Fluid handling – Self-proportioning or correlating systems – Self-controlled branched flow systems
Reexamination Certificate
2002-01-17
2004-01-27
Buiz, Michael Powell (Department: 3753)
Fluid handling
Self-proportioning or correlating systems
Self-controlled branched flow systems
C137S115130
Reexamination Certificate
active
06681794
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an unloading valve used in a hydraulic circuit for, e.g., a construction machine, and more particularly to an unloading valve having a spool which is formed with a flange for restricting slide movement of the spool and properly positioning the spool in its neutral state.
BACKGROUND ART
Various hydraulic control valves are conventionally used in a hydraulic circuit for, e.g., a construction machine, and one of those valves is an unloading valve. An unloading valve has the function of releasing the pressure in a hydraulic line when that pressure rises above the reference pressure in excess of a setting value, thereby keeping the pressure in the hydraulic line from rising above the reference pressure in excess of a certain value.
FIG. 5
shows one example of an unloading valve. In
FIG. 5
, the unloading valve has a valve body
2
in which a spool bore
3
is formed. A first pressure chamber
4
and a second pressure chamber
5
are formed at opposite ends of the spool bore
3
, and a reservoir port
6
is formed in an intermediate portion of the spool bore
3
. A spool
1
is slidably inserted in the spool bore
3
. A hydraulic fluid, of which pressure is to be controlled, is introduced to the first pressure chamber
4
, and a hydraulic fluid under a pressure as a reference for control is introduced to the second pressure chamber
5
. Further, a flange
7
is formed at an end of the spool
1
on the side of the second pressure chamber
5
, and serves as a stopper coming into contact with an abutment surface
8
provided in the valve body
2
, thereby preventing the spool
1
from slipping off. The flange
7
also functions as a spring receiver and supports a spring
10
disposed in the second pressure chamber
5
.
Further, a transverse hole
1
b
is bored in the spool
1
near its end on the side of the first pressure chamber
4
, and is communicated with the first pressure chamber
4
through a longitudinal hole
1
a.
Assuming in the above-described construction that the pressure of the hydraulic fluid introduced to the first pressure chamber
4
is P
1
, the pressure of the hydraulic fluid introduced to the second pressure chamber
5
is P
2
, and the pressing force of the spring
10
is Fk, the spool
1
of the unloading valve operates so as to satisfy the following hydraulic balance formula:
P
1
·
A=P
2
·
A+Fk
(1)
Herein, A represents, as shown in
FIG. 6
, an effective pressure bearing area of each of pressure bearing portions of the spool
1
, which are positioned in the first pressure chamber
4
and the second pressure chamber
5
. More specifically, on the side of the second pressure chamber
5
, a pressure bearing area dA of an annular portion of the flange
7
on one side thereof is the same as that of a corresponding portion of the flange
7
on the opposite side (i.e., the side facing the abutment surface
8
). Therefore, pressing forces imposed on those peripheral portions on both the sides of the flange are canceled and the pressure bearing area dA of the annular portion of the flange does not take part in the operation of the spool
1
.
Then, when the pressure P
1
rises in the formula (1) to such an extent that the differential pressure between the pressure P
1
and the pressure P
2
exceeds a hydraulic converted value (setting pressure) of the spring force Fk, the hydraulic balance expressed by the formula (1) is lost, whereupon the spool
1
moves to the left in the drawing and the hydraulic fluid in the first pressure chamber
4
is released to the reservoir port
6
through the longitudinal hole
1
a
. Thus, the unloading valve is opened and the pressure P
1
is lowered. As a result, the pressure P
1
is controlled to be held higher than the pressure P
2
by the setting pressure of the spring
10
.
Further,
FIG. 8
of U.S. Pat. No. 5,305,789 discloses an unloading valve provided with a flange having an outer diameter set as small as possible.
DISCLOSURE OF THE INVENTION
However, the above-described prior art has problems as follows.
The spool
1
of the unloading valve is, as described above, provided with the flange
7
. For the necessity of precisely positioning the spool in its neutral state, the flange
7
and the abutment surface
8
of the unloading valve body
2
, which comes into contact with the flange
7
, are both finished into high flatness. Because of such high flatness, the flange
7
and the abutment surface
8
tend to intimately contact with each other. Further, if the hydraulic fluid under a high pressure is present in the second pressure chamber
5
, a very small amount of the hydraulic fluid enters the interface between the flange
7
and the abutment surface
8
, whereby adhesion between the flange
7
and the abutment surface
8
is promoted. When the flange
7
and the abutment surface
8
are brought into such a tightly close contact condition, the pressure P
2
no longer acts upon one of the opposite sides of the flange
7
, i.e., the side of the flange
7
facing the abutment surface
8
. Hence, the pressure bearing area dA of the annular portion of the flange
7
becomes effective in pressure balance and the pressure bearing area on the side of the second pressure chamber
5
is increased from A to A+dA.
Accordingly, the hydraulic balance formula for the spool
1
is expressed by:
P
1
·
A=P
2
·(
A+dA
)+
Fk
(2)
Comparing the formula (1) and (2) with each other, it is seen that, as a result of the flange
7
coming into tightly close contact with the abutment surface
8
, the cracking pressure, at which the spool
1
starts moving to the left in the drawing and the unloading valve is opened, is increased by a value of P
2
·dA/A. Consequently, the proper operation of the unloading valve is impeded.
In the unloading valve shown in
FIG. 8
of U.S. Pat. No. 5,305,789, the outer diameter of the flange
7
is set as small as possible.
FIG. 7
shows a modification of the unloading valve shown in
FIGS. 5 and 6
, which has the flange
7
having the outer diameter set as small as possible based on the concept of the US Patent. In
FIG. 7
, character
7
A denotes a flange having a reduced outer diameter. With the flange
7
A having the reduced outer diameter, an increase in pressure bearing area of the flange
7
A, which occurs on the side facing the interior of the second pressure chamber
5
when the flange
7
A comes into tightly close contact with the abutment surface
8
, is reduced from dA to dA′ and a rise of the cracking pressure is also suppressed. It is however impossible to perfectly prevent a rise of the cracking pressure, which occurs upon the flange
7
A coming into tightly close contact with the abutment surface
8
. Reducing the outer diameter of the flange
7
A causes another problem in that the strength of the flange
7
A deteriorates.
For example, when an unloading valve is used in a hydraulic circuit for load sensing control (hereinafter referred to as “LS control”) of a hydraulic pump, the pressure P
1
is given by a delivery pressure of the hydraulic pump and the pressure P
2
is given by a load pressure of an actuator (maximum load pressure). Then, the unloading valve functions to hold the differential pressure between the pump delivery pressure and the maximum load pressure at a setting value. In that case, a high load pressure at a level of, e.g., 300 MPa may act momentarily in the second pressure chamber
5
at the startup of the actuator. This means that the flange
7
A of the spool
1
hits against the abutment surface
8
under an action of the high pressure of 300 MPa and is subjected to a great impact force. For that reason, there occurs a risk that the strength of the flange
7
A having the reduced outer diameter is deteriorated to such an extent as not withstanding the great impact force, and the flange
7
A is broken.
It is an object of the present invention to provide an unloading valve capable of preventing a rise of the cracking pressure, which occurs upon a flange coming into tightly close
Nishimura Yoshizumi
Nozawa Yusaku
Takahashi Kinya
Tougasaki Mitsuhisa
Buiz Michael Powell
Hitachi Construction Machinery Co. Ltd.
Krishnamurthy Ramesh
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