GB2353022A

GB2353022A - Pallet truck

Info

Publication number: GB2353022A
Application number: GB0014319A
Authority: GB
Inventors: Thomas Henry Chadwick
Original assignee: CHADWICK MATERIALS HANDLING LT
Current assignee: CHADWICK MATERIALS HANDLING LT
Priority date: 1999-06-11
Filing date: 2000-06-12
Publication date: 2001-02-14
Also published as: GB9913699D0; GB0014319D0

Abstract

A hand operated pallet truck (1, Fig.1) includes lifting forks (4) raisable by a hydraulically operated lifting cylinder (6) to which hydraulic fluid is fed by a pump (5) operable by a pumping handle (2). During a first phase of lifting a load with the forks (4) not yet engaging the load, the handle (2) is effective throughout its range of movement to raise the forks. During a second phase of lifting a load, however, the handle (2) automatically becomes ineffective to raise the forks (4) throughout an upper part of its range of movement. Thus movement of the handle (2) within its upper range of movement is easily accomplished during the second phase of lifting the load. In one embodiment (Fig 3A and 4), a dual area piston 9 and cylinder arrangement 18 having a passage 19 leading to the lifting cylinder has a bypass passage 16 leading to two one way valves 23,24 between it and the reservoir connected at 21. When the lifting cylinder is pressurised (load engaged), lowering of piston 9 opens one way valve 24 to vent the pressure back to the reservoir. Only after cylinder part 17 seals with cylinder stub 18 is the lifting cylinder further pressured to thereafter further raise the forks (4). In a further embodiment, Figs 5 and 6, a wedge shaped piston 9 coacts with a seal (27) and an inner spring loaded piston part (29) so as for cylinder pressure to raise the inner piston during downward movement, once the load is engaged, until seal (27) becomes effective to only thereafter further raise the forks (4) and load.

Description

2353022 Pallet Truck This invention relates to hand-operated pallet

trucks.

Hand-operated pallet trucks are well known pieces of equipment used for the movement of loads. Such loads may be conveniently arranged on a pallet and the pallet then lifted to move the load. In some cases, the load is not arranged on a pallet but the load incorporates an arrangement for the forks of the pallet truck to enter.

The forks of the pallet truck are manoeuvred beneath the load to be lifted and the pumping handle of the truck is pumped up and down about an arcuate path to raise the forks. The forks are raised to lift the pallet a sufficient distance above the ground so that the pallet may be moved on the truck. Conventionally, the part of the pallet engaged by the forks is between 95 and 120 mm off the ground when the pallet is resting on the ground. The truck is provided with wheels and the pumping handle is used to pull, push and steer the load into the desired position. The carrying capacity of the trucks can range between 1000 kg and 3000 kg. The usual capacity of the trucks is about 2000 kg.

In the usual operating procedure, the forks of the truck begin at a rest height above the ground of about 85 mm. The forks at that height are pushed under the load to be lifted and the handle is pumped to raise the forks. For the f orks to be easily located under the load, the base of the load to be lifted is a few centimetres above the rest height of the forks and the forks are lifted from their rest height until they begin to lift the load from the ground. The pumping of the forks during that first stage of lifting takes little effort. In the second stage of lifting, the forks are raised to a height of approximately 200 mm from the ground. During the second stage of lifting, the load is being raised from the ground and the pumping requires a much greater effort.

Ideally, the forks would be lifted quickly during a first stage, for example using only part of a first pumping stroke, or one or two pumping strokes, to raise the forks from their rest height of 85 mm. to a height at which the load begins to be lifted from the ground. Once the forks have begun to lift the load, it would be desirable for the forks to be lifted at a slower lifting speed so that several pump strokes are required to lift the load.

Various quick-lift systems have been used in the past but these have tended to involve complicated mechanisms, to be costly and/or not to provide a system that can be operated easily and efficiently during both the rapid lifting phase and the load-lifting phase.

A further problem with a conventional pallet truck arises from the difficulty of manoeuvring the truck when it is loaded. The handle of the truck is conventionally left in an upright position (and indeed is usually biased into that position). If the truck is left in a mode where pumping of the handle serves to raise the forks, then it may be very difficult to move the handle from its upright position, especially as from an ergonomic point of view it is difficult for an operator to apply much torque to the handle in that position. Consequently trucks are provided with a control lever that enables the hydraulic circuit to be changed so that the pump and the reservoir are in free fluid communication with one another. It is then an easy matter to move the handle for the purpose of manoeuvring the truck.

