GB2236091A - An extensible mast structure for a lift truck - Google Patents

Abstract

The structure has a fixed mast (11), a movable mast (13) mounted for up and down movement relative to the fixed mast effected by hydraulic jack 33 and a load carrying structure (20) movable up and down the movable mast by means of hydraulic jack 25. Each of the two jacks (25, 33) is supported vertically on the mast structure in a position which is offset from the longitudinal axis of the mast structure. Chains (28 and 36) are arranged to act as tension members which apply a pulling effect to substantially counterbalance the vertical asymmetric force applied by the associated jacks. Hydraulic feed from pipe (42) is through cylinder (34) and piston (35) of jack (33) to jack (25), the relative piston sizes being such that jack (33) only operates when jack (25) is fully extended. <IMAGE>

Description

LIFT TRUCKS AND EXTENSIBLE MAST STRUCTURES THEREFOR This invention relates to lift trucks and to extensible mast structures therefor.

More particularly the invention relates to an extensible mast structure for an industrial or agricultural lift truck, the mast structure comprising a fixed mast, at least one movable mast mounted for up and down movement relative to the fixed mast, and a load carrying structure, e.g. a fork structure, movable up and down on the movable mast, or the uppermost movable mast. Each mast preferably comprises two spaced parallel uprights connected by bracing members. The mast structure also has lift means both for raising the load carrying structure and for extending the mast structure. Preferably the load carrying structure is raised first in a manner known as full free lift, before any extension of the mast structure takes place.

It is known to provide a single lifting jack disposed centrally between spaced uprights of a mast structure which operates both full free lift of a load carrying structure and extension of the mast.

However, the positioning of the lift jack in this manner is disadvantageous because it acts to block the driver's view between the uprights of the mast structure.

To overcome this problem, it has been proposed to employ two spaced apart jacks, each being capable of performing in the same manner as the above-mentioned centrally mounted, single jack, i.e.

to effect full free lift of the load carrying structure and then extension of the mast structure.

Each dual operating jack is expensive and substantial in size. Thus duplication of a known arrangement is not an acceptable solution.

Another known proposal is to retain a centrally mounted jack for effecting full free lift of the load carrying structure and to provide a second asymmetrically mounted jack for extending the mast structure, each jack operating through associated tension members. The solution may be cheaper but the driver's vision is still seriously impaired because there is still a central jack.

In known arrangements in which a jack is supported on a movable section of the mast structure, hydraulic fluid is fed to the jack through a flexible hose.

According to one aspect of the invention there is provided an extensible mast structure for a lift truck, the mast structure comprising a fixed mast, a movable mast mounted for up and down movement relative to the fixed mast and a load carrying structure movable up and down on the movable mast, and lift means comprising a primary jack for raising the load carrying structure up the movable mast and a separate secondary jack for extending the mast structure, wherein each of the two jacks is supported vertically on the mast structure in a position which is offset from the longitudinal axis of the mast structure.

Preferably one or each jack has at least one associated tension member arranged to apply a pulling effect which substantially counterbalances the vertical asymmetric force applied by the respective jack.

It is also preferred that the movable mast comprises two spaced parallel uprights mounted for up and down movement relative to corresponding uprights of the fixed mast, and the primary jack is supported on one of the uprights at one side of the mast structure and the secondary jack is supported on one of the uprights at the other side of the mast structure.

Each jack is preferably disposed laterally outwardly of the angle of view of the driver between the mast uprights.

In an embodiment comprising a triple mast structure there is provided a fixed outer mast, an intermediate mast movable up and down on the outer mast, and an inner mast which is movable up and down on the intermediate mast and which supports the load carrying structure.

Preferably the primary jack is supported on the upright of the intermediate mast or the upright of the inner mast at said one side of the mast structure, and has a downwardly operating piston for raising the load carrying structure.

It is also preferred that the secondary jack is supported on the upright of the outer mast at said other side of the mast structure, and has an upwardly operating piston for extending the mast structure.

In one embodiment the primary jack is supported on the upright of the intermediate mast at said one side of the mast structure and has a downwardly operating piston and at least one associated tension member for raising the load carrying structure, the or each tension member being fixed at one end to the outer mast, passes under a pulley mounted on the end of the piston, over at least one pulley mounted at the top of the inner mast and is fixed at its other end to the load carrying structure.

In another embodiment the primary jack is supported on the upright of the inner mast at said one side of the mast structure and has a downwardly operating piston and at least one associated tension member for raising the load carrying structure, the or each tension member being fixed at one end to the inner mast, passes under a pulley mounted on the end of the piston, over at least one pulley mounted at the top of the inner mast, and is fixed at its other end to the load carrying structure.

