Abstract: A drive sprocket is rotatably mounted on the end of a shaft of a roller and is engaged with a drive chain. Coupled with the drive sprocket, to rotate with it, is a first drive member, having a frusto-conical axially directed annular drive transmission surface. This surface is in contact with the corresponding surface of a second drive member, which is keyed to the shaft to rotate therewith, yet slidable axially on the shaft. A spring is provided to bias the drive transmission surfaces into contact to transmit drive from the chain and sprocket to the roller via the drive members. Adjustment of the spring bias and/or the cone angle of the surfaces allows for alteration of the level of resistance to rotation of the roller at which the surfaces will slip and cease to transmit drive to the roller.