Such an arrangement has the disadvantage that an operator has to move the control lever to effect the adjustment of the hydraulic circuit and also there is an extra mechanism in the truck which may require servicing 5 and/or adjustment at intervals.

It is an object of the invention to provide a pallet truck which overcomes or mitigates at least some of the disadvantages described above.

According to a first aspect of the invention there is provided a hand operated pallet truck including lifting forks raisable by a hydraulically operated lifting cylinder to which hydraulic fluid is fed by a pump operable by a pumping handle, wherein, when the pump is operated by the pumping handle during a first phase of lifting a load and the forks have not engaged the load, the handle is effective throughout at least most of its range of movement to raise the forks and when the pumping handle is moved during a second phase of lifting the load and the forks have engaged the load, the handle automatically becomes ineffective throughout an upper part of its range of movement to raise the forks, whereby movement of the handle within its upper range of movement is easily accomplished.

By arranging the pallet truck as defined in the paragraph immediately above, it becomes possible to provide an arrangement in which, during a first phase of operation the pumping handle is moved easily because there is no load to be raised as the forks are raised and during a second phase of operation the handle remains easy to move over at least an upper part of its range of movement. Consequently it becomes possible to provide a truck in which the handle is readily movable even after engagement by the forks of a load.

Preferably during the first phase of lifting fluid displaced by a piston/cylinder pumping arrangement is passed along a first path into the lifting cylinder and during the second phase of lifting, during the upper part of the range of movement of the handle, fluid displaced by the piston/cylinder pumping arrangement is passed along a second path and does not enter the lifting cylinder.

In an embodiment of the invention fluid passed along the second path returns to the reservoir but another option, described in a second illustrated embodiment of the invention, is for fluid passed along the second path to enter a separate storage cavity, preferably a cavity in the piston.

Preferably the flow of the fluid along the second path is blocked until the pressure of fluid rises sufficiently to overcome an opposing resilient bias.

Preferably the pumping handle is movable from a generally upright position to a generally prone position and during the second phase of lifting of the load the handle is effective to raise the forks only when the handle is in a region close to the generally prone position. That is especially advantageous from an ergonomic point of view since it is much easier to apply the necessary force to the handle to effect pumping when the handle is closer to its prone position than its upright position.

Preferably, during the second phase of lifting the load, the handle is effective to raise the forks only over less than one third of the range of movement of the handle.

Preferably the handle is pivotably mounted for pivotal movement about a horizontal axis.

Also according to a first aspect of the invention there is provided a hand operated pallet truck including lifting forks raisable by a hydraulically operated lifting cylinder to which hydraulic fluid is fed by a pump operable by a pumping handle, wherein, when the pump is operated by the pumping handle during a first phase of lifting a load and the forks have not engaged the load, hydraulic fluid displaced by a piston/cylinder pumping arrangement is passed along a first path into the lifting cylinder to raise the forks and, when the pump is operated by the pumping handle during a second phase of lifting the load and the forks have engaged the load, hydraulic fluid displaced by the piston/cylinder pumping arrangement is diverted from the first path along a second path and does not enter the lifting cylinder.

In embodiments of the invention described below the transfer of the operation of the pumping handle from the first phase of lifting to the second phase of lifting is automatic and it is preferably a result of the increased pressure of the hydraulic fluid generated by the increased pressure required in the lifting cylinder to raise the forks. The truck may include a pressure operated valve arranged to be openable to allow flow of hydraulic fluid along the second path from the pump when the pressure of hydraulic fluid in the piston/cylinder pumping arrangement exceeds a predetermined level. The provision of such a pressure operated valve may provide a very simple way of diverting the hydraulic fluid flow from the first path to the second path.

A further valve may be provided for allowing hydraulic fluid to enter the piston/cylinder pumping arrangement when the pressure therein is reduced. The further valve and the pressure operated valve may be connected in parallel with one another providing two parallel paths in the same fluid passage. The further valve may be a one-way valve, placed in parallel to the pressure operated valve in the second path, and allowing flow of fluid only into the piston/cylinder pumping arrangement.

The pallet truck will conventionally include a hydraulic fluid reservoir and hydraulic fluid diverted from the first path along the second path may enter the reservoir.

