Screw thread

From Wikipedia, the free encyclopedia

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.
Converting rotary motion to linear motion, for lifting or moving objects, as in a leadscrew or screw jack

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. 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.

1 Standard threads
2 Engineering drawing
3 Manufacturing screw threads
4 Examples
5 See also
6 Notes
7 References
8 External links

Standards for machine screw threads have evolved since the early 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 due to fretting induced precession, 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, for example in gas welding the flammable gas supply uses left-handed threads.
In some instances, for example early Biro pens, to provide a "secret" method of disassembly.
In some applications of a leadscrew, for example the cross slide of a lathe, where it is desirable for the cross slide to move away from the operator when the leadscrew is turned clockwise.

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

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 Standard Pipe Taper (BSPT)
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 threads 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 remained in use until 2000 in some electrical installation accessories in Germany.

For a good summary of screw thread standards in current use in 1914, see Colvin FH, Stanley FA (eds) (1914): American Machinists' Handbook, 2nd ed. New York and London: McGraw-Hill, pp. 16-22.

The first historically important intra-company standardization of screw threads began with Henry Maudslay around 1800, when the modern screw-cutting lathe made interchangeable screws a practical commodity. During the next 40 years, standardization continued to occur on the intra-company and inter-company level. In 1841, Joseph Whitworth created a design that, through its adoption by many British railroad companies, became a national standard for the United Kingdom called British Standard Whitworth. During the 1840s through 1860s, this standard was often used in the United States and Canada as well, in addition to myriad intra- and inter-company standards. In April 1864, William Sellers presented a paper to the Franklin Institute in Philadelphia, proposing a new standard to replace the U.S.'s poorly standardized screw thread practice. Sellers simplified the Whitworth design by adopting a thread profile of 60° and a flattened tip (in contrast to Whitworth's 55° angle and rounded tip).^[1]^[2] The 60° angle was already in common use in America,^[3] but Sellers's system promised to make it and all other details of threadform consistent.

The Sellers thread, easier for ordinary machinists to produce, became an important standard in the U.S. during the late 1860s and early 1870s, when it was chosen as a standard for work done under U.S. government contracts, and it was also adopted as a standard by highly influential railroad industry corporations such as the Baldwin Locomotive Works and the Pennsylvania Railroad. Other corporations adopted it, and it soon became a national standard for the U.S.,^[4] later becoming generally known as the United States Standard. Over the next 30 years the standard was further defined and extended and evolved into a set of standards including National Coarse (NC), National Fine (NF), and National Pipe Taper (NPT).

During this era, various metric national thread standards were evolving in continental Europe, which usually employed 60° profiles. These were mostly unified in 1898 by the International Congress for the standardization of screw threads at Zurich, which defined the new international metric thread standards as having the same profile as the Sellers thread, but with metric sizes. Efforts were made in the early 20th century to convince the governments of the U.S., UK, and Canada to adopt these international thread standards and the metric system in general, but they were defeated with arguments that the capital cost of the necessary retooling would damage corporations and hamper the economy. (The mixed use of dualling inch and metric standards has since cost much, much more, but the bearing of these costs has been more distributed across national and global economies rather than being borne up front by particular governments or corporations, which helps explain the lobbying efforts.)

Problems with lack of interchangeability among American, Canadian, and British parts during World War II led to an effort to unify the inch-based standards among these closely allied nations, and the Unified Thread Standard was adopted by the Screw Thread Standardization Committees of Canada, the United Kingdom, and the United States on November 18, 1949 in Washington, D.C., with the hope that they would be adopted universally. (The original UTS standard may be found in ASA (now ANSI) publication, Vol. 1, 1949.) UTS consists of Unified Coarse (UNC), Unified Fine (UNF), Unified Extra Fine (UNEF) and Unified Special (UNS). The standard was not widely taken up in the UK, where many companies continued to use the UK's own British Association (BA) standard.

However, internationally, the metric system was eclipsing inch-based measurement units. In 1947, the International Organization for Standardization (interlingually known as ISO) had been founded; and in 1960, the metric-based International System of Units (abbreviated SI from the French Système international) was created. With continental Europe and much of the rest of the world turning to SI and the ISO metric screw thread, the UK gradually leaned in the same direction. The ISO metric screw thread is now the standard that has been adopted worldwide and has mostly displaced all former standards, including UTS. In the U.S., where UTS is still prevalent, over 40% of products contain ISO metric screw threads. The UK has completely abandoned its commitment to UTS in favour of the ISO metric threads, and Canada is in between. Globalization of industries produces market pressure in favor of phasing out minority standards. A good example is the automotive industry; U.S. auto parts factories long ago developed the ability to conform to the ISO standards, and today very few parts for new cars retain inch-based sizes, regardless of being made in the U.S.

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.^[5]

Page 23 of Colvin FH, Stanley FA (eds) (1914): American Machinists' Handbook, 2nd ed. New York and London: McGraw-Hill. Summarizes screw thread rolling practice as of 1914.

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 mixed with a liquid that will either dry or cure (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 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.

See also: Archimedes' screw

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

^ ASME 125th Anniversary: Special 2005 Designation of Landmarks: Profound Influences in Our Lives: The United States Standard Screw Threads
^ Wickham Roe 1916:248-249.
^ Wickham Roe 1916:249.
^ Wickham Roe 1916:249.
^ Wilson pp. 77–78 (page numbers may be from an earlier edition).

Wickham Roe, Joseph (1916). English and American Tool Builders. New Haven, CT, USA: Yale University Press. Reprinted by Lindsay Publications, Inc., Bradley, IL, USA. ISBN 978-0-917914-73-7.
Wilson, Bruce A. (2004). Design Dimensioning and Tolerancing, 4th ed., Goodheart-Wilcox. ISBN 1-59070-328-6.