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Buttress thread forms, also known as sawtooth thread forms or
breech-lock A breech-lock is a system for mounting camera lenses to camera bodies. The lens is attached to the camera by means of a rotating ring which is used to tighten the lens to the camera by friction. Other methods for mounting a lens to a camera incl ...
thread forms. are screw thread profiles with an asymmetric shape, having one square face and the other slanted. They are most commonly used for leadscrews where the load is principally applied in one direction. The asymmetric thread form allows the thread to have low friction and withstand greater loads than other forms in one direction, but at the cost of higher friction and inferior load bearing in the opposite direction. They are typically easier to manufacture than
square thread form The square thread form is a common screw thread profile, used in high load applications such as leadscrews and jackscrews. It gets its name from the square cross-section of the thread.Bhandari, p. 203. It is the lowest friction and most effic ...
s but offer higher load capacity than equivalently sized
Trapezoidal thread form Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are re ...
s.


Buttress thread characteristics

The buttress thread form is designed to handle axial thrust applied principally in one direction. The load-bearing thread face is perpendicular or at a slight slant (usually no greater than 7°) to the screw axis. The trailing face is slanted, often at 45°. The resulting thread form has the same low friction properties as a
square thread form The square thread form is a common screw thread profile, used in high load applications such as leadscrews and jackscrews. It gets its name from the square cross-section of the thread.Bhandari, p. 203. It is the lowest friction and most effic ...
but at about twice the
shear strength In engineering, shear strength is the strength of a material or component against the type of yield or structural failure when the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a materi ...
due to the long thread base. This thread form is also comparatively easy to machine on a
thread milling In manufacturing, threading is the process of creating a screw thread. More screw threads are produced each year than any other machine element. There are many methods of generating threads, including subtractive methods (many kinds of thread cu ...
machine, unlike the difficult-to-machine square thread form. It can also compensate for nut wear using a
split nut A split nut is a nut that is split lengthwise into two pieces (opposed halves) so that its female thread may be opened and closed over the male thread of a bolt or leadscrew. This allows the nut, when open, to move along the screw without the sc ...
, much like the trapezoidal
Acme thread form Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are re ...
.Bhandari, p. 204. Buttress threads have often been used in the construction of
artillery Artillery is a class of heavy military ranged weapons that launch munitions far beyond the range and power of infantry firearms. Early artillery development focused on the ability to breach defensive walls and fortifications during si ...
, particularly with the screw-type
breechblock A breechblock (or breech block) is the part of the firearm action that closes the breech of a breech loading weapon (whether small arms or artillery) before or at the moment of firing. It seals the breech and contains the pressure generated by t ...
. They are also often used in
vise A vise or vice (British English) is a mechanical apparatus used to secure an object to allow work to be performed on it. Vises have two parallel jaws, one fixed and the other movable, threaded in and out by a screw and lever. A vise grip is n ...
s, because great force is only required in one direction. It is obvious on inspection that a buttress thread with perpendicular face, operating in a split nut, generates minimal disengagement force when tightened in the normally loaded direction, and thus it is possible to derive quick release devices to, for example, allow rapid repositioning of the movable jaw of a vise without having to rotate the screw by many turns. A screw profile, such as acme, where the thrust face is not perpendicular to the axis, generates a significant disengagement force on a split nut, therefore a more robust controlling mechanism would be required. Quick release vices are readily available. It is not known whether any of them are currently using buttress screws. A
expired patent
for a clamp using a buttress thread exists an
this article
describes a vise whose screw thread is disengaged by reverse rotation, which is likely to use a buttress thread, however no currently manufactured devices of that nature have been found at this time (October 2018).


Types

The image gallery below shows some of the types of buttress threads. File:Buttress thread form.svg, Simple buttress thread formOberg, p.1817 File:ANSI 45-7 Buttress thread form.svg, The ANSI 45°/7° buttress thread form File:British 45-7 buttress thread form.svg, The British 45°/7° buttress thread form File:45-5 buttress thread form.svg, The 45°/5° buttress thread form File:Saw tooth buttress thread form.svg, The 30°/3° German "Sägegewinde" (saw tooth) buttress thread form


Mechanics


Buttress thread in oil field tubing

In oil field tubing, ''buttress thread'' is a pipe thread form designed to provide a tight hydraulic seal. The thread form is similar to that of
Acme thread Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are re ...
Figure 6. but there are two distinct threaded portions of differing diameters and profiles, the larger having a wedging profile, with a tapered sealing portion in between the larger and smaller diameters. High torque may be transmitted and longitudinal force is transmitted almost parallel to the axis. The thread is about the same strength as standard v threads.


See also

*
Leadscrew A leadscrew (or lead screw), also known as a power screw or translation screw,Bhandari, p. 202. is a screw used as a linkage in a machine, to translate turning motion into linear motion. Because of the large area of sliding contact between ...
*
Trapezoidal thread form Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are re ...


References


Bibliography

* * * *{{Citation , last = Timings , first = Roger Leslie , title = Newnes Mechanical Engineer's Pocket Book , publisher = Newnes , year = 2005 , edition = 3rd , url = https://books.google.com/books?id=IZdDk1cMlLcC , isbn = 978-0-7506-6508-7 , postscript =. Screws Thread standards Threading (manufacturing)