An optional feature of the invention employed in a first, but not a second, embodiment according to the first aspect of the invention is that the truck further includes, in addition to the first-mentioned piston/cylinder arrangement which is of a relatively large cross-section, a relatively small cross-section piston/cylinder pumping arrangement, and, when the pump is operated by the pumping handle during the second phase of lifting a load and the forks have engaged the load, hydraulic fluid is displaced by the relatively small cross-section piston/cylinder pumping arrangement and passed into the lifting cylinder to raise the forks. In that case, the continued operation of the pumping handle during the second phase of lifting can be arranged to have the effect or raising the forks, but at a slower rate than during the first phase of lifting. Because the piston/cylinder pumping arrangement during the second phase of lifting is of relatively small cross-section, the force that has to be applied to the pumping handle to generate a given lifting force is relatively small.

In principle, it is possible to provide the small and large cross-section piston/cylinder pumping arrangements separately from one another; it is preferred, however, that the relatively large cross-section piston/cylinder pumping arrangement and the relatively small cross-section piston/cylinder pumping arrangement are provided by the same pair of relatively displaceable members. Whilst other arrangements are of course possible, it is preferred that one member provides the cylinder of the relatively large cross-section piston/cylinder pumping arrangement and the piston of the relatively small cross-section piston/cylinder pumping arrangement and the other member provides the piston of the relatively large cross-section piston/cylinder pumping arrangement and the cylinder of the relatively small cross- section piston/cylinder pumping arrangement. Preferably the relatively large crosssection piston/cylinder pumping arrangement and the relatively small cross-section piston/cylinder pumping arrangement are arranged substantially co-axially.

Preferably the relatively small cross-section piston/cylinder pumping arrangement is operable over a part only of the range of movement of the pumping handle.

Preferably the relatively small cross-section pumping arrangement is operable over less than a third of the range of movement of the pumping handle; in an embodiment of the invention described below the relatively small cross-section pumping arrangement is operable over about a quarter of the range of movement of the pumping handle. The pumping handle will usually be movable from a generally upright position to a generally prone position and in that case the relatively small cross- section piston/cylinder pumping arrangement is preferably operable only when the handle is in a region close to the generally prone position. That is especially advantageous because, from an ergonomic point of view, it is much easier to apply the necessary force to the handle to effect pumping when the handle is closer to its prone position than its upright position.

It should be understood that the relatively large cross-section piston/cylinder pumping arrangement may be operable over substantially all of the range of movement of the pumping handle, including therefore the range of movement of the handle over which the relatively small cross-section piston/cylinder pumping arrangement is operable.

It will be understood that the terms "relatively large" cross-section and "relatively small" cross-section are used simply to indicate comparative sizes rather than any absolute size. Thus the relatively large crosssection piston/cylinder pumping arrangement operates over a larger cross-sectional area than the relatively small cross-section piston/cylinder pumping arrangement but that area is not necessarily especially large for a piston/cylinder pumping arrangement on a pallet truck. Preferably the relatively large cross-section piston/cylinder pumping arrangement operates over a cross- sectional area more than four times greater than the cross-sectional area over which the relatively small cross-section piston/cylinder pumping arrangement operates.

As will already have been understood the flow diverting arrangement of the first aspect of the invention may be employed with the relatively large and relatively small cross-section piston/cylinder pumping arrangements but can also be used with a single piston/cylinder arrangement. It is also possible, according to a second aspect of the invention to provide a hand operated pallet truck including lifting forks raisable by a hydraulically operated lifting cylinder to which hydraulic fluid is fed by a pump operable by a pumping handle, the pump including a relatively large cross-section piston/cylinder pumping arrangement and a relatively small cross-section piston/cylinder pumping arrangement, each of the arrangements being operably connected to the pumping handle. In that case the flow diverting arrangement of the first aspect of the invention is preferably provided.

The pallet truck according to the second aspect of the invention may include any of the features defined above with respect to the pallet truck of the first aspect of the invention.

The invention further provides a method of lifting a load using a hand operated pallet truck as defined above.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawings, of which:

Fig. I is a perspective view of part of a hand operated pallet truck and shows the handle of the truck in the starting position; Fig. 2 is a perspective view of part of the pallet truck of Fig. 1, showing the handle of the truck in a partly lowered position; Figs. 3A to 3F are diagrams of the piston and cylinder pumping arrangement of the pallet truck at various stages during raising of a load in a first embodiment of the invention; Fig. 4 is a sectional view of a valve assembly incorporated in the pallet truck in the first embodiment of the invention; Fig. 5 is a sectional view of a pump arrangement in a second embodiment of the invention; and Fig. 6 is a sectional view to a larger scale of a piston of the pump arrangement of Fig. 5.

