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The operator rotates the two long vertical bolts (via Bevel Gear ).]]
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 Screw s, machine screws, Nuts and Bolts .

  • Connecting Threaded Pipe s and Hose s 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 .


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 Spanner s 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 Turnbuckle s.

  • In some gas supply connections to prevent dangerous misconnections.

  • In antenna connectors on some Wireless LAN Access Point s, 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.

  • Some Gas applications


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

ISO standard threads


The most common threads in use are the ISO Metric Screw Thread s (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.


Other current standards

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




  • 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 Meter s and to mount optical lenses

  • Royal Microscopical Society (RMS) thread, a special 0.8"-36 thread used for Microscope Objective Lens es.

  • 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: ) is an old German 80° thread (DIN 40430) that remained in use until 2000 in some electrical installation accessories in Germany.



ENGINEERING DRAWING



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.


  • '''. 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 Spoke s 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 .



EXAMPLES


Examples of screw threads include:


See Also: Archimedes' screw




SEE ALSO

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


REFERENCES