG the drawing shows. The channel 82 is located above the channel 54 and is connected so that it a pressurized, incompressible fluid can receive from a source P, which to this serves to drive the piston 46 in the manner described by the control unit 56, as follows is described. The channel 83 lies between the channels 54 and 55 and serves as a drainage channel for the fluid displaced from chamber 49 and chamber 51 as caused by the operation of the control unit 56 is prescribed. The channels 84 and 85 are in communication with the channel 83 in the manner shown serve the ends of the sleeve 61 and spindle 62 with the same size and in their opposite effect to apply balanced forces. Pressurized fluid is also applied to a piston, and cylinder timing drive device 7o attached to the lower end of body 54. That

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In the channel 83 exiting fluid is through a non-illustrated check valve in the housing chamber 86 (Figure 3) returned »This check valve keeps the outlet pressure in the channel 83 to, for example, about 1.8 to 5.6 kp / cm, to thereby serve as a suitable pressure source for the drive 7o to serve.

The sleeve 61 delimits four circumferential grooves at the bottom there are openings leading into the interior of the sleeve. The valve spindle 62 has three circumferential grooves on, in which there are openings in the upper and in the lower groove, which through the hollow channel 87 stay in contact. The open end of the spindle 62 is closed by a threaded plug or the like.

The grooves and adjacent webs of the spindle 62 are with respect to the existing openings in the sleeve 61 with Distance separated in order to a) close off the openings leading to the channel 87 and those leading to the channels 54 and 55 Block sleeve openings as soon as the sleeve assumes the position shown in FIG. 7 with respect to the spindle 62 to furthermore b) a connection between the channels 82 and 54 and between the channels 55 and 83 as soon as the spindle 62 with respect to the sleeve 61, as can be seen from FIG. 8, has been moved downwards i and to c) a connection via the channels 82, 87 and 55 and between the channels 54 and 83 as soon as the spindle 62 with respect to the sleeve 61, as out Figure 9 can be seen, has been moved downwards.

The drive 7o has a cylinder 63 which is attached to the Body 54 is fastened in the manner shown and closed at its lower end by an end cap 64 is. An internally threaded end cover65 is on

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the cap 64 attached. The cylinder 63 takes one displaceable piston unit 66, which is provided with piston rods which are above and below the Extend cylinder. The upper rod is with the lower end of the valve spindle 62 through a side undercut slot holder device in connection, this connection being compressed by an axially Spring 72 is maintained.

The lower rod of the piston 66 is in a centering spring arrangement, a retaining ring 71, a under low tension spring 67 and a lower retaining ring 73 inserted and stands with these parts in connection, which all surround the lower piston rod and with the help of a washer 74 and a screw 75 are connected to this rod. The retaining rings and the spring sit in a sleeve 69 which is screwed into the cover 65 adjustably. A spring snap ring 71 holds the retaining ring B, the spring and the piston rod in a central position within the sleeve 69, the spring is further compressed when the piston rod in one direction or the other with respect to the sleeve is moved. By adjusting the sleeve 69 for the piston 66 and the valve stem 62 is a certain Rest position selected for the swash plate 16, such as can be seen below, a zero displacement position or corresponds to a selected displacement position. The setting of the sleeve is fixed by a locking or locking nut 76. A stopper 77 closes the top of the sleeve 69.

The piston unit 66 forms on each side of the piston a chamber 97 and a chamber 98 within the cylinder 63. The chamber 97 is connected via a line 1o1 a remote control unit 1oo in fluid communication, while the chamber 98 via a line 99 to

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the remote control unit a fluid connection Has.

The channel 84 becomes pressurized fluid steered through a formed channel 80 into a channel 89 and then from this to the ball check valves 78 passed into lines 99 and 1o1. The ball check valves shown are of springs 79, which of retaining plugs 81 are held, printed in the closed position. In the arrangement shown, the fluid flows then through a corresponding check valve into the chambers 97 or 98 and the remote control or drive unit 100 if the internal pressure in these elements is below that in channels 83, 84 and 85 Pressure drops.

As can be seen from Figure 7, the remote control or drive unit 1oo has a body 1o2, the one Forms bore which receives a reciprocating control slide 11o. An end cap 1o3 closes one end of the body 1o2 while a control lever cap 1o4 closes its other end. Retaining rings 1o7 and 1o8 are centered by tensioned Springs 1o5 and 1oB are held in abutment with shoulders at each end of the slide tb, thereby causing the slide Push 11o into its centered and neutral position. One end of the slide 11o extends into a closed position Chamber 113 while its other end runs into a closed chamber 114. One channel 118 in body 1o2 connects chamber 114 to the conduit 1o1, and a further channel 119 is provided, which connects the chamber 113 and the line manufactures.

In the control lever cap 1o4 there is a lateral one

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Bore that receives a rotatable drive shaft 115. The shaft 115 is attached to the slide 11o by an operating arm articulated so that the slide can be moved in one direction or the other by turning the shaft can. A hand-operated lever 116 is connected to the shaft 115 outside the cap 1o4, so that the slide 11o can be operated by hand in order to displace fluid either from the chamber 113 or from the chamber 114 in the desired manner and thereby the control 7o to be operated.