The secondary jack, in one embodiment, is supported on the upright of the outer mast at said other side of the mast structure, and has an upwardly operating piston and a pair of associated tension members for extending the mast structure, the tension members being spaced apart in the same manner as the mast uprights, and each tension member being fixed at one end to the outer mast and at the other end to the bottom of the inner mast, the tension member on the same side of the mast structure as the secondary jack passing over a pulley mounted on the end of the piston of the secondary jack, and the tension member on the other side of the mast structure passing over a corresponding pulley mounted at the top of the intermediate mast.Preferably adjacent the pulley on the end of the piston of the secondary jack is a second pulley mounted at the top of the intermediate mast and displaced outwardly of the first pulley, the associated tension member passing upwardly over the second pulley and then the first pulley in its passage between the outer mast and the inner mast.

In a further embodiment the secondary jack is supported on the upright of the outer mast at said other side of the mast structure, and has an upwardly operating piston and a pair of associated tension members for extending the mast structure, each tension member being fixed at one end to the outer mast, passing over a pulley mounted on the end of the piston of the secondary jack, and being fixed at the other end to the bottom of the inner mast.

In yet a further embodiment the secondary jack is supported on the upright of the outer mast at said other side of the mast structure, and has an upwardly operating piston and two pairs of associated tension members for extending the mast structure, each of the first pair of tension members being fixed at one end to the outer mast, passing over a pulley mounted on the end of the piston of the secondary jack, and being fixed at the other end to the bottom of the intermediate mast, and each of the second pair of tension members being fixed at one end to the outer mast, passing over a pulley at the top of the intermediate mast, and being fixed at the other end to the bottom of the inner mast, whereby on extension of the secondary jack the first pair of tension members will lift the intermediate mast relative to the outer mast and the second pair of tension members will lift the inner mast relative to the intermediate mast.

In one embodiment of a twin extensible mast, the primary jack is supported on the upright of the movable mast at said one side of the mast structure, and has a downwardly operating piston and at least one associated tension member for raising the load carrying structure, the or each tension member being fixed at one end to the movable mast, passes under a pulley mounted on the end of the piston, over at least one pulley mounted at the top of the movable mast, and is fixed at its other end to the load carrying structure, and the secondary jack is supported on the upright of the fixed mast at said other side of the mast structure, and has an upwardly operating piston for extending the mast structure.

The secondary jack may apply a direct lifting action on the movable mast. Alternatively, the secondary jack may have a pair of associated tension members for lifting the movable mast relative to the fixed mast, each tension member being fixed at one end to the fixed mast, passing over a pulley mounted on the end of the piston of the secondary jack, and being fixed at the other end to the bottom of the movable mast.

According to another aspect of the invention there is provided an extensible mast structure for a lift truck, the mast structure comprising a fixed mast, a movable mast movable up and down on the fixed mast, a load carrying structure mounted for up and down movement relative to the movable mast, and hydraulic lift means both for raising the load lifting structure relative to the movable mast and for extending the mast structure, wherein the lift means comprises a primary hydraulic jack having a piston and cylinder arrangement supported on the movable mast, for raising the load carrying structure, and for extending the mast structure a secondary hydraulic jack having piston and cylinder members, one of which members being connected to the fixed mast and the other member connected to the movable mast whereby extension of the secondary jack raises the movable mast, and there is provided feed means for hydraulic fluid to both jacks, the feed means passing through said one member of the secondary jack into said other member, and from said other member of the secondary jack to the primary jack. Such hydraulic fluid feed means avoids the necessity for any flexible hose despite the fact that the primary jack is supported on the movable mast.

Preferably the hydraulic fluid feed means includes a first pipe extending to the lower end of said one member of the secondary jack, and a second pipe extending between the upper end of said other end of the secondary jack to a connection point on a part of the primary jack which is attached to the movable mast.

It is also preferred that the jacks have selected operational cross-sectional areas which sequence the primary jack to operate to raise the load carrying structure before the secondary jack operates to extend the mast structure. The desired sequencing may be achieved by the piston of the primary jack having an effective cross-sectional area which is equal to or larger than the effective cross-sectional area of the piston of the secondary jack. Alternatively, the piston of the secondary jack may have an effective cross-sectional area which is larger than the effective cross-sectional area of the piston of the primary jack. In each case, no locks or latches are required to sequence the jacks to provide full free lift operation of the load supporting structure.

The secondary jack preferably has its cylinder mounted on the fixed mast and a piston which is upwardly operating for extending the mast structure, the upper end of the piston being connected to the movable mast.

Said movable mast may be an intermediate mast section, there being provided an upper movable mast which is movable up and down on the intermediate mast and which supports the load carrying structure.

In one embodiment of a triple mast the secondary jack has a pair of associated tension members for extending the upper movable mast relative to the intermediate mast, the tension members being spaced apart across the width of the mast structure, one of the members being fixed at one end to the fixed mast, passing over a pulley mounted on the end of the piston of the secondary jack, and being fixed at the other end to the bottom of the intermediate mast.

