US3530764A

US3530764A - Electro-hydraulic servomotor

Info

Publication number: US3530764A
Application number: US787166A
Authority: US
Inventors: Tamaki Tomita
Original assignee: Toyoda Koki KK
Current assignee: Toyoda Koki KK
Priority date: 1967-12-27
Filing date: 1968-12-26
Publication date: 1970-09-29
Anticipated expiration: 1987-09-29
Also published as: DE1816506A1

Description

United States Patent Inventor Tamaki Tomita Okazakl, Japan Appl. No. 787,166

Filed Dec. 26, 1968 Patented Sept. 29, 1970 Assignee Toyota Koki Kabushiki Kaisha Kariya, Aichi Prefecture, Japan a corporation of Japan Priority Dec. 27, 1967 Japan ELECTRO-IIYDRAULIC SERVOMOTOR 10 Claims, 5 Drawing Figs.

U.S. Cl 91/6.5,

Int. Cl. F0lb 3/00, 7 F0ll33/02,F15b9/l0 Field of Search 91/380,

368, 75 180 (cursory), 375 (Cursory) References Cited UNITED STATES PATENTS 2,527,458 10/1950 Schurr 91/380 I 2,960,071 1 1/1960 Rosebrook 91/380 2,984,217 5/1961 Cline; 91/380 3,310,284 3/1967 lnosa et al.,... 91/380 3,457,836 7/1969 Henderson 91/380 Primary Evamr'ner-Paul E. Maslousky Attorney- Hutchinson and Milans ABSTRACT: This invention relates to an electro-hydraulic servomotor wherein a hydraulic motor can exactly follow the rotation of an electric motor. No relative rotation occurs between a servovalve chamber and a valve spool closely mounted within said chamber. The valve spool is axially moved by the electric motor, thereby enlarging the opening degree of the oil ports of the hydraulic motor, and the rotation of said hydraulic motor moves said valve spool in the reverse direction to .said axial displacement of said valve spool, thereby controlling the opening degree of said oil ports of said hydraulic motor.

Petented Se t. 29, 1970 3,530,764

Sheet 1 of 2 PAM/\K 1' or I m Patented Sept. 29, 1970 Sheet FIG. 4

FIG. 5

uuu

'AMAhI VON I "A ELECTRO-I-IYDRAULIC SERVOMOTOR BACKGROUND OF THE INVENTION Recently with a spectacular advance in the automatic control of machine tools, and the like, a servomotor which can quickly respond to the r.p.m. indication and at the same time can withstand a heavy load is widely in demand.

As is well known, in an electric motor, like an electric pulse motor or a synchronous motor, the control of revolution is easy, but the motor cannot be used at high speed revolutions over a certain r.p.m. under a heavy load.

Meanwhile, in the case of a hydraulic motor, the motor may be used even at high speed revolutions under a heavy load, but it is not so easy for revolution control as the electric motor. Thus, both kinds of servomotors are found to have relative merits and demerits.

In view of this situation, the so-called electro-hydraulic servomotor has come into practical use, combining the good controllability features of the electric motor, such as an electric pulse motor or a synchronous motor, with the high speed and high output of the hydraulic motor.

The electro-hydraulicmotor, as is well known,works as follows: the revolution of the electric motor is converted into an axial displacement taking place atthe same time as the rotation of the valve spool or to a rotational displacement of the pilot valve. By either displacement, the oilp ort is opened to supply the pressurized oil to the hydraulic .motor or to discharge the oil out of the motor, thereby the revolution of the hydraulic motor is made to follow that of the electric motor.

The construction of the conventional electro-hydraulic motor, however, is such that either the valve spool orthe pilot valve is closely mounted within the valve chamber of the servovalve; thereby either the valve spool or the pilot valve is rotated by the electric motor; and thus eitherlthe valve spool or the pilot valve can rotate or slide in relation to the valve chamber of the servovalve. Under this construction, considerable power is needed to move the valve spool or the pilot valve.

The valve spool must be closely mounted within the valve chamber of the servovalve, or otherwisethe hydraulic motor will not be able to follow therotation of the electric motor with any degree of accuracy. Thus, great resistance should be overcome in order to realize a close mounting of either the valve spool or the pilot valve within'the valve chamber of the servovalve due to the relative rotation of either the valve spool or the pilot valve to thevalve chamber of the servovalve. Particularly when the oil viscosity is high, the resistance against the valve spool imposes anincreased torque on. the electric motor, resulting in a control failure of the electric motor.