A narrow circumferential slot 112 is around the center of the Slide 11o formed and can be brought into connection with small channels 1o9 and 111, which are located in the body 1o2 are located. The arrangement is such that when the lever 116 is released, the slide is in its Returns to the middle position, in which the line 1o9 is in communication with the line 111 through the slot 112. The system consisting of chambers 97, 9ff, 113 and 114 is located in this central position of the slide Equilibrium, with the valves 7B replacing any loss of fluid through leakage or the like. Ventilation valves, not shown, are used in the system to divert compressible fluids, such as air, from the system.

Although the control unit 7o has been described as a hydraulic unit, it can also be operated by hand or electrically operated unit. The hydraulic control unit described is suitable, for example for mining machines. A manual unit could be suitable for temporary use cases. One electrical unit would be particularly advantageous in connection with automatic control systems. You would be for example usable for modifications to electrical control units disclosed in U.S. Patents

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3 4o1 711 and 3 424 163 are described.

The operation of the control unit 7o with the help of Remote control or operating unit 1oo is explained below. Changes in fluid displacement of the pump 9oo by positioning the central valve spindle 62 are in view of the Figures 3-7 with reference to Figures B and 9 understandable.

The bracket 91 and the sleeve 61 are in Figures B and 9 is shown in the central or middle position, in which the swash plate 1B by the linkage of FIG is set perpendicular to the axis of the pump, with the fluid displacement by the pump corresponding Is zero. In Figure B, the control spindle has been moved downward, the movement of the piston 46, the Tilting curve 21 and the bracket 91 through the associated linkage has not started yet. The fluid from source P first passes into piston chamber 49 channels S2, Θ4 and 52 and begins. Fluid out to displace the chamber 51 through the channels 53, 55 and B4. As a result, the piston 46 and the start To move tilt control arm 39 downward, rotating the swash plate 1ß. Through the linkage of Figure 6 the movement of the tilt cam 21 causes a corresponding movement of the bracket 91 and a downward movement of the sleeve

61. The arrow on the right shows the direction of movement of the linkage. When the sleeve 61 moves downwards, the flow through the various openings is reduced and comes to a standstill as soon as the sleeve 61 has been brought into a position by the linkage which is determined by the control spindle 62. The swash plate 1B then remains fixed in this position with the aid of the piston 46 until another displacement position occurs the control spindle 62 is set when the remote control

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unit 1οο is operated. When the hand lever 116 of the unit 1oo is released, all rod parts return return to a neutral or other preselected position. The one on which the illustration in FIG. 9 is based Operation is the same as that of Figure θ, but takes place in the opposite direction.

The control device thus has a device for Positioning of the inlet link openings (control spindle) in such a way that fluid under pressure is fed to the corresponding channels, further an actuator to change the pump swashplate angle, and a feedback linkage, through which the swash plate position is transmitted to the feedback link (sleeve), which equalizes the control pressure on both sides of the actuation or drive unit, creating a further Movement of the pump swash plate is prevented. Therefore there is for every position of the input link (spindle) only a single pump swashplate position within the operational range in which mechanical jamming occurs.

Under normal conditions, for a given spindle position, the swash plate positions the sleeve via the feedback linkage so that the swash plate is held in this position. Any Disturbance upsets the pressure equilibrium at the ends of the pistons 46, causing a corrective movement of the swash plate which returns the system to the set position.

If the control spindle is moved, then through the change in the opening areas the pressure equilibrium disturbed in the chambers 49 and 51, whereby the swash plate is caused to move until the sleeve is again one

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Assumes a position in which the pressures are again in equilibrium are located. Thus, the swash plate angle and thus the pump displacement at each operating point, which is free of mechanical inhibitions or obstacles, for example stops, directly through the position controlled by the input spindle. The opening configuration and the backlash-free feedback linkage can be designed in this way ensure that this relationship is maintained with only very small deviations (deviations smaller than 2% of the full angle with control pressures of the source P of over 28.1 kp / cm ^). This type of follow-up servo control is generally very precise and stable and can also be so if the application requires it build it to work quickly.

Since the position of the servo spindle 62 is decisive for the pump displacement, the method for the control is decisive this spindle is very important. The method described here can be referred to as "fluid control" and is based on the displacement of an incompressible Fluid from one cylinder to another, thereby causing movement of a piston.

The control unit 1oo is the command or the "station ^a unit while the cylinder / piston arrangement is 7o the receiver unit. The refillable check valves 78 of the receiver unit permit filling of the cavities and lines connecting the transmitter to the receiver. The transmitter does not have shown Blow out plugs that allow air to be blown out so that the incompressibility of the fluid present in the system is ensured. The back pressure used for refilling must be high enough to open the check valves so that the incompressibility of the fluid is maintained (e.g. 1.76 to 5.62kp / cm), which means that any leakage

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outflow can be replaced.

