Screw thread

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Screw thread, used to convert torque into the linear force in the flood gate. The operator rotates the two long vertical bolts (via bevel gear).

Internal and external threads illustrated using a common nut and bolt.

The screw and nut pair can be used to convert torque into linear force. As the screw (or bolt) is rotated, the screw moves along its axis through the fixed nut, or the non-rotating nut moves along the lead-screw. A screw thread is a helical or tapered structure used to convert between rotational and linear movement or force.

Screw threads have several applications:

• Fasteners such as wood screws, machine screws, nuts and bolts.
• Connecting threaded pipes and hoses to each other and to caps and fixtures.
• The worm gear and similar mechanical connections, used particularly in adjustment mechanisms.

In all of these applications, the screw thread has two main functions:

• It converts rotary motion into linear.
• It prevents linear motion without the corresponding rotation.

In most applications, the thread pitch of a screw is chosen so that friction 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.

A screw thread may be thought of as an inclined plane wrapped around a cylinder or cone.

[edit] Standard threads

Standards for machine screw threads have existed since the mid nineteenth century, to facilitate compatibility between different manufacturers and users. Many of these standards also specified corresponding bolt head and nut sizes, to facilitate compatibility between spanners and other driving tools.

Nearly all threads are oriented so that a bolt or nut, seen from above, is tightened (the item turned moves away from the viewer) by turning it in a clockwise direction, and loosened (the item moves towards the viewer) by turning anticlockwise. This is known as a right-handed thread. Threads oriented in the opposite direction are known as left-handed.There are also self-tapping screw threads where no nut is required.

Left-handed threads are used:

• Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten, e.g. on a left-hand bicycle pedal.
• In combination with right-handed threads in turnbuckles.
• In some gas supply connections to prevent dangerous misconnections.
• In antenna connectors on some Wireless LAN access points, to prevent the connection of a non-FCC approved antenna.
• In some instances, for example early Biro pens, to provide a "secret" method of disassembly.

Unless stated otherwise, all standards below specify right-handed threads.

[edit] ISO standard threads

The most common threads in use are the ISO metric screw threads (M) and BSP threads also called G threads for pipes.

These were standardized by the International Organization for Standardization in 1947. Before that, there were separate metric thread standards used in France, Germany, and Japan, and the Swiss had a set of threads for watches.

[edit] Other current standards

In particular applications, largely for reasons of backwards compatibility, threads other than the ISO metric threads remain commonly used. These include:

• Unified Thread Standard, (UTS), which is still in common use in the United States and Canada. This standard includes:
  • Unified Coarse (UNC)
  • Unified Fine (UNF)
  • Unified Extra Fine (UNEF)
  • Unified Special (UNS)
  • National pipe thread (NPT)

• British Standard Whitworth (BSW), and for other Whitworth threads including:
  • British Standard Fine (BSF)
  • Cycle Engineers' Institute (CEI)
  • British standard pipe thread (BSP)
• British Association screw threads (BA)

• Camera case screws, used to mount a camera on a photographic tripod:
  • ¼″ British Standard Whitworth (BSW) used on almost all small cameras
  • ⅜″ BSW for larger (and some older small) cameras
• British Association screw threads (BA), used in moving coil meters and to mount optical lenses
• Royal Microscopical Society (RMS) thread, a special 0.8"-36 thread used for microscope objective lenses.
• Microphone stands:
  • ⅝″ 27 turns per inch (tpi) Unified Special thread (UNS, USA and the rest of the world)
  • ¼″ BSW (not common in the USA, used in the rest of the world)
  • ⅜″ BSW (not common in the USA, used in the rest of the world)
• Stage lighting suspension bolts (in some countries only; some have gone entirely metric, others such as Australia have reverted to the BSW threads, or have never fully converted):
  • ⅜″ BSW for lighter luminaries
  • ½″ BSW for heavier luminaries
• Panzergewinde (Pg) (also: Stahlpanzerrohr-Gewinde) is an old German 80° thread (DIN 40430) that is still in use in some electrical accessories, such as cord grips.

[edit] Engineering drawing

In American engineering drawings, ANSI Y14.6 defines standards for indicating threaded parts. Parts are indicated by their nominal diameter (the nominal outside diameter of the screw threads), number of threads per inch, and the class of the thread. For example, “.750-10UNC-2A” is male (A) with a nominal outside diameter of 0.750″, and 10 threads per inch; “.500-20UNC-1B” would be female (B) with a 0.500″ nominal diameter (i.e., the hole will be drilled less than that) and 20 threads per inch. An arrow points from this designation to the surface in question.^[1]

[edit] Manufacturing screw threads

Screw threads are normally manufactured by one of three methods:

• Cutting: The excess material is removed, with taps and dies for smaller diameters, or with a thread-cutting lathe for larger ones.

• Rolling: The material is extruded into a thread through mechanical pressure. Common for high-production threads of diameters typically smaller than one inch. A rolled thread is instantly recognizable because the thread has a larger diameter than the blank rod from which it has been made. The threads of bicycle spokes provide a perfect example. Rolled threads tend to be slightly stronger than cut threads.

• Casting: Material is either heated to a liquid (or rarely a gas), or dissolved in a liquid that will evaporate and allow the material to solidify (such as plaster or cement). Alternately, the material may be forced into a mould as a powder and compressed into a solid, as with graphite.

[edit] Examples

See also: Archimedes' screw

Examples of screw threads include:

• Fasteners:
  • Wood screws.
  • Nuts and bolts, machine screws, cap screws.
• C-clamps.
• Threaded pipe fittings.
• Threaded attachments used to mount equipment on stands, such as:
  • The case screw used to mount a camera on a photographic tripod.
  • The thread used to mount a microphone or microphone cradle on a microphone stand.

[edit] See also

See especially screw for more on standard machine screw threads and their history and on screw threads generally. See also:

• USS or United States Standard
• National Thread Form
• Acme Thread Form
• Buttress Thread Form
• National Pipe Thread Form
• Dryseal Pipe Threads Form
• Hose Coupling Screw Thread Form
• Metric: M Profile Thread Form
• Metric: MJ Profile Thread Form
• Joseph Whitworth
• British Association screw threads (BA)
• Buttress Thread Form

[edit] References

1. ^ Wilson, Bruce A. (2004). Design Dimensioning and Tolerancing, 4th ed., Goodheart-Wilcox. ISBN 1-59070-328-6.  pp. 77–78 (page numbers may be from an earlier edition).