JP2586563B2 - 2-speed lift mechanism for industrial vehicles - Google Patents

Description

The present invention relates to an industrial vehicle such as a forklift or a shovel loader, and more particularly, to two hydraulic pumps, a low pressure pump and a high pressure pump. The present invention relates to a two-speed lift mechanism for raising and lowering a lift cylinder at high speed and low speed in accordance with the magnitude of a load.

(Prior Art) Conventionally, this type of two-speed lift mechanism is configured as shown in FIG. As shown, the discharge oil sent from the low-pressure large-capacity pump 31 and the high-pressure small-capacity pump 32
It joins at the speed valve 33, and here, depending on the level of the load of the lift cylinder 34, it controls whether the discharge oil of the low-pressure large-capacity pump 31 is sent to the control valve 35 or drained to the tank 36. That is, the 2-speed valve 33 is a pilot-operated type. When the pressure in the circuit is equal to or lower than the set pressure (at a low load), the switching valve 38 is switched to the position A by closing the relief valve 37, and the two pumps 31, 32 The lift cylinder 34 is operated at a high speed with the total discharge amount, and when the pressure is equal to or higher than the set pressure (at a high load), the switching valve 38 is switched to the position B by opening the relief valve 37 to drain the discharge oil of the low-pressure large-capacity pump 31 to the tank 36. Then, the lift cylinder 34 is operated at a low speed by the discharge oil of the high-pressure small-capacity pump 32.

(Problems to be Solved by the Invention) However, in the above-described conventional example, when the degree of the load acting on the lift cylinder 34 (the weight of the load) corresponds to around the set pressure of the relief valve 37, the lift cylinder 34 is raised. Then, the relief valve 37 opens and closes due to the pressure fluctuation in the circuit at that time, that is, the switching valve 38 is switched between the position A and the position B.
Since the switching operation is repeatedly performed between the lift cylinder 34 and the position, there is a problem that the lifting speed of the lift cylinder 34 fluctuates, and the load makes the load unstable.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a two-speed lift mechanism for an industrial vehicle that can reliably switch the lift speed of a lift cylinder.

(Means for Solving the Problems) In order to solve the above problems, the present invention divides the discharge oil discharged from the low-pressure pump into a priority flow and a surplus flow by a flow dividing valve, and discharges the discharge oil on the surplus flow side. Is combined with the discharge oil discharged from the high-pressure hydraulic pump and sent to the lift cylinder in the two-speed lift mechanism of the industrial vehicle. When the pressure exceeds the set pressure at least once, it is switched to the open side to return the discharge oil to the tank, and once switched, it is re-excepted except when the pressure in the pipeline is below the set pressure and there is no discharge oil flow. Even if the pressure drops below the set pressure, this state is maintained, and when the pressure in the pipeline is lower than the set pressure and there is no discharge oil flow, an unloading valve that switches to the closed side and sends discharge oil to the lift cylinder side is installed. It is characterized by

(Operation) According to the present invention configured as described above, when the unloading valve is switched to the closed side,
During a low-load operation in which the surplus oil pipeline pressure is lower than the set pressure, the unloading valve is kept closed. Therefore, at this time, the discharge oil on the excess flow side sent from the low-pressure pump through the branch valve merges with the discharge oil of the high-pressure pump and is sent to the lift cylinder, and the lift cylinder is raised at a high speed.

On the other hand, during a high-load operation in which the pressure in the surplus oil pipeline is higher than the set pressure, the unloading valve is switched to the open side. Therefore, at this time, since the excess oil discharged from the low-pressure pump through the flow dividing valve is returned to the tank, the lift cylinder is raised at a low speed by the discharged oil sent from the high-pressure pump. Then, once the unloading valve is switched to the open side, the excess oil pipeline is maintained until the pressure in the excess oil pipeline is equal to or less than the set pressure and there is no discharge oil flow. The speed of the lift cylinder is maintained in a stable state even when the internal pressure changes near the set pressure and becomes lower than the set pressure.

(Example) Hereinafter, an example of the present invention will be specifically described with reference to the drawings. As shown in the figure, the high-pressure small-capacity pump 1 is connected to a control valve 3 for controlling a lift cylinder via a pipe line 2 so that all discharged oil is sent to the control valve 3. The control valve 3 is connected to the pipes 4 and 5
Is connected to the bottom side and the rod side of the lift cylinder 6. On the other hand, the low-pressure large-capacity pump 7 is connected to a flow divider 9 via a pipe 8, and the discharge oil from the flow divider 9 is transferred to a priority pipe 10 and an excess oil pipe 9.
It is diverted to 11 and so on. That is, as shown in FIG. 2, in the low rotation range from the idling operation of the engine to the predetermined rotation speed, all the hydraulic oil flows to the priority line 10 side, and at the high rotation beyond this low rotation range, the excess hydraulic oil is discharged. It is set so as to flow to the 2-speed valve 12 side. The priority pipeline 10 is used, for example, as a power steering circuit.

Thus, the two-speed valve 12 comprises a check valve 13 and an unloading valve 14, and the hydraulic oil flowing from the surplus oil line 11 is supplied to the flow to the check valve 13 and the flow to the unloading valve 14. The hydraulic oil sent to the check valve 13 side is combined with the discharge oil of the high-pressure small-capacity pump 1 via a pipe 17 and supplied to the control valve 3.