Figs. 1 and 2 show a hand pallet truck 1 having a handle 2 and a raisable forks frame 3 including forks 4 (only parts of which are shown), a pump arrangement 5 and a lift cylinder 6. Figure 1 shows the truck with the handle 2 in an upright starting position. The lifting forks 4 are lowered and are positioned beneath a load to be lifted. The handle 2 can then be pulled down in an arcuate movement (anticlockwise as viewed in Fig. 1) through an angle of up to about 90' and the lifting forks 4 thereby raised. The handle 2 is pumped up and down between the upright and the lower positions to raise the forks 4.

In both embodiments of the invention described below, the operation of the lifting mechanism is in some respects of conventional design. The handle 2 is pivotally mounted at 7 and each downward stroke of the handle 2 causes a roller (not shown) at the bottom of the handle to depress a pump piston 9 of the pump arrangement 5. When the downward stroke of the handle 2 is complete, the handle 2 is allowed to return to the upright position aided by a return spring (not shown) and the pump piston 9 is urged upwards by the return spring. The downward strokes of the handle 2 force hydraulic fluid into a lifting chamber in the lift cylinder 6 which causes a lift piston 11 to rise.

The rising of the lift piston 11 causes the forks 3 to rise.

In a conventional known hand pallet truck, each full downward movement of the handle 2 causes a given amount of hydraulic fluid to be transferred from the pump arrangement into the lifting cylinder, causing the forks to be lifted by a given amount, typically 9mm. Flow of hydraulic fluid into the lift cylinder is via a check valve which allows fluid to enter the lift cylinder but does not normally allow fluid to flow in the reverse direction. Similarly flow of hydraulic fluid into the pump arrangement is from a fluid reservoir via a check valve which allows fluid to leave the reservoir but does not normally allow return flow. The fluid reservoir is conveniently provided in the same casing as the lift cylinder. The handle 2 of the pallet truck is provided with a lever 12 which can be moved from a first position to a second position in which it acts on the check valve associated with the fluid reservoir and to a third position in which it also acts on the check valve associated with the lift cylinder. In the second position of the lever 12, fluid is free to flow to and from the reservoir as the pump arrangement 5 is actuated by the handle 2; thus movement of the handle is relatively free and does not have a pumping action; that state may be referred to as a neutral condition. In the third position of the lever 12, fluid is free to flow out of the lift cylinder back to the reservoir, allowing the forks 3 to drop under the weight of a load and their own weight.

In both embodiments of the invention described by way of example with reference to the drawings, somewhat different hydraulic arrangements are adopted, as will now be described.

Figs. 3A to 3F and Fig. 4 show a construction of the pump arrangement 5 for a first embodiment of the invention. The arrangement comprises a main cylinder 15 and a main piston 9. At the bottom of the main cylinder is an annular port 16. Also, and unconventionally, the main piston 9 is provided with a cylindrical cavity 17 extending upwards from its bottom face and the bottom of the main cylinder 15 is provided with a corresponding upstanding cylindrical projection 18. The projection 18 and the cavity 17 together define a smaller piston 18 and cylinder 17 arrangement co-axial with the larger piston 9 and cylinder 15 arrangement. An "0" ring may be provided in a groove towards the bottom of the smaller cylinder 17 5 to provide a seal between the piston 18 and the cylinder 17. A passageway 19 passes through the smaller piston 18.

Fig. 4 shows an additional valve assembly 20 that is provided in the first embodiment in a passageway that connects the annular port 16 directly to the hydraulic fluid reservoir. The valve assembly may be provided within the body of the pumping and lifting arrangement or as a separate component. The valve assembly 20 has a first port 21 connected to the reservoir and a second port 22 connected to the annular port 16. The valve assembly 20 contains two valves provided in parallel to one another: firstly there is a valve 23 comprising a ball valve member 23A and a spring 23B which prevents flow from the port 22 to the port 21 and then to the reservoir, but allows flow from the port 21 to the port 22 and then into the main cylinder 15 of the pump as soon as the presence for such flow is sufficient to overcome the force of spring 23B; secondly there is a valve 24 comprising a ball valve member 24A and a spring 24B which prevents flow from the port 21 to the port 22 and then into the main cylinder 15, but allows flow from the port 22 to the port 21 and then to the reservoir when the pressure for such flow is sufficient to overcome the force of spring 24B.

Whilst the force of spring 23B is not critical and is preferably as low as proves practical, the force of spring 24B is significant and affects the operation of the truck, as will be described below.

The operation of the truck will now be described, starting from the position in which the truck has been placed with its forks in a pallet to be lifted but prior to raising of the forks and with the handle 2 in an upright position.

