Kelly drive

A kelly drive is a type of well drilling device on an oil or gas drilling rig that employs a section of pipe with a polygonal (three-, four-, six-, or eight-sided) or splined outer surface, which passes through the matching polygonal or splined kelly (mating) bushing and rotary table.

The upper end of the kelly is screwed into the swivel, using a left-hand thread to preclude loosening from the right-hand torque applied below.

It generally is of steel-reinforced rubber construction but also assemblies of Chiksan steel pipe and swivels are used.

A rotary bushing fits around the flat sides to provide the torque needed to turn the kelly and the drill string.

Rollers in the bushing permit the kelly free movement vertically while rotating.