![Dubendorf floodgate](https://upload.wikimedia.org/wikipedia/commons/a/ab/Dubendorf_floodgate.jpg)
A screw thread, often shortened to thread, is a
helical
Helical may refer to:
* Helix, the mathematical concept for the shape
* Helical engine, a proposed spacecraft propulsion drive
* Helical spring, a coilspring
* Helical plc, a British property company, once a maker of steel bar stock
* Helicoil
A t ...
structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a
cylinder
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infin ...
or
cone
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines con ...
in the form of a helix, with the former being called a ''straight'' thread and the latter called a ''tapered'' thread. A screw thread is the essential feature of the
screw as a simple machine and also as a
threaded fastener
A fastener (US English) or fastening (UK English) is a hardware device that mechanically joins or affixes two or more objects together. In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or disman ...
.
The
mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for t ...
of a screw thread depends on its ''lead'', which is the linear distance the screw travels in one revolution. In most applications, the lead of a screw thread is chosen so that
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of t ...
is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied, as 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
elastic deformation
In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain ...
.
Applications
Screw threads have several applications:
* Fastening:
** Fasteners such as
wood screw
A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to fa ...
s, plastic screws, machine screws,
nuts, and
bolts.
** Connecting
threaded pipe
A threaded pipe is a pipe with screw-threaded ends for assembly.
Tapered threads
The threaded pipes used in some plumbing installations for the delivery of gases or liquids under pressure have a tapered thread that is slightly conical (in c ...
s and
hose
A hose is a flexible hollow tube designed to carry fluids from one location to another. Hoses are also sometimes called ''pipes'' (the word ''pipe'' usually refers to a rigid tube, whereas a hose is usually a flexible one), or more generally '' ...
s to each other and to caps and fixtures.
* Gear reduction via
worm drive
A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm wheel (which is similar in appearance to a spur gear). The two elements are also called the worm screw and worm gear. The terminolo ...
s
* Moving objects linearly by converting rotary motion to linear motion, as in the
leadscrew
A leadscrew (or lead screw), also known as a power screw or translation screw,Bhandari, p. 202. is a screw (simple machine), screw used as a linkage (mechanical), linkage in a machine, to translate Rotation, turning motion into linear motion. ...
of a
jack
Jack may refer to:
Places
* Jack, Alabama, US, an unincorporated community
* Jack, Missouri, US, an unincorporated community
* Jack County, Texas, a county in Texas, USA
People and fictional characters
* Jack (given name), a male given name, ...
.
* Measuring by correlating linear motion to rotary motion (and simultaneously amplifying it), as in a
micrometer Micrometer can mean:
* Micrometer (device), used for accurate measurements by means of a calibrated screw
* American spelling of micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
.
* Both moving objects linearly and simultaneously measuring the movement, combining the two aforementioned functions, as in a leadscrew of a
lathe
A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to c ...
.
In all of these applications, the screw thread has two main functions:
* It converts rotary motion into linear motion.
* It prevents linear motion without the corresponding rotation.
Design
Gender
Every matched pair of threads, ''external'' and ''internal'', can be described as ''male'' and ''female''. Generally speaking, the threads on an external surface are considered male, while the ones on an internal surface are considered female. For example, a
screw
A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to fa ...
has male threads, while its matching hole (whether in nut or substrate) has female threads. This property is called ''gender''. Assembling a male-threaded fastener to a female-threaded one is called ''mating''.
Handedness
![Right-hand grip rule](https://upload.wikimedia.org/wikipedia/commons/3/34/Right-hand_grip_rule.svg)
The helix of a thread can twist in two possible directions, which is known as ''handedness''. Most threads are oriented so that the threaded item, when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a
clockwise
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...
direction, and moves towards the viewer when it is turned counterclockwise. This is known as a ''right-handed'' (''RH'') thread, because it follows the
right-hand grip rule
In mathematics and physics, the right-hand rule is a common mnemonic for understanding orientation (vector space), orientation of Cartesian coordinate system, axes in three-dimensional space. It is also a convenient method for quickly finding t ...
. Threads oriented in the opposite direction are known as ''left-handed'' (''LH'').
By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads. Left-handed thread applications include:
* Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to applied torque or to
fretting induced precession. Examples include:
** The
left hand pedal on a
bicycle
A bicycle, also called a pedal cycle, bike or cycle, is a human-powered or motor-powered assisted, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A is called a cyclist, or bicyclist.
Bic ...
.
** The left-hand grinding wheel on a
bench grinder
A bench grinder is a benchtop type of grinding machine used to drive abrasive wheels. A pedestal grinder is a similar or larger version of grinder that is mounted on a pedestal, which may be bolted to the floor or may sit on rubber feet. These t ...
.
** The axle nuts, or less commonly,
lug nut
A lug nut or wheel nut is a fastener, specifically a nut, used to secure a wheel on a vehicle. Typically, lug nuts are found on automobiles, trucks (lorries), and other large vehicles using rubber tires.
Design
A lug nut is a nut fastener wi ...
s on the left side of some automobiles.
** The securing nut on some circular saw blades – the large
torque
In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
at startup should tend to tighten the nut.
** The spindle on
brushcutter and
line trimmer
A string trimmer, also known by the portmanteau strimmer and the trademarks Weedwacker, Weed eater and Whipper Snipper. is a garden tool for cutting grass, small weeds, and groundcover. It uses a whirling monofilament line instead of a blade ...
heads, so that the torque tends to tighten rather than loosen the connection
* In combination with right-hand threads in
turnbuckle
A turnbuckle, stretching screw or bottlescrew is a device for adjusting the tension or length of ropes, cables, tie rods, and other tensioning systems. It normally consists of two threaded eye bolts, one screwed into each end of a small metal f ...
s and clamping studs.
* In some gas supply connections to prevent dangerous misconnections, for example:
** In gas welding the flammable gas supply uses left-handed threads, while the oxygen supply if there is one has a conventional thread
** The
POL valve
A POL valve (originally for Prest-O-Lite) is a gas connection fitting used on Liquefied petroleum gas (LPG) cylinders.
