Buttress thread

Buttress thread form

The buttress thread form, also known as the breech-lock thread form,^[1] refers to two different thread profiles. One is a type of leadscrew and the other is a type of hydraulic sealing thread form. The leadscrew type is often used in machinery and the sealing type is often used in oil fields.

Buttress thread in machinery

In machinery, the buttress thread form is designed to handle extremely high axial thrust in one direction. The load-bearing thread face is perpendicular to the screw axis.^[2] or at a slight slant (usually no greater than 7°)^[3] The other face is slanted at 45°. The resulting thread form has the same low friction properties as a square thread form but at about twice the shear strength due to the long thread base. This thread form also is easy to machine on a thread milling machine, unlike the difficult to machine square thread form. It can also compensate for nut wear using a split nut, much like the Acme thread form.^[4]

Buttress threads have often been used in the construction of artillery, particularly with the screw-type breechblock.^[1] They are also often used in vises, because great force is only required in one direction.^[4]

Types

The image gallery below shows some of the types of buttress threads.

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  Simple buttress thread form^[1]

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  The ANSI 45°/7° buttress thread form^[2]

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  The British 45°/7° buttress thread form^[3]

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  The 45°/5° buttress thread form^[1]

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  The 33°/3° German "Sägegewinde" (saw tooth) buttress thread form^[1]

1. ^ ^{a b c} Oberg, p.1817
2. ^ Oberg, pp. 1819–1820.
3. ^ Timings, p. 127.

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^[5][6] the force is transmitted almost parallel to the axis and thread is about the same strength as standard v threads.

References

1. http://books.google.com/books?id=gYZIAAAAMAAJ&pg=PA141
2. Barnwell, p. 163.
3. US patent 5127784, David Eslinger, "Fatigue-resistant buttress thread", issued 1992-07-07 
4. Bhandari, p. 204.
5. US patent 6893057, M. Edward Evans, "Threaded pipe connection", issued 2005-05-17  Figure 6.
6. Oil field glossary entry for buttress thread

Bibliography

• Barnwell, George W. (1941), The new encyclopedia of machine shop practice, Wm. H. Wise & Company.
• Bhandari, V B (2007), Design of Machine Elements, Tata McGraw-Hill, ISBN 978-0-07-061141-2.
• Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry H. (2000), Machinery's Handbook (26th ed.), New York: Industrial Press Inc., ISBN 0-8311-2635-3.
• Timings, Roger Leslie (2005), Newnes Mechanical Engineer's Pocket Book (3rd ed.), Newnes, ISBN 978-0-7506-6508-7.