Hydraulic drive unit

BACKGROUND OF THE INVENTION

The present invention relates to a hydraulic drive unit for driving a hydraulic motor which is utilized as a traveling device in a construction machine. Particularly, the invention is concerned with a hydraulic drive unit capable of preventing the occurrence of cavitation while a hydraulic motor is OFF.

As this type of a hydraulic drive unit there is known, for example, such a hydraulic circuit as shown in FIG.

17

.

In this hydraulic circuit, a hydraulic motor M is connected to an oil pressure source comprising a pump and a tank via a pair of main circuits

1

and

2

, a counter-balancing valve

3

disposed halfway of the main circuits

1

and

2

, and ports P

1

and P

2

. Further, a return circuit

64

is disposed between the two main circuits

1

and

2

, a pair of check valves

5

and

6

are disposed halfway of the return circuit

64

so as to be capable of being opened and closed, and a low pressure circuit

7

is connected to the return circuit

64

at a position between the two check valves

5

and

6

. A leakage circuit from the motor M is connected to the low pressure circuit

7

, which in turn is connected to the tank side via tank ports T

1

and T

2

.

For rotating the hydraulic motor M in a forward direction for example, the pump port P

1

is connected to the pump side and the other pump port P

2

connected to the tank side through a change-over valve (not shown), allowing an oil pressure to be fed from the pump port P

1

. At this time, with a pilot pressure, the counter-balancing valve

3

changes over to its left-hand position, whereby an oil pressure is fed from the main circuit

1

to the hydraulic motor M, causing the motor to rotate in the forward direction. Return oil from the hydraulic motor M is returned to the tank side via the other main circuit

2

, counter-balancing valve

3

, and pump port P

2

.

When the change-over valve is changed over to its neutral position, the pilot pressure is extinguished, the counter-balancing valve

3

returns to its neutral position, and hence the supply of the pressure oil is stopped, with the result that the hydraulic motor M turns OFF.

However, at the beginning of turning OFF of the hydraulic motor M the motor rotates by the force of inertia. The hydraulic motor M sucks in oil in the main circuit

1

and discharges the oil into the other main circuit

2

, that is, performs a so-called pumping operation. Thus, the main circuit

1

becomes negative in pressure, the hydraulic motor M sucks in air in the main circuit

1

, and cavitation occurs, thereby generating a low-noise for the hydraulic motor M. Therefore, for preventing the occurrence of cavitation caused by vacuum action, oil is sucked in from the tank side by means of the low pressure circuit

7

and a hydraulic operating oil is fed to the hydraulic motor M via the return circuit

64

, check valve

5

and main circuit

1

.

In the above conventional hydraulic drive unit, when the hydraulic motor turns OFF, oil is supplied from the low pressure circuit

7

for preventing the occurrence of cavitation. In this connection, the low pressure circuit

7

is required to have a pressure and a flow rate both sufficient to effect the supply of oil.

However, since the low pressure circuit is a drain line, it is impossible to set its pressure so high, and for ensuring the required oil pressure and flow rate it is necessary to alter not only the hydraulic motor but also the circuit and the entire system used. However, when the hydraulic drive unit is used for a traveling device in a small-sized construction machine for example, it is impossible to alter even the circuit and system used in the machine body and it is the present situation that the improvement in deceleration feeling of the hydraulic motor and the prevention of low-noise occurrence due to cavitation are not effected to a satisfactory extent.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a hydraulic drive unit which, without the need of greatly altering the circuit configuration and the entire system, can prevent the occurrence of cavitation when a hydraulic motor is OFF and as a consequence thereof can also attain the improvement in the deceleration feeling of the hydraulic motor and the prevention of low-noise occurrence.

For achieving the above-mentioned object, according to one means adopted in the present invention there is provided a hydraulic drive unit comprising a pair of main circuits each connecting an oil pressure source with a hydraulic motor; a counter-balancing valve disposed halfway of the main circuits so as to be capable of being changed over from one position to another, said counter-balancing valve being provided with a pair of discharge passages and a pair of return passages, both of which are adapted to be opened and closed for the main circuits, a pair of first check valves disposed halfway of the discharge passages, a pair of orifices disposed halfway of the return passages, and a pair of branch passages which are connected to the discharge passages on upstream sides of the first check valves and selectively connected to the by-pass circuit; return circuits each connected to the main circuits in a position therebetween; a pair of second check valves disposed halfway of the return circuits; and a by-pass circuit connected to the return circuits in a position between the second check valves.

According to a still further means adopted in the present invention there is provided a hydraulic drive unit comprising a pair of main circuits each connecting an oil pressure source with a hydraulic motor and having first main circuits and second main circuits which are in parallel with each other; first check valves disposed halfway of the first main circuits; a counter-balancing valve disposed halfway of the second main circuits so as to be capable of being changed over from one position to another, said counter-balancing valve being provided with a pair of branch passages selectively opening and closing the second main circuits for the first by-pass circuit, a pair of return passages opened and closed for the second main circuits, and a pair of orifices disposed halfway of the return passages; return circuits each connected to the main circuits in a position there between; a pair of second check valves disposed halfway of the return circuits; and a first by-pass circuit connected to the return circuits in a position between the second check valves.

According to a still further means adopted in the present invention there is provided a hydraulic drive unit comprising a pair of main circuits each connecting an oil pressure source with a hydraulic motor and having first main circuits and second main circuits which are in parallel with each other; first check valves disposed halfway of the first main circuits; a counter-balancing valve disposed halfway of the second main circuits so as to be capable of being changed over from one position to another, said counter-balancing valve being provided with a pair of return passages opened and closed for the second main circuits, and a pair of orifices disposed halfway of the return passages; return circuits each connected to the main circuits in a position therebetween; a pair of second check valves disposed halfway of the return circuits; a first by-pass circuit connected to the return circuits in a position between the second check valves; second by-pass circuits connected to the main circuits in a position therebetween, in parallel with the return circuits; and a change-over valve disposed halfway of the second by-pass circuits and at the same time of changing-over thereof, selectively opening and closing the first by-pass circuit to one of the second by-pass circuits.

