Power plants – Pressure fluid source and motor – Condition responsive control of motive fluid flow
Reexamination Certificate
2002-08-30
2004-04-13
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Condition responsive control of motive fluid flow
C060S456000
Reexamination Certificate
active
06718763
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydraulic drive unit for driving a fan or the like.
2. Description of the Related Art
A radiator for the engine of construction machines and the like is cooled by a hydraulically operated fan. The hydraulically operated fan has a hydraulic pump as a hydraulic source and is rotated as a hydraulic motor is driven to rotate. The hydraulic pump is driven by the engine.
Lately, there are demands for operation of construction machines at a low noise level. Therefore, it is necessary to drive the hydraulically operated fan at a lower speed while securing adequate cooling performance.
To achieve it, it is necessary to control a flow rate flowing into the hydraulic motor so as to obtain the control characteristic LN
3
of FIG.
9
. The engine speed increases when controlled according to the control characteristic LN
3
, and the flow rate flowing into the hydraulic motor is kept at a fixed level when the engine speed increases and discharge flow rate Q of the hydraulic pump becomes prescribed flow rate Qc or more. Therefore, characteristic L
0
is obtained, indicating that when engine speed N becomes prescribed speed Nc or more, rotational speed NF of the fan is kept at a prescribed level as shown in FIG.
4
. Such a control characteristic is called as a flow control characteristic.
The flow control characteristic is obtained by adopting a variable-capacity hydraulic pump as the hydraulic pump and controlling a swash plate.
However, the variable-capacity hydraulic pump is generally expensive. Therefore, it is demanded to use a relatively inexpensive fixed-capacity hydraulic pump such as a gear pump to realize the flow control characteristic.
Therefore, the hydraulic circuit shown in
FIG. 10
has been conventionally used.
(Related Art 1)
Specifically, as shown in
FIG. 10
, fixed-capacity hydraulic pump
2
such as a gear pump is driven by an unshown engine to discharge pressure oil to oil passage
7
. The pressure oil discharged from the hydraulic pump
2
is supplied to hydraulic motor
1
through the oil passage
7
.
Throttle
41
is disposed on the oil passage
7
. The oil passage
7
is branched to oil passage
15
which is connected to an inlet port of control valve
20
. An outlet port of the control valve
20
is connected to tank
3
through oil passage
16
. The control valve
20
is provided with spring
20
c
. The upstream of the throttle
41
is connected to pilot port
20
d
through pilot oil passage
51
. The pilot port
20
d
is one of two pilot ports
20
d
,
20
e
of the control valve
20
and located on the side opposite to the side where spring
20
c
is disposed. The downstream of the throttle
41
is connected to the pilot port
20
e
, which is located on the same side where the spring
20
c
is disposed, through pilot oil passage
52
.
The structure of the control valve
20
of
FIG. 10
is shown in FIG.
11
. As shown in
FIG. 11
, the control valve
20
is a valve having a spool structure.
When it is assumed that a pressure on the upstream side of the throttle
41
is P
2
, a pressure on the downstream side thereof is P
3
, a sectional area of spool
21
is A and a spring force of the spring
20
c
is F, a balance of force acting on the spool
21
of the control valve
20
is ideally expressed by the following expression (1).
(
P
2
−
P
3
)·
A=F
(1)
Therefore, when the control valve
20
operates as indicated by the expression (1), force ((P
2
−P
3
)·A) which corresponds to pressure difference P
2
−P
3
before and after the throttle
41
and the prescribed spring force (F) of the spring
20
c
are mutually balanced and therefore a flow rate of the pressure oil flowing through the throttle
41
is kept at a prescribed constant level according to the prescribed spring force, and an ideal flow control characteristic indicated by LN
3
in
FIG. 9
is obtained.
However, the flow rate actually flowing into the hydraulic motor does not become constant, and the characteristic indicated by LN
1
is obtained, which indicates that the flow rate flowing into the hydraulic motor tends to increase according to an increase in pump discharge flow rate Q.
The reason is as follows. When the spool
21
of the control valve
20
opens to discharge the pressure oil to the tank
3
in
FIG. 11
, the pressure oil is discharged along streamline V which has a component parallel with the spool
21
. Therefore, a force called a flow force acts on the spool
21
of the control valve
20
in the same direction as that of the spring force F. The flow force increases according to an increase in flow rate of the pressure oil passing through the throttle
41
.
When it is assumed that the flow force is f, the balance of force acting on the spool
21
of the control valve
20
is indicated by the following expression (2).
(
P
2
−
P
3
)·
A=F+f
(2)
When the control valve
20
operates according to the expression (2), the spool
21
is pushed back by the flow force f in a direction that the opening of the spool
21
is closed. Therefore, as indicated by LN
1
in
FIG. 9
, the flow rate flowing into the hydraulic motor shows a tendency to increase according to the increase in pump discharge flow rate Q.
Thus, the related art 1 has drawbacks as described above.
(Related Art 2)
To remedy the drawbacks of the related art 1, it is tried to improve a notch shape or the like of the spool
21
so to remove the component possessed by the streamline V, which is parallel to the spool
21
. A control characteristic of the related art 2 is indicated by LN
2
in FIG.
9
.
According to the related art 2 (characteristic LN
2
), the problems of the related art 1 (characteristic LN
1
) are improved to some extent, but the flow rate flowing into the hydraulic motor still tends to increase according to the increase in pump discharge flow rate Q. Therefore, even when the engine speed becomes the prescribed speed or more, the rotational speed of the fan continues to increase, and its noise cannot be suppressed to a prescribed level. In other words, a desired target to suppress the noise to a predetermined level when the engine speed is at a prescribed level or more cannot be achieved.
The present invention was made in view of the above circumstances and provides a low-cost and low-noise hydraulic drive unit by making it possible to realize an ideal flow control characteristic by means of inexpensive hydraulic equipment.
SUMMARY OF THE INVENTION
A first aspect of the present invention is directed to a hydraulic drive unit comprising:
a hydraulic source which increases a discharge flow rate according to an increase in rotational speed;
a throttle through which pressure oil discharged from the hydraulic source passes;
hydraulic equipment which operates upon inputting the pressure oil having passed through the throttle; and
a control valve which controls the pressure oil passing through the throttle so that the flow rate passing through the throttle becomes a prescribed level when the rotational speed becomes a prescribed level or more, wherein:
a force for canceling a flow force produced by the control valve is applied to the control valve.
Specifically, as shown in
FIG. 2
, it is assumed that a pressure on the upstream side of second throttle
42
is P
1
, a pressure on the downstream side thereof is P
2
(pressure on the upstream side of the first throttle
41
), a pressure on the downstream side of the first throttle
41
is P
3
, the sectional area of the spool
21
is A, the spring force of the spring
20
c
is F, and the flow force is f. Then, a balance of force acting on the spool
21
of the control valve
20
is indicated by the following expression (3).
(
P
1
−
P
3
)·
A=F+f
(3)
Here, when it is assumed that the pressure difference P
1
−P
2
before and after the second throttle
42
is &Dgr;P
12
(see
FIG. 2
) to modify the expression (3), the following expression (4) is obtained.
&Dgr;
P
12
·
A+
(
P
2
−
P
3
)·
Maruta Kazuhiro
Yoshida Nobumi
Komatsu Ltd.
Leslie Michael
Look Edward K.
Varndell & Varndell PLLC
LandOfFree
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