Drill collars (usually straight drill collars) that have been machined with a reduced diameter at the box (up) end so that they may be more easily handled with open-and-close elevators. The elevators close around the reduced-diameter section, latch securely, and a shoulder on the elevators prevents the larger diameter end of the collar from passing through the elevators, so the collars can be lifted. If zip grooves are not used on the collars, special lifting subs must be threaded into each stand of collars for lifting, which is time-consuming and less efficient than zip grooves. The primary drawback to zip grooves is that they may reduce the life of the collar by putting an effective limit on how many times the collar threads may be recut.

Related Terms:

lifting sub