According to a still further means adopted in the present invention there is provided a hydraulic drive unit comprising a pair of main circuits each connecting an oil pressure source with a hydraulic motor; a counter-balancing valve disposed halfway of the main circuits so as to be capable of being changed over from one position to another, said counter-balancing valve being provided with a pair of discharge passages and a pair of return passages, both of which are adapted to be opened and closed for the main circuits, a pair of first check valves disposed halfway of the discharge passages, a pair of orifices disposed halfway of the return passages, and a pair of branch passages which are connected to the discharge passages on upstream sides of the first check valves and are selectively connected to the by-pass circuit; return circuits each connected to the main circuits in a position therebetween; a high pressure selection valve disposed halfway of the return circuits so as to be capable of being changed over from one position to another; and a by-pass circuit connected to the return circuits on a downstream side of the high pressure valve so as to be capable of being changed over from one position to another.

Preferably, auxiliary passages which communicate with a brake cylinder-side circuit so as to be capable of being opened and closed are connected to the branch passages of the counter-balancing valve.

Preferably, the counter-balancing valve consists of a spool type valve body inserted slidably into a valve hole of a valve body, said valve hole being provided with a land groove having a suitable width which opens and closes the by-pass circuit and the branch circuits in accordance with a stroke of the counter-balancing valve.

DESCRIPTION OF THE DRAWINGS

FIG. 1

is a circuit diagram for a hydraulic drive unit according to an embodiment of the present invention;

FIG. 2

is a front view in vertical section of a hydraulic drive unit based on the circuit illustrated in

FIG. 1

;

FIG. 3

is a front view in vertical section showing an operating state of the hydraulic drive unit illustrated in

FIG. 2

;

FIG. 4

is a front view in vertical section showing an operating state of the hydraulic drive unit illustrated in

FIG. 2

;

FIG. 5

is a circuit diagram for a hydraulic drive unit according to another embodiment of the present invention;

FIG. 6

is a front view in vertical section of a hydraulic drive unit based on the circuit illustrated in

FIG. 5

;

FIG. 7

is a circuit diagram for a hydraulic drive unit according to a further embodiment of the present invention;

FIG. 8

is a circuit diagram for a hydraulic drive unit according to a still further embodiment of the present invention;

FIG. 9

is a circuit diagram for a hydraulic drive unit according to a still further embodiment of the present invention;

FIG. 10

is a front view in vertical section of a hydraulic drive unit based on the circuit illustrated in

FIG. 9

;

FIG. 11

is a front view in vertical section showing an operating state of the hydraulic drive unit illustrated in

FIG. 110

;

FIG. 12

is a front view in vertical section showing an operating state of the hydraulic drive unit illustrated in

FIG. 10

;

FIG. 13

is a front view in vertical section of a hydraulic drive unit according to a still further embodiment of the present invention;

FIG. 14

is a front view in vertical section of a hydraulic drive unit according to a still further embodiment of the present invention;

FIG. 15

is a front view in vertical section showing an operating state of the hydraulic drive unit illustrated in

FIG. 14

;

FIGS.

16

(A), (B), (C), (D), and (E) are enlarged sectional views showing examples of land grooves; and

FIG. 17

is a conventional hydraulic circuit diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings.

FIG. 1

shows a hydraulic circuit for a hydraulic drive unit according to an embodiment of the present invention and

FIGS. 2

to

4

show a concrete structure of a hydraulic drive unit which utilizes the hydraulic circuit illustrated in FIG.

1

.

In the hydraulic circuit for the hydraulic drive unit shown in

FIG. 1

, an oil pressure source and a hydraulic motor M are connected together via a pair of main circuits

1

and

2

which are utilized on a discharge side and a return side selectively, a counter-balancing valve

3

is disposed halfway of the main circuits

1

and

2

so as to be capable of being changed over from one position to another, the counter-balancing valve

3

being provided with a pair of discharge passages

10

,

11

and a pair of return passages

12

a,

12

b

which discharge and return passages are adapted to be opened and closed for the main circuits

1

and

2

and also provided with a pair of first check valves

13

and

14

halfway of the discharge passages

10

and

11

, respectively, and a pilot circuit

17

provided with a spring

15

and an orifice

19

and a pilot circuit

18

provided with a spring

16

and an orifice

20

are disposed respectively on both sides of the counter-balancing valve

3

.

The counter-balancing valve

3

is further provided with a pair of branch passages

21

and

22

which are connected respectively to upstream sides of the first check valves

13

and

14

, return circuits

23

and

24

are connected to the main circuits

1

and

2

, respectively, and a pair of second check valves

25

and

26

which permit the flow of oil from the main circuits

1

and

2

are disposed halfway of the return circuits

23

and

24

, respectively.

Further, a by-pass circuit

27

is connected at one end thereof to the return circuits

23

and

24

at a position between the second check valves

25

and

26

, while the opposite end of the by-pass circuit

27

is opened and closed selectively for the paired branch passages

21

and

22

.

Orifices

28

and

29

are formed halfway of the paired return passages

12

a

and

12

b,

respectively. Pump ports P

1

and P

2

of the main circuits

1

and

2

respectively are connected to a pump side and a tank side selectively through a change-over valve.

The following description is now provided about the operation of the hydraulic drive unit based on the above hydraulic circuit.

When the pump ports P

1

and P

2

are connected to the pump side and the tank side respectively from a neutral position shown in

FIG. 1 through a

change-over valve, pressure oil is fed to one main circuit

1

to rotate the hydraulic motor M in the forward direction for example and return oil discharged as pressure oil from the motor M its returned to the tank side via the other main circuit

2

.

With pressure oil fed to the pump port P

1

, the counter-balancing valve

3

changes over to its left-hand position in the figure against the spring

16

by virtue of a pilot pressure in the pilot circuit

17

. As a result, the discharge passage

10

located on the left-hand position is connected to the main circuit

1

, the branch circuit

21

is connected to the by-pass circuit

27

, and the return passage

12

a

comes into communication with the other main circuit

2

. Consequently, a part of pressure oil from the pump port P

1

flows from the main circuit

1

to the discharge passage

10

, while the other part of pressure oil therefrom flows to the discharge passage

10

via return circuit

23

—second check valve

25

—return circuit

24

—by-pass circuit

27

—branch passage

21

. Then the pressure oil further opens the first check valve

13

and is fed to the hydraulic motor M via the main circuit

1

on the down stream side to rotate the motor in the forward direction. Return oil from the hydraulic motor M is returned to the tank side via the other main circuit

2

—return passage

12

a

—orifice

28

—pump port P

2

.

For driving the hydraulic motor M in the reverse direction, pressure oil is fed to the pump port P

2

and the pump port P

1

is brought into connection to the tank side. In this case, operations are merely reverse to the above operations.

