The present invention relates to a lifting chain and more particularly, but not exclusively, to a lifting chain of the kind that is used in the raising and lowering of a lifting fork carriage of a fork lift truck. 
In fork lift trucks the lifting fork carriage is raised or lowered by a mast assembly comprising at least one hydraulic jack having a sheave at its upper end over which is trained a lifting chain.  A lower end of the jack is fixed to the truck and a first end of the lift chain is connected to the lifting fork carriage by an anchor assembly.  The ether end of the chain is fixed to a stationary structure on the truck. 
It is well known for the mast assembly to have two pairs of lifting chains and sheaves, the pairs being connected to the carriage at laterally spaced locations. 
When the hydraulic jack is extended the sheave (or sheaves) is moved upwardly and the chain (or chains) is lifted so as to rise the carriage. 
A lifting chain of this kind generally comprises a plurality of interleaved links, each link being formed by a plurality of link plates of identical configuration disposed in a side-by-side relationship.  Overlapping ends of the interleaved link plates are mounted on a common pin.  The outermost link plates are normally connected to the pin with an interference fit between a pit hole in the plate and the pin whereas the remaining intermediate link plates are pivotally mounted on the pin with a clearance fit.  The degree of interference fit is normally sufficient to cold work the plate material locally at the interface between the pin and the pin hole.  It has now been realised that this causes plastic deformation and an artificial increase in the ability of the plate to resist fatigue loads. 
The particular number and arrangement of the intermediate link plates in comparison to outer link plates is selected according to the required lifting capacity of the chain.  Generally, these are at least two first groups of intermediate link plates offset from the outer link plates and connected thereto by the pins.  The first groups of intermediate link plates may comprise two or more aligned, adjacent plates, the number depending on the application.  In larger size chains there may be further groups of intermediate link plates.  Second groups of intermediate link plates are positions in-board of the first groups and are aligned with the outer link plates, third groups are disposed in-board of the second groups and aligned with the first groups, and so on. 
In order to manufacture a chain as cheaply as possible and to keep the number of different components of a single chain to a minimum it is conventional for all the plates (outer and intermediate) to be of the same thickness and shape having been produced by the same tool from a sheet of metal of constant thickness. 
Tests have established that, in use, the majority of the load on such chains is taken by the outermost link plates.  Accordingly, it has been known for the thickness of the outer link plates to be increased in comparison to the intermediate link plates so as to increase the lifting capacity of the chain. 
Further tests conducted by the applicant have established that, in fact, the closer the link plate is to the centre of the chain, the lower the loading.  In addition, as a result of the clearance fit with the pins, the central intermediate plates do not initially contribute to the bearing of the load during initial application of the load.  It is obviously undesirable for the load to be distributed unevenly in this way across a chain and for the outer plates to be under such stress. 
It is well known that the fatigue load capacity of a given lifting chain can be increased by replacing it with two separate side-by-side chains each having fewer links across its width than the original chain.  However, in many circumstances the substitution of a single relatively wide chain by two narrower chains is not possible owing to the limited space available.  Since there has to be a lateral clearance between the pins of the side-by-side chains there is generally insufficient room to make this substitution in a mast assembly of a fork lift truck. 
It is an object of the present invention to obviate or mitigate the aforesaid disadvantages or problems. 
According to the present invention there is provided a lifting chain having a first end for connection to a stationary structure and a second end for connection to an object to be moved, the chain comprising a plurality of interleaved link assemblies, each link assembly comprising outer link plates and a plurality of intermediate link plates, adjacent link assemblies being interconnected by a pin on which the outer link plates are non-rotatably mounted by means of an interference fit and the intermediate link plates are mounted so as to be rotatable relative thereto, characterised in that each outer link plate has a strength less than that of any intermediate link plate when measured in an unassembled state. 
The reduction in strength of the outer link plate can be achieved, for example, by reducing its thickness or xe2x80x9cheadxe2x80x9d area in comparison to that of an intermediate link plate, or using a different material or manufacturing method.  The term xe2x80x9cheadxe2x80x9d area is used herein in the description and claims to means the cross sectional area of the link taken along a radial line that passes through the centre of the pin and intersects the nearest outer edge of the chain link plate. 
Since it has been understood that a disproportionate amount of the lifted load is carried by the outer link plates, to date it has not been thought possible to reduce the strength of the outer link plates by, for example, reducing their thickness.  However, by directing the chain design towards achieving optimum fatigue resistance rather than increasing its tensile strength, it has been realised that by taking advantage of the enhanced fatigue performance (referred to above) provided by the interference fit between the outer link plates and the pin, a reduction in the strength of the outer plate (in the unassembled state) is possible. 
If the outer plate is thinner than in comparison to that of known lifting chains in which plates are of all the same thickness then a single relatively wide chain may be replaced by two or more narrower chains (thereby increasing the lifting capacity of the chain) without increasing the width required to accommodate the chains. 
In a preferred embodiment the outer link plates have a stiffness of 60% to 90% of the average stiffness across the chain. 
Preferably a first group of intermediate link plates have a stiffness of 70% to 95% of the average stiffness across the chain. 
In large chains a second group of intermediate link plates, in-board of the first group, may have a stiffness that is equal to the average stiffness and the stiffness of further groups of intermediate link plates is shared equally.