The oldest standard for such connections, it was developed by the Prest-O-Lite company, hence the name. It is still the mos ...
for
LPG cylinder
Liquefied petroleum gas (LPG or LP gas) is a fuel gas which contains a flammable mixture of hydrocarbon gases, specifically propane, propylene, butylene, isobutane and n-butane.
LPG is used as a fuel gas in heating appliances, cooking e ...
s
* In a situation where neither threaded pipe end can be rotated to tighten or loosen the joint (e.g. in traditional heating pipes running through several rooms in a building). In such a case, the
coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...
will have one right-handed and one left-handed thread.
* In some instances, for example early
ballpoint pen
A ballpoint pen, also known as a biro (British English), ball pen (Hong Kong, Indian and Philippine English), or dot pen ( Nepali) is a pen that dispenses ink (usually in paste form) over a metal ball at its point, i.e. over a "ball point". ...
s, to provide a "secret" method of disassembly.
* In artillery projectiles, anything that screws into the projectile must be given consideration as to what will happen when the projectile is fired, e.g., anything that screws into the base from the bottom of the projectile must be left hand threaded.
* In mechanisms to give a more intuitive action as:
** The leadscrew of the cross slide of a
lathe
A lathe () is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to c ...
to cause the cross slide to move away from the operator when the leadscrew is turned clockwise.
** The depth of cut screw of a "Bailey" (or "Stanley-Bailey") type metal
plane (tool)
A hand plane is a tool for shaping wood using muscle power to force the cutting blade over the wood surface. Some rotary power planers are motorized power tools used for the same types of larger tasks, but are unsuitable for fine-scale planing, ...
for the blade to move in the direction of a regulating right hand finger.
* Some
Edison base
Edison screw (ES) is a standard lightbulb socket for electric light bulbs. It was developed by Thomas Edison (1847–1931), patented in 1881, and was licensed in 1909 under General Electric's Mazda trademark. The bulbs have right-hand threaded ...
lamps and fittings (such as those formerly used on the
New York City Subway
The New York City Subway is a rapid transit system owned by the government of New York City and leased to the New York City Transit Authority, an affiliate agency of the state-run Metropolitan Transportation Authority (MTA). Opened on October 2 ...
or the pre-
World War I
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
Sprague-Thomson
Sprague-Thomson is the name of the first rolling stock on the Paris Métro made completely of metal. It replaced the mostly wooden M1.
History
Research before 1908
In light of the Paris Métro train fire of 1903, the Compagnie du chemin ...
rolling stock of the
Paris Metro
Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. Sin ...
) have a left-hand thread to deter theft, since they cannot be used in other light fixtures.
Form
![Different Thread Types Update](https://upload.wikimedia.org/wikipedia/commons/3/3a/Different_Thread_Types_Update.jpg)
The cross-sectional shape of a thread is often called its ''form'' or ''threadform'' (also spelled ''thread form''). It may be
square
In Euclidean geometry, a square is a regular quadrilateral, which means that it has four equal sides and four equal angles (90-degree angles, π/2 radian angles, or right angles). It can also be defined as a rectangle with two equal-length adj ...
,
triangular
A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices ''A'', ''B'', and ''C'' is denoted \triangle ABC.
In Euclidean geometry, any three points, when non- collinea ...
,
trapezoid
A quadrilateral with at least one pair of parallel sides is called a trapezoid () in American and Canadian English. In British and other forms of English, it is called a trapezium ().
A trapezoid is necessarily a Convex polygon, convex quadri ...
al, or other shapes. The terms ''form'' and ''threadform'' sometimes refer to all design aspects taken together (cross-sectional shape, pitch, and diameters), but commonly refer to the standardized geometry used by the screw. Major categories of threads include machine threads, material threads, and power threads.
Most triangular threadforms are based on an
isosceles triangle
In geometry, an isosceles triangle () is a triangle that has two sides of equal length. Sometimes it is specified as having ''exactly'' two sides of equal length, and sometimes as having ''at least'' two sides of equal length, the latter versio ...
. These are usually called ''V-threads'' or ''vee-threads'' because of the shape of the
letter V. For 60° V-threads, the isosceles triangle is, more specifically,
equilateral
In geometry, an equilateral triangle is a triangle in which all three sides have the same length. In the familiar Euclidean geometry, an equilateral triangle is also equiangular; that is, all three internal angles are also congruent to each othe ...
. For
buttress thread
Buttress thread forms, also known as sawtooth thread forms or breech-lock thread forms. are screw thread profiles with an asymmetric shape, having one square face and the other slanted. They are most commonly used for leadscrews where the load i ...
s, the triangle is
scalene
Scalene may refer to:
* A scalene triangle, one in which all sides and angles are not the same.
* A scalene ellipsoid, one in which the lengths of all three semi-principal axes are different
* Scalene muscles of the neck
* Scalene tubercle
The sc ...
.
The theoretical triangle is usually
truncated to varying degrees (that is, the tip of the triangle is cut short). A V-thread in which there is no truncation (or a minuscule amount considered negligible) is called a ''sharp V-thread''. Truncation occurs (and is codified in standards) for practical reasons—the thread-cutting or thread-forming tool cannot practically have a perfectly sharp point, and truncation is desirable anyway, because otherwise:
* The cutting or forming tool's edge will break too easily;
* The part or fastener's thread crests will have
burrs upon cutting, and will be too susceptible to additional future burring resulting from dents (nicks);
* The roots and crests of mating male and female threads need clearance to ensure that the sloped sides of the V meet properly despite error in pitch diameter and dirt and nick-induced burrs.
* The point of the threadform adds little strength to the thread.
In
ball screw
A ball screw (or ballscrew) is a mechanical linear actuator that translates rotational motion to linear motion with little friction. A threaded shaft provides a helical raceway for ball bearings which act as a precision screw. As well as bein ...
s, the male-female pairs have bearing balls in between.
Roller screws use conventional thread forms and threaded rollers instead of balls.
Angle
The included angle characteristic of the cross-sectional shape is often called the ''thread angle''. For most V-threads, this is standardized as 60
degrees, but any angle can be used.
The cross section to measure this angle lies on a plane which includes the axis of the cylinder or cone on which the thread is produced.
