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The term buttress thread is used to describe a specific screw thread design or profile. The defining characteristics of buttress thread profiles are almost perpendicular leading faces or edges and strongly slanted trailing faces. This design allows the buttress thread to exert considerable force in one direction of rotation and to be tensioned and released quickly with a minimum of friction and effort. These threads are most commonly used in the manufacture of the lead screws used in machinery and as hydraulic seals for viscous liquids such as oil.
Any threaded mechanism is designed to fulfill a specific purpose such as fastening items together in the case of a nut and bolt. A bolt will tighten up when turned in a certain direction and release that tension when turned in the other direction. The thread profile of that bolt will reflect this in the specific characteristics of the leading and trailing faces of its individual thread peaks. If any threaded item such as a bolt is cut through along its length and viewed from the side, the thread profile will resemble the teeth of a saw with a series of equally spaced peaks and troughs. The parts of those peaks facing away from the head of the bolt are known as the leading edges and those facing toward the head, the trailing edges.
A buttress thread is characterized by leading edges that are almost perpendicular to the axis or length of the bolt and trailing edges that are fairly sharply slanted. Typically a buttress thread leading edge is slanted at angles of 3° to 7° and the trailing edges at angles ranging from 33° to 45°. This means that thethread profile looks more like a succession of waves than the V shapes seen on a conventional fastener such as the nut and bolt. This unique thread pattern allows items cut with a buttress thread to exert a powerful force in one direction only and to release quickly with a minimum of effort.
The kind of thread can be an ideal option for use on machine lead screws, such as those used to open and close bench vices, where tensioning power is needed in one direction only. This principle also makes a buttress thread a good choice for use on the breech blocks of artillery weapons where a very strong lock-up is required to contain the rearward force exerted by burning propellants. The buttress design of thread is also very suitable for use in viscous liquid seals such as those used extensively in oil field piping. The threads powerful one-way parallel axis force allows for tightly sealed joints that can be removed and re-tensioned quickly and with little effort.
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