Terminology related to nuts and bolts
Presented below is a glossary of terminology on topics related
to nuts and bolts, threaded fasteners and tightening techniques.
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- ACORN NUT
- A nut (so-called because of its shape) that has a domed
top so that it prevents contact with the external thread.
- A/F - Across Flats
- A/F or AF is frequently used as an abbreviation for Across Flats. The term is used for the dimension across the opposite flats of a hexagon product such as a nut or bolt head. The across flats size of a fastener defines the spanner/wrench size or socket size needed to tighten the fastener. For metric fasteners, this dimension is standardized in ISO 272 Fasteners - Hexagon Products - Width Across Flats.
- AEROTIGHT NUT
- A torque prevailing nut of all metal construction. The
nut is slotted in two places which, after the nut has been
tapped, are bent slightly inwards and downwards. When the
nut is screwed onto the bolt thread the two slotted parts
are forced back to their original position. Their stiffness
causes the nut threads to bind onto the bolt threads and
thus provides a prevailing torque. Aerotight is a registered
trade mark of The Premier Screw and Repitition Co. Ltd of
Woodgate, Leicester, United Kingdom, LE3 5GJ.
- ANTI-FRICTION COATING
- AF coatings are dry lubricants consisting of suspensions
of solid lubricants, such as graphite, PTFE or molydbenum
disulphide of small particle size in a binder. Such coatings
can be applied to fastener threads to replace metallic coatings
such as zinc and cadmium and offer maintenance free permanent
lubrication. By careful selection of the lubricants, AF
coatings can be designed to meet specific applications.
The coatings are permanently bonded to the metal surface
and provide a lubricating film preventing direct metal to
- ANTI-SEIZE COMPOUND
- An anti-seize compound is used on the threads of fasteners
in some applications. The purpose of the compound depends
upon the application. It can prevent galling of mating surfaces
- such compounds are frequently used with stainless steel
fasteners to prevent this effect from occurring. In some
applications it is used to improve corrosion resistance
to allow the parts to be subsequently dis-assembled Thirdly,
it can provide a barrier to water penetration since the
threads are sealed by use of the compound.
- AUTOLOK NUT
- A torque prevailing nut of an all metal construction.
Covered by UK patent 1180842 the nut is marketed by GKN
Screws and Fasteners Limited.
- An intentional clearance between internal or external
thread and the design form of the thread when the thread
form is on it's maximum metal condition. Not all classes
of fit have an allowance. For metric threads the allowance
is called the fundamental deviation.
- ANAEROBIC ADHESIVE
- An adhesive which hardens in the absence of air, such
adhesives are often used as a thread locking medium.
- ANGLE CONTROLLED TIGHTENING
- A tightening procedure in which a fastener is first tightened
by a pre-selected torque (called the snug torque) so that
the clamped surfaces are pulled together, and then is further
tightened by giving the nut an additional measured rotation.
Frequently bolts are tightened beyond their
yield point by this method in order to ensure that a precise
preload is achieved. Bolts of short length can be elongated
too much by this method and the bolt material must be sufficiently
ductile to cater for the plastic deformation involved. Because
of the bolt being tightened beyond yield, its re-use is
- BASIC THREAD PROFILE
- This is the theoretical profile of external and internal
threads with no manufacturing tolerance applied.
- BEARING STRESS
- The surface pressure acting on a joint face directly as
a result of the force applied by a fastener.
- BIHEXAGON HEAD
- A bolt or screw whose cross section of its head is in
the shape of a 12 pointed star.
- BLACK BOLTS AND NUTS
- The word black refers to the comparatively wider tolerances
employed and not necessarily to the colour of the surface
finish of the fastener.
- A bolt is the term used for a threaded fastener, with
a head, designed to be used in conjunction with a nut.
- BREAKAWAY TORQUE
- The torque necessary to put into reverse rotation a bolt
that has not been tightened.
- BREAKLOOSE TORQUE
- The torque required to effect reverse rotation when a
pre-stressed threaded assembly is loosened.
- BRITISH STANDARD BRASS
- A specialist thread form based upon the Whitworth thread
and consisting of 26 threads per inch whatever the thread
- British Standard Fine. A thread form based upon the British
Standard Whitworth form but with a finer thread (more threads
per inch for a given diameter). This thread form was first
introduced in 1908, the thread form is specified in BS 84:
- British Standard Whitworth. A thread form developed by
Sir Joseph Whitworth in 1841. The thread form has rounded
roots and crests, the thread form is specified in BS 84:
1956. This thread form was superceded by the Unified thread
in 1948 and then the metric thread form.
