Apparatus for driving a device mounted on special-purpose vehicles

BACKGROUND OF THE INVENTION

This invention relates generally to an apparatus for driving devices mounted on special-purpose vehicles.

More specifically, the invention relates to an apparatus for driving devices mounted on special-purpose vehicles, in which the working speed of a device mounted on a special-purpose vehicle can be varied at will be manual operation and, on the other hand, kept at a constant value through automatic control irrespective of the rotational speed of an engine provided therefor.

More specifically, the invention relates to an apparatus for hydraulically driving a device mounted on a special-purpose vehicle, in which the working speed of the device is automatically controlled a specific value irrespective of the rotational speed of the engine provided for the chassis when the rotational speed of the chassis engine exceeds a preset value, while the working speed of the device is varied optionally in accordance with variation in the rotational speed of the engine when the rotational speed of the engine is less than a preset value.

In conventional special-purpose vehicles such as, especially, concrete mixer trucks, whether their drum driving system is mechanical (the direct transmission of power obtained from the transmission gear or crankshaft and the like by power extracting means to drum driving means) or hydraulic (the driving of the drum by coupling a hydraulic pump and a hydraulic motor respectively on the sides of the chassis engine and drum), the.rotational speed of their engine must be increased to add to its output when the vehicles are accelerated or are climbing an ascent, so that the rotational speed of their drum is increased proportionately. Thus, various problems have been caused, including an increased power consumption of engines, an irregular mixing of the aggregate contained in the mixer, and impairment of maneuverability.

SUMMARY OF THE INVENTION

The present invention aims at eliminating the abovedescribed disadvantages in prior special-purpose vehicles and the like.

An object of the invention resides in the provision of an apparatus for driving a device mounted on a special-purpose vehicle and the like wherein the working speed of the device can be varied optionally when manual operation is performed by operating a changeover valve, and the working speed of the machine is maintained at definite value irrespective of the rotational speed of the engine provided therefor during automatic control.

Another object of the invention is to provide an apparatus for driving a device mounted on a special-purpose vehicle and the like wherein the working speed of the machine is maintained at a definite value through automatic control irrespective of the rotational speed of the engine provided therefor in case the rotational speed of the chassis engine is more than a preset value, while the working speed of the device can be varied optionally according to the rotational speed of the engine in case the rotational speed of the engine is less than a preset value.

Still another object of the invention is to provide an apparatus for driving a device mounted on a special-purpose vehicle wherein a working fluid to be supplied to a servocylinder for the control of the working speed of the machine is extracted, not from a main fluid-flow pipeline, but from an auxiliary pipeline on a low pressure side such as a discharge pipe of an auxiliary pump, and therefore, even if a hydraulic pump and a hydraulic motor are reversely rotated and the direction of flow of the working fluid is reversed, adverse effects such as elevation in the temperature of the fluid or degradation in its quality are not caused due to the passage of high pressure fluid through an orifice provided at a branching portion of the pipeline for the working fluid to be supplied to said servo-cylinder. A still further object of the invention is to provide an apparatus for driving a device mounted on a special-purpose vehicle, such as a truck mixer, wherein a drum is rotated at a given speed in spite of increase in the rotational speed of the engine, thus insuring economy in fuel and horsepower and the supply of an always uniformly mixed aggregate. BRIEF DESCRIPTION OF THE DRAWING

These and other objects of the present invention as well as its features will become apparent from the ensuing description of the invention made with reference to the attached drawings, in which:

FIG. 1 is a hydraulic circuit diagram illustrating an embodiment of this invention;

FIG· 2 shows another embodiment in which a servo-cylinder and control valve in FIG. 1 are integrated;

FIG. 3 is a hydraulic circuit diagram illustrating still another embodiment of the invention;

FIG. 4 shows another example of an operating section shown in FIG. 3;