Meanwhile, it is conceivable to make the valve spool rotate or slide relative to the valve chamber of the servovalve by the rotating output of the hydraulic motor and provide such feedback as to nullify the axial displacement of the valve spool. Under this arrangement, however, the rotation of the valve spool would be synchronized with that of the hydraulic motor and accordingly the relative rotation of the valve spool to the valve chamber of the servovalve would be high-speed, or the absolute amount of this relative motion would be great. While the radial difference between the valve chamber of the servovalve and the valve spool, closely mounted within the valve chamber. is less than several microns, the valve spool and the valve chamber of the servovalve cannot be 'true circle. Moreover, the valve spool and the valve chamber of the ser vovalve cannot be precisely concentric. It follows, onone hand, that the valve spool would be worn out and accordingly it would be difficult to make the rotation of the hydraulic motor follow that of the electric motor with high accuracy,

The primary object of this invention is to provide an electrohydraulic motor in which no relative rotation takes place between a valve chamber and a valve spool closely mounted within said valve chamber. Only a relative displacement in the axial direction takes place between them, so that the valve spool can work smoothly.

Another object of this invention is to provide an electrohydraulic motor in which the oil port of the hydraulic motor is opened by a feeble or small torque of the electric motor. The rotation of the hydraulic motor can follow with high accuracy the rotation of the electric motor and a large torque of the hydraulic motor can be produced.

Still another object of this invention is to provide an electrohydraulic motor in which the valve spool is allowed to cause only an axial displacement, thereby preventing any heat generation through the shearing of the pressurized oil.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and effects of this invention will be obvious from the following description with reference to the attached drawings, in which:

FIG. 1 shows a longitudinal section of one embodiment of this invention;

FIG. 2 shows a sectional view of FIG. 1 taken along the line Il-ll;

FIG. 3 shows a sectional view ofFlG. 1 taken along the lineIII-III;

FIG. 4 shows a longitudinal scction of another embodiment of this invention; and

FIG. 5 shows a longitudinal section of still another embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 'In FIG. vI, 1 is an electric pulse motor, a synchronous motor, or other electric motor with controllable speed; 2 is a hydraulic motor; and 3 is a servovalve linking the electric motor 1 and the hydraulic motor2.

The hydraulic motor 2 shown here is a well known axial plunger type motor illustrated as an example. This motor comprises a valve plate 4 fixed to the, servovalve 3, a cylinder block '5 rotating in contact with said valve plate 4, an output shaft 9 splined to said cylinder block 5, an odd number of cylinders 6 provided within said cylinder block 5 axially disposed and circumferentially spaced around the axis of said output shaft 9, pistons 7 slidably mounted in the respective cylinders 6, and a rotatable inclined plate 8 which contacts the tips of said pistons 7.

As shown in FIG. 2, said valve plate 4 has two

arcuate circulation grooves

19 and 20 provided opposite to each other around said output shaft 9. One of said circulation grooves 19 communicates with the portIS of the servovalve 3, while the other 20 communicates with the port 16 of the servovalve 3 through passages not shown. The respective cylinders 6 provided in the cylinder block 5 communicate at one time with the circulation groove 19 and at another time with the circulation groove 20 according to the rotation of the cylinder block 5, whereby at any time some of the cylinders communicate with the circulation groove 19, while some of the cylinders communicate with the circulation groove 20.

Within the servovalve 3, a sleeve 10 is supported on bearings. The sleeve is rotatable but not shiftable in the axial direction. The left end of the sleeve 10 in FIG. 1 engages with a coupling 11 provided at the right end of the output shaft 9. Thus, the sleeve 10 can rotate together with the output shaft 9. The sleeve 10 has a valve chamber 10a bored therein. A valve spool 12 is mounted within this valve chamber 10a slidably in an axial direction. The valve spool12 engages with a key memberIOb screwed into the sleeve 10, whereby it can be shifted axially relative to the sleevelO, but cannot be rotated relative to the sleeve I0.

The right end of the valve spool 12 in FIG. 1 is provided with a male screw member 12a, which is threadedly received in the female screw member 18 connected to the rotating shaft 13 of the electric motor I. Said female screw 18 is rotatably supported by a thrust bearing within the block.