For turning OFF the hydraulic motor M during the above steady rotation, the change-over valve is returned to the neutral state to cut off the tank ports P

1

and P

2

from the oil pressure source. Consequently, there no longer is any pilot pressure from the pilot circuit

17

and the counter-balancing valve

3

is returned gradually to its neutral state in

FIG. 1

with the restoring force of the right-hand spring

16

. In this case, for a certain time the by-pass circuit

27

is in communication with the first branch passage

21

, so that the hydraulic motor M shifts in its rotation stopping direction while being slowed down. But at the beginning of turning OFF of the oil pressure source the motor M rotates by virtue of inertia, sucks in oil in the main circuit

1

and discharge the oil into the other main circuit

2

. Namely, the hydraulic motor M performs a so-called pumping operation for a certain time. For this reason, the main circuit

1

side is pressure-reduced and tends to become negative in pressure, and the internal pressure of the other main circuit

2

becomes high in the presence of the orifice

28

, thereby giving a braking force to the hydraulic motor M. The pressure oil passed through the orifice

28

acts on the right-hand second check valve

26

through the return circuit

24

, causing the valve

26

to open. Therefore, the pressure oil in the main circuit

2

is fed to the main circuit

1

via the second check valve

26

—return circuit

24

—by-pass circuit

27

—branch passage

21

—discharge passage

10

—first check valve

13

, and the pressure oil in the main circuit

1

is circulated to the hydraulic motor M, whereby the generation of a negative pressure in the main circuit

1

is prevented to prevent the occurrence of cavitation.

FIG. 2

illustrates a hydraulic drive unit based on the hydraulic circuit shown in

FIG. 1

according to the present invention. The details of its structure will be described below, in which the same components as in

FIG. 1

will be identified by the same reference numerals as in FIG.

1

.

Within a valve body

30

are formed a pair of main circuits

1

and

2

as passages which are connected to pump ports P

1

and P

2

and also connected to a hydraulic motor M. A valve hole

31

is formed in the valve body

30

so as to communicate with the main circuits

1

and

2

perpendicularly thereto and a counter-balancing valve

3

which changes over the main circuits

1

and

2

through opening and closing motions is inserted slidably into the valve hole

31

.

Further, a pair of return circuits

23

and

24

as passages communicating with the main circuits

1

and

2

respectively through the counter-balancing valve

3

and also through the valve hole

31

, as well as a by-pass circuit

27

as a passage connected to the return circuits

23

and

24

, are formed within the valve body

30

.

Halfway of the return circuits

23

and

24

are disposed a pair of second check valves

25

and

26

so as to be capable of being opened and closed, the second check valves

25

and

26

each comprising a poppet type valve body, a, and a spring, b.

The counter-balancing valve

3

comprises a hollow spool

32

, a discharge passage

10

and a return passage

12

a

both formed within the spool

32

, and a pair of first check valves

13

and

14

disposed respectively within the discharge passage

10

and the return passage

12

a

so as to be capable of being opened and closed, the first check valves

13

and

14

each comprising a valve body, C, and a spring, d. Further, in the spool

32

are formed a pair of branch passages

21

and

22

which permit the discharge passage

10

and the return passage

12

a

to be opened and closed for the by-pass circuit

27

upstream of the first check valves

13

and

14

.

The discharge passage

10

and the return passage

12

a

are used on the discharge side and the return side selectively. For example, when the spool

32

is on the right-hand side in

FIG. 2

, pressure oil is fed to the discharge passage

10

and the return passage

12

a

is connected to the tank side to let the pressure oil return.

Within the valve body

30

a pair of pressure chambers

33

and

34

are formed on both sides of the spool

32

. The pressure chambers

33

and

34

are in communication respectively with the main circuits

1

and

2

on the pump ports P

1

and P

2

through the pilot circuits

17

and

18

, with orifices

19

and

20

being formed in the pilot circuits

17

and

18

, respectively. Further, a pair of springs

15

and

16

are disposed respectively within the pressure chambers

33

and

34

and on both sides of the spool

32

.

The operation of this hydraulic drive unit will be described below with reference to

FIGS. 2

to

4

.

In

FIG. 2

, the counter-balancing valve

3

is held in a neutral state, pressure oil is not fed, and the hydraulic motor M is blocked by the first check valves

13

and

14

and is OFF. If in this state the pump port P

1

is connected to the pump side and the pump port P

2

is connected to the tank side, pressure oil is fed to the main circuit

1

. Consequently, pilot pressure acts on the left-hand pressure chamber

33

through the pilot circuit

17

and the spool

32

moves rightwards against the right-hand spring. In this state, one discharge passage

10

is open to the by-pass circuit

27

through the left-hand branch passage

21

and the right-hand return passage

12

a

is open to the return-side main circuit

2

through the right-hand branch passage

22

. In this case, a clearance gap regulated with an end notch is formed between the right-hand branch passage

22

and the main circuit

2

, and the clearance gap serves as an orifice

28

restricted by degrees. Once pressure oil is conducted to the main circuit

1

, a part of the pressure oil flows to the discharge passage

10

, while the other part of the pressure oil flows to the discharge passage

10

via return circuit

24

—by-pass circuit

27

—branch passage

21

after opening the second check valve

25

.

In this manner, when pressure oil is conducted into the left-hand discharge passage

10

, the pressure thereof causes the left-hand first check valve

13

to open, allowing the pressure oil to be conducted to the inflow-side main circuit

1

in the hydraulic motor M. Consequently, with the pressure oil in the main circuit

1

, the hydraulic motor M rotates in the forward direction, and the pressure oil returned from the hydraulic motor M is further returned to the tank via the other main circuit

2

—orifice

28

—return passage

12

a

—pump port P

2

.

For stopping the hydraulic motor M during the above operation, the pump ports P

1

and P

2

are cut off from the oil pressure source. At this time, however, in a certain time zone until complete return to the neutral position of the counter-balancing valve

3

for example, the spool

32

is still in its right-hand position as in

FIG. 4

though the spool

32

moves leftwards in degrees. With the gradual restricting of the orifice

28

, the left-hand first check valve

13

is opened, allowing the hydraulic motor M to shift to OFF while slowing down. As the hydraulic motor M begins t slow down, the pumping operation of the hydraulic motor M is performed, whereby pressure oil of one main circuit

1

is sucked in and the main circuit

1

is reduced in pressure, while the pressure oil is discharged to the other main circuit

2

, the inside of which becomes high in pressure in the presence of the orifice

28

, thereby giving a braking force to the hydraulic motor M. The pressure oil passed through the orifice

28

acts on the right-hand second check valve

26

through the return circuit

24

, causing the valve

26

to open. Consequently, the pressure oil present in the main circuit

2

is fed to the main circuit

1

circulatively via the return circuit

24

—second check valve

26

—by-pass circuit

27

—branch passage

21

—discharge passage

10

—first check valve

13

to prevent the internal pressure of the main circuit

1

from becoming negative and prevent the occurrence of cavitation. When this state is over, the spool

32

moves toward its original position under the action of the right-hand spring

16

and is restored to its state shown in

FIG. 2

, whereby the hydraulic motor M is blocked completely and turns OFF.

