allows relatively large sprockets to be used and in turn reduces the forces required of the chain on the sprocket teeth to develop a given level of torque at least the sprocket plates 18 on the outer sprocket assemblies 36 and preferably the sprocket plates on the inner sprocket assemblies 37 are able to be passed completely over their associated hubs 19 for purposes of removal and replacement the sprocket plates 18 can experience relatively high wear rates due to their operating environment and from time to time may need to be replaced both the inside and outside sprocket plates can be changed without removal of their associated hubs moreover removal and replacement of these plates can be readily accomplished because the machine screws 62 63 securing these plates on their respective hubs can be conveniently reached from the outside i e the space outward of the chain 14 with the convention that the conveyor rollers 11 are to the inside with the invention replacing each of the sprocket plates 18 is a simple matter of removing three screws 62 or 63 and separating the plate from its hub the need for breaking the hub loose from its fit on a shaft 12 is eliminated prior to assembly the screws 62 63 can be coated with a suitable protective sealant so that the risk of corrosion in the threaded holes in the sprocket plate 18.