Lead, pitch, and starts
![Helical screw single double triple quadruple start](https://upload.wikimedia.org/wikipedia/commons/1/1b/Helical_screw_single_double_triple_quadruple_start.png)
''Lead'' () and ''pitch'' are closely related concepts. They can be confused because they are the same for most screws. ''Lead'' is the distance along the screw's axis that is covered by one complete rotation of the screw thread (360°). ''Pitch'' is the distance from the crest of one thread to the next one at the same point.
Because the vast majority of screw threadforms are ''single-start'' threadforms, their lead and pitch are the same. Single-start means that there is only one "ridge" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360°), it has advanced axially by the width of one ridge. "Double-start" means that there are two "ridges" wrapped around the cylinder of the screw's body. Each time that the screw's body rotates one turn (360°), it has advanced axially by the width of two ridges. Another way to express this is that lead and pitch are parametrically related, and the
parameter
A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...
that relates them, the number of starts, very often has a value of 1, in which case their relationship becomes equality. In general, lead is equal to pitch times the number of starts.
Whereas metric threads are usually defined by their pitch, that is, how much distance per thread, inch-based standards usually use the reverse logic, that is, how many threads occur per a given distance. Thus, inch-based threads are defined in terms of ''threads per inch'' (TPI). Pitch and TPI describe the same underlying physical property—merely in different terms. When the inch is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a -20 thread has 20 TPI, which means that its pitch is inch ().
As the distance from the crest of one thread to the next, pitch can be compared to the
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of a
wave
In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (res ...
. Another wave analogy is that pitch and TPI are inverses of each other in a similar way that
period and frequency are inverses of each other.
Coarse versus fine
Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, where fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on a
saw
A saw is a tool consisting of a tough blade, wire, or chain with a hard toothed edge. It is used to cut through material, very often wood, though sometimes metal or stone. The cut is made by placing the toothed edge against the material and mo ...
or
file
File or filing may refer to:
Mechanical tools and processes
* File (tool), a tool used to ''remove'' fine amounts of material from a workpiece
**Filing (metalworking), a material removal process in manufacturing
** Nail file, a tool used to gent ...
, or between coarse grit and fine grit on
sandpaper
upright=1.35, Sheets of sandpaper with different grit sizes (40 (coarse), 80, 150, 240, 600 (fine)).
Sandpaper and glasspaper are names used for a type of coated abrasive that consists of sheets of paper or cloth with abrasive material glued to ...
.
![XK engine camshaft cover front stud-0822](https://upload.wikimedia.org/wikipedia/commons/e/e2/XK_engine_camshaft_cover_front_stud-0822.jpg)
The common V-thread standards (
ISO 261
The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.
Th ...
and
Unified Thread Standard
The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a w ...
) include a coarse pitch and a fine pitch for each major diameter. For example, -13 belongs to the UNC series (Unified National Coarse) and -20 belongs to the UNF series (Unified National Fine). Similarly, M10 (10 mm nominal outer diameter) as per ISO 261 has a coarse thread version at 1.5 mm pitch and a fine thread version at 1.25 mm pitch.
The term ''coarse'' here does not mean lower quality, nor does the term ''fine'' imply higher quality. The terms when used in reference to screw thread pitch have nothing to do with the tolerances used (degree of precision) or the amount of craftsmanship, quality, or cost. They simply refer to the size of the threads relative to the screw diameter.
Coarse threads are more resistant to stripping and cross threading because they have greater flank engagement. Coarse threads install much faster as they require fewer turns per unit length. Finer threads are stronger as they have a larger stress area for the same diameter thread. Fine threads are less likely to vibrate loose as they have a smaller helix angle and allow finer adjustment. Finer threads develop greater preload with less tightening torque.
Diameters
![Thread Profile Diameters](https://upload.wikimedia.org/wikipedia/commons/c/c9/Thread_Profile_Diameters.jpg)
There are three characteristic diameters (
⌀
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for ...
) of threads: ''major diameter'', ''minor diameter'', and ''pitch diameter'': Industry standards specify minimum (min.) and maximum (max.) limits for each of these, for all recognized thread sizes. The minimum limits for ''external'' (or ''bolt'', in ISO terminology), and the maximum limits for ''internal'' (''nut''), thread sizes are there to ensure that threads do not strip at the tensile strength limits for the parent material. The minimum limits for internal, and maximum limits for external, threads are there to ensure that the threads fit together.
Major diameter
The major diameter of threads is the larger of two extreme diameters delimiting the height of the thread profile, as a cross-sectional view is taken in a plane containing the axis of the threads. For a screw, this is its outside diameter (OD). The major diameter of a nut may not be directly measured, but it may be tested with go/no-go gauges.
The major diameter of external threads is normally smaller than the major diameter of the internal threads, if the threads are designed to fit together. But this requirement alone does not guarantee that a bolt and a nut of the same pitch would fit together: the same requirement must separately be made for the minor and pitch diameters of the threads. Besides providing for a clearance between the ''crest'' of the bolt threads and the ''root'' of the nut threads, one must also ensure that the clearances are not so excessive as to cause the fasteners to fail.
Minor diameter
![ISO and UTS Thread Dimensions](https://upload.wikimedia.org/wikipedia/commons/4/4b/ISO_and_UTS_Thread_Dimensions.svg)
The minor diameter is the lower extreme diameter of the thread. Major diameter minus minor diameter, divided by two, equals the height of the thread. The minor diameter of a nut is its inside diameter. The minor diameter of a bolt can be measured with go/no-go gauges or, directly, with an
optical comparator
An optical comparator (often called just a comparator in context) or profile projector is a device that applies the principles of optics to the inspection of manufactured parts. In a comparator, the magnified silhouette of a part is projected up ...
.
As shown in the figure at right, threads of equal pitch and angle that have matching minor diameters, with differing major and pitch diameters, may appear to fit snugly, but only do so radially; threads that have only major diameters matching (not shown) could also be visualized as not allowing radial movement. The reduced ''material condition'', due to the unused spaces between the threads, must be minimized so as not to overly weaken the fasteners.
In order to fit a male thread into the corresponding female thread, the female major and minor diameters must be slightly larger than the male major and minor diameters. However this excess does not usually appear in tables of sizes. Calipers measure the female minor diameter (inside diameter, ID), which is less than caliper measurement of the male major diameter (outside diameter, OD). For example, tables of caliper measurements show 0.69 female ID and 0.75 male OD for the standards of "3/4 SAE J512" threads and "3/4-14 UNF JIS SAE-J514 ISO 8434-2".
