GB2033968A

GB2033968A - Hydraulic Lifting Circuits

Info

Publication number: GB2033968A
Application number: GB7932648A
Authority: GB
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1978-09-25
Filing date: 1979-09-20
Publication date: 1980-05-29
Also published as: GB2033968B

Abstract

A hydraulic system for raising and lowering loads includes a pump (16) which is operable, preferably by an electric motor, to deliver fluid from a reservoir (18) to a remote jack (10) through a conduit (20) and a solenoid- operated check-valve (26) of an assembly (24) integral with jack (10) to effect raising of a load by the jack (10). The pump (16) is adapted to allow fluid to pass therethrough in the opposite sense and the system is arranged such that upon actuation of valve (26) fluid is returned from the jack (10) through a flow regulator (27) of assembly (24) and thence through the conduit (20) and the pump (16) to the reservoir (18) in order to lower the load. <IMAGE>

Description

SPECIFICATION Hydraulic Systems This invention relates to hydraulic systems and in particular to hydraulic systems for raising and lowering a load.

The invention is particularly applicable to hydraulic lifting systems which may be used in such applications as, for example, fork-lift trucks, scissor lifts and docking-bay ramps.

A further example of a lifting system to which this invention is applicable is a platform lifting system of the type which is mounted on a commercial vehicle for the purpose of raising and lowering loads to and from the deck of the vehicle to facilitate loading and unloading of the vehicle.

The platform of such a system is carried by a guide frame securely mounted on the vehicle and is raised and lowered with respect to the guide frame by means of a hydraulic arrangement which includes a hydraulic motor, such as a jack, that operates through a system of cables and pulleys so that movement of the piston of the jack out of and into the jack cylinder causes the platform to rise and descend respectively.

In known examples of this type of hydraulic lifting system it is common for the hydraulic motor to be mounted on the guide frame and for the hydraulic pump and its associated control apparatus to be located remote from the hydraulic motor, on, for example, the chassis of the vehicle for convenience of installation. The pump and control arrangement, which may comprise a modular pack, are coupled to the hydraulic motor by means of a hydraulic hose or conduit. In operation, hydraulic fluid is pumped under pressure from the reservoir through the hydraulic hose or conduit to the hydraulic motor to raise the platform. Upon lowering of the plafform, the fluid is returned through the hydraulic hose or conduit by the weight of platform and any load thereon to the control apparatus which diverts the fluid back into the reservoir.

It is one object of the present invention to provide an improved hydraulic system suitable for such platform lifting systems.

In the aforementioned platform lifting systems there is a possibility that in the event of a rupture or the like in the hydraulic hose or conduit and the subsequent loss of hydraulic fluid during raising and lowering of the platform, the platform could fall rapidly and cause damage or injury.

It is highly desirable from a safety point of view therefore that the hydraulic lifting system includes means substantially to prevent, or at least considerably reduce, the likelihood of this possibility from occurring in the event of hydraulic fluid leakage.

It is another object of the present invention to provide a hydraulic system suitable for such platform lifting systems that includes simple and effective safety means.

According to a first aspect of the present invention there is provided a hydraulic system for raising and lowering a load having a hydraulic motor for raising a said load in response to hydraulic fluid being supplied thereto from a reservoir, and a hydraulic pump connected to said hydraulic motor through a hydraulic fluid line, the said hydraulic pump being operable to pump hydraulic fluid therethrough in one direction from the reservoir to the hydraulic fluid line in order to effect raising of a said load by said hydraulic motor, and being adapted to allow hydraulic fluid to pass therethrough in the opposite direction, wherein the system is arranged such that hydraulic fluid is returned from the hydraulic motor through the said hydraulic fluid line and the hydraulic pump in said opposite direction to the reservoir in order to effect lowering of a said load.

The hydraulic pump may have an inlet and an outlet for hydraulic fluid in communication with the reservoir and the hydraulic fluid line respectively and is operable to draw hydraulic fluid into the said inlet and force the hydraulic fluid through the outlet, in which case the pump is preferably adapted to allow hydraulic fluid entering the outlet to pass therethrough to the inlet.