An inlet oil passage 14 is provided in the servovalve housing 3 and opens to the valve chamber a bored in the sleeve 10. At the neutral position of the valve spool 12, i.e., the halt condition of the electro-hydraulic servomotor, said passage 14 communicates with neither the port nor the port 16. When the valve spool 12 moves rightward from the position indicated in FIG. 1, the inlet oil passage 14 is opened to communicate with the port 15. As stated before, since the port 15 communicates with the circulation groove 19 (FIG. 2), the pressurized oil supplied from the passage 14 to the port 15 goes into the circulation groove 19 to be delivered to the cylinders 6 which communicate with the circulation groove 19.

When the valve spool 12 shifts leftward from the position of FIG. 1, the inlet passage 14 is opened to communicate with the port 16. Since the port 16 communicates with the circulation groove (FIG. 2), the pressurized oil supplied from the passage 14 to the port 16 reaches the groove 20, to be delivered to the cylinders 6 which communicate with the groove 20.

The servovalve 3 is further provided with an oil discharge passage 17, which opens to the valve chamber 10a bored in the sleeve 10. At the neutral position of the valve spool 12, said oil discharge passage 17 communicates with neither the port 15 nor the port 16.

When the valve spool 12 moves rightward from the position indicated in FIG. 1 the port 16 is opened to communicate with the oil discharge passage 17 and the pressurized oil within the cylinders 6 communicating with the groove 20 will drain out of the groove 20 and be discharged into the reservoir tank through the discharge passage 17 through the port 16.

When the valve spool 12 moves leftward from the position of FIG. 1, the port 15 is opened to communicate with the oil discharge passage 17 and the pressurized oil within the cylinders 6 communicating with the-groove 19 will pass through the groove 19 and be discharged into the reservoir tank via the port 15 and the discharge passage 17.

There are sealing members 29 of synthetic resin, etc. provided between the internal bore of the servovalve 3 and the external surface of the sleeve 10 to prevent the pressurized oil from leaking to the adjacent ports and passageways.

The performance of this electro-hydraulic servomotor is to be explained in the following.

An electric pulse motor is used at the electric motor 1. When an electric pulse is given to the electric motor 1, the rotating shaft 13 of said motor 1 begins to turn. Accordingly, the female screw member 18 turns along with the rotating shaft 13. When the electric pulse motor begins to turn, the valve spool 12 is at the neutral position shown in FIG. 1, with the output shaft 9 at rest. The sleeve 10 engaging with the output shaft 9 through the coupling 11 is also at rest. The valve spool 12 is retained against rotation relative to the sleeve 10 by means of the key member 10b. Therefore, when the female screw member 18 turns, the male screw member 120, which is threadedly engaged with said female screw member 18, is axially moved together with the valve spool 12. The motion of the valve spool 12 thereby is not a relative rotation to the sleeve 10 but merely a simple displacement in an axial direction. Thus.-tl1e screwed engagement of female screw member 18 with male screw member 12a is utilized as the means for converting the rotation of the electric motor 1 to an axial displacement of the valve spool 12. As the axial moving force of the valve spool 12 in this case is amplified, even a feeble torque of the electric motor 1 is enough to cause a very accurate displacement of the valve spool 12 with the least possible energy loss due to the viscosity of the pressurized oil and without the possibility of the pulse failing to be given to the electric motor,

If the axial displacement of the valve spool 12 is as indicated by the arrow in FIG. 1 rightward, the pressurized oil passage 14 communicates with the port 15 and the pressurized oil introduced at the passage 14 goes from the port 15 to the circulation, groove 19 to be delivered to the cylinders 6 communicating with said groove 19. As a result, the pistons 7 mounted in the cylinders 6 which communicate with said groove 19, tend to be projected outwardly of the cylinder block 5 and the tips of said pistons 7 push against the inclined plate 8, causing the cylinder block 5 to rotate in the same direction as the rotation of the rotating shaft 13.

When the cylinder block 5 is rotated, the pressurized oil in the cylinders 6 communicating with the circulation groove 19, is permitted to drain out through the groove 20 and through the port 16 to be discharged into the reservoir tank through the discharge passage 17, while the pistons 7 mounted in the cylinders 6 communicating with the circulation groove 20, are pushed by the inclined plate 8, upon the rotation of the cylinder block 5, back into the cylinders 6.

