chain preferably the fabric is a combination of fibers and a rubber composition the fibers preferably should be strong tough and elastic synthetic polyamide materials commonly known as nylon they are fashioned into strands and then woven together into a web the rubber is preferably carboxylated nitrel rubber the web is bonded to the rubber composition in a process which causes the fabric to take up a substantial amount of the rubber there is a predominance of the nylon web in one side of the combination in order to form a substantially smooth strong fabric surface and a predominance of the rubber adjacent the nylon web which may be shaped into protuberances in order to form a coarse nubby surface on the other side of the combination the material forming the fabric sheath 28 just described has been manufactured by the burrell belting company in skokie ill under the designation 200 rt a cross section of the sheath is shown in fig4 as seen in that figure the face 30 of the sheath 28 is relatively smooth being formed by the nylon web 32 which has become substantially impregnated with the rubber composition 34 the face 36 includes a large number of nubs 38 only some of which are numbered in the drawing to avoid confusion in order to provide a face which is coarse and spiked foam blocks 12 which the sheath 28 is wrapped around see fig1 and 2 are formed of foam panels which are preferably nine feet.