Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Reexamination Certificate
1999-06-28
2001-09-25
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
C060S484000, C060S486000
Reexamination Certificate
active
06293099
ABSTRACT:
DETAILED EXPLANATION OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydraulic circuit for a forklift, in particular it relates to the hydraulic circuit which can shift( lift) at least a fork for lift (hereinafter referred to “fork”) over a long stroke in high speed, or can inch position of the fork little by little or gradually, as the occasion demands.
2. Related Background Art
Generally, a forklift has at a front side a lifting device including the fork for lifting a load, and this lift is lifted vertically by a hydraulic circuit. The hydraulic circuit generally includes passages or routes extending from a tank to a lift cylinder, a hydraulic pump for supplying an operating oil (hereinafter referred to “oil”), an electric motor for driving the hydraulic pump, the lift cylinder having a piston connected to the fork to lift it vertically, and a flow controlling valve.
For lifting the load to a high position by lifting operation of the lifting device, it is desirable that the lift cylinder has long stroke, and for lifting the load to a high position in short time it is necessary that large amount of the oil is supplied to a bottom portion of the lift cylinder in short time to lift the fork in high speed. The lifting speed of fork is determined by the number of rotations of the electric motor and allowable flowing amount through the flow controlling valve, and it is sufficient to supply the large amount oil by the large-size hydraulic pump, when only increase of the fork lifting speed is considered.
However, in addition to the high-speed lifting of the fork, the operating characteristics of inching of the fork should be considered, in determining the number of rotations of the hydraulic pump and allowable flowing amount of the flow controlling valve. That is, for inching the fork, the hydraulic pump should supply the oil in high response even for the oil supplying of small amount. Thus, two conflicting characteristics, i.e. the characteristic to supply the small amount of oil in high response and the characteristic to supply the large amount of oil in short time, are required for the hydraulic circuit for lifting the fork.
By taking the above circumstances into consideration, there have been known some related art in which the lifting speed of fork is controlled in two steps. For example, in Japanese Unexamined (KOKAI) U.M. No.
56-84600
, as shown in
FIG. 5
, two routes
204
and
206
are provided between an oil tank and a lift cylinder
202
(check valve
216
is disposed between the routes
204
and
206
), and on each route a hydraulic pumps
208
or
210
, and a electric motor
212
or
214
are disposed. For shifting the piston
218
slowly only the hydraulic pump
208
and the electric motor
214
are operated, and for lifting the piston quickly both hydraulic pumps
208
,
210
and the hydraulic motors
212
,
214
are operated. However, since both of the electric pumps
212
and
214
are on-off controlled, a very small amount of oil is hardly supplied to the piston in high response, so inching of the piston
218
is difficult if only the electric motor
214
is driven for the inching.
In Japanese Unexamined (KOKAI) Patent No.
62-249897
, as shown in
FIG. 6
, on a main-route
226
extending from a lifting pump
22
2
to a lifting cylinder
224
a sub-route
228
is provided, and a switching valve
230
and a logic valve
232
are respectively disposed on the main-route
226
and the sub-route
228
. On a pilot route
234
extending from a pump
223
a remote-control valve
238
acting onto a pilot switching valve
236
co-operating with the switching valve
230
is disposed, and on a route
240
a pilot switching valve
242
acting onto the logic valve
232
is disposed. By controlling the oil flow in the pilot route by the remote control of the remote-control valve
238
, the pilot switching valve
236
, i.e. the switching valve
230
is switched, so that the oil flow in the route
240
is controlled.
When the fork is lifted in low speed, the switching valve
230
is switched to flow the oil only through the main route
226
. When the fork is shifted in high speed, the remote-control valve
238
is operated to switch the pilot switching valve
236
to thereby supply the oil through the route
240
. Thus, the logic valve
232
is opened to supply the oil also through the sub-route
228
. However, because the motor
222
is on-off controlled, a very small amount of oil is hardly supplied to the cylinder
224
, so inching of the fork is difficult, which is same as the hydraulic circuit in FIG.
5
.
In a hydraulic circuit shown in
FIG. 7
, on a route
254
extending from a tank
250
to a lift cylinder
252
a hydraulic pump
256
, a electric motor
258
which is chopper-controlled, a controlling valve
260
, and a flow regulator
270
are disposed. A first adjusting valve
274
in the controlling valve
260
has three positions
262
,
264
and
266
respectively corresponding to a lifting, lowering and neutral.
When the fork is lifted in high speed, the first adjusting valve
274
is switched to the position
262
, and the lever (not shown) is operated to make the number of rotation of the motor
258
maximum.
In this way, large amount of the oil is supplied to a bottom portion of the cylinder
252
through the route
254
. On the other hand, in inching the fork, the lever is operated to decrease the number of rotations of the motor
252
.
However, in this hydraulic circuit, the first adjusting valve
274
is switched corresponding to the high speed lifting or the inching of the fork, and the electric motor
258
is chopper-controlled to change the number of rotations. Such arrangement is convenient for the inching of the fork, but inconvenient for the high speed lifting of the fork. Here, for lifting the fork in high speed, the motor
258
and the first adjusting valve
274
need to be large-sized, which however makes the operating characteristic of inching inaccurate due to a flow force in the first adjusting valve
274
and a inertia of the electric motor
258
.
In this hydraulic circuit, in addition to the fork, a reach mechanism and tilt mechanism are provided, and they are controlled by a second adjusting valve
276
disposed side by side in the controlling valve
260
.
Further, in Japanese Unexamined (KOKAI) Patent No.
1-104599
, as shown in
FIG. 8
, on a first route
284
extending from a pump
280
to a lift cylinder
282
a controlling valve for lift
286
is disposed, and on a second route
290
branched from the first route
284
and extending to a reach cylinder
288
a restrictor
291
and a controlling valve for reach
292
is disposed, both of which cylinders are chopper controlled.
When both of the reach and lift are driven (
FIG. 8
shows this state), the controlling valve for reach
292
is changed to a position A
1
while the controlling valve for lift
286
is changed to a position B
1
and the pump P is rotated by the maximum speed. The oil is supplied to the reach cylinder only through the restriction
291
, so that the oil is supplied also to the lift cylinder
282
suitably.
When only the reach is driven, the controlling valve for reach
292
is changed to the position Al while the controlling valve for lift
286
is changed to the position B
2
, and the pump P is chopper-controlled to be rotated by the number of rotations smaller than the maximum number of rotations (duty ratio: 60 to 80%), for supplying the oil to the lift cylinder
282
without passing through the restriction
291
. Thus, the energy for driving the pump P is saved. When only the lift is driven, the controlling valve for reach
292
and the controlling valve for lift
282
are respectively changed to the position A
2
and B
1
, the pump P is rotated by the maximum number of rotations, and the oil is supplied to the lift cylinder
282
without passing through the restriction
291
.
In this hydraulic circuit, the pump P is chopper-controlled both when the lift cylinder
282
and the reach cylinder
288
are driven, and when only the lift cyl
Kabushiki Kaisha Toyoda Jidoshokki Seisakusho
Lazo Thomas E.
Look Edward K.
Morgan & Finnegan , LLP
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