Note the female threads are identified by the corresponding male major diameter (3/4 inch), not by the actual measurement of the female threads.
Pitch diameter
![Diagram of thread combinations of snug fit](https://upload.wikimedia.org/wikipedia/commons/f/fa/Diagram_of_thread_combinations_of_snug_fit.jpg)
The pitch diameter (PD, or ''D''
2) of a particular thread, internal or external, is the diameter of a cylindrical surface, axially concentric to the thread, which intersects the thread flanks at equidistant points, when viewed in a cross-sectional plane containing the axis of the thread, the distance between these points being exactly one half the pitch distance. Equivalently, a line running parallel to the axis and a distance ''D''
2 away from it, the "PD line," slices the ''sharp-V'' form of the thread, having flanks coincident with the flanks of the thread under test, at exactly 50% of its height. We have assumed that the flanks have the proper shape, angle, and pitch for the specified thread standard. It is generally unrelated to the major (''D'') and minor (''D''
1) diameters, especially if the crest and root truncations of the sharp-V form at these diameters are unknown. Everything else being ideal, ''D''
2, ''D'', & ''D''
1, together, would fully describe the thread form. Knowledge of PD determines the position of the sharp-V thread form, the sides of which coincide with the straight sides of the thread flanks: e.g., the crest of the external thread would truncate these sides a radial displacement ''D'' − ''D''
2 away from the position of the PD line.
Provided that there are moderate non-negative clearances between the root and crest of the opposing threads, and everything else is ideal, if the pitch diameters of a screw and nut are exactly matched, there should be no play at all between the two as assembled, even in the presence of positive root-crest clearances. This is the case when the flanks of the threads come into intimate contact with one another, before the roots and crests do, if at all.
However, this ideal condition would in practice only be approximated and would generally require wrench-assisted assembly, possibly causing the galling of the threads. For this reason, some ''allowance'', or minimum difference, between the PDs of the internal and external threads has to generally be provided for, to eliminate the possibility of deviations from the ideal thread form causing ''interference'' and to expedite hand assembly up to the length of engagement. Such allowances, or ''fundamental deviations'', as ISO standards call them, are provided for in various degrees in corresponding ''classes'' of fit for ranges of thread sizes. At one extreme, no allowance is provided by a class, but the maximum PD of the external thread is specified to be the same as the minimum PD of the internal thread, within specified tolerances, ensuring that the two can be assembled, with some looseness of fit still possible due to the margin of tolerance. A class called ''interference fit'' may even provide for negative allowances, where the PD of the screw is greater than the PD of the nut by at least the amount of the allowance.
The pitch diameter of external threads is measured by various methods:
* A dedicated type of
micrometer Micrometer can mean:
* Micrometer (device), used for accurate measurements by means of a calibrated screw
* American spelling of micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
, called a thread mic or pitch mic, which has a V-anvil and a conical spindle tip, contacts the thread flanks for a direct reading.
* A general-purpose micrometer (flat anvil and spindle) is used over a set of three wires that rest on the thread flanks, and a known constant is subtracted from the reading. (The wires are truly gauge pins, being ground to precise size, although "wires" is their common name.) This method is called the 3-wire method. Sometimes grease is used to hold the wires in place, helping the user to juggle the part, mic, and wires into position.
* An
optical comparator
An optical comparator (often called just a comparator in context) or profile projector is a device that applies the principles of optics to the inspection of manufactured parts. In a comparator, the magnified silhouette of a part is projected up ...
may also be used to determine PD graphically.
Classes of fit
The way in which male and female fit together, including
play
Play most commonly refers to:
* Play (activity), an activity done for enjoyment
* Play (theatre), a work of drama
Play may refer also to:
Computers and technology
* Google Play, a digital content service
* Play Framework, a Java framework
* Pla ...
and friction, is classified (categorized) in thread standards. Achieving a certain
class of fit requires the ability to work within tolerance ranges for dimension (size) and
surface finish
Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a surface from the perf ...
. Defining and achieving classes of fit are important for
interchangeability
Interchangeability can refer to:
*Interchangeable parts
Interchangeable parts are parts (components) that are identical for practical purposes. They are made to specifications that ensure that they are so nearly identical that they will fit into ...
. Classes include 1, 2, 3 (loose to tight); A (external) and B (internal); and various systems such as H and D limits.
Tolerance classes
Thread limit
Thread limit or pitch diameter limit is a standard used for classifying the tolerance of the thread pitch diameter for
taps. For imperial, H or L limits are used which designate how many units of 0.0005 inch over or undersized the pitch diameter is from its basic value, respectively. Thus a tap designated with an H limit of 3, denoted ''H3'', would have a pitch diameter 0.0005 × 3 = 0.0015 inch larger than base pitch diameter and would thus result in cutting an internal thread with a looser fit than say an H2 tap. Metric uses D or DU limits which is the same system as imperial, but uses D or DU designators for over and undersized respectively, and goes by units of .
Generally taps come in the range of H1 to H5 and rarely L1.
The pitch diameter of a thread is measured where the radial cross section of a single thread equals half the pitch, for example: 16 pitch thread = in = 0.0625in the pitch actual pitch diameter of the thread is measured at the radial cross section measures 0.03125in.
Interchangeability
To achieve a predictably successful mating of male and female threads and assured interchangeability between males and between females, standards for form, size, and finish must exist and be followed.
Standardization of threads is discussed below.
Thread depth
Screw threads are almost never made perfectly sharp (no truncation at the crest or root), but instead are truncated, yielding a final ''thread depth'' that can be expressed as a fraction of the pitch value. The UTS and ISO standards codify the amount of truncation, including tolerance ranges.
A perfectly sharp 60° V-thread will have a depth of thread ("height" from root to crest) equal to 0.866 of the pitch. This fact is intrinsic to the geometry of an equilateral triangle — a direct result of the basic
trigonometric functions
In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions) are real functions which relate an angle of a right-angled triangle to ratios of two side lengths. They are widely used in all ...
