GB2279061A

GB2279061A - Hydraulic lifting arrangement

Info

Publication number: GB2279061A
Application number: GB9408318A
Authority: GB
Inventors: Rudolf Hendrich
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 1993-05-25
Filing date: 1994-04-27
Publication date: 1994-12-21
Also published as: FR2705657A1; DE4317397A1; GB9408318D0

Description

le ' 1 HYDRAULIC LIFTING ARRANGEMENT 2279061 The invention concerns

hydraulic lifting arrangements, and particularly relates to an industrial truck with a load-supporting means having a first lifting cylinder and a second lifting cylinder connected in parallel with it, the lifting cylinders being connected to a delivery line by branch lines.

Load-supporting means suitable for industrial trucks, such as for example standard lifting frameworks designed to give the driver a clear view, often include two lifting cylinders connected in parallel. In many cases the lifting cylinders are driven by a common pump. The lifting speed is dependent on the delivery of the pump, which in turn is proportional to Its speed of revolution. Since the pump and the driving motor to which the pump is coupled can only produce a certain output (the product of the speed-dependent delivery and delivery pressure dependent on load), the revolutions of the pump are chosen for the maximum possible load and are thus limited, so that a certain lifting speed is pre-determined and thus a certain time is required for one working cycle. One working cycle consists of the lifting of the loadsupporting means and the lowering of the load-supporting means." Usually one of these operations is done with, and one without a load (e.g. when a load is being placed on a shelf or removed from a shelf).

The underlying object of the present invention is to provide an industrial truck of the kind described above which is improved with a view to an increase in the working capacity.

According to a first aspect of the invention, there is provided an industrial truck with a load-supporting means moved by means of a first lifting cylinder and a second lifting cylinder connected in parallel therewith, the lifting cylinders being connected to a delivery line by branch lines, characterised in that the branch line connected to the first lifting cylinder incorporates a valve which can be switched from a closed position to an open position by a signal derived from the delivery pressure of a common 2 pump. In the closed position the flow of fluid to the first lifting cylinder is cut. On the other hand, in the open position an unhindered flow is possible. According to the invention, in the unladen state of the load-supportinq means (low load) only one of the two lifting cylinders is used for lifting so that double the lifting speed is obtained with the same pump delivery. Here, the state of load of the load-liftinq means is detected by monitoring the delivery pressure. If a certain delivery pressure is exceeded due to loading of the load-supporting means, the valve also connects the lifting -cylinder not used for low load to the delivery line of the pump, thereby doubling the effective area on which the fluid under pressure acts, and increasing the loading capacity of the load-supporting means accordingly.

According to a first advantageous embodiment of the invention, it is proposed that the valve be embodied as a 3/2-way valve which is connected to the branch line and to a tank line and connects the first lifting cylinder with the delivery line or the tank line. Therefore, when the valve is not actuated, the first lifting cylinder is short-circuited with the tank. The loadsupporting means is lifted exclusively by the second lifting cylinder. In this process, the first lifting cylinder is also actuated from the outside, this lifting cylinder being able to suck fluid from the tank line.

The same effect is produced according to a second advantageous embodiment when the valve is embodied as a 2/2-way valve and a tank line is connected between the valve and the first lifting cylinder, which tank line incorporates a non-return valve which opens in the direction of the first lifting cylinder and is pre-loaded in the open position. The slight pre-loading in the open position means that even when the valve in the branch line is closed, the fluid sucked up can flow back into the tank when the first lifting cylinder is lowered without a load. If the load Is increased and a corresponding delivery pressure is present and the valve in the branch line is switched, the non-return valve is also in the closed 3 Ir ' position so that unintentional lowering of the load- supporting means is prevented.

According to a second aspect of the invention, there is provided an industrial truck with a load-supporting means and a lifting cylinder, the lifting cylinder connected to a delivery line of a pump, characterised in that the delivery line is permanently connected to the outlet side of a first pump and is selectively connected to the outlet of a second pump, a valve being disposed between the outlet of the second pump and the delivery line, which valve is switchable by a signal derived from the delivery pressure in the delivery line from an operative position connecting the second pump with the delivery line to a short-circuit position connecting the second pump with a tank line.

Thus, with this solution with a low load two pumps (or possibly more) are driven in order to obtain double the delivery and hence double the lifting speed. With a high load, with the same revolutions one of the pumps is short-circuited with the tank so as not to overload the driving motor of the pumps.

