Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of buttress thread. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
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.