The operator exerts pressure on the handle 2 to move the handle in a downward arcuate movement. As that movement begins the pumping arrangement 5 of the pumping unit is in the position shown in Fig. 3A. Arcuate movement of the handle 2 f irst moves the main piston 9 down in the main cylinder 15 to the position shown in Fig. 3B displacing hydraulic fluid from the main cylinder through the passageway 19 in the smaller piston 18 and past the conventional check valve into the lifting cylinder 6. thereby raising the forks which at this stage are not carrying any load. The force required to lift the forks is relatively small and consequently the pressure of the hydraulic fluid is relatively small during this phase of the operation with the result that the ball valve member 24A remains held in its closed position by the spring 24B and there is no flow through the port 16.

As the operator continues to move the handle 2 down towards its lowermost position the smaller piston 18 enters the smaller cylinder 17 as shown in Fig. 3C. In that condition, the pumping arrangement becomes a much smaller piston/cylinder arrangement comprising the piston 18 and cylinder 17 and therefore the amount of fluid entering the lifting cylinder 6 for a given downward movement of the main piston 9 becomes much smaller; put another way, the ratio of the area of the piston that is pumping to the area of the piston being lifted in the lifting cylinder 6 becomes much smaller. Consequently, the amount by which the forks 3 are raised for a given movement of the pumping handle 2 becomes much smaller.

As can be seen in Fig. 3C, during the f inal movement of the handle 2, hydraulic fluid at the bottom of the main cylinder 15 around the smaller piston 18 has to be displaced through the port 16. That occurs by the pressure of the fluid in the cylinder 15 rising until it is sufficient to press the valve member 24A against the bias of the spring 24B thereby opening the valve 24 and allowing the fluid to be returned to the reservoir. The bias of the spring 24B is arranged such that the valve opens at a stage where there is still only a relatively small force required to move the pumping handle, but that force is greater than the force required simply to raise the forks 4 when they are not loaded.

As the operator next begins to raise the handle, the situation in Fig. 3D is reached. To allow the main piston 9 to be raised under the action of the return spring, hydraulic fluid flows from the reservoir through the ball valve 23 and the port 16 into the main cylinder. Also hydraulic fluid flows from the reservoir past the check valve preventing return flow into the reservoir and through the passageway 19 into the small cylinder 17. Those flows continue until the raising of the handle 2 is stopped. As the handle 2 is then lowered, as shown in Fig. 3E, the parts assume the same condition as already described with reference to Fig. 3B and the forks 4 are again raised.

At some stage, the forks 4 are raised sufficiently far that they engage the load and there is then a much greater resistance to raising the forks 4. The stage at which this happens is of course dependent on the design of the hydraulic arrangement and certain physical dimensions of the truck and the load being raised. For the purposes of this description it will be assumed that the condition - is - is reached at the stage shown in Fig. 3E, that being a fairly typical stage (after one and a half downward strokes of the handle 2), but it should be understood that the condition may be reached sooner (for example, as early as after one half of one downward stroke) or later without affecting the usefulness of the invention.

Once the resistance to raising the forks 4 increases, the downward pressure exerted by an operator on the handle 2 is increased and therefore the pressure of hydraulic fluid in the main cylinder 15 is increased. Unless the load being lifted is unusually light, the pressure of the hydraulic fluid in the main cylinder 15 reaches a sufficient level that the ball valve 24 opens; the next downward movement of the handle is then operative to lower the main piston 9 and hydraulic fluid is driven from the main cylinder 15 through the port 16 and the ball valve 24 into the reservoir. Thus, at this stage the forks are not raised by downward movement of the handle 2.

Upon further downward movement of the handle 2 the smaller piston 18 enters the smaller cylinder 17 as shown in Fig. 3F. At that stage hydraulic fluid within the cylinder 17 is forced through the passageway 19 into the lifting cylinder 6, as previously described with reference to Fig. 3C, and the forks 4 are raised. Even though there may be a substantial load on the forks 4, the raising of the forks by the smaller piston/cylinder arrangement requires relatively little downward pressure of the handle 2.

Continued raising of the forks 4 is achieved by further reciprocating movement of the handle 2 but it will be appreciated that there is no need for the full range of movement of the handle to be used since it is only during the lowermost part of the movement (the last quarter of the arcuate movement of the handle 2), during which the smaller piston and cylinder arrangement is in operation, that the forks 4 are raised. Movement of the handle 2 in this lower region is much easier for an operator from an ergonomic point of view. A greater range of movement of the pumping handle 2 is not in any way harmful but merely results in hydraulic fluid in the main cylinder 15 flowing via the ball valve 24 into the reservoir and from the reservoir via the ball valve 23 into the main cylinder.