The delivery pressure and hence the load of the load-supporting means can be established by example by a pressure switch which is used as a signal generator in a system for electrical actuation of the valve. However, with a view to a reliable and advantageous design from the production standpoint, it is more expedient if the valve is loaded by the force of the spring in the direction of the closed position or operative position and Includes a control surface which is effective in the direction of the open position or short-circuit position and can be acted upon by pressure.

Exemplary embodiments of the invention are explalned in greater detail with reference to the accompanying diagrammatical Figures. in which:

Figure 1 shows the hydraulic circuit diagram for a first embodiment of the invention.

Figure 2 shows the hydraulic circuit diagram for a variant of Figure 1.

4 LP Figure 3 shows the hydraulic circuit diagram for a second embodiment of the invention.

In the embodiment shown in Figure 1 a pump 1 is driven by a driving motor (not shown in the Figure) and pumps fluid from a tank 2 into a delivery line 3. The delivery line 3 is connected through a valve block 4 by means of two branch lines 5 and 6 to a first lifting cylinder 7 and a second lifting cylinder 8, which serve to drive a load-supporting means (not shown in the Figures), for example a lifting mast of a lifting framework consisting of a static mast and a lifting mast.

The valve block 4 includes a control valve 4a and an upstream pressure balance 4b which throttles In intermediate positions and is acted upon in the opening direction by the pressure upstream of the control valve 4a via control pressure lines 24 and 25, and in the closing direction by the force of the spring 26 and by the pressure downstream of the control valve 4a or by the highest of the consuming unit pressures of further consuming units connected to the pump 1, this highest of the consuming unit pressures being established by shuttle valves 9. The pressure balance 4b has the task of draining fluid not required by the consuming units direct to the tank 2 so as not to have to "throttle away" any energy.

The branch line 5 which leads to the first lifting cylinder 7 incorporates a valve 10 embodied in this embodiment as a 3/2-way valve which in a closed position as seen in the Figure, closes the branch line 5 and connects the lifting cylinder 7 with the tank 2 by means of a tank line 11. In an open position of valve 10, the branch line 5 is connected with the lifting cylinder 7. A spring 12 urges valve 10 towards the closed position. A control surface 13, at which the pressure in the delivery line 3 downstream of the control valve 4a is effective, acts on valve 10 in the opening direction.

The hydraulic driving system Is completed by a lowering braking valve 14 disposed in the delivery line 3 and a non- return valve with a throttled bypass 15 for the second lifting cylinder 8 disposed In the branch line 6.

R ' The operation of the industrial truck according to the invention will now be described. When the pump 1 is switched on and the control valve 4a is actuated to lift the load-supporting means, fluid flows through the delivery line 3 to the two branch lines 5 and 6. Since the valve 10 is biased toward the closed position, the fluid initially only flows into the second lifting cylinder 8. If there is little or no load on the loadsupporting means operated by the lifting cylinder 8. the valve 10 remains closed and the fluid still only flows into the lifting cylinder 8. Given the fact that double the quantity of fluid flows to this lifting'cylinder per unit of time in comparison with that when the two lifting cylinders are supplied, the load-supporting means moves at double the speed in comparison with that when the two lifting cylinders are supplied with fluid. When the second lifting cylinder 8 is lifting, the first lifting cylinder 7, although not driven by fluid, also moves upwards due to the mechanical connection between the lifting cylinders 7 and 8 and the loadsupporting means. This draws fluid up into lifting cylinder 7 through the connection with the tank 2 through the tank line 11.

If the load-supporting means is heavily loaded, i.e. a large load has to be lifted, the delivery pressure in the delivery line 3 increases and eventually overcomes the preload in spring 12, causing the valve 10 to switch to the open position so that the pump 1 now supplies both lifting cylinders 7 and 8 with fluid. The speed of movement of the lifting cylinders 7 and 8, and thus the lifting speed of the load-supporting means, is halved with a constant fluid delivery rate from the pump 1, but greater lifting force is developed.

Figure 2 shows a variant of the embodiment in Figure 1, in which the valve 10 is embodied as a 2/2-way valve. The tank line 11 is connected to the branch line 5 between the valve 10 and the first lifting cylinder 7. To allow the fluid to be sucked up and released again unhindered at low load, the tank line 11 is provided with a non-return valve 16 which opens in the direction of the first lifting cylinder and is biased towards its open position.