FIG. 5 shows a still further embodiment of the invention; and,

FIG. 6 is a graph demonstrating the effects of the invention, in terms of relationship between the rotational speed of an engine and a drum. DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, reference numeral 1 designates a chassis engine mounted on a truck mixer or the like. Numeral 2 represents a variable capacity hydraulic pump which is coupled to the engine 1 through a power extracting means 4 fixed to a transmission gear 3 and supplies a hydraulic medium such as oil to a constant flowrate hydraulic motor 7 coupled to a mounted body 5 such as a concrete mixer drum through a reduction gear 6. Main oil flow pipes 8 and 9 connecting the pump 2 and the motor 7 have their pressure on the low pressure side determined at a definite value by means of a low-pressure regulating valve, with leakage therefrom being compensated for by means of an auxiliary pump 10. The discharge pressure of the auxiliary pump 10 is increased in proportion to the rotational speed of the engine 1. A restrictor 11 such as an orifice, Venturi tube or nozzle is inserted in a discharge pipeline of the auxiliary pump 10. In the case where the working oil on the low-pressure side constantly flows in one of the main oil flow pipes, the orifice 11 may be appropriately provided in such a main oil flow pipe.

A servo-cylinder 12 includes a cylinder 13 containing a piston 14 therein. The free end of a piston rod of the piston 14 is coupled to a cylinder block or swashplate 15 of the hydraulic pump 2 through a link mechanism 16. The tilting angle of the cylinder block 15 is controlled in relation to the sliding movement of the piston 14. The bore of the cylinder 13 is divided into two oil chambers A and B by means of the piston 14. Low-pressure oil on the low-pressure side is supplied from the discharge pipeline of the auxiliary pump 10 through a pipeline branching from the discharge pipeline ahead of the orifice 11. 17 is a changeover valve built into the piston 14.

The changeover valve 17 is in the form of a spool valve axially slidable within the bore of the piston 14, and is provided with two cylindrical parts of larger diameter as shown in FIG. 1 with the left-hand cylindrical part being formed with at least one axial hole therethrough. The piston 14 is formed with a chamber C therein. In the condition depicted in FIG. 1, the right-hand cylindrical part closes both passages for providing communication between the chamber B and chambers A and C. When the valve 17 is moved to the right from the position shown in FIG. 1, the chambers A and B remain disconnected while the chambers B and C are placed into communication. On the other hand, when the valve 17 is moved to the left from the position illustrated in FIG. 1, the chambers A and B are placed into communication while the chambers B and C remain disconnected. A spool rod 18 for the operation of the changeover valve 17 is coupled with a manually operable lever

3,603,084 through a link mechanism 19. A stopper 21 which limits the rightward movement of the changeover valve 17 includes a cylinder 22 and a piston 23 slidable within the cylinder 22. An extensible adjusting rod 24 extends from the free end of the piston rod of the piston 23, with the free end of the rod 24 5 being abutted on the end face of the rod 18 during automatic control. The pressure receiving area of the piston 23 on the side of the oil chamber D is larger than the pressure receiving area of the changeover valve 17 on the side of the oil chamber C. 10

A control valve generally designated by the numeral 25 is disposed midway in a pipeline connecting the oil chamber C provided in the piston 14 with an oil reservoir. The control valve 25 incorporates a spool 26, a compression spring 27 disposed on one side thereof, and a pressure operation ³chamber 28 provided on the other side. The pressure operation chamber 28 is made to accept the pressure oil on the low pressure side through a changeover valve 30, and the force which the spool 26 receives from the pressure oil on the low 2θ pressure side is balanced with the force with which the spool 26 is pushed to the right by means of the spring 27. The external end of the spring 27 is coupled with the cylinder block 15 through link mechanisms 29 and 16, with the tilting motion of said cylinder block 15 of the hydraulic pump 2 being fed back 25 to the spring 27 through the link mechanisms 16 and 29.

The chamber C is communicated with the oil reservoir when the spool 26 is shifted to the right, and the chamber C is placed in communication with the pressure operation chamber 28 when the spool 26 is shifted to the left, thereby 30 receiving the pressurized oil from the pressure operation chamber 28. The changeover valve 30 is adapted to effect selective changeover between manual operation and automatic control, and is capable of leading the working oil on the low-pressure side from the auxiliary pump 10 into the oil 35 chamber D of the stopper 21 as well as into the pressure operation chamber 28.