It should be noted that even once the load has been raised and is being carried by the forks 4, it still requires relatively little force to move the pumping handle over its range of movement. This is highly advantageous in facilitating the manoeuvring of the pallet truck when it is carrying a load. As described above, in a conventional truck the lever 12 is advantageously provided with a second position (neutral condition) in which fluid is free to move between the pumping arrangement and the reservoir so that the handle can easily be moved, but in the example of the invention just described that is not necessary.

When it is desired to lower the forks 4, that is carried out in the conventional way by moving the lever 12 to what is referred to above as the third position, in which fluid is free to flow out of the lifting cylinder past the check valve that normally blocks such flow and into the reservoir. Thus the lever 12 of the truck illustrated in the drawings can simply have two positions: one usual position in which the lifting operation described above takes place and the lever 12 does not influence the operation of any valve; and another "lowering" position in which the check valve that normally prevents hydraulic fluid flow out of the lifting cylinder 6 is displaced so as to allow such flow. Because the lever 12 is now required only to control the check valve associated with the lifting cylinder, the position of that check valve can be altered to suit other considerations.

Figs. 5 and 6 show a construction of the pump arrangement 5 for a second embodiment of the invention (in this case there is no additional valve assembly of the kind shown in Fig. 4). The pump arrangement 5 comprises a main cylinder 15 and a main piston 9. At the bottom of the main cylinder 15 is a passageway 25. An upper seal 26 and a lower seal 27 are provided in the interior of the cylinder 15 to provide seals between the main piston 9 and the cylinder 15. The piston 9 shown in Figs. 5 and 6 is of special construction and

incorporates within it a cylindrical cavity 28 in which an inner piston 29 with an associated seal 29a is mounted. An air chamber is defined in the cavity 28 above the inner piston 29 and an air vent 30 vents the air chamber to the atmosphere. A compression spring 31 in the cavity 28 biases the inner piston downwards against a raised portion 32 of a plug fixed to and forming an integral part of the body of the piston 9. Passages 33 provide a path of fluid communication between the exterior of the main piston 9 and the part of the cavity 28 below the inner piston 29. The limit of upward movement of the piston 29 is defined by a stop 34 at the top of the cavity 28.

The operation of the pump arrangement of Figs. 5 and 6 is generally similar to that of the first embodiment of the invention already described and therefore, in the description below, it is the differences in operation that will mainly be described.

Starting from the position in which the truck has been placed with its forks in a pallet to be lifted but prior to raising of the forks, an operator exerts pressure on the handle 2 to move the handle in a downward arcuate movement. That moves the main piston 9 down in the main cylinder 15; assuming the handle has started in a fairly upright position, the piston 9 will be above the seal 26 and therefore only the seal 27 is effective. While there is no load on the forks, however, the pressure of the fluid in the cylinder 15 remains relatively low and the pressure of that fluid is insufficient to lift the inner piston 29 against the bias of the spring 31. Consequently the piston 9 operates as a conventional solid piston and, as it moves down the main cylinder 15, fluid is expelled from the cylinder through the passageway 25 to the lifting cylinder, thereby raising the forks. On the return upward movement of the handle, fluid is drawn from the reservoir into the main cylinder 15. Control of the flow of fluid between the cylinder 15, the lifting cylinder 6 and the reservoir is achieved via a valve arrangement as on a conventional pallet truck. In this case the valve shown in Fig. 4 is not employed.

When the forks are raised sufficiently to encounter a load, the pressure of the hydraulic fluid in the cylinder 15 rises during downward movement of the piston. The operation of the pump then depends upon whether the main piston 9 is above or below the position at which it comes into sealing engagement with the lower seal 27: if it is below that position (at the bottom end of its range of movement) then the piston 9 and cylinder 15 act in a conventional manner and fluid is expelled through the passageway 25 and enters the lifting cylinder 6 as the piston moves down; if, however, the piston 9 is above that position, then the increased pressure of the hydraulic fluid is sufficient to raise the inner piston 29 against the bias of the spring 31 and fluid passes through the passages 33 into the part of the cavity 28 below the inner piston 29 instead of being expelled through the passageway 25. Thus during downward movement of the handle 2 in the upper portion of its range of movement, the forks are not raised and relatively little force is required to lower the handle, whilst during downward movement of the handle in the lower portion of its range of movement, a greater force is required to lower the handle.