In another embodiment of a triple mast the secondary jack has a pair of associated tension members spaced apart across the width of the mast structure, each tension member being fixed at one end to the fixed mast, passing over a respective pulley mounted at the top of the intermediate mast, and being fixed at the other end to the bottom of the intermediate mast.

Preferably the primary jack has its cylinder connected to the movable mast, and has a piston which is downwardly operating and at least one associated tension member for raising the load carrying structure.

It is also preferred that the or each tension member associated with the primary jack is fixed at one end to the fixed mast and at its other end to the load carrying structure, and passes under a pulley mounted on the end of the piston of the primary jack.

The load carrying structure is preferably a fork structure.

The invention also provides a lift truck, for example an industrial or agricultural lift truck, having a mast structure as defined above.

By way of example, specific embodiments in accordance with the invention will be described with reference to the accompanying diagrammatic drawings in which: Figure 1 is an elevation of a triple mast structure for an industrial lift truck indicating the lowered and full free lift positions of the fork structure, as viewed from the driver's position; Figure 2 shows the mast structure of Figure 1 fully extended; Figure 3 is a section along line 3-3 in Figure 2; Figures 4 and 5 illustrate another embodiment of a triple mast structure both contracted and fully extended, as viewed from the driver's position; Figures 6 and 7 are similar views to Figures 4 and 5 of a third embodiment of a triple mast structure; Figures 8 and 9 illustrate a twin mast structure capable of full free lift, the mast structure being shown both contracted and extended, as viewed from the driver's position; and Figures 10 and 11 are similar views to Figures 8 and 9 of another embodiment of a twin mast structure.

This example concerns an industrial lift truck having a truck body on which a driver sits looking forwards, and a triple extensible mast mounted on the front of the body. The truck body is supported on ground wheels, at least one of which is driven through drive connections from an electric motor mounted on the body. All these features are well known and need not be described further.

The invention is concerned only with the construction and operation of the mast and the fork or other load carrying structure carried thereon.

Referring to Figures 1 to 3, the extensible mast 10 comprises three separately constructed portions 11, 12, 13 which are approximately of equal height and which are at least partially nested together to form a compact unit having a central longitudinal axis. Each portion of the mast respectively comprises a pair of vertical uprights 14, 15, 16 spaced apart and braced together by horizontal struts 17, 18, 19 at their top and bottom ends. The outer mast portion 11 is mounted on the truck body. The intermediate and inner mast portions 12, 13 respectively are mounted for relative vertical movement on the outer and intermediate portions respectively. A load lifting carriage 20 having load supporting forks 21, or other load carrying structure, is mounted for relative vertical movement on the inner mast portion 13.For this purpose, in this embodiment, the intermediate and inner mast uprights 15, 16 are of I-section and the intermediate uprights 15 carry rollers 22 for running in the outwardly facing channels of the inner uprights 16.

Similarly the carriage 20 carries rollers (not shown) for running in the inwardly facing channels of the inner uprights 16. The outer mast uprights 14 are of inwardly facing channel section and carry rollers 24 for running in the outwardly facing channels of the intermediate uprights 15. Alternatively, the outer mast uprights 14 may be of I-section like the intermediate uprights and the inner uprights.

Likewise, if desired, the intermediate uprights and/or the inner uprights may have a section which is equivalent to an I-section but which is not a strict I-section.

For raising the load lifting carriage 20 there is provided on one side of the mast structure a primary hydraulic jack 25 comprising a cylinder 26 and a downwardly extending piston 27. The cylinder 26 is bolted adjacent its upper end to one of the uprights of the intermediate mast portion 12 which, in this embodiment, is the left-hand intermediate mast upright 15 as viewed in Figure 1. The jack is thereby supported vertically on the mast structure in a position which is offset from the longitudinal axis of the mast structure. The jack is also substantially aligned with the associated intermediate upright 15 and disposed laterally outwardly of the angle of view of the driver between the mast uprights.

A chain, cable or other flexible tension member, which, in this embodiment, is a chain 28, is fixed at one end to the left-hand upright 14 of the outer mast 11, passes under a pulley 29 on the lower end of the piston 27 and over a pulley 30 on the strut 19 at the upper end of the inner mast 13, and is fixed at its other end to the centre of the load lifting carriage 20. In between the pulleys 29, 30, the chain 28 passes over at least one intermediate pulley 31 mounted on the bracing strut 18 at the upper end of the intermediate mast 12. The effective path of the chain 28 is thereby displaced from the left-hand upright of the intermediate mast to the centre of the carriage 20 whilst being maintained substantially vertical.By this means, the chain 28 is arranged to apply a pulling effect on the carriage 20 which, during operation of the primary jack 25 to raise the carriage, substantially counterbalances the downward asymmetric force applied by the primary jack.

