Mast for industrial truck

Abstract

 An industrial truck, for example, a fork lift truck, has a telescopic mast which consist of two pairs of uprights. The drawings shows in plan view one upright of each pair (1,2); they are located at one side of the truck and a similar pair are located at the other side of the truck. The uprights (1) are located outside the uprights (2) and are fixed. The uprights (2) move up the uprights (1) by means of roller bearings (not shown). A ram (9) of the displacement type associated with the uprights (1,2) on one side of the truck, together with a similar ram associate with those on the other side of the truck, is responsible for extending the mast (and for moving the load carrier up and down the mast).
The cylinders of the rams are made in one piece with the respective uprights (1), preferably by means of an extrusion, or could be welded thereto. Because the cylinders are made in this way, the upright is strengthened and the cylinder is secured to it more rigidly. This in turn permits a smaller overlap between the ram and the ram rod when the ram is fully extended, and this increases the so-called 'free lift' of the truck.

Description

[0001] The invention relates to masts for industrial trucks.

[0002] Extensible masts are known in which a first and second pair of uprights are movable relative to each other to extend the mast. Typically the ram for extending the mast is located between the uprights but in order to improve the driver's forward view, it has been proposed to emply two rams and locate them immediately adjacent to uprights; this leaves the space in the centre between the uprights free.

[0003] The invention provides amast for an industrial truck, which comprises a first pair and a second pair of uprights movable relative to each other to extend the mast, a load carrier movable up and down the mast, a pair of rams for extending the uprights and moving the load carrier, wherein the cylinder of each ram and the respective upright of one of the pairs of uprights are rigidly secured to each other and the rams are of the displacement type.

[0004] By rigidly securing each cylinder to an upright and using cylinders of the displacement type, the improved visibility referred to above is obtained, while at the same time the upright wuth which the cylinder is associated is strengthened and the cylinder itself is more rigidly secured than hitherto so that the overlap needed between the ram rod and cylinder when the ram is fully opened is reduced. Previously greater overlap was needed because the cylinder of the ram was only secured to its adjacent upright at one end to accompdate misalignment between the ram rod and cylinder. The use of a displacement ram, that is, one where there is clearance between the ram rod diameter and the internal diameter of the cylinder, the hydraulic fluid seal being at the mouth of the cylinder, permits a small amount of misalignment of ram rod and cylinder due to flexing of the gland which seals the fluid in the ram at the mouth of the cylinder. This in turn is sufficient to accomodate any misalignment consequent upon rigidly securing the cylinder to the upright.

[0005] Advantageously, one pair of uprights is fixed and located outside the other pair which are movable relative to the fixed pair and the cylinders are rigidly secured to the fixed outer pair of uprights. The cylinders will then be located on the outer side of the outer uprights.

[0006] The cylinders and respective uprights may be welded to each other, but are advantageously formed in one piece by producing the pieces by extrusion.. The material for the pieces may be steel. If necessary, the length of the cylinder can be trimmed by machining the top off the part defining the cylinder.

[0007] If desired a third pair of uprights may be provided.

[0008] The invention also provides an industrail truck having a mast of any of the forms defined above.

[0009] A duplex mast industrial truck contructed in accordance with the invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a rear view of the mast showing it extended and retracted; and

Figure 2 is a horizontal section through one side of the mast.

[0010] Referring to the drawings, the industrial truck has a mast comprising a first, outer, pair of uprights 1 and a second, inner, pair of uprights 2. From Figure 2, it will be seen- that the inner uprights 2 are of I-section and the outer uprights 1, are of U-section. Rollers (not shown) are provided in the space between the two sections. The outer uprights 1 are joined by cross-pieces 3,5. The mast is pivotally mounted about a horizontal axis (not shown) at its lower end and the brackets 4 provide a further axis 4a to which a second ram (not shown) is applied in order to tilt the mast about the pivot at its lower end. The inner uprights 2 have cross pieces 6,7. A load carrier 8 has cross-pieces 8a, 8b which carry load-supporting forks (not shown).