6 1W When the valve 4a is moved to select lowering of the load, delivery line 3 is connected through valve 4a to tank line 11. The pressure in delivery line 3 below valve 14 falls, and valve 14 moves to a throttlinq position. Fluid drains from cylinder 8 through the throttled bypass of valve 15, through the throttle valve 14, and to the tank 2 via valve 4a. Fluid drains from cylinder 7 through the valve 10 (now open) and the tank line 11, and the load-supporting means is lowered.

Figure 3 shows a second embodiment of the invention. Here, the delivery line 3 is connected on the inlet side to a first pump 1 and to a second pump 17. Between the second pump 17 and the delivery line 3 there is a valve 18 which can be switched by a signal derived from the delivery pressure in the delivery line 3 from an initial position connecting the second pump 17 with the delivery line 3 against the force of a spring 19 to a short-circuit position in which the second pump 17 is connected with a tank line 11. Thus, in this embodiment, the output of both pumps I and 17 is used when the load on the load-supporting means is low in order to obtain double the delivery and hence double the lifting speed. When the load is high, at constant revolutions the second pump 17 is shortcircuited with the tank 2, and thus runs at a minimum power requirement. This allows the majority of the driving effort of motor 20 to be directed to pump 1, producing high output pressure without the risk of overloading the driving motor 20 of the pumps 1 and 17.

The circuit of Figure 3 shows two lifting cyclinders, but it is to be understood that a single lifting cylinder, or a plurality of lifting cylinders, may be connected to the output of the pumps I and 17. Likewise, while Figure 3 shows only two pumps I and 17, it is to be understood that a plurality of pumps such as 17 may have their outputs connected to the delivery line 3 via respective valves 18 of varying pie-load, so as to be sequentially short-circuited as the pressure in delivery line 3 rises.

7

Claims

1. An industrial truck with a load-supporting means moved by means of a first lifting cylinder and a second lifting cylinder connected in parallel therewith, the lifting cylinders being connected to a delivery line by branch lines, characterised in that the branch line (5) connected to the first lifting cylinder (7) incorporates a valve (10) which can be switched from a closed position to an open position by a signal derived from the delivery pressure of a common pump (1).

2. An industrial truck according to claim 1, characterised in that the valve (10) is embodied as a 3/2-way valve which is connected to the branch line (5) and to a tank line (11) and connects the first lifting cylinder (7) with the delivery line (3) or the tank line (11).

3. An industrial truck according to claim 1, characterised in that the valve (10) is embodied as a 2/2-way valve and a tank line (11) is connected between the valve (10) and the first lifting cylinder (7), which tank line incorporates a non-return valve (16) which opens in the direction of the first lifting cylinder (7) and is pre- loaded in the opening position.

4. An industrial truck according to one of claims 1 to 3, characterised in that the valve (10; 18) Is resiliently biased in the direction of the closed position. and has a control surface (13) which can be acted upon by pressure to urge the valve towards the open position.

5. An industrial truck with a load-supporting means and a lifting cylinder, the lifting cylinder connected to a delivery line of a pump, characterised in that the delivery line (3) is permanently connected to the outlet side of a first pump (1) and is selectively connected to the outlet of a second pump (17), a valve (18) being disposed between the outlet of the second pump (17) and the delivery line (3), which valve is switchable by a signal derived from the delivery pressure In the delivery line (3) from an operative position 8 C connecting the second pump (17) with the delivery line (3) to a short-circuit position connecting the second pump (17) with a tank line (11).

6. An industrial truck according to claim 5., characterised in that the valve (10; 18) is resiliently biased in the direction of the operative position, and has a control surface (13) which can be acted upon by pressure to urge the valve towards the short-circuit position.

7. An industrial truck according to claim 5 or claim 6, wherein a plurality of second pumps are connected via respective valves to the delivery line.

8. An industrial truck according to claim 7, wherein the respective valves of the second pumps are arranged to short-circuit their pumps at a respective predetermined pressures sensed in the delivery line.

9. An industrial truck substantially as described herein with reference to Figure 1 or Figure 2 of the accompanying drawings.

10. An industrial truck substantially as described herein with reference to Figure 3 of the accompanying drawings.