The hydraulic pump may be arranged to be driven in a first sense to deliver hydraulic fluid under pressure to the hydraulic fluid line and may be adapted to be driven in the opposite sense by hydraulic fluid returning from the hydraulic motor through the hydraulic fluid line.

The pump is preferably driven in the first sense by an electric motor. Clutch means may be disposed between the electric motor and the pump to inhibit rotation thereof whilst the pump is being driven in the opposite sense.

Alternatively, the electric motor may be arranged to drive the pump in the opposite sense to control the rate at which hydraulic fluid passes therethrough.

The hydraulic pump may comprise a part of a hydraulic unit which has a fluid port connected to the hydraulic fluid line. In this case, the unit may also include the reservoir and a pressure relief valve which is disposed between the pump and the hydraulic fluid line.

Preferably the system includes a hydraulic assembly disposed between said hydraulic fluid line and said hydraulic motor which has valve means for controlling the flow of hydraulic fluid to and from the said hydraulic motor. The valve means may have a first operative state in which it acts as a non-return valve to allow hydraulic fluid to flow from the hydraulic fluid line to the hydraulic motor and a second operative state in which it allows hydraulic fluid to flow from the hydraulic motor to the said hydraulic fluid line.

The valve means conveniently comprises a solenoid check valve which is energisable to change from its first state to its second state.

The hydraulic assembly may also include means, for example a flow regulator, for limiting the rate of flow of hydraulic fluid from the hydraulic motor, which preferably is disposed between the valve means and the hydraulic motor. The hydraulic assembly may be mounted on the hydraulic motor and in direct fluid communication with the hydraulic motor.

The hydraulic fluid line and hydraulic motor may comprise a conduit and a jack respectively.

According to a second aspect of the present invention, there is provided a vehicle including a hydraulic system in accordance with the first aspect of the present invention for use in loading and unloading the vehicle.

A hydraulic system for raising and lowering a load in accordance with the present invention and in the form of a platform lifting system for a commercial vehicle will now be described by way of example with reference to the accompanying drawing which is a schematic circuit diagram of the hydraulic system.

The hydraulic lifting system generally comprises a hydraulic jack mounted on the guide frame (not shown in the drawings) of a moveable platform arrangement carried on a commercial vehicle, whose piston is coupled to a system of cables and pulleys connected to the platform so that reciprocal movement of the piston out of and into the cylinder of the jack causes the platform to rise and descend respectively relative to the guide frame. The hydraulic jack is of the single-acting kind and hydraulic fluid is supplied to the jack to extend it from a hydraulic fluid reservoir by a pump which is disposed remote from the jack on the chassis of the vehicle.

Referring to Figure 1, the hydraulic lifting system includes a jack 10 having a cylinder 11 and a piston 12 mounted for slidable movement within the cylinder 11. A rod 1 3 carried by the piston 12 extends through an end of the cylinder 11 and is connected via cables to the moveable platform (not shown) on the vehicle. Hydraulic fluid is supplied to the jack 10 through a port 1 5 in the end of the cylinder 11.

The hydraulic circuit for the jack 10 includes a hydraulic gear pump 1 6 which is driven by an output shaft of electric motor 1 7 and acts to draw hydraulic fluid from a reservoir 1 8 and deliver the fluid to a conduit 20. A pressure relief valve 21 is connected between the output from the pump 1 6 and the conduit 20 to divert hydraulic fluid back to the reservoir 18 should the pressure of the fluid in the conduit 20 exceed a predetermined value.

The gear pump 1 6 is of generally conventional form and includes a pair of inter-meshing gears which are mounted in a chamber for rotation about respective axes. Upon rotation of the gears by the electric motor 17, hydraulic fluid is drawn into an inlet region of the pump 16 and forced through an outlet region. It has been found that some gear pumps, when not being driven by their electric motor, are capable of being driven in the opposite sense by hydraulic fluid supplied under pressure to the outlet region of the pump.More specifically, when hydraulic fluid is supplied under pressure to the outlet region of the pump and the electric motor is unenergised, the inter-meshing gears of the gear pump are driven by the fluid in a direction opposite to their normal direction of rotation, that is when being driven by the electric motor, and hydraulic fluid passes to the inlet region of the pump. The gear pump 16 which is employed in the hydraulic lifting system is one which is capable of being driven in the opposite sense in this manner. An example of such a pump is that supplied by Smiths Industries Hydraulics Company Limited under the reference G2.3.