. It is independent of measurement units (inch vs mm). However, UTS and ISO threads are not sharp threads. The major and minor diameters delimit truncations on either side of the sharp V.
The nominal diameter of Metric (e.g. M8) and Unified (e.g. in) threads is the theoretical major diameter of the male thread, which is truncated (diametrically) by of the pitch from the dimension over the tips of the "fundamental" (sharp cornered) triangles. The resulting flats on the crests of the male thread are theoretically one eighth of the pitch wide (expressed with the notation ''p'' or 0.125''p''), although the actual geometry definition has more variables than that. A full (100%) UTS or ISO thread has a height of around 0.65''p''.
Threads can be (and often are) truncated a bit more, yielding thread depths of 60% to 75% of the 0.65''p'' value. For example, a 75% thread sacrifices only a small amount of strength in exchange for a significant reduction in the force required to cut the thread. The result is that
tap and die
Taps and dies are tools used to create screw threads, which is called threading. Many are cutting tools; others are forming tools. A tap is used to cut or form the female portion of the mating pair (e.g. a nut). A die is used to cut or form the ...
wear is reduced, the likelihood of breakage is lessened and higher cutting speeds can often be employed.
This additional truncation is achieved by using a slightly larger
tap drill in the case of female threads, or by slightly reducing the diameter of the threaded area of workpiece in the case of male threads, the latter effectively reducing the thread's
major diameter. In the case of female threads, tap drill charts typically specify sizes that will produce an approximate 75% thread. A 60% thread may be appropriate in cases where high tensile loading will not be expected. In both cases, the
pitch diameter is not affected. The balancing of truncation versus thread strength is similar to many engineering decisions involving the strength, weight and cost of material, as well as the cost to machine it.
Taper
Tapered threads are used on fasteners and pipe. A common example of a fastener with a tapered thread is a
wood screw
A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to fa ...
.
The
threaded pipe
A threaded pipe is a pipe with screw-threaded ends for assembly.
Tapered threads
The threaded pipes used in some plumbing installations for the delivery of gases or liquids under pressure have a tapered thread that is slightly conical (in c ...
s used in some plumbing installations for the delivery of fluids under pressure have a threaded section that is
slightly conical. Examples are the
NPT and
BSP series. The seal provided by a threaded pipe joint is created when a tapered externally threaded end is tightened into an end with internal threads. For most pipe joints, a good seal requires the application of a separate sealant into the joint, such as
thread seal tape
Thread seal tape (also known as PTFE tape, Teflon tape, or plumber's tape) is a polytetrafluoroethylene (PTFE) film tape commonly used in plumbing for sealing pipe threads. The tape is sold cut to specific widths and wound on a spool, making it ...
, or a liquid or paste pipe sealant such as
pipe dope
Pipe dope is any thread lubricant, thread sealing compound, or anaerobic chemical sealant that is used to make a pipe thread joint leak proof and pressure tight. It is also referred to as "thread compound" or "pipe thread sealant." Although com ...
.
History
The screw thread concept seems to have occurred first to
Archimedes
Archimedes of Syracuse (;; ) was a Greek mathematician, physicist, engineer, astronomer, and inventor from the ancient city of Syracuse in Sicily. Although few details of his life are known, he is regarded as one of the leading scientists ...
, who briefly wrote on spirals as well as designed several simple devices applying the screw principle.
Leonardo da Vinci
Leonardo di ser Piero da Vinci (15 April 14522 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, Drawing, draughtsman, engineer, scientist, theorist, sculptor, and architect. While his fame initially res ...
understood the screw principle, and left drawings showing how threads could be cut by machine. In the 1500s, screws appeared in German watches, and were used to fasten suits of armor. In 1569
Besson invented the
screw-cutting lathe
A screw-cutting lathe is a machine (specifically, a lathe) capable of cutting very accurate screw threads via single-point screw-cutting, which is the process of guiding the linear motion of the tool bit in a precisely known ratio to the rotatin ...
, but the method did not gain traction and screws continued to be made largely by hand for another 150 years. In the 1800s, screw manufacturing began in England during the
Industrial Revolution
The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
. In these times, there was no such thing as standardization. The bolts made by one manufacturer would not fit the nuts of another.
Standardization
Standardization
Standardization or standardisation is the process of implementing and developing technical standards based on the consensus of different parties that include firms, users, interest groups, standards organizations and governments. Standardization ...
of screw threads has evolved since the early nineteenth century to facilitate compatibility between different manufacturers and users. The standardization process is still ongoing; in particular there are still (otherwise identical) competing metric and inch-sized thread standards widely used.
[ISO/TC/ 1 Business Plan]
2007-03-05, Version 1.3. Table 3: The market share of each screw thread, p. 7. Standard threads are commonly identified by short letter codes (M, UNC, etc.) which also form the prefix of the standardized designations of individual threads.
Additional product standards identify preferred thread sizes for screws and nuts, as well as corresponding bolt head and nut sizes, to facilitate compatibility between
spanners (wrenches) and other tools.
ISO standard threads
The most common threads in use are the
ISO metric screw thread
The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. They were one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.
Th ...
s (M) for most purposes, and
BSP threads (R, G) for pipes.
These were standardized by the
International Organization for Standardization
The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...
(ISO) in 1947. Although metric threads were mostly unified in 1898 by the International Congress for the standardization of screw threads, separate metric thread standards were used in France, Germany, and Japan, and the Swiss had a set of threads for watches.
Other current standards
In particular applications and certain regions, threads other than the ISO metric screw threads remain commonly used, sometimes because of special application requirements, but mostly for reasons of
backward compatibility
Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially i ...
:
*
Unified Thread Standard
The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a w ...
(UTS), is the dominant thread standard used in the United States and Canada. It is defined i
ANSI/ASME B1.1 Unified Inch Screw Threads, (UN and UNR Thread Form) In some cases products are still made according to the old ''American National Standard Series'', which has slightly different specifications, and has been technically obsolete since 1949. The old national standard is compatible with the newer unified standard, but is long out of date. This unified standard includes:
** Unified Coarse (UNC), the successor to the obsolete National Coarse (NC) thread.
** Unified Fine (UNF), the successor to the obsolete National Fine (NF) thread.
** Unified Extra Fine (UNEF)
** Unified Special (UNS)
*
National pipe thread
American National Standard Pipe Thread standards, often called national pipe thread standards for short, are United States national technical standards for screw threads used on threaded pipes and pipe fittings. They include both tapered and stra ...
