The power sub is the simplest and lowest cost hydraulic drilling tool. As the name implies, a power sub is a drill pipe sub which is hydraulically powered. Since it is rotated by hydraulic power, it also has torque arms to react the torque. Like ordinary drill pipe subs, the power sub has top and bottom API threaded connections.
The power sub does not include a heavy thrust bearing to carry the hook load, or a high pressure wash pipe & packing seal arrangement to conduct drilling fluids while rotating. To provide these functions, the power sub must be screwed into the bottom of an oilfield rotary swivel. The load-carrying rating of the power sub is limited by the swivel it is suspended from.
So, a power swivel is the combination of a power sub and a rotary swivel into one short assembly. The result is a hydraulically-powered rotary swivel called a power swivel.