The gear pump 16, motor 17, reservoir 18 and pressure-relief valve 21 conveniently comprise a modular pack, indicated by the dotted lines 22 in Figure 1, so that hydraulic fluid is delivered from the reservoir 1 8 to the conduit 20 without passing through external hydraulic hoses or conduits. The gear pump 16 and the relief valve 21 for example comprise block structures having respective surfaces in which their outlet and inlet ports are provided, the two structures being secured together such that the surfaces are clamped together with the outlet port of the pump 1 6 being aligned with the inlet port of the relief valve 21. In this way, the possibility of hydraulic fluid leakage occurring between the reservoir 1 8 and the conduit 20 is substantially reduced.

The modular pack 22 is mounted on the chassis of the vehicle and coupled via the conduit 20 to the remotely positioned jack 10.

The end of the conduit 20 remote from the modular pack 22 is connected to an inlet 23 of a hydraulic assembly 24 mounted on the bottom of the jack cylinder 11 so as to form an integral part of the jack 10. The outlet 25 of the assembly 24 communicates directly with the interior volume of the jack cylinder 11 through the port 1 5.

The assembly 24 comprises a solenoid check valve, shown schematically at 26, in series with a flow regulator 27. The check valve 26 is of the type which includes a poppet that is biassed by a spring against a valve seat so that in normal operation with the solenoid unenergised, hydraulic fluid can only flow from the conduit 20 to the flow regulator 27. Upon energisation of the solenoid, the poppet of the check valve 26 is lifted from its seat so that hydraulic fluid can flow from the regulator 27 to the conduit 20. The flow regulator 27 serves to limit the rate of flow of hydraulic fluid out of the jack cylinder 11.

Operation of the hydraulic lifting system will now be described, and for this it will be assumed that initially the piston 1 2 of the jack 10 is in its retracted position and that the check valve 26 of the hydraulic assembly 24 is in its normal position with the associated solenoid unenergised.

Upon the supply of electrical current to the motor 1 7 by manual actuation of an associated switch (not shown) the gear pump 1 6 draws hydraulic fluid from the reservoir 18 into its inlet region and impels that fluid through its outlet region to the conduit 20. Upon reaching the hydraulic assembly 24, the fluid opens the check valve 26 and flows through the flow regulator 27 into the jack cylinder 11 via the port 1 5. Hydraulic fluid entering the cylinder 11 extends the jack 10 and the rod 1 3 operates through the system of cables and pulleys as previously described to raise the platform with respect to its guide frame.

The guide frame may include a limit switch which is actuated by the platform to de-energise the electric motor 1 7 once the platform has reached a predetermined height. Alternatively, the electric motor 1 7 may be de-energised manually.

Upon deenergisation of the motor 1 7, and the subsequent loss of fluid flow in the conduit 20, the check valve 26 closes to prevent hydraulic fluid flowing from the jack cylinder 11 thereby to maintain the jack 10 in its extended position.

In the event of, for example, the platform being obstructed during the course of its travel or the piston 12 of the jack 10 reaching its maximum possible extension without the limit switch operating, fluid pressure in the conduit 20 increases, and, as a result, the pressure-relief valve 21 responds to allow hydraulic fluid to escape to the reservoir 1 8.

In order to lower the platform from its raised position, the solenoid of the check valve 26 in the assembly 24 is energised so that hydraulic fluid is allowed to flow therethrough in the reverse direction. The weight of the load on the platform, or simply the weight of the platform by itself if unloaded, urges the piston 12 into the cylinder 11 and forces hydraulic fluid out of the cylinder 11 through the port 1 5 and the flow regulator 27 back into the conduit 20, the regulator 27 serving to limit the rate at which fluid is allowed to escape from the cylinder 11 and therefore the rate of descent of the platform.

