Knuckle thread

API threads taper at 3/4 inch of diameter per foot of length.

Many knuckle threads follow neither the DIN nor API standards, and are custom designed and fabricated for the particular application.

This thread form's good debris tolerance is the source of its use in oilfields, where it provides a leak-free connection in field use.

Other applications use a knuckle thread's rounded edges to reduce the stress on softer materials at a point of connection.

Some linear actuators use a knuckle thread to reduce the wear of the steel leadscrew against a plastic sliding nut.

Knuckle threads have very wide rounded crests and roots. These DIN knuckle threads also use a steep thread angle.
This leadscrew's smooth rounded knuckle thread protects the actuator's white plastic nut from wear.