FIG. 5

illustrates a hydraulic circuit for a hydraulic drive unit according to another embodiment of the present invention. In this hydraulic circuit, in addition to the circuit components used in the embodiment illustrated in

FIG. 1

, a second by-pass circuit is provided in parallel and a change-over valve is provided in the second by-pass circuit. In this embodiment, therefore, the same components as in the embodiment illustrated in

FIG. 1

will be identified by the same reference numerals as in FIG.

1

.

In the hydraulic circuit shown in

FIG. 5

, an oil pressure source and a hydraulic motor M are connected together via a pair of main circuits

1

and

2

capable of being utilized on a discharge side and a return side selectively, a counter-balancing valve

3

is disposed halfway of the main circuits

1

and

2

so as to be capable of being changed over from one position to another, the counter-balancing valve

3

being provided with a pair of discharge passages

10

,

11

and a pair of return passages

12

a,

12

b

which discharge and return passages are adapted to be opened and closed for the main circuits

1

and

2

and also provided with a pair of fist check valves

13

and

14

disposed halfway of the discharge passages

10

and

11

respectively, and a pilot circuit

17

provided with a spring

15

and an orifice

19

and a pilot circuit

18

provided with a spring

16

and an orifice

20

are disposed respectively on both sides of the counter-balancing valve

3

.

Return circuits

23

and

24

are connected between the paired main circuits

1

and

2

and a pair of second check valves

25

and

26

which permit the flow of oil from the main circuits

1

and

2

are disposed halfway of the return circuits

23

and

24

, respectively. A first by-pass circuit

27

is connected to the return circuits

23

and

24

at a position between the second check valves

23

and

24

. Further, second by-pass circuits

40

and

41

are connected between the main circuits

1

and

2

in parallel with the return circuits

23

and

24

, and a change-over valve

42

adapted to be changed over from one position to another with an internal pressure of one of the main circuits

1

and

2

is disposed halfway of the second by-pass circuits

40

and

41

. With operation of the change-over valve

42

the first by-pass valve

27

is opened and closed selectively for the second by-pass circuits

40

and

41

.

The change-over valve

42

is provided with a pair of by-pass ports

43

and

44

, and a spring

45

and a pilot circuit

47

connected to the main circuit

1

, as well as a spring

46

and a pilot circuit

48

connected to the main circuit

2

, are disposed respectively on both sides of the change-over valve

42

, with orifices

49

and

50

being formed within the pilot circuits

47

and

48

, respectively. The constructions and operations of the main circuits

1

and

2

, counter-balancing valve

3

, return circuits

23

and

24

, by-pass circuit

27

, and hydraulic motor M are substantially the same as in the previous embodiment illustrated in FIG.

1

.

While the hydraulic motor M is OFF, the change-over valve

42

is held in its neutral position shown, and when oil is fed to a pump port P

1

, the change-over valve

42

moves rightwards with a pilot pressure provided from the main circuit

1

and the pilot circuit

47

and the by-pass port

43

provides a connection between the second by-pass circuit

40

and the first by-pass circuit

27

, while the other second by-pass circuit

41

is closed. In this state, if pump ports P

1

and P

2

are cut off from the oil pressure source, the hydraulic motor M shifts in its OFF direction while being decelerated for a certain time and the pumping operation through the hydraulic motor M is performed to reduce the internal pressure of one main circuit

1

, while the internal pressure of the other main circuit

2

becomes high. As a result, the check valve

26

is opened through the right-hand return circuit

24

and the high-pressure oil present in the main circuit

2

is introduced into the first by-pass circuit

27

and is further introduced into the main circuit

1

through the by-pass port

43

in the change-over valve

42

and further through the second by-pass circuit

40

, thereby preventing the occurrence of a negative pressure in the main circuit

1

and also preventing the occurrence of cavitation.

FIG. 6

shows a hydraulic drive unit based on the hydraulic circuit illustrated in

FIG. 5

according to the present invention. Since a basic structure thereof is the same as in the embodiment illustrated in

FIG. 1

, the same components as in

FIG. 1

are identified by the same reference numerals as in FIG.

1

and explanations thereof will be omitted.

Within a valve body

30

are formed a pair of passages

53

and

54

which are connected to main circuits

1

and

2

, respectively, and which are also connected to return circuits

23

and

24

through second check valves

25

and

26

, respectively. A valve hole

51

is in communication with the passages

53

and

54

perpendicularly thereto and the change-over valve

42

constituted by a spool

52

is inserted slidably into the valve hole

51

. The change-over valve

42

and the valve hole

51

are disposed in parallel with the return circuits

23

and

24

and the change-over valve

42

functions to connect a by-pass circuit

27

to the passages

53

and

54

selectively. Pilot circuits

47

and

48

are in communication with valve hole portions of the valve hole

51

located on both sides of the change-over valve

42

, with the opposite ends of the pilot circuits

47

and

48

being open to the passages

53

and

54

, respectively. The passages

53

and

54

are common oil passages for the circuits

23

,

24

and the circuits

40

,

41

, shown in FIG.

5

. Annular grooves serving as by-pass ports

43

and

44

are formed in the spool

52

.

The operation of this hydraulic drive unit will be described below.

When pressure oil is fed to a pump port P

1

, it is then fed to the hydraulic motor M through the main circuit

1

and is also fed to the passage

53

. Consequently, with a pilot pressure from the pilot circuit

47

, the spool

52

of the change-over valve

42

moves rightwards in FIG.

