The square thread form is a common screw thread profile, used in high load applications such as leadscrews 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 moderately and heavy weights, such as vehicles; to raise and lower the horizontal stabilizers of aircraft; and as adjustable supp ...

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 ( Acme or metric trapezoidal). Due to the lack of a

thread angle The thread angle of a screw is the included angle between the thread flanks, measured in a plane containing the thread axis.. This is a defining factor for the shape of a screw thread A screw thread, often shortened to thread, is a helical str ...

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 Taps and dies are tools used to create screw threads, which is called threading. 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 used to cut or form the ...

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 ISO is the most common abbreviation for the International Organization for Standardization. ISO or Iso may also refer to: Business and finance * Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007 * Iso ...

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 with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

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)