Upon rotation of the cylinder block 5, the output shaft 9, splined to said block 5 is rotated, thereby causing the rotation of the sleeve 10 through the coupling 11. Since the valve spool 12 is retained against rotation relative to the sleeve 10 by means of the key member 10b, the rotation of the sleeve 10 is accompanied by that of the valve spool 12.

When the electric pulse ceases to be given to the electric motor 1 and in consequence the motor 1 comes to a halt, the still continuing rotation of the cylinder block 5 and the output shaft 9 maintains the rotation of the valve spool 12. The r0tating male screw member 12a will therefore move leftward to,

the original neutral position of FIG. 1 within the female screw member 18 which has now stopped rotating with the electric motor. The valve spool cuts the communication of the oil passage 14 with the port 15 and that of the oil discharge passage 17 with the port 16. Thereupon, the rotation of both the cylinder block 5 and the output shaft 9 stops, no relative rotation between the valve spool 12 and sleeve 10 taking place even at this time.

if under continuous rotation of the electric motor 1 at constant speed the rotation of the output shaft 9 is slower than that of the rotating shaft 13, the valve spool 12 makes an axial displacement in the direction of enlarging the opening degrees of the

ports

15 and 16 and increasing the flow of pressurized oil to the hydraulic motor and as the result the supply and discharge of oil to the cylinders 6 are increased to promote increased rotation of the cylinder block 5. When the rotation of the output shaft 9 is faster than that of the rotating shaft 13, the valve spool 12 makes an axial displacement in the direction of narrowing the opening degrees of the

ports

15 and 16 and decreasing the flow of pressurized oil to the motor and as the result the supply and discharge of oil to the cylinders 6 are decreased to reduce the speed of rotation of the cylinder block 5. Thus, the speed of rotation of the output shaft 9 is at all times controlled to follow the speed of the rotating shaft of the electric motor 1.

FIG. 4 illustrates another embodiment according to this in vention, in which the identical parts to FIG. 1 are denoted by the identical symbols.

in the embodiment of FIG. 4, the servovalve 3 is bored to form a valve chamber 3a, within which the valve spool 21 is mounted slidably in an axial direction. The valve spool 21 engages with a key member 30 screwed into the body of the servovalve 3. Said valve spool is therefore axially movable relative to the internal bore of the servovalve 3, but not rotatable. The valve spool 21 has an axial through hole 28 bored therein and into this through hole 28 a working shaft 22 is inserted rotatable relative to the valve spool 21. At both ends of the valve spool 21 there are installed

bearings

23 and 24 which rotatably support said working shaft 22. Outside of said

bearings

23 and 24 there are

flanges

22a and 22b fixed to the working shaft 22. Thus, the working shaft 22 can both rotate and make displacement in an axial direction. While the valve spool 21 moves axially therewith, the valve spool 21 does not rotate.

At the right end of the working shaft 22 there is provided the male screw member 220, which is threadedly received in the female screw member 13a formed on the rotating shaft of the electric motor 1. At the left end of the working shaft 22a gear 26 is rigidly mounted and this gear meshes with and is axially slidable relative to a wide gear 25 fixed to the output shaft In the embodiment of FIG. 4, when an electric input like the electric pulse is imparted to the electric motor 1, the rotating shaft 13 of the electric motor begins to rotate. Accordingly, the female screw member 13a turns along with the rotating shaft 13. When the electric pulse motor begins to rotate the cylinder block 5 and the output shaft 9 splined to said block 5 are at rest, so that the working shaft 22 on which is rigidly mounted the gear 26 meshing with the gear 25 rigidly mounted on the output shaft 9 does not rotate at first. Therefore, as the female screw member 13a turns,:the male screw member 22c, cooperatively engaged with said female screw member 13a, will be axially displaced together with the working shaft 22.

When the working shaft 22 is displaced in an axial direction, the valve spool 21 disposed between the

flanges

22a and 22b is also displaced axially together with the working shaft 22 and the oil passage 14 and the oil discharge passage 17 are made to communicate either with the port or with the port 16, thereby causing a rotation of the cylinder block 5.