, used in North America for several purposes.
** National Pipe Taper (NPT)
** National Pipe Taper Fuel (NPTF), also known as Dryseal, a better sealing version of NPT.
** National Pipe Taper Railing fittings (NPTR)
** National Pipe Straight Coupling (NPSC)
** National Pipe Straight Mechanical (NPSM)
** National Pipe Straight Locknut (NPSL)
** National Pipe Straight Hose coupling (NPSH)
*
British Standard Whitworth
British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, devised and specified by Joseph Whitworth in 1841 and later adopted as a British Standard. It was the world's first national screw thread standard, and is the basis ...
(BSW), and for other Whitworth threads including:
**
British Standard Fine British Standard Fine (BSF) is a screw thread form, as a fine-pitch alternative to British Standard Whitworth (BSW) thread.
It was used for steel bolts and nuts on and in much of Britain's machinery, including cars, prior to adoption of Unified, an ...
(BSF)
** Cycle Engineers' Institute (CEI) or
British Standard Cycle British Standard Cycle (BSC or BSCy) is a British Imperial screw thread standard. Unlike other major British imperial thread standards (British Standard Whitworth and British Standard Fine) the thread runs at a 60 degrees rather than a 55 degrees a ...
(BSC)
*
British standard pipe thread
British Standard Pipe (BSP) is a set of technical standards for screw threads that has been adopted internationally for interconnecting and sealing pipes and fittings by mating an external (male) thread with an internal (female) thread. It has b ...
(BSP) which exists in a taper and non taper variant; used for other purposes as well
** British Standard Pipe Taper (BSPT)
*
British Association screw threads
British Association screw threads, or BA screw threads, are a set of small screw threads, the largest being 0BA at 6 mm diameter. They were, and to some extent still are, used for miniature instruments and modelling.
They are unusual in that ...
(BA), primarily electronic/electrical,
moving coil meter
An ammeter (abbreviation of ''Ampere meter'') is an instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. For direct measurement, the ammeter is connected in series with the circuit ...
s and to mount optical lenses
* British Standard Buttress Threads (BS 1657:1950)
* British Standard for
Spark Plugs
A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air ...
BS 45:1972
*
British Standard Brass British Standard Brass or British Brass Thread is an imperial unit based screw thread. It adopts the Whitworth thread form with a pitch of 26 threads per inch and a thread angle of 55 degrees for all diameters. It is often wrongly called ''British S ...
a fixed pitch 26 TPI thread
*
Glass Packaging Institute
The Glass Packaging Institute (GPI) is the North American trade association for the glass container industry, headquartered in Arlington, Virginia. Through GPI, glass container manufacturers advocate job preservation and industry standards, and pr ...
threads (GPI), primarily for glass bottles and vials
*
Power screw
A leadscrew (or lead screw), also known as a power screw or translation screw,Bhandari, p. 202. is a screw used as a linkage in a machine, to translate turning motion into linear motion. Because of the large area of sliding contact between th ...
threads
**
Acme thread form
Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews (power screws). They offer high strength and ease of manufacture. They are typically found where large loads are r ...
**
Square thread form
The square thread form is a common screw thread profile, used in high load applications such as leadscrews and jackscrews. It gets its name from the square cross-section of the thread.Bhandari, p. 203. It is the lowest friction and most effi ...
**
Buttress thread
Buttress thread forms, also known as sawtooth thread forms or breech-lock thread forms. are screw thread profiles with an asymmetric shape, having one square face and the other slanted. They are most commonly used for leadscrews where the load i ...
*
Royal Microscopical Society
The Royal Microscopical Society (RMS) is a learned society for the promotion of microscopy. It was founded in 1839 as the Microscopical Society of London making it the oldest organisation of its kind in the world. In 1866, the society gained it ...
(RMS) thread, also known as ''society thread'', is a special 0.8-inch diameter × 36 thread-per-inch (TPI) Whitworth thread form used for
microscope objective lens
In optical engineering, the objective is the optical element that gathers light from the object being observed and Focus (optics), focuses the ray (optics), light rays to produce a real image. Objectives can be a single Lens (optics), lens or mirr ...
es.
*
Microphone stand
A microphone stand is a free-standing mount for a microphone. It allows the microphone to be positioned in the studio, on stage or on location without requiring a person to hold it.
The most basic microphone stand is a ''straight stand''. It u ...
s:
** -inch 27
threads per inch
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 ...
(TPI) Unified Special thread (UNS, USA and the rest of the world)
** -inch BSW (not common in the US, but used in the rest of the world)
** -inch BSW (not common in the US, but used in the rest of the world)
*
Stage lighting
Stage lighting is the craft of lighting as it applies to the production of theater, dance, opera, and other performance arts. suspension bolts (in some countries only; some have gone entirely metric, others such as
Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
have reverted to the BSW threads, or have never fully converted):
** -inch BSW for lighter luminaires
** -inch BSW for heavier luminaires
* Tapping screw threads (ST) – ISO 1478
* Aerospace inch threads (UNJ) – ISO 3161, controlled root radius on male threads for greater fatigue strength with larger minor diameter on female threads to clear radius.
* Aerospace metric threads (MJ) – ISO 5855
* Tyre valve threads (V) – ISO 4570
* Metal bone screws (HA, HB) – ISO 5835
*
Panzergewinde
The Stahlpanzerrohrgewinde (, "steel conduit thread") standard for screw threads, more often called by the shortened Panzergewinde (), was a technical standard created in Germany and subsequently used in Switzerland, Austria, and other neighbori ...
(Pg) (German) is an old German 80° thread (DIN 40430) that remained in use until 2000 in some electrical installation accessories in Germany.
* Fahrradgewinde (Fg) (English: bicycle thread) is a German bicycle thread standard (per DIN 79012 and DIN 13.1), which encompasses a lot of CEI and BSC threads as used on cycles and mopeds everywhere (http://www.fahrradmonteur.de/fahrradgewinde.php)
*
Edison base
Edison screw (ES) is a standard lightbulb socket for electric light bulbs. It was developed by Thomas Edison (1847–1931), patented in 1881, and was licensed in 1909 under General Electric's Mazda trademark. The bulbs have right-hand threaded ...
