Power plants – Pressure fluid source and motor – Pulsator
Reexamination Certificate
2001-09-10
2003-06-24
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Pulsator
Reexamination Certificate
active
06581379
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pressure intensifying apparatus for a hydraulic cylinder, and, more particularly, to an apparatus which is adapted for use for a core driving cylinder in a die cast machine or an injection molding machine and deals with the phenomenon that a hydraulic fluid is compressed to push the core back at the time of casting or applying pressure.
2. Description of the Related Art
A die cast machine employs a casting scheme of injecting a molten metal or semimolten alloy to which a molding pressure is applied by an injection cylinder into a cavity whose shape corresponds to the shape of a product and which is formed by assembling a movable mold and the core to a fixed mold, via a runner. The pressure molding can provide castings of relatively complicated shapes with high precision.
The conventional core driving hydraulic cylinders usually use a hydraulic circuit as shown in FIG.
10
.
Referring to the diagram, a hydraulic cylinder
200
comprises a cylinder tube
201
, a rod cover
202
, a head cover
203
, a piston
204
and a rod
205
. A 4-port, 3-position switching valve
210
allows the supply and discharge of a hydraulic fluid to and from a rod-side port
206
and a head-side port
207
via meter-in circuits
208
and
209
. The hydraulic cylinder
200
is designed in such a manner that a core
211
attached to the piston rod
204
,
205
is fitted in and pulled out from a predetermined hole or the like, which is formed in a mold, in the advancing/reversing step. A pilot check valve
212
is disposed between the head-side port
207
of the hydraulic cylinder
200
and the meter-in circuit
209
, and its pilot pressure comes from the rod-side port
206
.
With the structures of the hydraulic cylinder
200
and the hydraulic circuit, first, the switching valve
210
is set to the state shown in
FIG. 10
, and then the hydraulic fluid is supplied from the head-side port
207
to move the piston rod
204
,
205
forward from the backward limit. This causes the core
211
to enter the mold to a predetermined position. When the core
211
reaches the insertion limit, the pressure of a head-side cylinder chamber
213
of the hydraulic cylinder
200
becomes the rated pressure of a hydraulic pump
214
. With the pressure balanced, the piston
204
stops, closing the open pilot check valve
212
.
As a result, the head-side cylinder chamber
213
of the hydraulic cylinder
200
is sealed tightly and the piston rod
204
,
205
is locked. Then, a molten metal is injected into the cavity under high pressure (molding pressure) and is left in that state for solidification, which completes casting.
When casting is completed, the switching valve
210
is switched to the opposite supply and discharge position, and the hydraulic fluid is supplied from the rod-side port
206
to move the piston rod
204
,
205
backward. In this case, the pilot pressure acts on the pilot check valve
212
to open the closed valve
212
, and the piston rod
204
,
205
returns to the original backward limit.
Thereafter, the cavity is opened for removal of the casting.
When a molten metal is injected into the cavity with the core
211
pressed into the mold to its positional limit, the molding pressure of the molten metal is applied to the core
211
and the force FL equivalent to the molding pressure × the pressure receiving area acts on the core
211
. This applies a large push-back load on the piston rod
204
,
205
of the hydraulic cylinder
200
.
In this case, as the pilot check valve
212
is closed, the piston rod
204
,
205
is held locked. The generation of such a push-back load normally raise no problems. Because the load is generally intensive reactive force, which is several times the rated pressure of the hydraulic pump
214
, however, a very large pressure is applied to the hydraulic fluid in the head-side cylinder chamber
213
.
Specifically, the pressure of the hydraulic fluid becomes PLa=FL/Ah where FL is the push-back load on the core
211
and Ah is the pressure receiving area of the piston
204
. Given that the rated pressure of the hydraulic pump
214
is Pp, the pressure of the hydraulic fluid increases by &Dgr;Pa=PLa−Pp after the core
211
is moved forward to the positional limit and the pilot check valve
212
is closed.
The hydraulic fluid is not a complete incompressive fluid, but has a compressibility, very small though it is.
Accordingly, given that the volume of the head-side cylinder chamber
213
is Va and the compressibility of the hydraulic fluid is &bgr;, the hydraulic fluid is compressed by &Dgr;V=&Dgr;Pa×Va×&bgr;, with the result that the piston rod
204
,
205
is pushed back by (&Dgr;V/Ah).
The push-back quantity is naturally decided depending on the conditions of the respective elements relating to the aforementioned equation. In actuality, the quantity is of such magnitude that it may be almost ignored with respect to the process length of the normal hydraulic cylinder since the compressibility of the hydraulic fluid: &bgr; is low.
However, in recent years, high accuracy of dimension has been required of die cast products, and products having no secondary operation needed are often demanded of auto-parts, parts for electric equipment, and the like, and an error in size caused by the aforementioned push-back cannot be ignored in some cases.
As measures against the above problem, there is a case in which a mold and the like are designed in consideration of the above-mentioned error beforehand. The specification of the hydraulic cylinder adapted to the die cast machine is not uniformly provided, a case in which the change in the specification is made must be assumed, and the above measures are unrealistic in view of the point that the aforementioned error changes depending on the condition of the hydraulic cylinder.
It is an object of the present invention is to provide an apparatus wherein a small-sized pressure intensifying cylinder designed rationally is additionally provided to a working cylinder is added and the pressure intensifying cylinder is controlled adaptively in an advancing step or reversing step, whereby solving the aforementioned problem.
SUMMARY OF THE INVENTION
The first invention relates to a pressure intensifying apparatus for a hydraulic cylinder comprising: a working cylinder; a pressure intensifying cylinder for allowing a pressure intensifying chamber to communicate with a head-side cylinder chamber of the working cylinder; a switching valve for controlling supply and discharge of a hydraulic fluid with respect to a head-side port and a rod-side port of the working cylinder; a first pilot check valve which is disposed in a head-side supply and discharge circuit for connecting the head-side port of the working cylinder to the switching valve and whose pilot pressure is a pressure of a rod-side supply and discharge circuit for connecting the rod-side port of the working cylinder to the switching valve; a check-valve equipped sequence valve which is disposed in a connection circuit for connecting a drive chamber side of the pressure intensifying cylinder to a portion of the head-side supply and discharge circuit closer to the switching valve than the first pilot check valve, and switches from a closed state to an open state with a rated pressure of a hydraulic pump for supplying the hydraulic fluid to the head-side supply and discharge circuit via the switching valve; and a second pilot check valve whose pilot pressure is the pressure of the rod-side supply and discharge circuit, the working cylinder, the pressure intensifying cylinder, the switching valve, the first pilot check valve, the check-valve equipped sequence valve and the second pilot check valve being connected in series in such a way as to satisfy K>PLa/Pp where K is a pressure intensifying ratio of the pressure intensifying cylinder, Pp is the rated pressure of the hydraulic pump and PLa is a pressure in the head-side cylinder chamber when a maximum load occurr
Kitamura Hiroshi
Komatsu Satoshi
Nomura Kazushi
Kershteyn Igor
Nambu Co., Ltd.
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