Acme thread form

Acme threads are a common screw thread profile, which offers high strength with ease of machining and assembly. Typically found where large loads or accuracy is required as in vises or the lead screw of a lathe. Standardized variations include multiple-start threads, left-hand threads, and self-centering threads which are less likely to bind under lateral forces.

Threads formed around a shaft are used to translate rotational motion into linear motion. In this way fasteners such as screws and bolts convert the torque of a screwdriver or wrench into a compressional holding force. The linear motion is also used to move or position mechanical components, such as a tool post on a metal-working lathe, or to raise or lower a garage door.

Precursors to the Acme Thread

The most common profile for threads in a fastener is a V-form; the threads, if sliced parallel to the shaft would describe a series of Vs, representing the apex of the threads and the valleys between them. V-form threads are narrower and weaker closer to the apex, which makes them susceptible to shearing off under loads which would otherwise not cause the screw or nut body to fail.

Jack screws for moving large structures, bench vises and C-clamps are common examples of the type of screw application which require high strength, and square threads were developed to address this issue. Square threads are cut with a square profile, with a zero degree thread angle which provides a stronger, more robust bearing shoulder for the rotating shaft, and subsequently far more strength than a V-form thread. However, square threads are difficult to machine.

The Acme Profile

The Acme thread was developed as a compromise, to offer better strength than a V-form while overcoming the problems with the square thread form.

The Acme thread form uses a 29 degree thread angle with flat apex and valley.

External links

* [http://www.mech.uwa.edu.au/DANotes/threads/intro/intro.html Threaded Fasteners]
* [http://www.efunda.com/designstandards/screws/screws_intro.cfm Screw Thread Forms]
* [http://www.gewinde-normen.de/en/index.html International Thread Standards]
* [http://www.roymech.co.uk/Useful_Tables/Cams_Springs/Power_Screws.html Comparision with other power threads]
* [http://www.diracdelta.co.uk/science/source/a/c/acme%20thread/source.html Drawing of an Acme thread]
* [http://www.gewinde-normen.de/en/trapezoidal-coarse-thread.html Trapezoidal coarse thread]
* [http://www.gewinde-normen.de/en/trapezoidal-fine-thread.html Trapezoidal fine thread]