On the other side of the mast structure from the primary jack 25, i.e. on the right-hand side as viewed in Figure 1, there is provided a secondary hydraulic jack 33 for extending the mast. This secondary jack has a cylinder 34 and an upwardly extending piston 35. The cylinder 34 is bolted adjacent its upper end to the right-hand upright 14 of the outer mast 11 and will be seen from Figure 2 to have a stroke substantially equal to the full height of the outer mast. The secondary jack 33 is thereby supported vertically on and substantially aligned with the right-hand outer mast upright.

Thereby, like the primary jack 25, the secondary jack 33 is mounted in a position which is offset from the longitudinal axis of the mast structure and disposed laterally outwardly of the angle of view of the driver between the mast uprights.

A pair of chains 36, 37 or other tension members are arranged spaced apart across the mast structure. The right-hand chain 36, as viewed in Figure 1, is mounted at one end to the top of the outer mast 11 and at its other end to the bottom of the inner mast 13. In between, the chain 36 extends over a pair of adjacent pulleys 38, 39, both pulleys being mounted in series on the bracing strut 18 at the upper end of the intermediate mast 12 and the first pulley 38 also being mounted at the top of the piston 35 of the secondary jack. Extension of the secondary jack 33 will thereby effect a direct lifting action on the intermediate mast 12.The purpose of the second pulley 39 is to displace outwardly the effective path of the chain 36 relative to the secondary jack 33 so as to substantially counterbalance the upward asymmetric force applied by the secondary jack to lift the intarmediate mast. A similar effect could be achieved by replacing pulley 38 on the piston 35, by a pulley of sufficiently larger diameter.

The other chain 37 follows a similar path on the left-hand side of the mast structure as viewed in Figure 1, but only requires a single pulley 40 of the same diameter as the pulley 38. The pulley 40 is mounted on the bracing strut 18 at the upper end of the intermediate mast.

It will be appreciated that the joint pulling effect of the chains 36, 37 to raise the inner mast is displaced substantially equally on opposite sides of the longitudinal axis of the mast structure.

The jacks 25, 33 are connected in series to a common hydraulic supply, which is fed through pipe 42 (Figure 2) to the bottom end of the cylinder 34 of the secondary jack 33, vertically upwardly through the piston 35 of the secondary jack, and then laterally via connection pipe 41 to the top end of the cylinder 26 of the primary jack 25. The piston 27 of the primary jack 25 has an effective cross-sectional area which is equal to or larger than the effective cross-sectional area of the piston 35 of the secondary jack 33 to ensure that the primary jack is extended first and only when it is fully extended is the secondary jack operated. This arrangement is advantageous because it avoids the need for locks and latches which are known in other arrangements for sequencing the operation of two jacks.

Extension of the primary jack 25 downwardly results in full free lift of the load lifting carriage 20 to its chain line position in Figure 1, since the chain 28 will cause the carriage to rise at twice the rate of the downward extension of the piston 27. At this stage of the operation, no movement of any of the mast sections has taken place. The secondary jack 33 will then extend upwardly to effect its direct lifting action of the intermediate mast 12 relative to the outer mast 11, and the chains 38, 39 will simultaneously raise the inner mast 13 relatively to the intermediate mast at twice the rate of the lifting action of the secondary jack. Upward movement of the intermediate and inner masts will continue until, at the end of the stroke of the jack 33, the masts are located as shown in Figure 2, the carriage 20 having remained at the top of the inner mast throughout the whole extension of the mast. Reverse operation of the secondary jack 33 will contract the mast, followed by reverse operation of the primary jack 25 to lower the carriage 20 to its full line position in Figure 1.

When the mast structure is being extended, the off-centre pushing effect of the single, asymmetrically mounted secondary jack 33 on the intermediate mast 12 is at least partially counterbalanced by the outwardly displaced pulling effect of the chain 36 on the same side of the mast structure as the secondary jack to raise the inner mast. Similarly, during raising of the load lifting carriage 20, the off-centre action of the asymmetrically mounted primary jack 25 is transmitted by the chain 28 to the centre of the carriage 20.

Hence, the illustrated arrangement of chains 28, 36, 37 combine with the asymmetrically mounted jacks 25, 33 to provide a mast structure having an acceptable out of balance condition.

Figures 1 and 2 clearly illustrate the improved vision to the driver through the mast structure which results from the jacks 25, 33 being spaced apart across the mast structure by a distance whereby each jack is disposed laterally outwardly of the driver's angle of view between the uprights of the mast, the primary jack in this embodiment being on the left and the secondary jack being on the right. When the mast 10 is fully contracted, and the carriage 20 lowered, only the carriage lifting chain 28 is present to partially obscure the driver's vision between the spaced uprights of the mast.

Figures 4 to 7 illustrate two other embodiments of triple mast structure which likewise employ an asymmetric jack arrangement in which primary and secondary jacks 25, 33 are spaced apart one on either side of the mast structure and disposed laterally outwardly of the angle of view of the driver between the uprights 16 of the mast structure. As before, in each of these additional embodiments, extension of the primary jack effects full free lift of the load lifting carriage 20 and the subsequent extension of the secondary jack raises the mast structure.