Since during the lowering mode of operation of the hydraulic system, the electric motor 1 7 remains unenergised, the pressure of hydraulic fluid in the conduit 20 and the outlet region of the pump 1 6 due to the weight of the platform acting on the piston 12 idly drives the gears of the pump 1 6 in their opposite direction so that hydraulic fluid passes through the pump 16 back into the reservoir 1 8. In this way, the hydraulic fluid which has been pumped from the reservoir 1 8 through the conduit 20 to the jack 10 during the raising operation of the platform is returned through the conduit 20 and the idling pump 16 to the reservoir 18 during the lowering operation of the platform.

A second limit switch may be provided on the guide frame and arranged to be actuated by the platform to denergise the solenoid of the check valve 26 once the platform has reached a lower predetermined position thereby to prevent further escape of the fluid from the cylinder 11.

Alternatively, de-energisation of the solenoid of the check valve 26 may be accomplished manually. The hydraulic system is then ready for its next raising operation.

In the event of a rupture or the like occurring in the conduit 20 or a power failure resulting in deenergisation of the motor 1 7 during a raising operation of the hydraulic lifting system, the check valve 26 of the hydraulic assembly 24 ensures that the jack is maintained at its current position. If a rupture or the like of the conduit 20 were to occur during a lowering operation, the flow regulator 27 of the assembly 24 would ensure that the platform is lowered at a controlled predetermined rate. The platform may be stopped at any time by de-energisation of the solenoid valve 26.

Furthermore, should a failure of the electrical power supply to the solenoid of the check valve 26 occur during a lowering operation of the hydraulic system, normal operation of the check valve 26 is automatically resumed in the circuit between the cylinder 11 and the conduit 20 so that the piston 12 of the jack 10 is maintained in the position reached immediately preceding the power failure.

In the embodiment described above, hydraulic fluid rotates, in idling manner, the inter-meshing gears of the gear pump 1 6 during the lowering operation of the platform. As a result of the reverse operation of the gear pump 16, the electric motor 1 7 is also turned in its opposite sense. The inertia of the armature of the electric motor 1 7 serves to control the rate at which the gear pump 1 6 is driven by the hydraulic fluid and therefore supplements the operation of the flow regulator 27 to control the rate at which the platform descends.

In some circumstances it may be undesirable to allow an electric motor which is designed to operate in one direction only to be rotated in the opposite direction if such an electric motor is used to drive the pump 16. To overcome this problem, a one-way clutch mechanism may be coupled between the output from the electric motor and the input of the pump 16 so that drive of the electric motor in the reverse direction is prevented.

It is envisaged that other types of pumps may be used in the hydraulic arrangement provided that the pump employed is capable of being driven in the reverse direction in response to pressure of hydraulic fluid at its output region so as to allow hydraulic fluid to pass from the conduit 20 through the pump to the reservoir 18 during the lowering operation.

In an alternative embodiment, the electric motor 1 7 may be a reversible electric motor in which case the motor 1,7 may be energised during the lowering operationsof the platform to drive the inter-meshing gears of the pump 16 in their opposite sense at a predetermined or variable speed to control the rate of descent of the platform.

Various modifications may be made to the embodiment described without exceeding the scope of the invention. For example, although the hydraulic system has been described with reference to the lifting and lowering of platforms carried by commercial vehicles, it is visualised that the system may be used for other lifting purposes. Furthermore, the jack may be of the double-acting kind with the appropriate switching valves incorporated in the hydraulic assembly 24.

The jack 10 may be replaced by other forms of hydraulic motors.

Claims

1. A hydraulic system for raising and lowering a load having a hydraulic motor for raising a said load in response to hydraulic fluid being supplied thereto from a reservoir, and a hydraulic pump connected to said hydraulic motor through a hydraulic fluid line, the said hydraulic pump being operable to pump hydraulic fluid therethrough in one direction from the reservoir to the hydraulic fluid line in order to effect raising of a said load by said hydraulic motor and being adapted to allow hydraulic fluid to pass therethrough in the opposite direction, wherein the system is arranged such that hydraulic fluid is returned from the hydraulic motor through the said hydraulic fluid line and the hydraulic pump in said opposite direction to the reservoir in order to effect lowering of a said load.

