Abstract: A mechanism for converting rotary motion into translatory motion including a threaded shaft, an internally threaded sleeve surrounding the shaft and a plurality of balls in a helical passage defined between the sleeve and shaft by the threads, comprises a channel for transferring the balls between opposite ends of the passage which includes a longitudinal bore in the sleeve and, in each of two identical annular end plates secured to either end of the sleeve, an arcuate recess formed by a central bore portion of increased radius, and a linear groove having opposite ends communicating with the recess and the sleeve bore. The end plates which may be moulded from self-lubricating plastics material may have stops adjacent the inner ends of the groove which project inwardly between adjacent threads of the groove to guide the balls into the groove. The ends of the sleeve bore may be chamfered and the outer end walls of the grooves inclined to aid transfer of the balls between the bore and groove.