When the rotation speed of the hydraulic motor 7 is to be regulated by manual operation, for instance, when a concrete material is to be charged into the concrete drum 5 or 40 discharged therefrom, the changeover valve 30 is moved to the left from the position shown in FIG. 1 so that the oil chamber D in the stopper 21 and the operation chamber 28 in the control valve 25 are simultaneously placed in communication with the oil reservoir. Accordingly, in this instance, the ⁴³piston 23 can be moved in a right-hand direction and also the spool 26 is moved to the right due to the biasing force of the spring 27. Thus, the oil chamber C of the servo-cylinder 12 is placed in communication with the oil reservoir, so that the piston 14 in the servo-cylinder 12 may readily follow the ^3υmovement of the lever 20, with the result that the tilting angle of the cylinder-block 15 can be varied at will.

To describe the foregoing operation in more detail, the spool rod 18 is moved to the right through the link mechanism $$ 19 when the lever 20 is moved to the left; simultaneously the changeover valve 17 is also moved to the right so that the chambers B and C are interconnected. In this instance, the piston 14 is pushed by the working oil in the chamber A and shifted to the right until the chambers B and C are discon- ^() nected by means of the changeover valve 17. Conversely, when the lever 20 is moved to the right, the changeover valve 17 is shifted to the left and the chambers A and B are interconnected while the chambers B and C are left disconnected between each other, with the piston 14 being then shifted to 65 the left under pressure due to the difference in area between the right and left pressure-receiving surfaces of the piston 14.

Supposing that the drum 5 is turned forwardly when the lever 20 is moved to the left from its neutral position and drum 5 is reversely turned when the lever 20 is moved to the right, 70 the tilting angle of the cylinder block 15 of the pump 2 is increased on the forward rotation side and the drum 5 is rotated faster, as the lever 20 moves on to the left. On the other hand, when the lever 20 is moved to the right for discharging raw concrete from the drum 5, the cylinder block 15 is tilted in the 75 opposite direction and the drum 5 increases in its rotational speed in the direction of the discharge.

The rotation of the mixer drum 5 may be stopped by returning the lever 20 to its neutral position, thereby making the tilting angle of the cylinder block 15 of the hydraulic pump 2 zero, and stopping the flow of oil to and from the pump 2.

When the vehicle runs while raw concrete is being stirred, it is desirable that the quantity of discharge of the pump 2 be maintained at a given value regardless of the rotational speed of the engine. The maintenance of such a given value is effected through automatic control in the present invention. When the changeover valve 30 is turned to the right as illustrated in FIG. 1, the working oil from the auxiliary pump 10 is fed into the chamber D of the stopper 21 and into the pressure operation chamber 28 of the control valve 25. Consequently, the piston 23 in the stopper 21 is shifted to the left under pressure until the piston 23 abuts against the left hand end of the cylinder 22. This causes a leftward movement of the spool rod 18 and, in turn, a leftward movement of the valve 17 relative to the piston 14. When the valve 17 moves to the left relative to the piston 14, the chambers A and B are placed in communication and hydraulic fluid in the chamber A flows into the chamber B. Since the pressure-receiving area of the piston 14 on the side of the chamber B is greater than that of the piston on the side of the chamber A, the piston is moved to the left until communication between the chambers A and B is prevented. As long as the working oil in chamber D acts on the piston 23, the valve 17 maintains its axial position thus reached and, hence, the piston 14 connected via the link mechanism 16 to the cylinder block 15 of the pump 2 also maintains its axial position to keep the tilting angle of the block 15 constant at normal condition.

In a preferred example, the rotational speed of the engine at this instant is approximately 500 r.p.m. while the speed of the drum 5 is 3 r.p.m. The length of the adjusting rod 24 may be variable, and the angle of the cylinder block 15 can be set at will be regulating the length of the rod 24.

