GB2316671A - Control apparatus for a lifting platform - Google Patents

Abstract

Automatic control apparatus to maintain a constant operator working height for a lifting platform 34 as the platform 34 is loaded. Platform raising and lowering is by a hydraulic cylinder 14, the cylinder's hydraulic system 16 being connected to resilient means such as an air bag 24. As the platform 34 is loaded, fluid in the hydraulic system 16 compresses the air bag 24, the platform 34 lowers and a constant operator working height is maintained. By varying the air bag pressure when the unloaded platform 34 is at the operator working height, the load at which the platform 34 reaches its lowest, unloading, height can be varied. Through the addition of a secondary cylinder 45 connected between the load cylinder 14 and the air bag 24, the load required for the unloading height to be reached can be further modified, as can the unloading height itself, by altering the bore and/or stroke of the secondary cylinder 45 relative to those of the load cylinder 14. Platforms having chain hoists (46, Fig. 3) as well as scissor arms are illustrated.

Description

LIFTING APPARATUS AND METHOD The present apparatus relates to lifting apparatus, and an associated method of lifting. More particularly, the invention relates to apparatus capable of raising or lowering a variable load.

Typically in the manufacturing industry including, for example, food processing plants and vegetable canning plants, mass produced goods flow off production lines. The goods are transferred onto pallets in order that they may be transferred to a warehouse for storage, dispatch etc using pallet trucks or forklift trucks.

At present in many factories, operators wait at the end of production lines and are required to stack heavy loads onto the pallet. As the pallet requires loading at floor level, this job requires a significant amount of bending and manual operators are prone to injury problems.

It is estimated that there are at least 110,000 manual handling injuries at work per year in the UK alone. This costs the UK between 110 and 130 million annually. In many countries, legal limits have been set for such operators regarding the maximum loads which may be lifted repeatedly.

This problem has been addressed to some extent by the use of scissor tables and stackers. Conventional stackers and scissor tables use hydraulic cylinders to power the raising/lowering mechanism. In the case of a scissor table, the cylinders work on the scissor arms or, in the case of a stacker, the cylinder pushes on the chain wheel to incrementally raise or lower the loading platform.

In these arrangements, the height of a loading platform, on which the pallet is mounted, is raised initially to the required loading height. As the pallet is loaded, the operator can lower the load platform manually, either by pulling a lever, or by pushing a button. In this way the loading height for the operator remains constant, and bending is reduced.

One drawback with this type of system is that adjustment of the load height is a manual operation, and the operator must break off from loading the pallet in order to lower the height of the load platform.

Automatic self-levelling system have been proposed in which the load platform is mounted on an arrangement of springs, or a similar resilient arrangement. The mounting is calibrated to move the load platform incrementally from an initial loading height to a desired off-loading height, for a given maximum load.

One of the problems with this apparatus is that although the lowering of the load platform is responsive to the weight of the load, a different spring arrangement must be used for each type of load in order that the pallet is in its fully lowered positioned when the pallet is fully loaded, and thus to provide the operator with a constant working height. In view of this, the same arrangement cannot readily be used for tasks requiring a wide variation in the nature and maximum weight of the load. The installation of a different spring arrangement is a time consuming operation, and requires the apparatus to be out of action whilst the modifications are made.

Furthermore, because the loading platform is resiliently mounted, when the loaded pallets are removed for transporting, the height of the load platform rises incrementally as the load is reduced. Thus the assistance of a stacker or fork lift is required to remove the pallets.

Additionally, the height control mechanism including the spring arrangement is located beneath the load platform. This has the effect that the lowest off-loading height position is limited by the bulk of the height control mechanism. As the load platform cannot be lowered to ground level, a fork lift truck, or similar apparatus must be employed to transport the loaded pallet. A fork lift truck is a relatively expensive item, and must be operated by a trained operator. Therefore the total cost of operating this self-levelling system is significantly greater than the more basic systems described above.

A need therefore exists for a constant rate lowering, or lifting, system which may be readily adapted for varying types of loads; and/or for a system which provides a self-levelling load height; and/or for such a system in which the off-load height may be varied.

It is an object of the present invention to provide improvements in relation to one or more problems discussed herein; and/or to provide improvements generally.

