The square thread form is a common

screw thread A screw thread is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a ''straight'' thread and t ...

profile, used in high load applications such as

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 ...

s and

jackscrew A jackscrew, or screw jack, is a type of jack that is operated by turning a leadscrew. It is commonly used to lift moderate and heavy weights, such as vehicles; to raise and lower the horizontal stabilizers of aircraft; and as adjustable suppor ...

s. It gets its name from the square cross-section of the thread.Bhandari, p. 203. It is the lowest friction and most efficient thread form, but it is difficult to fabricate.

Advantages & disadvantages

The greatest advantage of square threads is that they have a much higher intrinsic efficiency than trapezoidal threads (

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or metric trapezoidal). Due to the lack of a

thread angle In mechanical engineering, the thread angle of a screw is the included angle between the Threading (manufacturing), thread flanks, measured in a plane containing the thread axis.. This is a defining factor for the shape of a screw thread. Standar ...

there is no radial pressure, or bursting pressure, on the nut. This also increases the nut life. The greatest disadvantage is the difficulty in machining such a thread. The single-point cutting tools or

taps and dies In the context of threading, taps and dies are the two classes of tools used to create screw threads. Many are cutting tools; others are forming tools. A tap is used to cut or form the female portion of the mating pair (e.g. a nut). A die is us ...

used to cut the thread cannot have efficient rake and relief angles (because of the square form), which makes the cutting slow and difficult. Square threads also cannot carry as much load as a trapezoidal thread, because the root of the square thread is smaller. Also, there is no way to compensate for wear on the nut, so it must be replaced when worn out.

Classification

Square threads are defined as follows by

ISO The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. Me ...

standards: :

Sq\,60 \times 9

where ''Sq'' designates a square thread, ''60'' is the nominal diameter in millimeters, and ''9'' is the pitch in millimeters. When there is no suffix it is a single start thread. If there is a suffix then the value after the multiplication sign is the

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

and the value in the parentheses is the pitch. For example: :

Sq\,60 \times 18 (P9) LH

would denoted two starts, as the lead divided by the pitch is two. The "LH" denotes a left hand thread.Bhandari, p. 205.

Mechanics

References

Notes

Bibliography

*{{Citation , last = Bhandari , first = V B , title = Design of Machine Elements , publisher = Tata McGraw-Hill , year = 2007 , url = https://books.google.com/books?id=f5Eit2FZe_cC , isbn = 978-0-07-061141-2. Screws Threading (manufacturing)