6

and the first by-pass circuit

27

is connected to the passage

53

through the valve hole

51

and the annular groove serving as the by-pass port

43

. If in this state the supply of pressure oil is stopped to turn OFF the hydraulic motor M, then in a certain time zone the internal pressure of the main circuit

2

becomes high and causes the second check valve

26

to open through the passage

54

. Further, the pilot pressure is exerted on the right-hand side of the change-over valve

42

through the pilot circuit

48

, causing the change-over valve

42

to move leftwards. But in the initial stage of this operation the by-pass port

43

is open and so is the check valve

26

, so that the high-pressure oil provided from the main circuit

2

is fed to the main circuit

1

via passage

54

—check valve

26

—return circuit

24

—by-pass circuit

27

—by-pass port

43

—passage

53

—discharge passage

10

and first check valve

13

in the counter-balancing valve

3

to prevent the occurrence of cavitation in the main circuit

1

.

FIG. 7

is a circuit diagram for a hydraulic drive unit according to a further embodiment of the present invention, which is a slight modification of the main circuits shown in FIG.

1

. Main circuits

1

and

2

used in this embodiment are provided halfway with parallel first main circuits

1

a,

2

a

and second main circuits

1

b,

2

b,

and first check valves

13

and

14

are disposed in the first main circuits

1

a

and

2

a,

respectively. In other words, in

FIG. 1

, the first check valves

13

and

14

of the counter-balancing valve

3

are provided in the first main circuits

1

a

and

2

a,

respectively.

More specifically, in the circuit diagram for a hydraulic drive unit illustrated in

FIG. 7

, an oil pressure source and a hydraulic motor M are connected together via a pair of main circuits

1

and

2

which can be utilized on a discharge side and a return side selectively. The main circuits

1

and

2

have parallel first main circuits

1

a,

2

a

and second main circuits

1

b,

2

b,

first check valves

13

and

14

are disposed halfway of the first main circuits

1

a

and

2

a,

respectively, and a counter-balancing valve

3

is disposed halfway of the second main circuits

1

b

and

2

b

so as to be capable of being changed over from one position to another. The counter-balancing valve

3

is provided with a pair of branch passages

21

,

22

and a pair of return passages

12

a,

12

b

which branch and return passages are adapted to be opened and closed for the second main circuits

1

b

and

2

b,

and further provided with restrictions

28

and

29

. A spring

15

and a pilot circuit

17

connected to the second main circuit

1

b,

as well as a spring

16

and a pilot circuit

18

connected to the second main circuit

2

b,

are disposed respectively on both sides of the counter-balancing valve

3

. Further, return circuits

23

and

24

are connected to the main circuits

1

and

2

respectively, and a pair of second check valves

25

and

26

which permit the flow of oil from the main circuits

1

and

2

are disposed halfway of the return circuits

23

and

24

. A by-pass circuit

27

is connected at one end thereof to the return circuits

23

and

24

at a position between the second check valves

25

and

26

and the opposite end of the by-pass circuit

27

is opened and closed selectively for the paired branch circuits

21

and

22

. For example, when pressure oil is fed to one main circuit

1

, the counter-balancing valve

3

changes over to its left-hand position in

FIG. 7

, so that pressure oil is fed to the hydraulic motor M via the first main circuit la and the first check valve

13

and return oil from the hydraulic motor M is returned to the tank side via the other main circuit

2

, return passage

12

a

and restriction

28

. Other structural points, as well as functions and effects, are the same as in the embodiment illustrated in FIG.

1

.

FIG. 8

is a circuit diagram for a hydraulic drive unit according to a still further embodiment of the present invention. This embodiment, like the embodiment illustrated in

FIG. 7

, is a slight modification of the main circuits

1

,

2

and counter-balancing valve

3

used in the hydraulic circuit of FIG.

5

. In this embodiment, main circuits

1

and

2

are provided with first main circuits

1

a,

2

a

and second main circuits

1

b,

2

b,

and the first check valves

13

and

14

shown in

FIG. 5

are disposed in the first main circuits

1

a

and

2

a,

respectively. More specifically, in the hydraulic circuit for a hydraulic drive unit illustrated in

FIG. 8

, an oil pressure source and a hydraulic motor M are connected together via a pair of main circuits

1

and

2

which can be utilized on a discharge side and a return side selectively. The main circuits

1

and

2

are provided with parallel first main circuits

1

a,

2

a

and second main circuits

1

b,

2

b,

and first check valves

13

and

14

are disposed halfway of the first main circuits

1

a

and

2

a,

respectively. A counter-balancing valve

3

is disposed halfway of the second main circuits

1

b

and

2

b

so as to be capable of being changed over from one position to another, the counter-balancing valve

3

being provided with a pair of return passages

12

a

and

12

b

which are adapted to be opened and closed for the second main circuits

1

b

and

2

b,

respectively, and also provided with restrictions

28

and

29

. A spring

15

and a pilot circuit

17

connected to the second main circuit

1

b,

as well as a spring

16

and a pilot circuit

18

connected to the second main circuit

2

b,

are disposed respectively on both sides of the counter-balancing valve

3

. Further, return circuits

23

and

24

are connected to the main circuits

1

and

2

and a pair of second check valves

25

and

26

which permit the flow of oil from the main circuits

1

and

2

are disposed halfway of the return circuits

23

and

24

. A first by-pass circuit

27

is connected to the return circuits

23

and

24

at a position between the second check valves

25

and

26

and second by-pass circuits

40

and

41

are connected between the paired main circuits

1

and

2

in parallel with the return circuits

23

and

24

. A change-over valve

42

adapted to be changed over its position with the internal pressure of one of the main circuits

1

and

2

is disposed halfway of the second by-pass circuits

40

and

41

, and in accordance with operation of the change-over valve

42

the first by-pass circuit

27

is opened and closed selectively for the second by-pass circuit

40

or

41

.

In this hydraulic circuit, for example when pressure oil is fed to the main circuit

1

, it is fed to the hydraulic motor M via the first main circuit

1

a

and the first check valve

13

, while return oil is returned to the tank side via the other main circuit

2

, return passage

12

a

and restriction

28

. Other structural points, as well as functions and effects, are the same as in the embodiment illustrated in FIG.

5

.

In each of the above embodiments, for example in the hydraulic drive unit shown in

FIG. 2

, a land groove may be formed in the valve hole

31

as in a hydraulic drive unit illustrated in

FIG. 14

which will be described later.

To be more specific, there may be adopted a construction wherein the counter-balancing valve

3

is inserted into the valve hole

31

of the valve body

30

, a land groove is formed in the inner periphery of the valve hole

31

, and the by-pass circuit

27

and the branch circuit

21

are opened and closed through the land groove in accordance with the stroke of the counter-balancing valve

3

.