Similarly, the jacks 25, 33 have selected operational cross-sectional areas which sequence the primary jack 25 to operate to raise the load carrying structure 20 before the secondary jack 33 operates to extend the mast structure 10.

Referring to Figures 4 and 5, the arrangement of the primary jack 25 and secondary jack 33 is similar to the embodiment of Figures 1 to 3. The primary jack 25 is supported on one of the intermediate mast uprights 15, but displaced laterally outwardly from the respective upright compared with the position of the jack in the previous embodiment. The secondary jack 33 is similarly supported on the upright 14 of the outer mast portion 11 at the other side of the mast structure and again displaced outwardly relative to the upright compared with its position in the embodiment of Figures 1 to 3.

Hydraulic fluid to the jacks is likewise fed via pipe 42 (Figure 9) to the bottom end of the cylinder 34 of the secondary jack 33, up through the piston 35 of the secondary jack and then across the top of the intermediate mast portion to the top of the cylinder 26 of the primary jack 25 in the same manner as shown in Figure 3.

Also, as in the previous embodiment, the piston 27 of the primary jack 25 has an effective cross-sectional area which is equal to or larger than the effective cross-sectional area of the piston 35 of the secondary jack 33, to achieve the desired sequencing of the jacks without the need for locks and latches.

Each jack has an associated double chain arrangement. In the case of the primary jack 25, chain 50 is fixed at one end to the top of the outer mast upright 14 on the same side as the primary jack, passes under pulley 52 on the piston of the primary jack, over pulley 70 mounted at the top of the intermediate mast portion 12 adjacent the upper end of the primary jack, under and over a pair of pulleys 71, 72 mounted respectively towards the bottom and at the top of the inner mast portion 13 on the same side as the primary jack, and has its other end fixed to the load lifting carriage.The other chain 51, follows a similar path but is transferred to the other side of the mast structure by pulley 73 on the inner mast portion at the same height as pulley 71, before passing upwardly and over its respective pulley 74 at the top of the inner mast portion on the same side of the mast structure and having its other end fixed to the carriage 20.

Chain 55 associated with the secondary jack 33 has one end fixed to the outer mast upright 14 adjacent the secondary jack, passes over a pulley 75 on the end of the piston 35 of the secondary jack and has its other end fixed to the bottom of the inner mast portion 13 on the same side of the mast structure as the secondary jack. The other chain 56 follows a similar path but is transferred to the other side of the mast structure by a second pulley 74 mounted on the top of the intermediate mast portion 12 at the side adjacent the primary jack 25.

In this embodiment, when the mast structure 10 is fully contracted and the carriage 20 lowered, (Figure 4), the driver again has substantially unimpaired vision between the spaced uprights of the mast structure. Indeed, the double chain arrangement 50, 51 for the primary jack 25 gives the driver improved vision even compared with the embodiment of Figures 1 to 3 because of the absence of a central chain. Moreover, even the chains 50, 51 may be arranged within the profile of the uprights 16 of the inner mast 13 so that they lie outside the driver's angle of view between the uprights.

Figures 6 and 7 illustrate an embodiment in which the primary jack 25 is supported at one side of the mast structure 10 on the inner mast portion 13, and the secondary jack 33 is supported on the outer mast portion 11 at the other side of the mast structure.

Associated with the primary jack 25 are two chains 50, 51 or other tension members. Chain 50 is fixed at one end to the top of the outer mast upright 14 on the same side of the mast structure as the primary jack, passes under a pulley 52 mounted on the piston 27 of the primary jack, then over a pulley 53 mounted at the top of the adjacent inner mast upright 16, and has its other end fixed to the carriage 20.

The other chain 51 follows a similar path but is transferred from the respective pulley 53 to a second pulley 54 at a position near to the other inner mast upright 16. Extension of the asymmetrically mounted primary jack 25 thereby lifts the carriage 20 through the associated chains 50, 51.

Also, in this embodiment, two double chain arrangements are associated with the secondary jack 33. In the case of the first pair of chains 80, 81, each chain has one end fixed to a respective outer mast upright 14, passes over a respective pulley 82 at the top of the intermediate mast portion on the same side of the mast structure, and has its other end fixed to the bottom of the inner mast portion 13. The second chain arrangement comprises chains 83, 84. Chain 83 is fixed at one end to the outer mast upright 14 on the same side of the mast structure as the secondary jack 33, passes over pulley 57 on the end of the piston 35 of the secondary jack, and is fixed at its other end to the bottom of the intermediate mast portion.The other chain 84 follows substantially the same path but is transferred to the other side of the mast structure by a pair of pulleys 85, 86 mounted on the spaced apart uprights respectively of the outer mast, the chain passing under pulley 85 and over pulley 86 before having its other end fixed to the outer mast upright at the side of the mast structure remote from the secondary jack.