On the other hand, the unloading valve 14 is a two-position type switching valve.
15 and a relief valve 16.
6 are each operated by pilot pressure. That is, the switching valve 15 is connected to the pilot lines 18, 1
Pilot pressure is received on both sides of the opening direction and the closing direction via 9, and the relief valve 16 receives pilot pressure via a common pilot line 19 with the closing side of the switching valve 15. Note that the common pilot line 19
20 and a throttle 21 are respectively inserted into the inlet side of the switching valve 15, and the outlet side of the switching valve 15 is connected to a tank 23 via a pipe 22. Further, the pilot line 19 communicates with the tank 23 via the bypass passage 24 downstream of the throttle 20 when the switching valve 15 is switched to the open position.

Since the present embodiment is configured as described above, the flow rate of the hydraulic oil sent to the lift cylinder 6 when the control valve 3 is switched to the ascending side is as shown in FIG. That is, the discharge oil from the high-pressure small-capacity pump 1 driven by the engine is always sent directly to the control valve 3 via the pipe 2, but the low-pressure large-capacity pump 7
Priority is given to the entire discharge oil in the low engine speed range.
Since it flows to the 10 side, the amount of oil sent to the lift cylinder 6 at this time is only the oil discharged from the high-pressure small-capacity pump 1.

On the other hand, when the engine is running at high speed, the excess of the discharge oil of the low-pressure large-capacity pump 7 flows to the two-speed valve 12. When the pressure in the circuit is lower than the set pressure of the relief valve 16, that is, when the load acting on the lift cylinder 6 is low and the load is low, both the switching valve 15 and the relief valve 16 of the unloading valve 14 maintain the closed state. Therefore, the hydraulic oil joins with the discharge oil of the high-pressure small-capacity pump 1 via the check valve 13 and the pipeline 17. Therefore, at this time, the discharge oil amount (excluding the priority flow rate) of the low-pressure large-capacity pump 7 is sent to the lift cylinder 6 as well as the discharge oil amount of the high-pressure small-capacity pump 1, so that the lift cylinder 6 is raised at a high speed.

On the other hand, when the load is high, the pressure in the circuit exceeds the set pressure of the relief valve 16, so that the pilot pressure acting through the pilot line 19 opens the relief valve 16, and this opening operation causes the pressure on the pilot line 19 side to rise to the pilot line 18 side. , The switching valve 15 is switched to the open side. That is, the unloading valve
14 operates and low-pressure large-capacity pump 7 to 2-speed valve
Since all the hydraulic oil sent to 12 is returned from the switching valve 15 to the tank 23 through the pipe 22, the lift cylinder 6 is raised at a low speed by the discharge oil from the high-pressure small-capacity pump 1. .

When the switching valve 15 is switched to the open side, the pilot line 19 communicates with the tank 23 through the bypass passage 24.
The pressure is maintained higher than the side pressure, and the switching valve 15 remains open. That is, once the unloading valve 14 is operated to the unloading side, the pressure in the circuit is equal to or lower than the set pressure of the relief valve 16 and the low speed of the engine reduces
If the flow of hydraulic oil to the speed valve 12 does not stop at all, it will not return. Therefore, even when the load corresponds to the pressure around the set pressure of the relief valve 16 during the lifting operation of the lift cylinder 6, the lift cylinder 6 is raised at a stable low speed.

The applicable range of the two-speed lift mechanism of the present invention is not limited to a forklift, but may be applied to any industrial vehicle as long as it is necessary to control the lift cylinder in two stages, high speed and low speed. Also, the capacities of the high-pressure and low-pressure pumps 1, 7 may be the same.

(Effects of the Invention) As described in detail above, according to the two-speed lift mechanism of the present invention, once the lift speed of the lift cylinder is switched to low speed, the low speed state can be maintained unless the engine speed is reduced to low speed. In the case where a load corresponding to the vicinity of the set pressure of the relief valve is applied to the lift cylinder, it is possible to obtain a stable ascending operation by preventing a change in the ascending speed of the lift cylinder, which is a conventional problem, thereby improving operability of work. This is a great help in planning.

[Brief description of the drawings]

FIG. 1 is a hydraulic circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing hydraulic oil flow to a lift cylinder, and FIG. 3 is a hydraulic circuit diagram showing a conventional example. 1 high-pressure small-capacity pump 3 control valve 6 lift cylinder 7 low-pressure large-capacity pump 9 flow divider 12 2-speed valve 13 check valve 14 unloading valve 15 Switching valve 16 ... Relief valve

Claims (1)

(57) [Claims] A discharge oil discharged from a low pressure pump is divided into a priority flow and a surplus flow by a flow dividing valve, and the discharge oil on the surplus flow side is merged with discharge oil discharged from a high pressure hydraulic pump. 2 of industrial vehicles that are sent to the lift cylinder
In the speed-lift mechanism, when the pressure in the excess oil pipeline at least once exceeds the set pressure, it is switched to the open side to return the excess oil to the excess oil pipeline that guides the excess flow-side discharge oil, and the discharge oil is returned to the tank and switched once. After that, even if the pressure in the pipeline is lower than the set pressure and the pressure drops below the set pressure again except when the flow of the discharged oil is lost, the state is maintained. A two-speed lift mechanism for an industrial vehicle provided with an unloading valve that is switched to the closed side when it runs out and sends discharge oil to the lift cylinder side.