The control valve 25 operates as follows. In the event of increase in the speed of the engine, the discharge quantity of the hydraulic pump 2 together with the discharge quantity of the auxiliary pump 10 is increased, with a result that the pressure of the working oil on the low-pressure side is increased, and the spool 26 is shifted to the left under the pressure against the biasing force of the spring 27. In this instance, the pressure oil in the operation chamber 28 flows into the chamber C and the piston 14 is shifted to the left due to the pressure of the pressure oil fed into the chamber C since the changeover valve 17 is prevented from movement in a right-hand direction due to the operation ofthestopper21.

When the piston 14 is shifted to the left, the chamber C is placed in communication with the chamber B, and the angle of the cylinder block 15 of the pump 2 is decreased, with the quantity of discharge of the pump 2 also decreasing as a result thereof. In accompaniment with the shifting to the left of the piston 14, the motion of the cylinder block 15 is transmitted to the spring 27 through the link mechanism 29. The spring 27 pushes back the spool 26 in the right-hand direction against the oil pressure of the pressure operation chamber 28 until the discharge quantity of the hydraulic pump 2 is restored to a preset value.

Decrease in the rotational speed of the engine entails the lowering of the discharge pressure of the auxiliary pump 10, so that the spool 26 is shifted to the right due to the force of the spring 27 and the chamber C is placed in communication with the oil reservoir. Thus, the pressure oil in the chamber C is exhausted and a movement to the right of the piston 14 relative to the valve 17 occurs. The cylinder block 15 accordingly increases in its tilting angle, increasing the discharge quantity of the pump 2 until the quantity is maintained at its preset value.

FIG. 6 is a graphical representation of relationship between the speed of the engine and that of the drum. When the speed of the engine is more than N2, the speed of the drum is maintained constantly at a set value n. In this figure, such a con3,603,084 stant value is illustrated by the straight line 0B. The maintenance of such a constant value is insured through the automatic control, as previously described, in the present invention. Desirably, however, the rotational speed of the drum should be maintained at less than the set value n depending upon the speed of the engine in case it is less than N2. This brings about the following advantages.

The consumption of fuel can be lessened since there is less power consumption when the speed of the engine is low. Moreover, the climbing capability of the vehicle is increased to such a degree that a climb which has been made only at a first speed may be made at second speed. The acceleration is also improved by a degree corresponding to the amount of power consumption which has been dispensed with. Furthermore, because of the minor horsepower consumption at the time of low speed of the engine at the moment of the start of the vehicle, troubles such as engine failure are seldom caused, and performance at the time of starting and stopping is likewise improved.

Such a rotation speed of the drum is represented by the straight line CO in FIG. 6. A^ described above, since manual operation is enabled by the switching of the changeover valve 30, the rotation of the drum can be regulated at will in the case of the charging of aggregate or the discharging of raw ₂₅concrete and also its forward and backward rotations can be selected arbitrarily.

In FIG. 2, which depicts another embodiment of the present invention, the servo-cylinder 12 and control valve 25 shown in FIG. 1 are built into the same main body 32. Numeral 33 is a 30 piston having its piston rod coupled with a cylinder block 15 of the hydraulic pump 2, and 34 is a changeover valve built into the piston 33 and, each corresponds to the piston 14 and changeover valve 17 in FIG. 1, so that the piston 33 is operated following the displacement of the changeover valve 35 34 as in the embodiment illustrated in FIG. 1. A cylinder 35 slidably mounted in a valve body 32 has one of its ends coupled with a manual operation lever 20 through a link mechanism.

The cylinder 35 is provided with a hollow chamber 36, and 40 a piston 40 slidably disposed in the hollow chamber is provided with an enlarged portion 37 having an orifice 38, and its piston rod 39 is formed integrally with the changeover valve 34. The enlarged portion 37 is pressed to the left by means of a compression spring 41 interposed between the enlarged portion 37 and the opening of the hollow chamber 36. The position of the piston 40 illustrated in FIG. 2 suggests a state when the rotation number of the engine is less than N2.