Hydraulic fluid to the jacks 25, 33 is fed from a supply on the truck through a supply pipe (not shown) which is connected to separate flexible hoses extending, in known manner, via hose reels or over pulleys to the top of the primary jack 25 and to the bottom of the secondary jack 33.

Also, in this embodiment, it will be appreciated that the stroke of the secondary jack 33 is half the vertical movement of the intermediate mast 12. Accordingly, when the secondary jack is extended the intermediate mast 12 will rise at twice the rate of the piston 35. In these circumstances, the desired sequencing of the jacks to provide full free lift of the carriage 20 followed by extension of the mast, is achieved by the piston 33 of the secondary jack having an effective cross-sectional area which is larger than the effective cross-sectional area of the piston 27 of the primary jack 25.

In operation, extension of the primary jack 25 causes the associated chains 50, 51 to effect full free lift of the carriage 20. Subsequent extension of the secondary jack 33 causes its associated chains 80, 81 and 83, 84 to raise the mast structure, the load lifting carriage remaining at the top of the inner mast portion 13. It will be appreciated that the chains 83, 84 raise the intermediate mast portion 12 relative to the outer mast portion, and through this movement the chains 80, 81 raise the inner mast portion 13 relative to the intermediate mast portion 12. Likewise, reverse contraction of the secondary jack 33 followed by contraction of the primary jack 25 allows the associated chains to lower the mast structure 10 and subsequently the load lifting carriage 20.

Each of the embodiments described above concerns a triple mast structure having a primary jack supported on one of the uprights at one side of the mast structure and a secondary jack supported at the other side of the mast structure. The asymmetrically mounted jacks 25, 33 are also spaced apart across the mast structure by a distance whereby each jack is disposed laterally outwardly of the angle of view of the driver between the mast uprights. At the same time, the chain arrangements associated with both asymmetrically mounted jacks 25, 33 are disposed centrally on the longitudinal axis of the mast structure or equally on opposite sides of the longitudinal axis. Furthermore, in each embodiment, when the mast structure is fully contracted and the carriage lowered, the driver has at least substantially unimpaired vision between the uprights of the mast structure.

A similar arrangement is also applicable in the case of a twin mast. The embodiment of a twin mast shown in Figures 8 and 9 is, in effect, a modification of the triple mast structure illustrated in Figures 6 and 7 having the intermediate mast portion and the chains 80 and 81 omitted. The twin mast structure 10 has inner mast uprights 15 mounted for up and down movement on outer mast uprights 14.

Also chains 83, 84 associated with the secondary jack 34 are fixed at one end to the outer mast portion 11, pass over the pulley 57 on the end of the piston of the secondary jack and are each fixed at the other end to the bottom of the inner mast portion 13.

Also, in this embodiment of a twin mast structure, to provide for the desired sequencing of the jacks, the piston 27 of the primary jack 25 has an effective cross-sectional area which is equal to or larger than the effective cross-sectional area of the piston 35 of the secondary jack 33.

Chains 50, 51 for lifting the carriage 20 are disposed equally on opposite sides of the longitudinal axis of the mast structure 10 and their combined pulling effect substantially counterbalances the downward asymmetric force applied by the primary jack 25. Similarly, the chains 83, 84 for raising the inner mast 12 substantially counterbalance the upward asymmetric force applied by the secondary jack 33. Reverse contraction of the secondary jack 33 followed by contraction of the primary jack 25 allows the associated chains to lower the inner mast portion 12 and subsequently the carriage 20. An acceptable out of balanced lifting and lowering of the carriage and the mast is thereby achieved. Moreover, when the mast is contracted and the carriage lowered (Figure 8), the driver again has substantially unimpaired vision between the spaced uprights of the mast structure.

Figures 10 and 11 show another embodiment of a twin mast in which the lift means are closely similar to the arrangement of Figures 1 to 3. The primary jack 25 is mounted on the upright 15 of the movable inner mast 12 at one side of the mast structure, and has a single associated chain 28 which is fixed at one end to the fixed outer mast 11, passes under pulley 29 on the end of the piston 27 of the primary jack, over pulley 30 at the top of the movable mast, and is fixed at its other end centrally of the carriage 20. Between the pulleys 29, 30, the chain 28 passes over and under intermediate pulleys 90, 91 mounted on the transverse strut 17 of the outer mast in order that the chain should extend substantially parallel to the longitudinal axis of the mast.

The secondary jack 33 is mounted on the upright 14 of the fixed mast 11 at the other side of the mast structure, and has the upper end of its piston 35 attached to the movable mast. Extension of the secondary jack thereby applies a direct lifting force to extend the mast structure without the need for an associated tension member. The primary jack simultaneously provides a reaction upward force and thereby an acceptable out of balanced lifting of the movable mast 12 is again achieved.