Incandescent light bulb
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxida ...
holder screw thread
*
Fire hose connection (
NFPA standard 194)
*
Hose Coupling Screw Threads (ANSI/ASME B1.20.7-1991
2003
File:2003 Events Collage.png, From top left, clockwise: The crew of STS-107 perished when the Space Shuttle Columbia disintegrated during reentry into Earth's atmosphere; SARS became an epidemic in China, and was a precursor to SARS-CoV-2; A des ...
for garden hoses and accessories
*
Löwenherz thread, a German metric thread used for measuring instruments
*
Sewing machine thread
History of standardization
![Survey results on use of SAE standards Horseless Age v37 n9 1916-05-01 p353](https://upload.wikimedia.org/wikipedia/commons/4/4a/Survey_results_on_use_of_SAE_standards_Horseless_Age_v37_n9_1916-05-01_p353.png)
The first historically important intra-company standardization of screw threads began with
Henry Maudslay
Henry Maudslay ( pronunciation and spelling) (22 August 1771 – 14 February 1831) was an English machine tool innovator, tool and die maker, and inventor. He is considered a founding father of machine tool technology. His inventions were a ...
around 1800, when the modern
screw-cutting lathe
A screw-cutting lathe is a machine (specifically, a lathe) capable of cutting very accurate screw threads via single-point screw-cutting, which is the process of guiding the linear motion of the tool bit in a precisely known ratio to the rotatin ...
made interchangeable V-thread machine screws a practical commodity.
[Quentin R. Skrabec, Jr. (2005). "The Metallurgic Age: The Victorian Flowering of Invention and Industrial Science". p. 169. McFarland] During the next 40 years, standardization continued to occur on the intra- and inter-company levels.
pp. 9–10.
/ref> No doubt many mechanics of the era participated in this zeitgeist; Joseph Clement
Joseph Clement (13 June 1779 – 28 February 1844) was a British engineer and industrialist, chiefly remembered as the maker of Charles Babbage's first difference engine, between 1824 and 1833.
Biography
Early life
Joseph Clement was born on ...
was one of those whom history has noted.
In 1841, Joseph Whitworth
Sir Joseph Whitworth, 1st Baronet (21 December 1803 – 22 January 1887) was an English engineer, entrepreneur, inventor and philanthropist. In 1841, he devised the British Standard Whitworth system, which created an accepted standard for scr ...
created a design that, through its adoption by many British railway companies, became a standard for the United Kingdom and British Empire called British Standard Whitworth
British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, devised and specified by Joseph Whitworth in 1841 and later adopted as a British Standard. It was the world's first national screw thread standard, and is the basis ...
. During the 1840s through 1860s, this standard was often used in the United States as well, in addition to myriad intra- and inter-company standards. In April 1864, William Sellers
William Sellers (September 19, 1824 – January 24, 1905) was a mechanical engineer, manufacturer, businessman, noted abolitionist, and inventor who filed more than 90 patents, most notably the design for the United States standard screw thread ...
presented a paper to the Franklin Institute
The Franklin Institute is a science museum and the center of science education and research in Philadelphia, Pennsylvania. It is named after the American scientist and statesman Benjamin Franklin. It houses the Benjamin Franklin National Memori ...
in Philadelphia
Philadelphia, often called Philly, is the largest city in the Commonwealth of Pennsylvania, the sixth-largest city in the U.S., the second-largest city in both the Northeast megalopolis and Mid-Atlantic regions after New York City. Sinc ...
, proposing a new standard to replace the US' 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).
pp. 248–249.
/ref> The 60° angle was already in common use in America,
p. 249.
/ref> 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
The Baldwin Locomotive Works (BLW) was an American manufacturer of railroad locomotives from 1825 to 1951. Originally located in Philadelphia, it moved to nearby Eddystone, Pennsylvania, in the early 20th century. The company was for decades t ...
and the Pennsylvania Railroad
The Pennsylvania Railroad (reporting mark PRR), legal name The Pennsylvania Railroad Company also known as the "Pennsy", was an American Class I railroad that was established in 1846 and headquartered in Philadelphia, Pennsylvania. It was named ...
. Other firms adopted it, and it soon became a national standard for the U.S., later becoming generally known as the United States Standard thread
United States Standard thread (USS thread), also known as Sellers Standard thread, Franklin Institute thread and American Standard thread, is a standard for inch based threaded fasteners and washers.
The USS standard is no longer supported. It, t ...
(USS thread). 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).''
Meanwhile, in Britain, the British Association screw threads
British Association screw threads, or BA screw threads, are a set of small screw threads, the largest being 0BA at 6 mm diameter. They were, and to some extent still are, used for miniature instruments and modelling.
They are unusual in that ...
were also developed and refined for small instrumentation and electrical equipment. These were based on the metric Thury thread
Thury may refer to:
People
*Pierre de Thury ( -1410), said the cardinal Maillezais, a French cardinal of the Avignon Obedience, see Council of Pisa
*György Thury (1519-1571), a Hungarian nobleman ( hu)
*Louis-Pierre Thury (died in 1699), French m ...
, but like Whitworth etc. were defined using Imperial units
The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed thro ...
.
During this era, in continental Europe, the British and American threadforms were well known, but also various metric
Metric or metrical may refer to:
* Metric system, an internationally adopted decimal system of measurement
* An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement
Mathematics
In mathema ...
thread standards were evolving, which usually employed 60° profiles. Some of these evolved into national or quasi-national standards. They 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
Capital costs are fixed, one-time expenses incurred on the purchase of land, buildings, construction, and equipment used in the production of goods or in the rendering of services. In other words, it is the total cost needed to bring a project to a ...
of the necessary retooling would drive some firms from profit to loss and hamper the economy. (The mixed use of dueling 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.)
Sometime between 1912 and 1916, the Society of Automobile Engineers (SAE) created an "SAE series" of screw thread sizes reflecting parentage from earlier USS and American Society of Mechanical Engineers (ASME) standards.