When the rotation speed of the engine exceeds N2, the pressure oil from an auxiliary pump 10 flows into the hollow chamber 36 so that a pressure difference is created on both sides of the orifice 38. This pressure difference has a tendency for pressing the piston 40 in a right-hand direction, with the piston 40 being then shifted to the right together with the changeover valve 34 until the force due to said pressure difference is balanced with the force of the spring 41. The rightward movement of the valve 34 relative to the piston 33 causes the oil to be led through a line 36α onto the surface of the piston 33 to flow through passages 33α and 33b into the chamber B so that the piston 33 is moved to the right following the movement of the valve 34 while allowing the oil in the chamber A to be exhausted through passages 33c and 33d into the reservoir, with the result that the quantity of discharge of ferentiates the apparatus shown in FIG. 3 from that in FIG. 1 is that the control valve 25 as a feedback mechanism is not provided. Instead, the apparatus in FIG. 3 is equipped with a spring 23s disposed in the cylinder 22 of the stopper 21 for pressing the piston 23 to the right in FIG. 3 and with a spring 17s disposed in the oil chamber C provided in the piston 14 of the servo-cylinder 12 for pressing the changeover valve 17 to the right.

For manual operation of the apparatus, the changeover valve 30 is switched to the right as viewed in FIG. 3, so that an oil chamber D of the stopper 21 is placed in communication with an oil reservoir and the piston 23 is constantly held at the right end position by means of the spring 23s. When the . manual operation lever 20 is turned to the left in the above condition, a lever 50 is shifted to the right through a link mechanism 19 and also the changeover valve 17 is shifted to the right by virtue of the biasing force of the spring 17s with a pin 51 provided on an end of the spool rod 18 maintaining engagement with the lever 50. By the shifting to the right of the changeover valve 17, the chambers B and C are placed in communication with the oil in chamber B being exhausted through the chamber C into the reservoir, whereby the piston 14 is shifted to the right under the pressure of oil supplied into the chamber A from the auxiliary pump 10 until the chambers B and C are disconnected by means of the changeover valve

When the manual operation lever 20 is turned to the right, the changeover valve 17 is shifted to the left by being pressed by the lever 50 against the force of the spring 17s. Upon the leftward shift of the changeover valve, the chambers A and B are placed in communication and, since the pressure oil in the chamber A flows into the chamber B and acts on the face of the piston 14 on the side of the chamber B, which is larger than the face of the piston 14 on the side of the chamber A, the piston 14 is shifted leftwardly in the figure.

As described above, the piston 14 is shifted in both directions by the operation in both directions of the manual operation lever, so that the tilting angle of the cylinder block 15 of the hydraulic pump 2 can be varied optionally through the link mechanism 16 interconnecting the piston 14 and cylinder block 15. Thus the quantity of discharge of the hydraulic pump 2 can be varied arbitrarily for the same reason as that described with reference to FIG. 1.

For the automatic control of the apparatus illustrated in FIG. 3, the manual operation lever 20 is turned to the left to a considerable degree, thereby disengaging the lever 50 from the pin shaft 51, and is bound in that condition by adequate binding means not shown in the drawing. Thereafter, by turning the changeover valve 30 to the left, the pipeline of the pressure oil from the auxiliary pump 10 is placed in communication with the chamber D disposed on the right-hand side of the piston 23 of the stopper 21. The piston 23 pressed by the pressure oil is therefore shifted in a left-hand direction against the force of the spring 235, and the free end of a regulatable rod 24 of the piston 23 contacts the adjacent end of the spool rod 18. Increase in the rotation speed of the engine 1 entails increase in the quantity of discharge from the hydraulic pump 60 2, and simultaneously the discharge of the auxiliary pump 10 also increases. The pressure oil discharged from the auxiliary pump 10 has its pressure increased by means of an orifice 11 and flows into the oil chamber D of the stopper 21, shifting the piston 23 in a left-hand direction against the force of the the pump 2 is kept at a constant value as in the case of the em- 65 spring 23.r. Thus the changeover valve 17 is shifted to the left nrsri 1 m λ r, t η« μ λ _ a. _ T'Tz' « rr -t «· · . * bodiment explained with reference to FIG. 1. If the cylinder 35 is secured at an appropriate position by operating the lever 20, this apparatus can be controlled automatically so as to hold the discharge quantity of the pump 2 at an arbitrarily set value.

