Screw thread facts for kids

Internal and external threads of a common nut and bolt

An Archimedes' screw, used to raise water up

Gears from a five-speed + reverse gearbox from the 1600 Volkswagen Golf (2009).

A screw thread, often shortened to thread, is a helical structure. It 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.

The tightening of a screw (hardware) to a bolt is like driving a wedge into a gap until it sticks fast through friction and slight plastic deformation (how the surfaces of the two objects slightly squash together).

A screw thread is one of the six simple machines which give mechanical advantage. In this dynamic (moving) use, screw threads are set up as gears. Most of the steam engines of the 19th century turned steam into motion. By using gears with screw threads, the direction of movement can be changed. A very complicated modern version is the way the movement of pistons in a car engine is turned into movement of the car wheels. The illustration of transmission gears gives an idea of how screw threads are used in the transmission.

The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one revolution.

Images for kids

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  Screw thread, used to convert torque into the linear force in the flood gate. The operator rotates the small vertical bevel gear in the center. Through mechanical advantage this eventually causes the horizontal bevel gears (at far left and far right, with threaded center holes) to rotate. Their rotation raises or lowers the two long vertical threaded shafts - as they are not free to rotate.

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  Right- and left-handed screw threads

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  Lead and pitch for two screw threads; one with one start and one with two starts

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  Up to four starts are labeled with different colors in this example.

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  Camshaft cover stud threaded ¹⁄₄-20 UNC (left, for aluminium cylinder head) and ¹⁄₄-28 UNF (right, for steel nut; from a 1960s Jaguar XK engine)

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  The three diameters that characterize threads

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  Variants of snug fit. Only threads with matched PDs are truly snug, axially as well as radially.

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  An example of M16, ISO metric screw thread

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  Graphic representation of formulas for the pitches of threads of screw bolts

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  A good summary of screw thread standards in current use in 1914 was given in Colvin FH, Stanley FA (eds) (1914): American Machinists' Handbook, 2nd ed, New York and London, McGraw-Hill, pp. 16–22. USS, metric, Whitworth, and BA standards are discussed. The SAE series was not mentioned—at the time this edition of the Handbook was being compiled, they were either still in development or just newly introduced.

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  A table of standard sizes for machine screws as provided by the American Screw Company of Providence, Rhode Island, USA, and published in a Mechanical Engineers' Handbook of 1916. Standards seen here overlap with those found elsewhere marked as ASME and SAE standards and with the later Unified Thread Standard (UTS) of 1949 and afterward. One can see the theme of how later standards reflect a degree of continuation from earlier standards, sometimes with hints of long-ago intracompany origins. For example, compare the 6–32, 8–32, 10–24, and 10–32 options in this table with the UTS versions of those sizes, which are not identical but are so close that interchange would work.

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  Survey results on the use of SAE standards (including screw size standards), reported in the journal Horseless Age, 1916