In this case, both by-pass circuit

27

and branch circuit can open widely through the land groove, thus permitting sufficient flow of the hydraulic actuating oil and thereby permitting an effective prevention of the occurrence of cavitation.

FIG. 9

illustrates a hydraulic circuit according to a still further embodiment of the present invention and

FIGS. 10

to

13

illustrate a concrete structure of a hydraulic drive unit which utilizes this hydraulic circuit.

In the hydraulic circuit for a hydraulic drive unit illustrated in

FIG. 9

, as in the embodiment illustrated in

FIG. 1

, an oil pressure source and a hydraulic motor M are connected together via a pair of main circuits

1

and

2

which are utilized on a discharge side and a return side selectively, a counter-balancing valve

3

is disposed halfway of the main circuits

1

,

2

and so as to be capable of being changed over from one position to another, the counter-balancing valve

3

being provided with a pair of discharge passages

10

,

11

and a pair of return passages

12

a,

12

b

which discharge and return passages are adapted to be opened and closed for the main circuits

1

and

2

and also provided with a pair of first check valves

13

and

14

disposed halfway of the discharge passages

10

and

11

, respectively, and a spring

15

and a pilot circuit

17

having an orifice

19

, as well as a spring

16

and a pilot circuit

18

having an orifice

20

, are disposed respectively on both sides of the counter-balancing valve

3

.

Further, a pair of branch passage

21

,

22

and a pair of auxiliary circuits

131

,

132

, which are connected upstream of the first check valves

13

and

14

, are provided in the counter-balancing valve

3

. Return circuits

23

and

24

are connected to the main circuits

1

and

2

, respectively, and a high pressure selection valve

4

is disposed halfway of the return circuits

23

and

24

, the high pressure selection valve

4

being adapted to change over its position with a pilot pressure provided from the main circuit

1

or

2

and permitting the flow of oil from the main circuit

1

or

2

located on the return side.

A by-pass circuit

27

is connected at one end thereof to the return circuits

23

and

24

on a downstream side of the high pressure selection valve

4

and the opposite end of the by-pass circuit

27

is opened and closed selectively for the paired branch passages

21

and

22

.

The high pressure selection valve

4

has a neutral position and two right and left positions and is provided with pilot circuits

23

a

and

24

a

which are opposed to springs

151

and

152

disposed on both sides and which receive pilot pressures from the main circuits

1

and

2

. The high pressure selection valve

4

is also provided with passages

153

and

154

which come into communication with the by-pass circuit

27

at the time of change-over of the return circuits

23

and

24

. Further provided is a brake cylinder

105

in a brake mechanism which controls the hydraulic motor M. The brake cylinder

105

is connected to drain lines T

1

and T

2

via circuit

133

, counter-balancing valve

3

and circuit

134

. Upon changing over of the counter-balancing valve

3

the drain lines T

1

and T

2

are cut off and the brake cylinder

105

is connected to the auxiliary circuit

131

or

132

.

Orifices

29

and

28

are formed halfway of the paired return passages

12

a

and

12

b,

respectively. Further, pump ports P

1

and P

2

of the main circuits

1

and

2

respectively are connected to the pump side and the tank side selectively through a change-over valve.

The following description is now provided about the operation of the hydraulic circuit configured as above.

When the pump ports P

1

and P

2

are connected from the neutral position shown in

FIG. 9

to the pump side and the tank side, respectively, through the change-over valve, pressure oil is fed to one main circuit

1

to rotate the hydraulic motor M in the forward direction for example and return oil discharged from the motor M is returned to the tank side through the other main circuit

2

.

When pressure oil is fed to the pump port P

1

, the high pressure selection valve

4

changes over to its left-hand position against the spring

152

by virtue of a pilot pressure from the pilot circuit

23

a.

Likewise, with a pilot pressure from the pilot circuit

17

, the counter-balancing valve

3

changes over to its left-hand position in the figure against the spring

16

. As a result, the discharge passage

10

located at the left-hand position is connected to the main circuit

1

, the branch passage

21

is connected to the by-pass circuit

27

, the auxiliary passage

131

connects to the circuit

133

, and the return passage

12

a

comes into communication with the main circuit

2

. Consequently, the pressure oil from the pump port P

1

is fed to the hydraulic motor M via discharge passage

10

—first check valve

13

—main circuit

1

, causing the hydraulic motor M to rotate in the forward direction. Return oil from the hydraulic motor M is returned to the tank side via the main circuit

2

—return passage

12

a

—orifice

29

—pump port P

2

.

On the other hand, the oil conducted to the auxiliary circuit

131

is introduced into the brake cylinder

105

via the circuit

133

to release the brake. For rotating the hydraulic motor M in the reverse direction, pressure oil is fed to the pump port P

2

and the pump port P

1

is connected to the tank side. In this case there are performed operations merely reverse to the above operations.

For stopping the hydraulic motor M during the above steady rotation, the change-over valve is returned to its neutral state to cut off the pump ports P

1

and P

2

from the oil pressure source. As a result, there no longer is any pilot pressure from the pilot circuit

17

and the counter-balancing valve

3

is returned gradually to its neutral state shown in

FIG. 9

with the restoring force of the right-hand spring

16

. In this case, for a certain time, the by-pass circuit

27

is in communication with the first branch passage

21

. Consequently, the hydraulic motor M shifts to its turning OFF direction while being slowed down. But at the beginning of turning OFF of the oil pressure source the motor still rotates by virtue of inertia. Thus, the pumping operation of the hydraulic motor M is performed for a certain time and the pressure of the main circuit

1

side is reduced and tends to become negative, while the internal pressure of the other main circuit

2

becomes high in the presence of the orifice

29

, thereby giving a braking force to the hydraulic motor M. The pressure oil passed through the orifice

29

acts as a pilot pressure via the pilot circuit

24

a,

causing the high pressure selection valve

4

to change over to its right-hand position and connecting the return circuit

24

to the by-pass circuit

27

. As a result, the pressure oil present in the main circuit

2

is fed to the main circuit

1

via the return circuit

24

—by-pass circuit

27

—branch passage

21

—discharge passage

10

—first check valve

13

and the pressure oil in the main circuit

1

is circulated to the hydraulic motor M, whereby the occurrence of a negative pressure in the main circuit

1

and the occurrence of cavitation are prevented. Pressure oil is also fed to the brake cylinder

105

via the auxiliary passage

131

and circuit

133

to keep the brake released.

