as shown in fig1 a mineral mining installation comprises a mineral winning machine in the form of a shearer 20 mounted on a scraper - chain conveyor 21 disposed alongside a mineral face ( not shown ). the conveyor 21 is constructed , as is shown , from a series of individual channel sections or pans 22 arranged end - to - end and a scraper - chain conveyor assembly 23 which is circulated along the pans 22 to transfer material detached from the mineral face by the shearer 20 . the shearer 20 is moved back and forth along the conveyor 21 by a reactive - track haulage system disposed at the goaf side ofthe conveyor 21 . this haulage system , known per se , is composed of a toothed sprocket wheel 1 rotatably mounted on the main body of the shearer 20 and engaging with a chain 4 fitted into a carrier and guide means 24 . the carrier and guide means 24 is itself composed of sections mounted on a support structure 25 at the goaf side of the pans 22 . the drive wheel 1 is driven by drive means of the shearer 20 to propel the shearer 20 along the chain 4 and the conveyor 21 . a tongue or lug 26 depends from the shearer body to engage beneath a shoulder 27 of the guide and hold the sprocket wheel 1 down in constant meshing engagement with the chain 4 . fig2 to 4 and 5 to 7 depict constructional forms for the chain 4 and drive wheel 1 representing embodiments of the invention . as shown , in both versions , the drive wheel 1 has nine teeth 2 with cycloidal involute flanks 3 . the associated chain 4 is composed of horizontal links 5 into which the teeth 2 are received and vertical links 6 which engage between the teeth 2 . the links 5 are basically of conventional elongate oval shape constructed from round , i . e . circular cross - sectional material , while the links 6 are of special design . the links 6 are smaller in length along the longitudinal axis 7 of the chain 4 than the links 5 . the links 6 each have a main generally rectangular upstanding body . in the embodiment shown in fig2 to 4 , the main body of each link 6 merges with a flange - like base or foot portion 12 . the sides of the base portion 12 are curved as shown in fig3 . the base portions 12 of the links 6 serve to locate the chain 4 in a channel ( not shown ) of the guide 24 and prevent the chain 4 from lifting out of the guide 24 . in the simpler version shown in fig5 to 7 , the links 6 do not have the base portions 12 and as shown in fig1 one or both sides of the links 5 are simply held in a pocket of the guide 24 . in both embodiments , the main body of each link 6 has an elongate opening 8 oriented in the direction of the longitudinal axis 7 of the chain 4 . this opening 8 receives the looped end regions of a pair of adjacent horizontal links 5 . the opening 8 has a length just slightly greater than twice the diameter of the links 5 with clearance just enough to permit the requisite articulation . the ends of the opening 8 are curved to match the curvature of the links 5 in lateral and vertical senses so that the links 5 are supported with large contact areas over their outer end regions . the vertical distance of the opening is also just sufficient to accommodate the links 5 with slight clearance . the main body of each link 6 has outer end faces 9 which co - operate with the flanks 3 of the teeth 2 and these faces 9 extend normal to the axis 7 and , during use , substantially vertically . the end faces 9 merge with a generally rounded upper face . the end faces 9 extend over a distance larger than the diameter of the links 5 and terminate at the upper ends above the uppermost apex plane 11 of the links 5 . this ensures adequate rolling contact for the flanks 3 of the teeth 6 and this despite variation in the distance between the axis 7 of the chain 4 and the rotational axis 10 of the wheel 1 due to clearances and tolerances of manufacture . typically , the variation can be up to 10mm and is determined primarily by the means 26 , 27 holding down the wheel 1 . the flanks 3 of the teeth 2 roll over the faces 9 of the links 6 of the chain 4 in succession and subject the chain 4 to traction and compressive forces . predominantly normal faces only act between the flanks 3 and the faces 9 of the links 6 in the direction of the axis 7 of the chain 4 to propel the machine and no transverse forces are created to any significant extent even if the aforementioned variation in the distance between the axis 7 and the axis 10 occurs . frictional forces are minimized and undue wear is precluded .