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A screw thread is an inclined plane wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. More screw threads are produced each year than any other machine element. [1] Threads are generally produced according to one of the many standards of thread systems.
United States Standard thread (USS thread), also known as Sellers Standard thread, [1] Franklin Institute thread [1] and American Standard thread, [2] is a standard for inch based threaded fasteners and washers. [3] The USS standard is no longer supported. It, together with the SAE fastener standard, was incorporated into the Unified Thread ...
The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries.
ISO 965 (ISO general purpose metric screw thread—tolerances) is an International Organization for Standardization (ISO) standard for metric screw thread tolerances. [1] It specifies the basic profile for ISO general purpose metric screw threads (M) conforming to ISO 261. [2] The tolerance system refers to the basic profile in accordance with ...
The term fine adjustment screw typically refers to screws with threads from 40–100 TPI (Threads Per Inch) (0.5 mm to 0.2 mm pitch) and ultra fine adjustment screw has been used to refer to 100–254 TPI (0.2 mm to 0.1 mm pitch). These screws are most frequently used in applications where the screw is used to control fine motion of an object.
Coarse threads install much faster as they require fewer turns per unit length. Finer threads are stronger as they have a larger stress area for the same diameter thread. Fine threads are less likely to vibrate loose as they have a smaller helix angle and allow finer adjustment. Finer threads develop greater preload with less tightening torque. [5]
The major minus pitch technique also works for inch-based threads, but you must first calculate the pitch by converting the fraction of threads-per-inch (TPI) into a decimal. For example, a screw with a pitch of 1/20 in (20 threads per inch) has a pitch of 0.050 in and a 1 ⁄ 13 in pitch (13 threads per inch) has a pitch of 0.077 in.
The thread pitch increases with diameter in steps specified on a chart. The Whitworth thread system was later to be adopted as a British Standard to become British Standard Whitworth (BSW). An example of the use of the Whitworth thread are the Royal Navy's Crimean War gunboats.