FIG. 10

illustrates a hydraulic drive unit based on the hydraulic circuit shown in FIG.

9

. The details of its structure will be described below, in which the same components as in

FIG. 9

are identified by the same reference numerals as in FIG.

9

.

Within a valve body

30

are formed a pair of main circuits

1

and

2

as passages connected to oil pressure source-side pump ports P

1

, P

2

and also connected to a hydraulic motor M. A valve hole

140

communicating with the main circuits

1

and

2

perpendicularly thereto is formed in the valve body

30

and a counter-balancing valve

3

which changes over the main circuits

1

and

2

for opening and closing motions is inserted slidably into the valve hole

140

.

Within the valve body

30

are further formed a pair of return circuits

23

and

24

as passages communicating respectively with the main circuits

1

and

2

via the counter-balancing valve

3

and the valve hole

140

, as well as a by-pass circuit

27

as a passage connected to the return circuits

23

and

24

.

A spool type high pressure selection valve

4

is inserted movably halfway of the return circuits

23

and

24

. In the body of the high pressure selection valve

4

are formed left and right pilot circuits

23

a,

24

a,

and springs

151

and

152

are mounted on the left and right sides, respectively, of the valve body.

In the counter-balancing valve

3

there are provided a hollow spool

141

, a discharge passage

10

and a return passage

12

a

both formed within the spool

141

, and a pair of first check valves

13

and

14

each comprising a valve body and a spring, the first check valves

13

and

14

being inserted into the discharge passage

10

and the return passage

12

a

respectively so as to be capable of being opened and closed. In the spool

141

are formed a pair of branch passages

21

and

22

which cause the discharge passage

10

and the return passage

12

a

to be opened and closed for the by-pass circuit

27

upstream of the first check valves

13

and

14

.

The discharge passage

10

and the return passage

12

a

are used on the discharge side and the return side selectively. For example, when the spool

141

is at its right-hand position in

FIG. 10

, pressure oil is fed to the discharge passage

10

and the return passage

12

a

is connected to the tank side for the return of pressure oil.

Within the valve body

30

, a pair of pressure chambers

143

and

144

are formed on both sides of the spool

32

and are in communication respectively with pump port P

1

-, P

2

-side main circuits

1

and

2

via pilot circuits

17

and

18

, with orifices

19

and

20

being formed within the pilot circuits

17

and

18

, respectively. Further, a pair of springs

15

and

16

are mounted within the pressure chambers

143

and

144

, respectively, and on both sides of the spool

141

.

The operation of this hydraulic drive unit will be described below with reference to

FIGS. 10

to

13

.

In the state shown in

FIG. 10

, the counter-balancing valve

3

is held in its neutral state, pressure oil is not fed, and the hydraulic motor M is blocked by the first check valves

13

and

14

and is OFF. In this state, if the pump ports P

1

and P

2

are connected to the pump side and the tank side, respectively, as in

FIG. 10

, pressure oil is fed to the main circuit

1

, so that the pilot pressure acts on the left-hand pressure chamber

143

through the pilot circuit

17

and the spool

141

moves rightwards against the right-hand spring

16

, passes its position shown in FIG.

11

and reaches its position shown in FIG.

12

. In this state, one discharge passage

10

opens to the by-pass circuit

27

through the left-hand branch passage

21

and the right-hand return passage

12

a

opens to the return-side main circuit

2

through the right-hand branch passage

22

. In this case, between the right-hand branch passage

22

and the main circuit

2

there is formed a clearance gap regulated with an end notch of the land of the spool

141

, and the clearance gap serves as an orifice

28

.

On the other hand, when pressure oil is conducted into the left-hand discharge passage

10

, the oil pressure thereof causes the left-hand first check valve

13

to open, allowing the pressure oil to be conducted to the main circuit

1

on the inflow side of the hydraulic motor M. The pressure oil in the main circuit

1

is also conducted to the pilot circuit

23

a

through the return circuit

23

, causing the high pressure selection valve

4

to move rightwards to connect the by-pass circuit

27

to the return circuit

23

. Then, the pressure oil conducted to the by-pass circuit

27

is conducted to the brake cylinder

105

through the circuit

133

. Therefore, the hydraulic motor M rotates in the forward direction with the pressure oil in the main circuit

1

and the pressure oil returned from the hydraulic motor M is returned to the tank via the other main circuit

2

—orifice

28

—branch passage

22

—return passage

12

a

—pump port P

2

.

For stopping the hydraulic motor M during the above operation, the pump ports P

1

and P

2

are cut off from the oil pressure source. In this case, however, in a certain time zone until complete return of the counter-balancing valve

3

to its neutral position for example, the spool

141

still occupies the right-hand position, as shown in

FIG. 12

though the spool

141

moves leftwards in degrees. With the gradual restricting of the orifice

28

, the left-hand first check valve

13

located on the left-hand side is opened, allowing the hydraulic motor M to shift to an OFF state while slowing down. Once the hydraulic motor M begins to slow down, the pumping operation of the hydraulic motor M is performed, whereby pressure oil of one main circuit

1

is sucked in and the main circuit

1

is reduced in pressure, while the pressure oil is discharged to the other main circuit

2

, the inside of which becomes high in pressure in the presence of the orifice

28

, thereby giving a braking force to the hydraulic motor M. The pressure oil passed through the orifice

28

acts on the right-hand side of the high pressure selection valve

4

from the return circuit

24

through the pilot circuit

24

a,

causing the valve

4

to move leftwards. Therefore, connecting the return circuit

24

to the by-pass circuit

27

, as shown in

FIG. 12

, the pressure oil present in the main circuit

2

is circulated to the main circuit

1

via he return circuit

24

—by-pass circuit

27

—branch passage

21

—discharge passage

10

—first check valve

13

, thereby preventing the occurrence of a negative pressure in the main circuit

1

and preventing the occurrence of cavitation. After this state, the spool

141

moves toward its original position under the action of the right-hand spring

16

and is restored to its state shown in

FIG. 9

, so that the hydraulic motor M is blocked completely and turns OFF.

FIG. 13

illustrates a hydraulic drive unit according to a still further embodiment of the present invention, which is not provided with a brake mechanism and hence not provided with the circuit

133

connected to the brake cylinder

105

. Other structural points, a well as functions and effects, are the same as in the embodiment illustrated in FIG.

9

.

FIGS. 14

to

16

illustrate a hydraulic drive unit according to a still further embodiment of the present invention.

This hydraulic drive unit is basically the same as the hydraulic drive unit illustrated in FIG.