Hydraulic fluid is fed to the jacks in series in the same manner as described above in respect of the embodiments of Figures 1 to 3 and Figures 4 and 5. The jacks are likewise also sequenced by selected operational cross-sectional areas, the piston 35 of the primary jack 33 having an effective cross-sectional area which is twice or greater than twice the effective cross-sectional area of the piston 35 of the secondary jack 37, thereby avoiding the need for locks and latches.

When the mast is contracted, the driver's view between the uprights is obstructed only by the central carriage lifting chain 28.

The invention is not restricted to the specific details of the embodiments described above. For example, in each embodiment, the mounting of the jacks 25, 33 and the associated chains or other tension members may be reversed so that the secondary jack 33 is on the left-hand side of the mast structure and the primary jack 25 is on the right-hand side of the mast structure, as viewed in the drawings.

Also, although each of the above described embodiments illustrates a mast structure primarily intended for an industrial lift truck, the invention is equally applicable to equivalent mast structures for agricultural lift trucks.

Claims (30)

1. An extensible mast structure for a lift truck, the mast structure comprising a fixed mast, a movable mast mounted for up and down movement relative to the fixed mast and a load carrying structure movable up and down on the movable mast, and lift means comprising a primary jack for raising the load carrying structure up the movable mast and a separate secondary jack for extending the mast structure, wherein each of the two jacks is supported vertically on the mast structure in a position which is offset from the longitudinal axis of the mast structure.

2. A mast structure as claimed in Claim 1, wherein one or each jack has at least one associated tension member arranged to apply a pulling effect which substantially counterbalances the vertical asymmetric force applied by the respective jack.

3. A mast structure as claimed in Claim 1 or Claim 2, wherein the movable mast comprises two spaced parallel uprights mounted for up and down movement relative to corresponding uprights of the fixed mast, and the primary jack is supported on one of the uprights at one side of the mast structure and the secondary jack is supported on one of the uprights at the other side of the mast structure.

4. A mast structure as claimed in Claim 3, wherein each jack is disposed laterally outwardly of the angle of view of the driver between the mast uprights.

5. A mast structure as claimed in Claim 3 or Claim 4, wherein there is provided a fixed outer mast, an intermediate mast movable up and down on the outer mast, and an inner mast which is movable up and down on the intermediate mast and which supports the load carrying structure.

6. A mast structure as claimed in Claim 5, wherein the primary jack is supported on the upright of the intermediate mast or the upright of the inner mast at said one side of the mast structure, and has a downwardly operating piston for raising the load carrying structure.

7. A mast structure as claimed in Claim 5 or Claim 6, wherein the secondary jack is supported on the upright of the outer mast at said other side of the mast structure, and has an upwardly operating piston for extending the mast structure.

8. A mast structure as claimed in Claim 5, wherein the primary jack is supported on the upright of the intermediate mast at said one side of the mast structure and has a downwardly operating piston and at least one associated tension member for raising the load carrying structure, the or each tension member being fixed at one end to the outer mast, passes under a pulley mounted on the end of the piston, over at least one pulley mounted at the top of the inner mast and is fixed at its other end to the load carrying structure.

9. A mast structure as claimed in Claim 5, wherein the primary jack is supported on the upright of the inner mast at said one side of the mast structure and has a downwardly operating piston and at least one associated tension member for raising the load carrying structure, the or each tension member being fixed at one end to the inner mast, passes under a pulley mounted on the end of the piston, over at least one pulley mounted at the top of the inner mast, and is fixed at its other end to the load carrying structure.

10. A mast structure as claimed in any one of Claims 5 to 9, wherein the secondary jack is supported on the upright of the outer mast at said other side of the mast structure, and has an upwardly operating piston and a pair of associated tension members for extending the mast structure, the tension members being spaced apart in the same manner as the mast uprights, and each tension member being fixed at one end to the outer mast and at the other end to the bottom of the inner mast, the tension member on the same side of the mast structure as the secondary jack passing over a pulley mounted on the end of the piston of the secondary jack, and the tension member on the other side of the mast structure passing over a corresponding pulley mounted at the top of the intermediate mast.

11. A mast structure as claimed in Claim 10, wherein adjacent the pulley on the end of the piston of the secondary jack is a second pulley mounted at the top of the intermediate mast and displaced outwardly of the first pulley, the associated tension member passing upwardly over the second pulley and then the first pulley in its passage between the outer mast and the inner mast.

12. A mast structure as claimed in any one of Claims 5 to 9, wherein the secondary jack is supported on the upright of the outer mast at said other side of the mast structure, and has an upwardly operating piston and a pair of associated tension members for extending the mast structure, each tension member being fixed at one end to the outer mast, passing over a pulley mounted on the end of the piston of the secondary jack, and being fixed at the other end to the bottom of the inner mast.

