Buttress thread

An asymmetric thread that takes heavy load in one direction only.

Definition

One flank is nearly perpendicular to the axis and the other is at 45°, so the thread behaves like a ratchet: it carries very high thrust in one direction and very little in the other. It is used where the load is unidirectional and heavy — shell cases, breech mechanisms, jacks and some pressure vessels.

Because it only works one way, orientation on the drawing is critical. A buttress thread installed backwards has almost no capacity.

Common misuse

Specifying a buttress thread without stating which direction it faces. The drawing is ambiguous and the part may be made backwards.