Screw Thread Forms
Screw threads can be classified according to their form.
V-Thread: These are triangular threads with flanks that typically form 60° with each other. The crests and roots are sharp, but in some cases, as a small flat portion due to limitations in fabrication.
American National Thread: Formerly known as the United States Standard Screw Thread, the American National Thread is a more standardized version of the V-thread which has specific dimensions to the flatness of the crests and roots of the threads. This form replaced the V-thread for general use.
British Whitworth Thread: This was the British counterpart of the American National Thread.
Unified Thread: This thread form replaced the American National Thread along with thread standards from Canada and Britain. This was developed to allow interchangeability of parts. Unified threads still have the V-shape profile but with rounded or flat crests and roots. The Unified Thread Standard (UTS) consists of series, namely, Unified Fine (UNF), Unified Coarse (UNC), Unified Extra Fine (UNEF), and Unified Special (UNS).
Metric Thread: This thread form was developed to transition from the imperial-based measurement into the metric system. This was brought by the ISO, displacing the UTS thread form.
Square Thread: Square threads are special-purpose threads used for power transmission. Theoretically, they are the ideal thread for mechanisms and drive applications due to the perpendicularity of the load-bearing faces or flanks with the axis. However, this form is not practical due to manufacturing limitations.
Acme Thread: This thread form is a modification of the square thread. The acme thread is characterized as having a trapezoidal form with a narrower root than its crest. Acme threads are stronger and easier to machine than square threads.
Buttress Thread: In this thread form, one flank is perpendicular or with a slight angle with the axis while the other has a 45° angle. This thread form is designed to transmit high loads in one direction.
Knuckle Thread: Knuckle threads have highly rounded crests and roots with a flank angle of 30°. The rounded profile allows debris to be shifted to not interfere with the meshing of the threads.
