# Square thread form

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Screw thread profile with a square cross-section

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Dimensions of a square thread form

The **square thread form** is a common [screw thread](/source/Screw_thread) profile, used in high load applications such as [leadscrews](/source/Leadscrew) and [jackscrews](/source/Jackscrew). It gets its name from the square cross-section of the thread.[1] 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](/source/Trapezoidal_threads) ([Acme](/source/Acme_thread_form) or metric trapezoidal). Due to the lack of a [thread angle](/source/Thread_angle) there is no radial pressure, or [bursting pressure](/source/Bursting_pressure), on the nut. This also increases the nut life.[1]

The greatest disadvantage is the difficulty in machining such a thread. The [single-point cutting tools](/source/Tool_bit) or [taps and dies](/source/Tap_and_die) 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.[1]

## Classification

Square threads are defined as follows by [ISO](/source/International_Organization_for_Standardization) standards:

- S q 60 × 9 {\displaystyle 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](/source/Lead_(engineering)) and the value in the parentheses is the pitch. For example:

- S q 60 × 18 ( P 9 ) L H {\displaystyle 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.[2]

Standard pitches for metric diameters[2] Nominal diameter [mm] Pitch [mm] 22, 24, 26, 28 5 30, 32, 36 6 40, 44 7 48, 50, 52 8 55, 60 9 65, 70, 75, 80 10 85, 90, 95, 100 12

## Mechanics

Main article: [Leadscrew mechanics](/source/Leadscrew#Mechanics)

## References

### Notes

1. ^ [***a***](#cite_ref-bhandari203_1-0) [***b***](#cite_ref-bhandari203_1-1) [***c***](#cite_ref-bhandari203_1-2) Bhandari, p. 203.

1. ^ [***a***](#cite_ref-bhandari205_2-0) [***b***](#cite_ref-bhandari205_2-1) Bhandari, p. 205.

### Bibliography

- Bhandari, V B (2007), [*Design of Machine Elements*](https://books.google.com/books?id=f5Eit2FZe_cC), Tata McGraw-Hill, [ISBN](/source/ISBN_(identifier)) [978-0-07-061141-2](https://en.wikipedia.org/wiki/Special:BookSources/978-0-07-061141-2).

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Adapted from the Wikipedia article [Square thread form](https://en.wikipedia.org/wiki/Square_thread_form) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Square_thread_form?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.