Apparatus is provided for converting rotational motion to linear motion which includes a screw shaft and nut surrounding the screw shaft, each having a helical thread. A plurality of threaded planetary rollers is interposed between and in contact with the threads of the screw shaft and nut. A secondary locking thread-locking threaded path system is formed on two adjacent or all of the screw shaft, nut and planetary rollers to provide stable rotation and to maintain the planetary rollers in place. The screw shaft can have the same or a different number of starts than the number of starts on the nut.