In one aspect of the present invention there is provided control apparatus for lifting apparatus having a load platform, wherein the control apparatus enables the platform to be incrementally lowered, or raised, between an initial position and a pre-determined off-loading position, the predetermined position being reached once the platform is fully loaded.

In another aspect of the invention there is provided control apparatus for lifting apparatus having a load platform, wherein the control apparatus is adapted to provide a constant working height for an operator, irrespective of the weight of the load to be lowered, or raised.

In a further aspect of the invention there is provided control apparatus for lifting apparatus in which the off-load height may be flush, or substantially flush, with the ground level.

According to the present invention there is provided control means for lifting apparatus comprising at least one primary hydraulic cylinder for moving a load platform, said control means including pressure variation means for control and/or variation of the hydraulic pressure within the system as the weight of the load on the load platform is varied.

As with conventional apparatus, the operating principle is to automatically lower (or raise) a load platform as the weight of the load thereon increases, thereby maintaining a constant loading height for the operator. By providing control means including pressure variation means according to the present invention, the effective hydraulic pressure within the primary cylinder may be varied according to the type of load to be lifted.

By increasing the hydraulic pressure within the system, the distance moved by the load platform when given weight is loaded thereon is reduced proportionally. Conversely, by decreasing the hydraulic pressure within the system, the distance the platform moves when a load of the same weight is added, is reduced.

It has been found that use of such control apparatus may be used to provide a self-levelling table or stacker in which a chosen rate of lowering of the load platform may be achieved between an initial height and a pre-determined off-loading height, and which provides a constant working height for an operator, irrespective of the weight of the load. Such apparatus has been found to be more versatile than existing lifting apparatus and to minimise operator injury potential, by eliminating bending.

Furthermore, because the height of the load platform is varied by controlling the hydraulic pressure within the lifting mechanism of the apparatus, the bulk of the control apparatus is greatly reduced, when compared to the prior art spring arrangement. This has the advantage that the control apparatus does not need to be positioned below the load platform, and the minimum off-loading height is not limited by the presence of the height control mechanism.

In one embodiment, the pressure variation means comprises resilient means capable of exerting, transmitting or accepting a variable force on, to or from the hydraulic fluid within the system. The resilient means may comprise an air bag, or similar arrangement, such as an accumulator, in which the air pressure may be increased or decreased.

In a further embodiment, the resilient means comprises a combination of springs and air bag/s.

The pressure variation means may communicate force to or from the hydraulic system through a secondary cylinder connected to the primary hydraulic cylinder(s) of the lifting apparatus.

The pressure variation means may exert a force, or work, on the secondary cylinder, which is in turn transmitted to the primary cylinder, and vice versa.

By varying the pressure at the top of the secondary cylinder, which in the case of the air bag is achieved by varying the air pressure within the bag, and in the case of springs is achieved by selecting different types or quantity, it is possible to make the lifting apparatus load sensitive over a range of loads.

For example, the load platform on a stacker with a particular pressure setting may be lowered progressively to reach a predetermined off-loading height as the weight of the load is added to a full load weight of 1000kg. By reducing the air pressure in the air bag, the same apparatus would operate in the same manner with a lighter load, still reaching the pre determined off-loading height when fully loaded with 150kg.

The advantage of this embodiment is that existing apparatus may be readily adapted to include control apparatus according to the present invention.

The load range of the apparatus may be varied by changing the cylinder bore and/or stroke of the secondary cylinder.

By varying the relative fluid capacities of the primary and secondary cylinders, the off-loading height of the apparatus for a given maximum load may be varied.

By selecting a fluid capacity of the secondary cylinder below that of the primary lift cylinder(s), an off-loading height above floor level may be obtained when the load platform is fully loaded. This has been found to aid movement of the loaded apparatus, when in the form of a stacking truck, and/or prevents perimeter off-load in the case of a scissor table.

In certain applications, an off-loading height of approximately 50mm from the floor is particularly convenient.

In other applications, it is desirable that the off-loading height is flush, or substantially flush with ground level. The loaded pallet may the be transported with a hand pallet truck, rather than requiring the use of a fork lift truck, with the attendant cost implications.

In a further aspect of the invention there is provided lifting, or lowering, apparatus including control apparatus of the previously described embodiments. The apparatus may comprise a scissor table, or alternatively may be in the form of a stacker.