When the cylinder block 5 rotates, the output shaft 9 splined with said block 5 also rotates and as the result the working shaft 22 will be rotated through the meshing gears and 26. If there is a speed difference between the rotation of the working shaft 22 and that of the rotating shaft 13 of the electric motor 1, the male screw member 220 threaded into the female screw member 13a will shift the valve spool 21 together with the working shaft 22 in the axial direction, thereby changing the opening degrees of

ports

15 and 16 and controlling the flow of pressurized oil, with the result that the rotation of the output shaft 9 can follow that of the rotating shaft 13 ofthe electric motor 1.

The valve spool 21 is prevented by the key member from being rotated. As there are

bearings

23 and 24 inserted between the valve spool 21 and the working shaft 22, the valve spool 21 will not rotate relative to the valve chamber 3a ofthc servovalve 3, but instead will merely be axially displaceable whenever the working shaft 22 is turned by the gear 26 and makes an axial displacement.

In the embodiment of FIG. 4, as compared with that of FIG. I, the composition of the servovalve 3 is simplified and the equipment is made compact. With the working shaft provided within the valve spool 21, the sealing of pressurized oil is also simplified. Moreover the working shaft 22 is reduced both in diameter and in weight, so with the load on the electric motor I is greatly reduced. It is thus possible to make the rotating shaft 13 of the electric motor 1 turn correctly following the frequency of the electric input.

FIG. 5 illustrates still another embodiment of this invention, the identical parts to FIG. 4 being denoted by the identical symbols. In the embodiment of FIG. 5, the rotating shaft 13 of the electric motor I has fixed thereon a wide gear 27, which meshes with a gear 26 fixed to the working shaft 22. The male screw member 22c provided at the left end of the working shaft 22 is threadedly received in the female screw member 31, formed at the right end of the output shaft 9.

In the embodiment of FIG. 5 when an electric input like the electric pulse is given to the electric motor I, the rotating shaft 13 of the electric motor I begins to turn. Through the

gears

27 and 26 the working shaft 22 is rotated. When the electric pulse motor begins to turn, both the cylinder block 5 and the output shaft 9 splined therewith are at rest. The rotation of the working shaft 22 causes the male screw member 22c, threaded into the female screw member 31, to be axially displaced along with the working shaft 22.

When the working shaft 22 is shifted, while rotating, in the axial direction. the valve spool 21 disposed between the

flanges

22a and 22b, will likewise be displaced axially along with the working shaft 22, thereby supplying and discharging the pressurized oil to the cylinders 6 of the cylinder block 5 to rotate the cylinder block 5. The valve spool 21 being retained against rotation by means of the key member 30, the valve spool 21 does not rotate relative to the valve chamber 3a of the servovalve 3, but merely makes an axial displacement therein as illustrated in the embodiment of FIG. 4.

If there is a speed difference between the rotation of the output shaft 9 and that of the working shaft 22, the male screw member 22c, threaded into the female screw member 31, causes the valve spool 21 to be axially displaced together with the working shaft 22, thereby changing the degree of opening of the ports 15 and I6 and thus controlling the flow of pressurized oil, with the result that the rotation of the output shaft 9 can follow that of the rotating shaft 13 of the electric motor 1.

As explained above, this invention makes it possible for the rotation of the hydraulic motor to follow with high accuracy the rotation of the electric motor by converting the rotation of the electric motor with controllable angle or speed of rotation to an axial displacement of the valve spool and effecting the supply and discharge of pressurized oil to the hydraulic motor through this axial displacement of the valve spool. Particu- I larly, according to this invention, no relative rotation is permitted to occur between the axially moving valve spool and the sleeve mounted about said valve spool or the valve chamber of the servovalve. In consequence the valve spool can be very accurately moved by an extremely small or feeble rotating torque. Therefore, the response of the valve spool is extremely rapid, making it possible for the hydraulic motor to follow the rotational angle or speed of the electric motor with extremely high accuracy. Besides, there being no relative rotation between the valve chamber and the valve spool mounted closely within the valve chamber, the temperature rise of the pressurized oil is small.

Thus, unlike the conventional electro-hydraulic servomotors of the prior art, the servomotor of the present invention is free from the inconveniences and disadvantages of being affected by the viscosity of the pressurized oil requiring a large power consumption in order to move the valve spool. In this invention there is no possibility of the pulse failing to be given to the electric motor.

