Screw threads

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge 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]

The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one revolution.^[2] In most applications, the lead of a screw thread is chosen so thatfriction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight plastic deformation.

The most used screw thread forms are those having symmetrical sides with inclined at equal angles. The Unified, the Whitworth and the Acme forms fall into this category. Symmetrical threads are easy to manufacture and to inspect compare to non-symmetrical threads. They are widely used on all type of mass-produced general-purpose thread fasteners. In addition to being used as fasteners, certain threads are used to move or drive machine parts against heavy loads; thus they require a stronger thread system. The most widely use translation thread forms are the Square and the Acme. Square thread is the most efficient, but it is also the hardest to manufacture due to its parallel sides. Another disadvantage is they cannot be adjusted to compensate for wear. The Acme thread, although less efficient, is easier to manufacture and can be adjusted.

DefinitionsAllowance is the prescribed difference between the design (maximum material) size and the basic size. Basic Size is the nominal size of the screw thread being produced. The tolerance is applied to the basic size to determine the maximum and minimum acceptable dimension. Thread Classes are used to specify the amounts of tolerance and allowance. Classes 1A, 2A and 3A apply to external threads; classes 1B, 2B and 3B apply to internal threads.

Unified Screw Threads Unified Thread Series - These are groups of diameter-pitch combinations differentiate from each other by the numbers of threads per inch applied to a specific diameter. The various diameter-pitch combinations of eleven standard series are shown here. Selected combinations are shown here.

Coarse Thread Series - The UNC/UNRC is the most commonly used in the bulk production of bolts, screws, and nut for general engineering applications. It is also used for threading into lower tensile strength materials such as cast iron, mild steel and to softer materials as brass and aluminum to prevent stripping of the internal threads.

Fine Thread Series - The UNF/UNRF series is also suitable for production of bolts, screws, and nuts for other applications. The external thread of this series has greater contact area than comparable sizes of the Coarse series. They are suitable when the resistance of stripping both external and internal threads equals or exceeds the tensile load carrying capacity of the screw.

Extra Fine Thread Series - The UNEF/UNREF series is applicable when even finer pitches of threads are required. They are very useful for short lengths of engagement, where their fine thread increase the contact area in a short screw.

Constant Pitch Series The UN series provides a comprehensive range of diameter pitch combinations where the Coarse, Fine, and Extra-Fine series do not satisfy the requirement of the design. They are available with 4, 6, 8, 12, 16, 20, 28 and 32 threads per inch. More details regarding the 8, 12, and 16 thread series are provided below: The 8-UN series coarse thread is used with large diameter (greater than 1 inch) and high-pressure applications, such as high-pressure joint bolts. The 12-UN series is also used for large diameters with a medium pitch thread. Originally intended for use in pressure vessels, it is currently used as a fine pitch series for diameter larger than 1½ inch. The 16-UN series is used for large diameters with fine pitch thread. It is often use as a nut retainer and can be considered as an extra fine pitch series for diameter larger than 1 11/16 inches.