Drive circuit for fluid motor

Details Drive circuit for fluid motor Drive circuit for fluid motor

2002-03-14

2004-03-02

Look, Edward K. (Department: 3745)

Power plants

Pressure fluid source and motor

Servo-motor having externally operated control valve sets...

C092S012200, C091S506000

Reexamination Certificate

active

06698196

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a drive circuit for a fluid motor having a swash plate which is tiltable between positions of high-speed and low-speed rotation.
A related drive circuit for a fluid motor is shown in FIG.
10
. This drive circuit is comprised of a fluid motor
12
having a swash plate
11
which is tiltable between positions of high-speed and low-speed rotation; a reduction gear
10
coupled to the fluid motor
12
and adapted to reduce the output rotational speed of the fluid motor
12
before outputting it; a tilting piston
13
which, when a high-pressure fluid is introduced to it, pushes and tilts the swash plate
11
to the position of high-speed rotation, and which, when set under a tank pressure, allows the swash plate
11
to tilt to the position of low-speed rotation; a changeover valve
18
which is interposed midway in a connecting passage
17
for connecting the tilting piston
13
and a selector valve
16
for selectively obtaining the high-pressure fluid from main circuits
14
and
15
of the fluid motor
12
, and which is changed over between a high-speed position K for introducing to the tilting piston
13
the high-pressure fluid obtained by the selector valve
16
and a low-speed position L for discharging the fluid acting on the tilting piston
13
to a drain passage
22
; a first pilot passage
19
for introducing into the changeover valve
18
a constant-pressure pilot fluid for a direction in which the changeover valve
18
is changed over to the high-speed position K; and a second pilot passage
21
for introducing into the changeover valve
18
the high-pressure fluid selectively obtained from the main circuits
14
and
15
by a counterbalance valve
20
, for a direction in which the changeover valve
18
is changed over to the low-speed position L.
In the above-described drive circuit for the fluid motor
12
, during a normal load, the fluid force based on the constant-pressure pilot fluid in the first pilot passage
19
is higher than the fluid force based on the high-pressure fluid (the high-pressure fluid selectively obtained from the high pressure-side main circuits
14
and
15
) in the second pilot passage
21
, so that the changeover valve
18
has been changed over to the high-speed position K. Consequently, the high-pressure fluid from the main circuits
14
and
15
obtained by the selector valve
16
causes the tilting piston
13
to project through the connecting passage
17
, thereby holding the swash plate
11
at the position of high-speed rotation.
Next, when the load acting on the above-described fluid motor
12
increases, the pressure in the main circuits
14
and
15
on the high-pressure side increases, so that the fluid force based on the high-pressure fluid in the second pilot passage
21
becomes higher than the fluid force based on the constant-pressure pilot fluid in the first pilot passage
19
. Consequently, the changeover valve
18
is changed over to the low-speed position L, so that the fluid acting on the tilting piston
13
is discharged to the drain passage
22
, and the swash plate
11
is tilted to the position of low-speed rotation while pushing the tilting piston
13
. As a result, the fluid motor
12
rotates at low speed with a large torque, and the pressure within the high pressure-side main circuits
14
and
15
declines.
However, with such a drive circuit for a fluid motor, since the fluid acting on the tilting piston
13
is instantly discharged into the drain passage
22
simultaneously as the changeover valve
18
is changed over to the low-speed position L, the swash plate
11
is suddenly tilted to the position of low-speed rotation, and the pressure within the high pressure-side main circuits
14
and
15
declines. Consequently, the output torque of the fluid motor
12
instantly changes from a small torque with high-speed rotation to a large torque with low-speed rotation, with the result that there are problems in that a shock can be imparted to an operator to deteriorate an operational feeling, and a large load can be imparted to the drive circuit itself and shorten its life.
In addition, if the swash plate
11
is suddenly tilted to the position of low-speed rotation and the pressure within the high pressure-side main circuits
14
and
15
declines substantially as described above, the fluid force based on the constant-pressure pilot fluid in the first pilot passage
19
becomes higher than the fluid force based on the high-pressure fluid in the second pilot passage
21
, and the changeover valve
18
is changed over again to the high-speed position K. However, when the changeover valve
18
is thus changed over to the high-speed position K, the pressure within the high pressure-side main circuits
14
and
15
rises again, so that the changeover valve
18
is changed over again to the low-speed position L. Hence, there is a problem in that the changeover valve
18
is repeatedly changed over between the high-speed position K and the low-speed position within a short time, i.e., hunting due to pressure fluctuations of the fluid motor
12
can occur.
SUMMARY OF THE INVENTION
The object of the invention is to provide a drive circuit for a fluid motor which is capable of improving the operational feeling and prolong the life of the drive circuit while preventing hunting at the time of changeover of the changeover valve to the low-speed position.
The above object can be attained by a drive circuit for a fluid motor including a fluid motor having a swash plate which is tiltable between positions of high-speed and low-speed rotation; a tilting piston which, when a high-pressure fluid is introduced thereto, pushes and tilts the swash plate and tilts it to the position of high-speed rotation, and which, when set under a tank pressure, allows the swash plate to tilt to the position of low-speed rotation; a changeover valve which is interposed midway in a connecting passage for connecting the tilting piston and a selector valve for selectively obtaining the high-pressure fluid from two main circuits of the fluid motor, and which is changed over between a high-speed position for introducing to the tilting piston the high-pressure fluid obtained by the selector valve and a low-speed position for discharging the fluid acting on the tilting piston to a drain passage; a first pilot passage for introducing into the changeover valve a constant-pressure pilot fluid for a direction in which the changeover valve is changed over to the high-speed position; and a second pilot passage for introducing into the changeover valve the high-pressure fluid selectively obtained from the main circuits, for a direction in which the changeover valve is changed over to the low-speed position, the drive circuit for a fluid motor characterized in that:
an additional pilot passage is provided for introducing from the connecting passage between the changeover valve and the tilting piston into the changeover valve an additional pilot fluid for the direction in which the changeover valve is changed over to the low-speed position, and that a first narrow passage for allowing the drain passage and the connecting passage extending from the changeover valve on a tilting piston side to communicate with each other and a second narrow passage for allowing the connecting passage extending from the changeover valve on the tilting piston side and the communicating passage extending from the changeover valve on the selector valve side to communicate with each other are provided midway in a stroke in which the changeover valve is changed over from the high-speed position to the low-speed position.
When the two main circuits are under low pressure and the rotation of the fluid motor is stopped, the constant-pressure pilot fluid is only being introduced into the changeover valve through the first pilot passage, so that the changeover valve remains changed over to the high-speed position.
Next, when the high-pressure fluid is supplied to either one of the main circuits, the fluid motor rotates. At this tim

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