10

and based on the hydraulic circuit of

FIG. 9. A

different point is that a land groove

60

is formed in the valve hole

140

.

The structure and operation of this hydraulic drive unit will be described below, in which the same components as in

FIG. 10

will be identified by the same reference numerals as in FIG.

10

and explanations thereof will be omitted.

Within a valve body

30

are formed pump ports P

1

and P

2

on an oil pressure source side and a pair of main circuits

1

and

2

which are connected to a hydraulic motor M. A valve hole

140

which communicates with the main circuits

1

and

2

perpendicularly thereto is formed in the valve body

30

and a valve body of a spool type counter-balancing valve

3

which changes over the main circuits

1

and

2

through opening and closing motions is inserted slidably into the valve hole

140

. A land groove

60

having a suitable width is formed centrally in the inner periphery of the valve hole

140

. When the valve body of the counter-balancing valve

3

is in a neutral state, the land groove

60

is closed with the valve body, while upon stroke of the valve body in one direction, the valve body becomes open in accordance with the stroke and a by-pass circuit

27

and a branch passage

21

or

22

come into communication with each other through the land groove

60

.

All the other structural points are the same as in hydraulic drive unit illustrated in FIG.

10

.

In the state of

FIG. 14

the counter-balancing valve

3

is held in a neutral state, pressure oil is not fed, and the hydraulic motor M is blocked by first check valves

13

and

14

and is OFF. In this state, if the pump ports P

1

and P

2

are connected to the pump side and the tank side, respectively, pressure oil is fed to the main circuit

1

. Consequently, a pilot pressure acts on a left-hand pressure chamber

143

through a pilot circuit

17

, so that a spool

141

moves rightwards against a right-hand spring

16

. In this state, a discharge passage

10

located on one side opens to the by-pass circuit

27

through the left-hand branch passage

21

and the land groove

60

, while a return passage

12

a

located on the other right-hand side opens to the return-side main circuit

2

through the right-hand branch passage

22

. In this case, between the right-hand branch passage

22

and the main circuit

2

there is formed a restriction

28

by the land of the spool

141

.

When pressure oil is conducted into the left-hand discharge passage

10

, the oil pressure thereof causes the left-hand first check valve

13

to open, allowing the pressure oil to be conducted to the main circuit

1

on the inflow side of the hydraulic motor M. The pressure oil in the main circuit

1

is also conducted to a pilot circuit

23

a

through a return circuit

23

, causing a high pressure selection valve

4

to move rightwards to connect the by-pass circuit

27

to the return circuit

23

. The pressure oil conducted to the by-pass circuit

27

is also conducted to a brake cylinder

105

through a circuit

133

. Consequently, the hydraulic motor M rotates in the forward direction with the pressure oil in the main circuit

1

and the pressure oil returned from the hydraulic motor M is returned to the tank via the other man circuit

2

—orifice

28

—branch passage

22

—return passage

12

a

—pump port P

2

.

For stopping the hydraulic motor M during the above operation, the pump ports P

1

and P

2

are cut off from the oil pressure source. In this case, however, in a certain time zone until complete return of the change-over valve to its neutral position for example, the spool

141

still occupies its right-hand position as in

FIG. 15

though the spool

141

moves leftwards in degrees. With the gradual restricting of the orifice

28

, the left-hand first check valve

13

located on the left-hand side is opened, allowing the hydraulic motor M to shift to an OFF state while slowing down. Once the hydraulic motor M begins to slow down, the pumping operation of the hydraulic motor M is performed, whereby one main circuit

1

is reduced its pressure, while the internal pressure of the other main circuit

2

becomes high in the presence of the orifice

28

, thereby giving a braking force to the hydraulic motor M. The pressure oil passed through the orifice

28

acts on the right-hand side of the high pressure selection valve

4

from a return circuit

24

through a pilot circuit

24

a,

causing the valve

4

to move leftwards and connecting the return circuit

24

to the by-pass circuit

27

. Consequently, as shown in

FIG. 15

, the pressure oil present in the main circuit

2

is circulated to the main circuit

1

via the return circuit

24

—by-pass circuit

27

—land groove

60

—branch passage

21

—discharge passage

10

—first check valve

13

to prevent the internal pressure of the main circuit

1

from becoming negative and prevent the occurrence of cavitation. In this case through the land groove

60

the by-pass circuit

27

and the branch passage

21

are widely open and communicate with each other, allowing a large amount of the hydraulic actuating oil to flow and thereby preventing the oil flow from becoming deficient. After this state, the spool

141

moves toward its original position under the action of the right-hand spring

16

and is restored to its state shown in

FIG. 14

, whereby the hydraulic motor M is blocked completely and turns OFF.

FIG. 16

shows examples of the land groove

60

.

A land groove shown in FIGS.

16

(A) and (B) is an annular groove formed in the inner periphery of the valve hole

140

in the valve body

20

and having a suitable width in the longitudinal direction. It is substantially the same as that shown in FIG.

14

.

A land groove shown in FIG.

16

(C) comprises two or more annular grooves, a, b, formed spacedly from each other in the longitudinal direction.

A land groove

60

shown in FIGS.

16

(D) and (E) comprises two annular grooves, a, b, and a lateral groove, c, which provides communication between the two annular grooves, a, b.

The land groove

60

is not specially limited insofar as it can provide communication in a large flow path area between the by-pass circuit

27

and the branch circuits

21

,

22

.

The following effects are attained by the present invention. (1) According to the inventions defined in the appended claims, when turning OFF the pump, the pressure oil in the return-side main circuit is circulated to the feed-side main circuit, so that sufficient pressure and flow rate are ensured, whereby the occurrence of cavitation caused by a negative pressure can be prevented and it is possible to improve the deceleration feeling of the hydraulic motor and prevent the occurrence of a noise. (2) The hydraulic drive unit according to the present invention is applicable to a traveling device in a small-sized construction machine without using a large circuit or modifying the internal structure of the system concerned for the replenishment of oil and is superior in machinability and assemblability because the counter-balancing valve is utilized as it is and only some by-pass circuits are provided.

Number	Date	Country
11-369905	Dec 1999	JP
2000-230445	Jul 2000	JP
2000-230446	Jul 2000	JP

Number	Date	Country
57479	May 1977	JP
58-128506	Aug 1983	JP
58-191402	Dec 1983	JP

Hydraulic drive unit

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (3)

US Referenced Citations (1)

Foreign Referenced Citations (3)