2. A hydraulic system according to Claim 1, wherein the said hydraulic pump has an inlet and an outlet for hydraulic fluid in communication with the reservoir and the hydraulic fluid line respectively and is operable to draw hydraulic fluid into the said inlet and force the hydraulic fluid through the said outlet, and wherein the pump is adapted to allow hydraulic fluid entering the said outlet to pass therethrough to the said inlet of the hydraulic pump.

3. A hydraulic system according to Claim 1 or Claim 2, wherein the said hydraulic pump is arranged to be driven in a first sense to deliver hydraulic fluid under pressure to said hydraulic fluid line from the reservoir and is adapted to be driven in the opposite sense by hydraulic fluid returning from the hydraulic motor through the hydraulic fluid line.

4. A hydraulic system according to Claim 3, wherein the said hydraulic pump is coupled to an electric motor which is energisable to drive the pump in the said first sense.

5. A hydraulic system according to Claim 4, wherein clutch means are disposed between the electric motor and the said pump to inhibit rotation of the electric motor whilst the pump is being driven in the opposite sense.

6. A hydraulic system according to Claim 4, wherein the electric motor is arranged to drive the hydraulic pump in the said opposite sense so as to control the rate at which hydraulic fluid passes therethrough from the hydraulic fluid line to the reservoir.

7. A hydraulic system according to any one of the preceding claims, wherein the said hydraulic pump comprises a gear pump.

8. A hydraulic system according to any one of the preceding claims, wherein said hydraulic pump comprises part of a hydraulic unit having a fluid port connected to said hydraulic fluid line.

9. A hydraulic system according to Claim 8, wherein said hydraulic unit includes a pressure relief valve disposed between the said hydraulic pump and the said port.

1 0. A hydraulic system according to Claim 8 or Claim 9, wherein said hydraulic unit includes said reservoir and wherein said reservoir is in direct communication with the hydraulic pump.

11. A hydraulic system according to any one of the preceding claims, including a hydraulic assembly disposed between said hydraulic fluid line and said hydraulic motor which has valve means for controlling the flow of hydraulic fluid to and from the said hydraulic motor.

12. A hydraulic system according to Claim 11, wherein said valve means has a first operative state in which it acts as a non-return valve to allow hydraulic fluid to flow from the hydraulic fluid line to the hydraulic motor and a second operative state in which it allows hydraulic fluid to flow from the hydraulic motor to the said hydraulic fluid line.

13. A hydraulic system according to Claim 12, wherein said valve means comprises electricallyoperable valve means which is adapted to change from its first operative state to its second operative state upon electrical energisation thereof, and to change from its second operative state to its first operative state upon deenergisation thereof.

14. A hydraulic system according to Claim 13, wherein said electrically-operable valve means comprises a solenoid-operated check valve.

1 5. A hydraulic system according to any one of Claims 11 to 14, wherein said hydraulic assembly further includes means for limiting the rate of flow of hydraulic fluid from the hydraulic motor to the hydraulic fluid line.

1 6. A hydraulic system according to Claim 15, wherein said flow-limiting means is disposed in the hydraulic fluid path between said valve means of the hydraulic assembly and the hydraulic motor.

1 7. A hydraulic system according to Claim 1 5 or Claim 1 6 wherein said flow-limiting means comprises a flow regulator.

18. A hydraulic system according to any one of Claims 11 to 17, wherein said hydraulic assembly is mounted on the hydraulic motor.

1 9. A hydraulic system according to Claim 18, wherein said hydraulic assembly is in direct fluid communication with the hydraulic motor.

20. A hydraulic system according to any one of the preceding claims, wherein the said hydraulic fluid line comprises a hydraulic fluid conduit.

21. A hydraulic system according to any one of the preceding claims, wherein the said hydraulic motor comprises a jack.

22. A hydraulic system for raising and lowering a load substantially as hereinbefore described with reference to the accompanying drawing.

23. A vehicle including an hydraulic system as claimed in any one of the preceding claims for use in loading and unloading the vehicle.