During return upward movement of the piston 9 from its extreme bottom position, fluid is first drawn into the cylinder 15 through the passageway 25 from the reservoir but once the piston 9 has risen to a height where the lower seal 27 is ineffective fluid in the cavity 28 is driven back out of the piston 9 under the action of the spring 31 and the higher fluid pressure in the cavity 28.

Thus the pump arrangement of Figs. 5 and 6 provides a lifting effect over the entire range of movement of the handle 2 whilst there is no significant load on the forks and then, when a load is applied, automatically converts to a state where the handle is easily movable over all of an upper range of movement (and is not effective over that range of movement to raise the forks) and requires a greater force to be moved over a lower range of movement where the handle remains effective to raise the forks. The force required to lower the handle and raise the forks when the forks are loaded is greater but it is applied only when the handle is approaching a prone position, where a large force can be applied by an operator much more easily.

Thus, it will be seen that a pallet truck according to either of the described embodiments can provide an operator with the following advantages:

(i) a first rapid lifting phase that automatically ends when the forks encounter a load to be lifted; (ii) a second lifting phase that can be carried out with the effort exerted when the pumping handle is in more of a prone position than an upright position; the lower position of the pumping handle during pumping to lift a load is highly advantageous from an ergonomic point of view; (iii) the automatic facility during or at the end of the lifting process to move the pumping handle along its arc of movement with very little resistance, even when a heavy load is being carried by the forks, thereby facilitating manoeuvring of the truck.

Claims

Claims

1. A hand operated pallet truck including lifting forks raisable by a hydraulically operated lifting cylinder to which hydraulic fluid is fed by a pump operable by a pumping handle, wherein, when the pump is operated by the pumping handle during a first phase of lifting a load and the forks have not engaged the load, the handle is effective throughout at least most of its range of movement to raise the forks and when the pumping handle is moved during a second phase of lifting the load and the forks have engaged the load, the handle automatically becomes ineffective throughout an upper part of its range of movement to raise the forks, whereby movement of the handle within its upper range of movement is easily accomplished. 2. A pallet truck according to claim 1, in which during the first phase of lifting fluid displaced by a piston/cylinder pumping arrangement is passed along a first path into the lifting cylinder and during the second phase of lifting, during the upper part of the range of movement of the handle, fluid displaced by the piston/cylinder pumping arrangement is passed along a second path and does not enter the lifting cylinder. 3. A pallet truck according to claim 2, in which fluid passed along the second path enters a cavity in the piston. 4. A pallet truck according to claim 3, in which f low of the fluid along the second path is blocked until the pressure of fluid rises sufficiently to overcome an opposing resilient bias. S. A pallet truck according to any preceding claim, in which the pumping handle is movable from a generally upright position to a generally prone position and during the second phase of lifting of the load the handle is effective to raise the forks only when the handle is in a region close to the generally prone position. 6. A pallet truck according to any preceding claim, in which during the second phase of lifting of the load the handle is effective to raise the forks only over less than one third of the range of movement of the handle. 7. A pallet truck according to any preceding claim in which the handle is pivotably mounted for pivotal movement about a horizontal axis. 8. A hand operated pallet truck including lifting forks raisable by a hydraulically operated lifting cylinder to which hydraulic fluid is fed by a pump operable by a pumping handle, wherein, when the pump is operated by the pumping handle during a first phase of lifting a load and the forks have not engaged the load, hydraulic fluid displaced by a piston/cylinder pumping arrangement is passed along a first path into the lifting cylinder to raise the forks and, when the pump is operated by the pumping handle during a second phase of lifting the load and the forks have engaged the load, hydraulic fluid displaced by the piston/cylinder pumping arrangement is diverted from the first path along a second path and does not enter the lifting cylinder.

