1 and carries the two auxiliary rolls or roller assemblies 18 these are identical in construction and each contacts a respective one of the drive rolls 15 each roller assembly 18 comprises a roller bearing having an annular outer race 38 an annular inner race 39 and bearings 40 positioned between the inner and outer races the bearing is mounted coaxially about the shaft 14 on an annular rubber portion 41 a metal pin 42 abuts the radially inner surface of the inner race 39 and extends through the rubber portion 41 and an aperture 43 in the shaft 14 into the shaft a moulded plastics housing 44 is mounted within the shaft 14 and comprises a central tubular portion 45 integral with end portions 46 each of which has a bore 47 communicating with the tubular portion 45 a pair of light emitting diodes 48 are mounted in the inner ends of the bores 47 while a pair of phototransistors 49 are mounted at the outer ends of the bores 47 for clarity only portions of the connecting wires from the light emitting diodes 48 and the phototransistors 49 have been illustrated in fact these wires will pass along and out of the shaft 14 to monitoring circuitry to be described below and to facilitate assembly all wires extend from the same end of the shaft each portion 46 of the housing 44 also has an.