It has been found that a table or stacker including control means of the present invention is the suitable for stacking cases of goods onto conventional pallets, as the self levelling capability reduces loading time and increases operator efficiency. The pallet may be loaded/unloaded onto the apparatus without, in many cases, the need for additional handling equipment.

Conversely, the apparatus may be used for unloading purposes.

A fully loaded pallet may be placed on the load platform, and as the load is reduced, the height of the platform rises incrementally to reach full height when the load is removed.

Embodiments of the invention will now be described, by way of example only, with reference to the following illustrative drawings in which: Figure 1 is a diagrammatic view of control apparatus according to the present invention; Figure 2 is a diagrammatic view, of lifting apparatus including control apparatus similar to that shown in Figure 1, the lifting apparatus comprising a scissor table; and Figure 3 is a diagrammatic view of lifting apparatus comprising a stacker.

Figure 1 illustrates the basic elements of the hydraulic lifting mechanism 10 of lifting apparatus. The lifting mechanism 10 comprises at least one primary hydraulic cylinder 12, including a reciprocable piston 14, which is connected by a hydraulic line 16 to provide an oil connection to a manual pump 18. The pump 18 provides the pressure within the system.

In the case of a scissor table (illustrated in more detail in Figure 2), the piston 14 works on the scissor arms of the lifting apparatus, or, in the case of a stacker (illustrated in more detail in Figure 3), the cylinder pushes on a chain wheel to incrementally raise or lower a load platform, in the conventional manner.

According to the present invention, the control apparatus includes pressure variation means 20 connected to the hydraulic lifting mechanism 10 by a secondary connection 22.

The pressure variation means 20 forms a levelling unit to adapt conventional apparatus and provide the feature of load sensitivity. The pressure variation means 20 comprises an air bag 24 connected, via safety valve 26, to an air inlet valve 28, which in turn is connected to an air supply (not shown).

The air pressure within the air bag may be varied using air valve 28, and the pressure may be monitored by pressure gauge 30.

The system 20 only needs to be connected to the external air supply for short periods of time to increase the pressure in the air bag 24 for higher loads. Generally, the apparatus is not connected to the air supply in use.

The pressure within the air bag 24 acts on the hydraulic fluid within connection 22 via action on cylinder 45, which in turn effects the pressure within the primary hydraulic arrangement 10, and vice versa. By control and variation of the pressure in the air bag 24, different total loads may be accommodated as explained in detail below.

As shown in Figure 2, the pressure variation means 20 of Figure 1 may be used in conjunction with a scissor table 32 having a load platform 34. In this figure, the hand pump 18 of Figure 1 has been replaced with a pump motor 36. The piston 14 works on the scissor arm arrangement 38 to incrementally raise or lower the load platform 34.

In use, an empty pallet is placed on the load platform 34 and raised, using lifting mechanism 10, to the required working height. The pallet is loaded by an operator, and as the weight on the platform increases, the hydraulic fluid within the connections 16 and 22 is compressed.

As the pallet is loaded, the increasing weight of the load tends to cause the piston 14 to retract within the cylinder 12 to lower the height of the load platform 34, such that the effective working height for the operator remains constant.

This lowering effect is opposed by force on the piston 14 due to the hydraulic fluid within the system, being under pressure by air bag 24.

In conventional apparatus, the hydraulic pressure within the system would need to be varied by manual alteration of the pressure within lifting mechanism 10, by variation of the setting of the pump motor 36 (or the hand pump 18 of Figure 1) However, in the present arrangement, as the weight of the load increases, the hydraulic fluid in connection 22 works via cylinder 45 on the resilient air bag 24, and tries to compress it. In this way the increased pressure within the system due to the increased weight of the load is "absorbed" by the air bag 24 which is compressed, and the piston 14 is able to retract into the cylinder 12 by a pre-determined amount, resulting in the automatic lowering of the load platform height.

The resistance provided by the pressure variation means 20, and thus the distance moved by piston 14, is determined by the initial air pressure within the bag 24. The higher the total load, the higher the setting of pressure in the air bag 24 needs to be in order to provide a constant working height for the operator.

As the weight of the load increases, the hydraulic pressure acting on the bag 24 from the hydraulic fluid forces the air bag 24 to become fully compressed or contracted. At this point, in the described embodiment, the height of the load platform 34 is at its lowest position. The initial pressure within the air bag 24 is chosen such that the air bag 24 is in its fully contracted state when the weight on the load platform 34 corresponds to a full load.