13. A mast structure as claimed in any one of Claims 5 to 9, wherein the secondary jack is supported on the upright of the outer mast at said other side of the mast structure, and has an upwardly operating piston and two pairs of associated tension members for extending the mast structure, each of the first pair of tension members being fixed at one end to the outer mast, passing over a pulley mounted on the end of the piston of the secondary jack, and being fixed at the other end to the bottom of the intermediate mast, and each of the second pair of tension members being fixed at one end to the outer mast, passing over a pulley at the top of the intermediate mast, and being fixed at the other end to the bottom of the inner mast, whereby on extension of the secondary jack the first pair of tension members will lift the intermediate mast relative to the outer mast and the second pair of tension members will lift the inner mast relative to the intermediate mast.

14. A mast structure as claimed in Claim 3 or Claim 4, wherein the primary jack is supported on the upright of the movable mast at said one side of the mast structure, and has a downwardly operating piston and at least one associated tension member for raising the load carrying structure, the or each tension member being fixed at one end to the movable mast, passes under a pulley mounted on the end of the piston, over at least one pulley mounted at the top of the movable mast, and is fixed at its other end to the load carrying structure, and the secondary jack is supported on the upright of the fixed mast at said other side of the mast structure, and has an upwardly operating piston for extending the mast structure.

15. A mast structure as claimed in Claim 14, wherein the secondary jack applies a direct lifting action on the movable mast.

16. A mast structure as claimed in Claim 14, wherein the secondary jack has a pair of associated tension members for lifting the movable mast relative to the fixed mast, each tension member being fixed at one end to the fixed mast, passing over a pulley mounted on the end of the piston of the secondary jack, and being fixed at the other end to the bottom of the movable mast.

17. An extensible mast structure for a lift truck, the mast structure comprising a fixed mast, a movable mast movable up and down on the fixed mast, a load carrying structure mounted for up and down movement relative to the movable mast, and hydraulic lift means both for raising the load lifting structure relative to the movable mast and for extending the mast structure, wherein the lift means comprises a primary hydraulic jack having a piston and cylinder arrangement supported on the movable mast, for raising the load carrying structure, and for extending the mast structure a secondary hydraulic jack having piston and cylinder members, one of which members being connected to the fixed mast and the other member connected to the movable mast whereby extension of the secondary jack raises the movable mast, and there is provided feed means for hydraulic fluid to both jacks, the feed means passing through said one member of the secondary jack into said other member, and from said other member of the secondary jack to the primary jack.

18. A mast structure as claimed in Claim 17, wherein the hydraulic fluid feed means includes a first pipe extending to the lower end of said one member of the secondary jack, and a second pipe extending between the upper end of said other end of the secondary jack to a connection point on a part of the primary jack which is attached to the movable mast.

19. A mast structure as claimed in Claim 17 or Claim 18, wherein the jacks have selected operational cross-sectional areas which sequence the primary jack to operate to raise the load carrying structure before the secondary jack operates to extend the mast structure.

20. A mast structure as claimed in Claim 19, wherein the desired sequencing of the jack is achieved by the piston of the primary jack having an effective cross-sectional area which is equal to our larger than the effective cross-sectional area of the piston of the secondary jack.

21. A mast structure as claimed in Claim 19, wherein the desired sequencing of the jacks is achieved by the piston of the secondary jack having an effective cross-sectional area which is larger than the effective cross-sectional area of the piston of the primary jack.

22. A mast structure as claimed in any one of Claims 17 to 21, wherein the secondary jack has its cylinder mounted on the fixed mast and a piston which is upwardly operating for extending the mast structure, the upper end of the piston being connected to the movable mast.

23. A mast structure as claimed in any one of Claims 17 to 22, wherein said movable mast is an intermediate mast section, there being provided an upper movable mast which is movable up and down on the intermediate mast and which supports the load carrying structure.

24. A mast structure as claimed in Claim 23, wherein the secondary jack has a pair of associated tension members for extending the upper movable mast relative to the intermediate mast, the tension members being spaced apart across the width of the mast structure, one of the members being fixed at one end to the fixed mast, passing over a pulley mounted on the end of the piston of the secondary jack, and being fixed at the other end to the bottom of the intermediate mast.

25. A mast structure as claimed in Claim 23, wherein the secondary jack has a pair of associated tension members spaced apart across the width of the mast structure, each tension member being fixed at one end to the fixed mast, passing over a respective pulley mounted at the top of the intermediate mast, and being fixed at the other end to the bottom of the intermediate mast.

26. A mast structure as claimed in any one of Claims 17 to 25, wherein the primary jack has its cylinder connected to the movable mast, and has a piston which is downwardly operating and at least one associated tension member for raising the load carrying structure

27. A mast structure as claimed in Claim 26, wherein the or each tension member associated with the primary jack is fixed at one end to the fixed mast and at its other end to the load carrying structure, and passes under a pulley mounted on the end of the piston of the primary jack.

28. A mast structure as claimed in any one of the preceding claims, wherein the load carrying structure is a fork structure.

29. An extensible mast structure for a lift truck substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

30. An industrial or agricultural lift truck having a mast structure as claimed in any one of the preceding claims.