Definition(s)
Top Drive or Power Swivel
A top drive (frequently also referred to as a power swivel) is a piece of equipment that serves the following functions: rotating the drill string (formerly undertaken by the rotary table); providing a conduit for drilling mud (formerly undertaken by the rotary swivel); disconnecting/connecting pipe (formerly undertaken by the iron roughneck); closing in the drill pipe by an integrated kelly valve (formerly undertaken by the kelly valve in connection with the rotary table); lifting/lowering drill string by use of standard elevator (formerly undertaken by the hook by using same kind of elevator). Top drives may be either electrically or hydraulically driven. If they are hydraulically driven, several hydraulic motors are normally used. Elevator links and elevators are not regarded as a part of the top drive (standard drilling equipment).
Source: API STD 689, Collection and Exchange of Reliability and Maintenance Data for Equipment, First Edition, July 2007. Global Standards