While the above descriptions are concerned with the preferred embodiment of this invention, it will be evident to those skilled in the art that various changes and modifications may be made therein without departing from the invention; and the appended claims intend to cover all such changes and modifications as fully within the true spirit and scope of the invention.

lclaim:

1. An electro-hydraulic servomotor comprising an electric motor having a drive shaft and rotatable in response to electric signals, a hydraulic rotary motor having an output shaft, a source of fluid pressure, servovalve means controlled by said electric motor for operating said hydraulic rotary motor through control of the supply of fluid pressure from saidsource, said servovalve means having a valve chamber provided with ports for the supply and discharge of the pressurized fluid to said hydraulic motor, a spool valve disposed within said chamber and retained from relative rotation therewith but axially shiftable within said chamber to control the selection and degree of opening of said ports, a screw threaded connection means associated with said spool valve and rotatable under the influence of relative rotation of the drive shaft of said electric motor and the output shaft of said hydraulic motor to cause axial displacement of said spool valve whereby the hydraulic motor is effectively controlled to follow very closely the rotational speed of the electric motor.

2. An electro-hydraulic servomotor as defined in claim 1, wherein said electric motor is an electric pulse type motor rotatable through a predetermined angle by responding to electric pulses.

3. An electro-hydraulic servomotor as defined in claim 1, wherein said hydraulic rotary motor is of the hydraulic plunger type motor.

4. An electro-hydraulic servomotor as defined in claim 1, wherein said valve chamber is formed within a sleeve rotatably mounted within the body of the servovalve means and connected at one end to the output shaft of said hydraulic rotary motor with said spool valve slidably mounted within said sleeve and retained from relative rotation with respect to said sleeve.

5. An electro-hydraulic servomotor as defined in claim 4, wherein said screw threaded connection means is interposed between the drive shaft of said electric motor and the spool valve.

6. An electro-hydraulic servomotor as defined in claim 4, wherein said sleeve is provided with a port for introducing fluid under pressure from said fluid pressure source, a first pair of ports for supplying said fluid tosaid hydraulic rotary motor and a second pair of ports for discharging said fluid from said hydraulic rotary motor, and wherein said spool valve is provided with spaced lands thereon to simultaneously control said fluid supplied to said hydraulic motor and discharged therefrom by controlling said ports.

7. An electro-hydraulic servomotor as defined in claim 1, wherein said spool valve slidably mounted within said valve chamber is keyed against rotation relative to the body of said servovalve means and is provided with an axially extending working shaft rotatably mounted therethrough and axially slidable along with said spool valve, said working shaft having gear means at one end thereof slidingly engaging gear means carried by the output shaft of said hydraulic rotary motor, and wherein said screw threaded connection means is interposed between the other end of said working shaft and the drive shaft of said electric motor so that when the electric motor is rotated said spool valve is shifted axially through said working shaft with respect to said valve chamber to control pressure fluid to and from said hydraulic rotary motor.

8. An electro-hydraulic servomotor as defined in claim 7, wherein said body of said servovalve means is provided with a port for introducing fluid under pressure from said fluid pressure source, a first pair of ports for supplying said fluid to said hydraulic rotary motor and a second pair of ports for discharging said fluid from said hydraulic rotary motor, and wherein said spool valve is provided with spaced lands thereon to simultaneously control said fluid supplied to said hydraulic motor and discharged therefrom by controlling said ports.

9. An electro-hydraulic servomotor as defined in claim 1, wherein said spool valve slidably mounted within said valve chamber is keyed against rotation relative to the body of said servovalve means and is provided with an axially extending working shaft rotatably mounted therethrough and axially slidable along with said spool valve, said working shaft having gear means at one end thereof slidingly engaging gear means carried by the drive shaft of said electric motor, and wherein said screw threaded connection means is interposed between the other end of said working shaft and the output shaft of said hydraulic rotary motor, so that when said electric motor is rotated the spool valve is shifted axially through said working shaft with respect to said valve chamber to control pressure fluid to and from said hydraulic rotary motor.

10. An electro-hydraulic servomotor as defined in claim 9, wherein said body of said servovalve means is provided with a port for introducing fluid under pressure from said fluid pressure source, a first pair of ports for supplying said fluid to said hydraulic rotary motor and a second pair of ports for discharg ing said fluid from said hydraulic rotary motor, and wherein said spool valve is provided with spaced lands thereon to simultaneously control said fluid supplied to said hydraulic motor and discharged therefrom by controlling said ports.