the side plates 18 of an adjacent interconnected link 17 prevents relative horizontal movement of those links 17 in a direction parallel to the pivot axes 33 thereof the meshing chains 13 14 are locked against relative horizontal movement in a direction at right angles to the pivot axes of the links 17 by virtue of the interlocking engagement between the upper edge surface 23 of the transverse rail 20 of each link element 17 in one of the chains 13 14 and the lower edge surface 24 of the rail 20 of an opposite adjacent link element 17 in the other of the chains 13 14 whilst relative horizontal movement in a direction parallel to the pivot axes of the links 17 is restrained by the mating engagement between the raised abutment surfaces 34 at opposite ends of each rail 20 of a link element 17 in one chain and associated inner faces 35 of tooth formations 30 of an opposite link element 17 in the other chain the interlocking engagement between the link elements 17 of the meshing chains 13 14 is effective in providing an extremely rigid structure which is capable of resisting significant deformation forces the pivotal interconnection between adjacent links 17 of each chain 13 14 is made by horizontally aligned projecting cylindrical bosses 36 integrally moulded with the side plate portions 26 the bosses engaging in co operable aligned holes 39 formed in the opposed side plates 18 adjacent the upper front corners thereof of an adjacent link 17 the offset portions 26 permit the side plates 18 of adjacent connected links 17.