FIG. 3 illustrates still another embodiment of the present invention. Corresponding portions between the apparatus shown in FIG. 1 and that illustrated in FIG. 3 are designated by the same numerals that were used in FIG. 1, in order to _____ _________________.___„_{lc x a} clarify differences therebetween. A primary point which dif- 75 nies a decrease in the discharge quantity of the hydraulic through the regulatable rod 24 and spool rod 18.

By the leftward shifting of the changeover valve 17, the chambers A and B are placed in communication and the piston 14 is shifted to the left due to the difference in the pressing forces of pressure oils in the chambers A and B. The leftward shift of the piston 14 decreases the angle of the cylinder block. 15 through the link mechanism 16 so that the discharge quantity of the pump 2 is decreased.

Also, since decrease in the speed of the engine 1 accompa3,603,084 pump 2 and a decrease in the discharge quantity of the auxiliary pump Ϊ0, the oil pressure within the chamber D of the stopper 21 is also decreased, so that the piston 23 is shifted in a right-hand direction by the pressing force of the spring 23s. In cooperation therewith, the changeover valve 17 is shifted to the right by the pressure of the spring 17s in the chamber C and also the piston 14 is shifted to the right by the pressure of the oil in the chamber A. The rightward shift of the piston 14 increases the angle of the cylinder block 15 through the link mechanism 16, thereby increasing the discharge quantity of the pump 2. Thus, in the present apparatus, the discharge of pressure oil from the pump 2 is maintained constant when the changeover valve 30 is switched to the position of automatic control, so that it will be understood that the rotation speed of the hydraulic motor 7 is held at a preset value.

FIG. 4 illustrates a modified example of an operating section of the manual operation lever and stopper. As shown in this figure, a link 56 is pivoted by means of a pivot 57 at the front end of the spool rod 18 of the servo-cylinder 12. One end of the link 56 is coupled with a link mechanism 52 related with the manual operation lever 20 by means of a pivot 58 and the other end is coupled with an end portion of a piston rod 54 of a piston 53 in the stopper 21 by means of a pivot 59. The piston 53 is maintained at a position where the force of a · spring 55 and the pressure of the pressure oil flowing into the oil chamber D from the auxiliary pump are balanced. In this apparatus, manual operation is made while the piston 53 is pressed against the right-hand end wall of the cylinder of the stopper 21 by the force of the spring 55, and the motion of the manual operation lever 20 is transmitted to the spool rod 18 through the link 56 which is pivotable around the pivot 59. During automatic control, on the other hand, the lever 20 is fastened at an adequate position by means of a stopper 26 and the motion of the piston 53, the position of which is determined by the pressure of the pressure oil in the chamber D and that of the spring 55, is transmitted to the spool rod 18 through the link 56 swingable around the pivot 58.

The stopper 26 may be constituted in such a manner that the rotation speed of the hydraulic motor 7 is automatically 40 controlled at predetermined values by fastening the manual operation lever at several different positions. Furthermore, the opening of the orifice 11 may be made variable, in order to vary the opening of the orifice 11 each time a fastened position of the lever 20 is changed. Further, it will be easily understood by anyone versed in the art of the field of this application that the leverage of the link 56 may be varied by changing a position where said link is pivoted to the spool rod 18.

FIG. 5 shows a still further embodiment of the present invention, in which the parts corresponding to those illustrated ⁵⁰in FIG. 1 are designated by the same numerals as in FIG. 1. The characteristic features of the apparatus shown in FIG. 5 are that the changeover valve in the servo-cylinder is provided separately from the servo-cylinder and that the stopper of the spool rod is formed integrally with the changeover valve. Through such an arrangement, only one feedback mechanism suffices and the use of the servo-cylinder which requires a complex structure is eliminated.