When the spool 11o is moved, it displaces fluid βμε of one end chamber through the connected Leads therethrough into an end cavity. Because the fluid is essentially incompressible is, the piston 78 of the receiver 7o is thereby caused to move in a volume proportional to the displacement Measure to move. Preferably the areas are equal so that the receiver moves the same amount moves like the transmitter. Of course, as the receiver piston moves, it pushes the fluid away from the opposite side back through the connected cables to the next side of the transmitter unit. if at any time the pressure on either side of the trapped control volume under the The counterpressure prevailing in the servo device drops, the check valves open to allow the control fluid to replenish.

The receiver piston 66 is via a backlash-free connection connected to the servo input spindle 62. Therefore, the position of the pump swash plate 18 becomes remotely controlled by the position of the piston of the transmitter unit.

Furthermore, it should be pointed out that in the operating or drive unit 1oo a neutral position is built in, in which the end chambers are connected to one another through the narrow slot of the slide 11o. If the slide moves by approximately ήτ 2 mm, for example, the openings are closed and the working stroke begins.

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Claims (12)

Claims Iy Servo-controlled hydraulic power transmission device with a body in which the rotating piston drum and a tilting cam forming a swash plate are located, which is rotatably mounted in a curved bearing seat formed in the body, and with several Pistons, which are axially movably mounted in the piston barrel, bear against the swash plate and are reciprocable by rotating the piston barrel with respect to the swash plate by a pair of hydraulic drive elements (44, 45, 46) which are hinged opposite the tilting cam (21) and are operable to tilt the cam from a position in which there is minimal fluid displacement caused by the power transmission device in a position of maximum fluid displacement pivot through the power transmission device, further by a servo control valve device (56), which with the drive members (44, 45, 46) is connected and the fluid supplies and from each drive member removes each drive element, thereby to position the tilt cam (21) selectively, wherein the servo control valve device (56) has an externally operated spool valve (62) which is pressurized Fluid supplies one of the drive elements (44, 45, 46) while there is the outflow of the fluid from the other drive element (44, 45, 46), and wherein the servo control valve (56) also has a driver sleeve valve (61) which is mechanically linked to the tilting cam (21) by a linkage and can thus be actuated is that it shuts off the flow of fluid through the spool valve (62) as soon as the tilt curve (21) is pivoted into a position which corresponds to a position determined by the slide valve (62), wherein 409820/0683 the linkage is geometrically designed so that between the fluid displacement through the power transmission device and the corresponding position of the Slide valve (62) has an essentially linear relationship. 2. Hydraulic power transmission device according to claim 1, characterized in that each drive element (4o, 45, 46) has a hydraulic piston and cylinder device having. 3. Power transmission device according to claim 1, characterized by a fluid-operated control device (7o), which positions the slide valve (62). 4. Power transmission device according to claim 3, characterized characterized in that the control device (7o) has a remote-controlled hydraulic positioning device (1oo) having. 5. Power transmission device according to claim 3, characterized characterized in that the control device (7o) has a double-acting piston and cylinder unit (63, 66). 6. Power transmission device according to claim 4, characterized in that the drive elements (44, 45, 46) is provided with a double head piston device (46) which is articulated to the tilting cam (21). 7. Power transmission device according to claim 3, characterized in that the control device (7o) to this effect works that, as soon as no external actuating forces act on them, the tilting cam (21) in one position which causes a preselected fluid displacement through the device. 409820/0683 JtO 8. Power transmission device according to claim 4, characterized in that the hydraulic positioning device C1oo) a body (1o2) having a first chamber (113) and a second chamber (114) as well as a piston (11o) which is reciprocally supported between the chambers (113, 114) and so is operable to move fluid into one of the chambers while fluid from the other of the two chambers is displaced. 9. Power transmission device according to claim 8, characterized by means (112) for fluid communication between the chambers (113, 114) as soon as the positioning device (1oo) is in a neutral position. 10. Power transmission device according to claim 3, characterized by an electrically driven control device which can be actuated to control the slide valve (62) positioned. 11. Power transmission device according to claim 1, characterized characterized in that the two hydraulic drive elements (44,45, 46) the tilting cam (21) from a position in which a maximum fluid displacement through the transfer device takes place therethrough in a first direction of flow, via a position of minimum fluid displacement to a position of maximum fluid restriction pivot in a second direction which is opposite to the first flow direction. 12. Power transmission device according to claim 1, comprising a fluid-driven control device which controls the slide valve adjusted, and which consists of a control device with piston and cylinder, which is attached to the control element a servo control valve can be connected, characterized in that the control device (63, 66) has two control 40y820 / 0683 having chambers (97, 98} inversely variable volume, further comprising a check valve means (7B) which can be connected and operable to a source of pressurized fluid, that it feeds into the Ste _(uerkammern (97, 98) fluid, the drive device (1oo) consisting of piston and cylinder has two drive chambers (113, 114) of inversely variable volume, so that each of the control chambers (97, 98) can be connected to a corresponding drive chamber (113, 114) by a line (99, 1o1), that the control device (63, 66) is provided with a device for equalizing the fluid pressure in the control chambers (97, 98) and the drive chambers (113, 114) as soon as the control device (63, 66) assumes a neutral position, and that the Control device (7o) has an element which centers this control device in a selected position as soon as the fluid pressure between the control chambers (97, 98) is equalized chen will. £ 09820/0683 Empty soap