When the pallet is fully loaded, the pallet is held at approximately 125mm from the floor because the fluid volume in the cylinder 45 is less than the fluid volume in cylinder 14. The pressure within the system is then released using a lowering valve associated with the drive unit 36. The scissor table may then be off loaded by a conventional hand or motorised pallet truck. This is not possible with the springmounted prior art systems, in which the minimum off loading height is determined by the bulk of the control apparatus.

Thus, in apparatus including the levelling unit of the present invention, once the pressure within the system is released, the load platform remains stationary allowing for ready unloading. In contrast, in prior art spring-loaded platforms, the platform rises as the load is removed, and an additional stacker or fork lift truck is required to remove the pallets.

The load range of the apparatus can be varied by changing the cylinder bore and/or stroke of the cylinders, and/or capacity of the air bag. In addition, by varying the relative fluid capacities of the primary and secondary cylinders, the offloading height of the apparatus for a given maximum load may be varied.

Figure 3 illustrates the control apparatus 20 providing a levelling unit for a wheeled stacker 40 having a load platform 42.

In this embodiment, a chain hoist is provided to raise and lower the load platform 42. The piston 14 of the cylinder 12 carries a pulley 44 over which a chain 46, attached to platform 42, may pass. The levelling unit functions in the same manner as described in relation to Figure 2.

When the pallet is fully loaded, the stacker 40 holds it at approximately 50mm from the floor, and the stacker 40 may be wheeled to the stacking area. Once in place, the pressure in the system may be released, and the pallet lowered fully to the floor.

In this way the chain hoist, driven by the hydraulic piston 14 provides linear travel of the platform 42 irrespective of the load thereon.

In both of the described embodiments the role of the apparatus can be reversed and used for off loading purposes i.e. a fully loaded pallet can be placed on the load platform 34,42 with the system pressurised. As the load is removed, the load platform rises, and a constant working height is maintained.

Claims (15)

1. Control apparatus for lifting apparatus having a load platform, wherein the control apparatus enables the platform to be incrementally lowered, or raised, between an initial position and a pre-determined off-loading position, the predetermined position being reached once the platform is fully loaded.

2. Control apparatus for lifting apparatus having a load platform, wherein the control apparatus is adapted to provide a constant working height for an operator, irrespective of the weight of the load to be lowered, or raised.

3. Control apparatus according to claim 1 or claim 2 wherein the off-load height may be flush, or substantially flush, with the ground level.

4. Control apparatus according to any one of the preceding claims, said lifting apparatus comprising at least one primary hydraulic cylinder for moving a load platform; and said control means including pressure variation means for control and/or variation of the hydraulic pressure within the system as the weight of the load on the load platform is varied.

5. Control apparatus according to claim 4 wherein said pressure variation means comprises resilient means capable of exerting, transmitting or accepting a variable force on, to or from the hydraulic fluid within the system.

6. Control apparatus according to claim 5, wherein said resilient means comprises an air bag, or similar arrangement, such as an accumulator, in which the air pressure may be increased or decreased.

7. Control apparatus according to claims 5 or 6 wherein said resilient means comprises a combination of springs and air bag/s.

8. Control apparatus according to any one of claims 4 to 7 wherein said pressure variation means communicates force to or from the hydraulic system through a secondary cylinder connected to the primary hydraulic cylinder(s) of the lifting apparatus.

9. Control apparatus according to claim 8 wherein said pressure variation means is capable of exerting a force, or working, on the secondary cylinder, which is in turn transmitted to the primary cylinder, and vice versa.

10. Control apparatus according to claim 8 or claim 9 wherein load range of the apparatus may be varied by changing the cylinder bore and/or stroke of the secondary cylinder.

11. Control apparatus according to any one of claims 8 to 10 wherein the fluid capacity of the secondary cylinder is less than of the primary lift cylinder(s).

12. Control apparatus substantially as hereinbefore described with reference to the accompanying drawings.

13. Lifting, or lowering, apparatus including control apparatus according to any one of the preceding claims.

14. Apparatus according to claim 13 comprising a scissor table or a stacker.

15. Lifting or lowering apparatus as hereinbefore described with reference to the accompanying drawings.