A hydraulic cylinder 61 is divided into an oil chamber E and oil chamber F by means of a piston 64. A piston rod 62 of the piston 64 coupled with the cylinder block 15 of the pump 2 through a link mechanism 60. The tilting of the cylinder block 15 is fed back to a valve cylinder 70 of a changeover valve 63 through a link mechanism 69. An operating stem 66 of a valve 65 spool 65 slidable in the interior of the valve cylinder 70 is constantly urged in a left-hand direction in the drawing by means of a compression spring 68 located in a spring chamber 67. The pipeline for the pressure oil from auxiliary pump 10 communicates with the oil chamber E in the hydraulic cylinder 61 70 and also with an oil chamber G provided on a left-hand side of the changeover valve 63 through the changeover valve 30. The elasticity of the spring 68 mounted in the spring chamber 67 should preferably be of such a strength that the motion of the manual operation lever 20 is transmitted to the operating 75 rod 66 without accompanying the compression of the spring 68.

When the present apparatus is switched to manual operation by shifting the changeover valve 30, to the right-hand position from that shown in FIG. 5 the motion of the manual operation lever 20 is transmitted to the valve spool 65 through the link mechanism 19, spring chamber 67 and operating rod 66. The rightward shift of the valve spool 65 causes the oil chamber F to communicate with the oil reservoir, so that the piston 64 is shifted to the right by the oil pressure of the oil in the chamber E. The rightward shift of the piston 64 decreases the angle of the cylinder block 15 of the pump 2 through the link mechanism 60, or tilts it to the side of reverse rotation, and shifts the valve cylinder 70 of the changeover valve 63 to the right through the link 69. As the rightward shift of the valve cylinder proceeds, the communication of the oil chamber F to the oil reservoir is intercepted and, in cooperation therewith, the rightward shift of the piston 64 is stopped. Conversely, when the valve spool 65 is shifted in a left-hand direction, the oil chambers E and F are placed in communication through the changeover valve 63, and the piston 64 is shifted in a left-hand direction due to a difference in areas at which the piston 64 receives pressure. The leftward shift of the piston 64 has a tendency to increase the tilting angle of cylinder block 15 of the pump 2 on its forward rotation side and also shifts the valve cylinder 70 of the changeover valve 63 in a left-hand direction until the intercommunication between the oil chamber E and oil chamber F is broken.

In switching the present apparatus to automatic control by means of the changeover valve 30, the manual operation lever 20 and link mechanism 19 are firstly fastened by adequate binding means of the type, for example, as shown at 26 in FIG. 4. When the changeover valve 30 has been switched to the position of automatic control, the pressure of the pressure oil flowing into the oil chamber G from the auxiliary pump 10 is balanced with the force of the spring 68 in the spring chamber 67. Increase in the rotation speed of the engine 1 causes increase in the oil pressure in the Oil chamber G so that the valve spool 65 is shifted in a right-hand direction while the valve cylinder 70 is shifted in a left-hand direction. The result is that the oil chamber F is placed in communication with the oil reservoir and the piston 64 is shifted in a right-hand direction so that the angle of the cylinder block 15 is decreased toward the side of reverse rotation. Oppositely, when the rotation speed of the engine 1 is decreased, the valve spool 65 is shifted in a left-hand direction by the force of the spring 68 so that the chambers E and F are placed in communication, and the angle of the cylinder block 15 is increased by the leftward shift of the piston 64. Thus it will be understood that the present apparatus is capable of maintaining the rotation speed of the hydraulic motor 7 at a preset value through automatic control.

The present invention has been described in the foregoing with reference to its several embodiments, which, however, are not intended to limit the invention. Other embodiments and their modifications are to be included in this invention to an extent not deviating from the scope of fair meaning of the appended claims.