9. A pallet truck according to claim 8, including a pressure operated valve arranged to be openable to allow flow of hydraulic fluid along the second path from the pump when the pressure of hydraulic fluid in the piston/cylinder pumping arrangement exceeds a predetermined level. 10. A pallet truck according to claim 9, in which a further valve is provided for allowing hydraulic fluid to enter the piston/cylinder pumping arrangement when the pressure therein is reduced. 11. A pallet truck according to claim 10, in which the further valve and the pressure operated valve are connected in parallel with one another in the same fluid passage. 12. A pallet truck according to claim 8 or 9, in which the pallet truck includes a hydraulic fluid reservoir and hydraulic fluid diverted along the second path enters the reservoir. 13. A pallet truck according to any of claims 8 to 12, further including, in addition to the first-mentioned piston/cylinder pumping arrangement which is of a relatively large cross-section, a relatively small cross- section piston/cylinder pumping arrangement, wherein, when the pump is operated by the pumping handle during the second phase of lifting a load and the forks have engaged the load, hydraulic fluid is displaced by the relatively small cross-section piston/cylinder pumping arrangement and passed into the lifting cylinder to raise the forks. 14. A pallet truck according to claim 13, in which the relatively large cross-section piston/cylinder pumping arrangement and the relatively small cross- section piston/cylinder pumping arrangement are provided by the same pair of relatively displaceable members. 15. A pallet truck according to claim 14, in which one member provides the cylinder of the relatively large cross-section piston/cylinder pumping arrangement and the piston of the relatively small cross-section piston/cylinder pumping arrangement and the other member provides the piston of the relatively large cross-section piston/cylinder pumping arrangement and the cylinder of the relatively small cross-section piston/cylinder pumping arrangement. 16. A pallet truck according to claim 14 or 15, in which the relatively large cross-section piston/cylinder pumping arrangement and the relatively small cross-section piston/cylinder pumping arrangement are arranged substantially co-axially. 17. A pallet truck according to any one of claims 13 to 16, in which the relatively small cross-section piston/cylinder pumping arrangement is operable over a part only of the range of movement of the pumping handle. 18. A pallet truck according to claim 17, in which the relatively small cross-section piston/cylinder pumping arrangement is operable over less than one third of the range of movement of the pumping handle. 19. A pallet truck according to claim 17 or 18, in which the pumping handle is movable from a generally upright position to a generally prone position and the relatively small cross-section piston/cylinder pumping arrangement is operable only when the handle is in a region close to the generally prone position. 20. A pallet truck according to any one of claims 17 to 19, in which the relatively large cross-section piston/cylinder pumping arrangement is operable over substantially all of the range of movement of the pumping handle. 21. A pallet truck according to any one of claims 13 to 20, in which the relatively large cross-section piston/cylinder pumping arrangement operates over a cross- sectional area more than four times greater than the cross-sectional area over which the relatively small cross-section piston/cylinder pumping arrangement operates.

22. A hand operated pallet truck including lifting forks raisable by a hydraulically operated lifting cylinder to which hydraulic fluid is fed by a pump operable by a pumping handle, the pump including a relatively large cross-section piston/cylinder pumping arrangement and a relatively small cross-section piston/cylinder pumping arrangement, each of the arrangements being operably connected to the pumping handle.

23. A pallet truck according to claim 22, in which the relatively large cross-section piston/cylinder pumping arrangement and the relatively small cross-section piston/cylinder pumping arrangement are provided by the same pair of relatively displaceable members.

24. A pallet truck according to claim 23, in which one member provides the cylinder of the relatively large cross-section piston/cylinder pumping arrangement and the piston of the relatively small cross-section piston/cylinder pumping arrangement and the other member provides the piston of the relatively large cross-section piston/cylinder pumping arrangement and the cylinder of the relatively small cross-section piston/cylinder pumping arrangement.

25. A pallet truck according to claim 23 or 24, in which the relatively large cross-section piston/cylinder pumping arrangement and the relatively small cross-section piston/cylinder pumping arrangement are arranged substantially co-axially.

26. A pallet truck according to any one of claims 22 to 25, in which the relatively small cross-section piston/cylinder pumping arrangement is operable over a part only of the range of movement of the pumping handle.

27. A pallet truck according to claim 26, in which the relatively small cross-section piston/cylinder pumping arrangement is operable over less than one third of the range of movement of the pumping handle. 28. A pallet truck according to claim 26 or 27, in which the pumping handle is movable from a generally upright position to a generally prone position and the relatively small cross- section piston/cylinder pumping arrangement is operable only when the handle is in a region close to the generally prone position. 29. A pallet truck according to any one of claims 26 to 28, in which the relatively large cross-section piston/cylinder pumping arrangement is operable over substantially all of the range of movement of the pumping handle. 30. A pallet truck according to any one of claims 22 to 29, in which the relatively large cross-section piston/cylinder pumping arrangement operates over a crosssectional area more than four times greater than the cross-sectional area over which the relatively small cross-section piston/cylinder pumping arrangement operates. 31. A hand operated pallet truck substantially as herein described with reference to and as illustrated by the accompanying drawings. 32. A method of lifting a load using a hand operated pallet truck according to any preceding claim.