[0011] The mast assembly is connected to the industrial truck via the horizontal pivotal axis at its lower end at the further axis 4 a on which the tilt ram acts.

[0012] In accordance with the invention, the outer uprights 1 each have a cylinder 9 rigidly secured to them, in this case by being formed integrally with them. The member is formed from steel by extrusion. After extrusion an upper section of the part of the extrusion defining the cylinder is machined away, because it is desired that the cylinder length should be less than the length of the uprights (when retracted) in order to obtain a certain amount of 'free lift' of the load carrier: this results from extension of the ram up to the point where it becomes level with the top of the retracted mast.

[0013] The lower end of the cylinder is closed by welding a metal plug into the bottom of the cylinder in the usual fashion, and a gland housing 9b is secured to the top of the cylinder after the ram rod has been inserted. Pressurised fluid is fed into the cylinder by fitting a feed boss (not shown) into the cylinder below the gland housing.

[0014] A ram rod 9a is displaceable along the cylinder 9 by hydraulic fluid pressure. There is a slight gap between the exterior diameter of the ram rod and the interior diameter of the cylinder. A sealing gland (not shown) is positioned at the mouth of the cylinder. It is to be noted that since the ram is a displacement ram rather than a piston-and-cylinder (where the ram rod is in actual contact with the walls of the cylinder), the finish produced by the extrusion is sufficiently good and there is no need for an additional machining step.

[0015] A pulley 10 is secured to the top of the ram rod and chains (not shown) connect the load carrier 8 to the cross-piece 7 of the inner uprights. When the ram is pressurised, the load carrier 8 first travels up the inner uprights for a limited stroke and then the inner uprights extend relative to the outer uprights.

[0016] The advantages of the arrangement of the invention are that the driver has an unimpeded view between the masts that the strength of the outer mast uprights is improved by the integral cylinder and that the cylinder is more securely fixed than hitherto, enabling less overlap between ram rod and cylinder to be built in when the ram is fully extended. The advantage of this is that, for a given overall length of extended ram, the length of the retracted ram can be reduced. This permits the use of a shorter ram and this in turn leads to a greater extension of the ram being possible before the ram rises above the height of the retracted masts. Thus the lift of the load carrier 8, the so-called free lift' before anything projects beyond the height of the retracted mast, is increased. This would not have been possible previously because a ram of the piston and cylinder type was used. Misalignment could have occured between the piston and cylinder as the ram was extended and the cylinder had therefore to be mounted in such a way that this could be accomodated, for example, by being pivotally mounted. By contrast, misalignment in the arrangement of the invention is accomodated by means of slight deformation of the gland at the mouth of the cylinder.

[0017] As an alternative to forming the cylinder of each ram and its respective upright in one piece by extrusion, the parts may be welded together, for example, at a number of points spaced along its length.

[0018] Although the invention has been described in relation to a duplex mast, it is applicable to a triple mast (having a third pair of uprights) and although the free lift of the load carrier is limited, that is, projection beyond the retracted mast takes place before the load carrier has travelled to the top of the mast, the invention is also applicable to this arrangement (so called 'full free lift') as well.

Claims

1. A mast for an industrial truck, which comprises a first pair and a second pair of uprights movable relative to each other to extend the mast, a load carrier movable up and down the mast, a pair of rams for extending the uprights and moving the load carrier, wherein the cylinder of each ram and the respective upright of one of the pairs of uprights are rigidly secured to each other, and the rams are of the displacement type.

2. A mast as'claimed in claim 1, wherein one pair of uprights is fixed and located outside the other pair which are movable relative to the fixed pair, and the cylinders are rigidly secured to the outer pair of uprights.

3. A mast as claimed in claim 1 or claim 2, wherein the cylinders and respective uprights are formed in one piece with each other by extrusion.

4. A mast as claimed in claim 2 or 3, wherein the material for the pieces is steel.

5. A mast substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

6. An industrial truck having a mast as claimed in any one of the preceding claims.

Drawing