During the late 19th and early 20th centuries, engineers found that ensuring the reliable interchangeability of screw threads was a multi-faceted and challenging task that was not as simple as just standardizing the major diameter and pitch for a certain thread. It was during this era that more complicated analyses made clear the importance of variables such as pitch diameter and surface finish.
A tremendous amount of engineering work was done throughout World War I
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
and the following interwar period
In the history of the 20th century, the interwar period lasted from 11 November 1918 to 1 September 1939 (20 years, 9 months, 21 days), the end of the World War I, First World War to the beginning of the World War II, Second World War. The in ...
in pursuit of reliable interchangeability. Classes of fit were standardized, and new ways of generating and inspecting screw threads were developed (such as production thread-grinding machines and optical comparator
An optical comparator (often called just a comparator in context) or profile projector is a device that applies the principles of optics to the inspection of manufactured parts. In a comparator, the magnified silhouette of a part is projected up ...
s). Therefore, in theory, one might expect that by the start of World War II, the problem of screw thread interchangeability would have already been completely solved. Unfortunately, this proved to be false. Intranational interchangeability was widespread, but international interchangeability was less so. 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
The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a w ...
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.
)
, image_skyline =
, image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
, 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 widely taken up in the UK, although a small number of companies continued to use the UK's own British standards for Whitworth (BSW), British Standard Fine (BSF) and British Association (BA) microscrews.
However, internationally, the metric system was eclipsing inch-based measurement units. In 1947, the ISO was founded; and in 1960, the metric-based International System of Units
The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...
(abbreviated ''SI'' from the French ''Système International'') was created. With continental Europe and much of the rest of the world turning to SI and 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 is slowly displacing all former standards, including UTS. In the U.S., where UTS is still prevalent, over 40% of products contain at least some ISO metric screw threads. The UK has completely abandoned its commitment to UTS in favour of ISO metric threads, and Canada is in between. Globalization
Globalization, or globalisation (Commonwealth English; see spelling differences), is the process of interaction and integration among people, companies, and governments worldwide. The term ''globalization'' first appeared in the early 20t ...
of industries produces market pressure in favor of phasing out minority standards. A good example is the automotive industry
The automotive industry comprises a wide range of company, companies and organizations involved in the design, Business development, development, manufacturing, marketing, and selling of motor vehicles. It is one of the world's largest 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.
Even today, over a half century since the UTS superseded the USS and SAE series, companies still sell hardware with designations such as "USS" and "SAE" to convey that it is of inch sizes as opposed to metric. Most of this hardware is in fact made to the UTS, but the labeling and cataloging terminology is not always precise.
Engineering drawing
In American engineering drawing
An engineering drawing is a type of technical drawing that is used to convey information about an object. A common use is to specify the geometry necessary for the construction of a component and is called a detail drawing. Usually, a number of ...
s, ANSI Y14.6 defines standards for indicating threaded parts. Parts are indicated by their nominal diameter (the nominal major diameter of the screw threads), pitch (number of threads per inch), and the class of fit for the thread. For example, “.750-10 UNC-2A” is male (A) with a nominal major diameter of 0.750 inches, 10 threads per inch, and a class-2 fit; “.500-20 UNF-1B” would be female (B) with a 0.500-inch nominal major diameter, 20 threads per inch, and a class-1 fit. An arrow points from this designation to the surface in question.[Wilson pp. 77–78 (page numbers may be from an earlier edition).]
Generation
There are many ways to generate a screw thread, including the traditional subtractive types (for example, various kinds of cutting ingle-pointing, taps and dies, die heads, milling molding; casting ie casting, sand casting forming and rolling; grinding; and occasionally lapping
Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine.
Lapping often follows other subtractive processes with more aggressive material removal as a first step ...
to follow the other processes); newer additive techniques; and combinations thereof.
Inspection
Another common inspection point is the straightness of a bolt or screw. This topic comes up often when there are assembly issues with predrilled holes as the first troubleshooting point is to determine if the fastener or the hole is at fault. ASME B18.2.9 "Straightness Gage and Gaging for Bolts and Screws" was developed to address this issue. Per the scope of the standard, it describes the gage and procedure for checking bolt and screw straightness at maximum material condition (MMC) and provides default limits when not stated in the applicable product standard.
See also
* Anti-seize compound
* British Standard Cycle British Standard Cycle (BSC or BSCy) is a British Imperial screw thread standard. Unlike other major British imperial thread standards (British Standard Whitworth and British Standard Fine) the thread runs at a 60 degrees rather than a 55 degrees a ...
* Dryseal Pipe Threads Form
* Filter thread
In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a gl ...
* Metric: M Profile Thread Form
* National Thread Form
* Nut (hardware)
A nut is a type of fastener with a screw thread, threaded hole. Nuts are almost always used in conjunction with a mating bolt (fastener), bolt to fasten multiple parts together. The two partners are kept together by a combination of their threads ...
* Tapered thread
* Tap and die
Taps and dies are tools used to create screw threads, which is called threading. Many are cutting tools; others are forming tools. A tap is used to cut or form the female portion of the mating pair (e.g. a nut). A die is used to cut or form the ...
* Thread angle
The thread angle of a screw is the included angle between the thread flanks, measured in a plane containing the thread axis.. This is a defining factor for the shape of a screw thread
A screw thread, often shortened to thread, is a helical stru ...
* Thread pitch gauge
A thread gauge, also known as a screw gauge or pitch gauge, is used to measure the pitch or lead of a screw thread.
Thread pitch gauges are used as a reference tool in determining the pitch of a thread that is on a screw or in a tapped hole. T ...
* Thread protector
A thread protector is used to protect the threads of a pipe during transportation and storage. Thread protectors are generally manufactured from plastic or steel and can be applied to the pipe manually or automatically (by machine).
Thread pro ...
* Thread-locking fluid
Thread-locking fluid or threadlocker is a single-component adhesive, applied to the threads of fasteners such as screws and bolts to prevent loosening, leakage, and corrosion.
Most thread-locking formulas are methacrylate-based and rely on the ...
Notes
References
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External links
International Thread Standards
NASA RP-1228 Fastener Design Manual
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Fasteners
Screws
Threading (manufacturing)