- BUMP THREAD
- A modified thread profile patented and trade mark of the
Bosco Tool Inc. The thread form has a small projection at
the pitch diameter that eliminates the clearance from the
thread assembly on both flanks. By doing this it is claimed
that resistance to vibration loosening is significantly
- CADMIUM ELECTROPLATING
- Coating of threaded fasteners with cadmium can provide
the parts with excellent corrosion resistance. The appearance
of the coating is bright silver or yellow if subsequently
passivated. The friction values associated with this coating
are also comparatively low. A chromate conversion coating
is frequently applied to the surface to improve corrosion
resistance. Cadmium is not now frequently used because of
the environmental and worker health problems associated
with the coating process and should not be used in applications
above 250C or when contact with food is possible.
- CLAMPING FORCE
- The compressive force which a fastener exerts on the joint.
- CLASS OF FIT
- The Class of Fit is a measure of the degree of fit between
mating internal and external threads. Three main Classes
of Fit are defined for metric screw threads :
- FINE: This has a tolerance class of 5H for internal threads
and 4h for external threads.
- MEDIUM: This has a tolerance class of 6H for internal
threads and 6g for external threads.
- COARSE: This has a tolerance class of 7H for internal
threads and 8g for external threads.
- For Unified threads, a similar designation as for metric
threads is used. The thread classes used are 1A, 2A and
3A for external threads and 1B, 2B and 3B for internal threads.
- CLEVELOC NUT
- A torque prevailing nut of all metal construction. The
collar of the nut is elliptical in cross section and it
is this that provides the flexible locking element. The
nut is pre-lubricated to reduce the torque needed when tightening
and to minimise galling.
- COEFFICIENT OF FRICTION
- A dimensionless number representing the ratio of the friction
force to normal force. Typically for threaded connections
it is between 0.10 to 0,18 but can vary significantly depending
upon the materials used and whether a lubricant has been
used. In relation to threaded fasteners, the coefficent of friction can be further sub-divided into the coefficient of friction between the threads and the coefficient of friction under the nut face. There is in general a difference in values between the two coefficients due to typically the contact surfaces being different. For example, a zinc plated nut on a zinc plated bolt, the thread coefficent of friction would be due to zinc plating contacting zinc plating. The nut face coefficient of friction would be due to zinc plating contacting the joint surface finish.
- A term used to describe the undesirable practice of mixing
fasteners from different batches that are the same size
and grade in the same container.
- CONELOC NUT
- The Coneloc is an all metal prevailing torque type self-locking nut. The
locking action is achieved by localised precision deformation of the thread
in the cone section on top of the nut. When the nut is tightened onto the
bolt, the thread is gripped on the flanks providing the locking action. Coneloc is a trade mark of NUTS BOLTS Limited
- CONE PROOF LOAD
- This is an axial applied force applied to a nut when it
is seated on a cone shaped washer which has an included
angle of 120 degrees. Failure in this test is usually due
to the nut splitting. The intention of the test is to introduce
a nut dilation operation which will assess the potential
detrimental effects of surface discontinuities. This type
of test is sometimes applied to nuts which are intended
for high temperature service.
- Creep is deformation with time when a part is subjected
to constant stress. Metals creep can occur at elevated temperature
however with gasket materials it can occur at normal ambient
temperatures. Creep resistance is an important property
of gasket materials. Gasket materials are designed to flow
under stress to fill any irregularities in the flange surface.
The amount of creep sustained tends to increase with temperature.
. However once the tightening is completed it is important
that no further flow occurs since such deformation will
lead to a reduction in bolt extension and subsequently the
stress acting on the gasket. If this stress is reduced to
below a certain minimum, which depends upon the type and
construction of the gasket and the operating temperature,
a high rate of leakage can be anticipated to occur.
- DECOMPRESSION POINT
- The point at which there is zero pressure at the joint
interface as a result of forces applied to the joint. If
the applied force is increased beyond the decompression
point, a gap will form at the interface. Analytically, a
criteria of joint failure is often taken as when the applied
force on the joint reaches the decompression point. This
is because forces acting on the bolt(s) can dramatically
increase at this point. Loading beyond this point can also
result in fretting at the interface that will lead to bolt
tension loss that will subsequently lower the decompression
point. This process can continue until bolt failure does
occur. The failure can be by fatigue or other mechanism
but the underlying cause was loading of the joint beyond
the decompression point. It is for this reason that it is
frequently taken as a failure criteria in analysis work.
- A high performance surface coating that can be applied
to fasteners. The coating consists of passivated zinc flakes
that are stoved onto the metal surface. The coating can
be coloured and eliminates the risk of hydrogen embrittlement
associated with electroplated metal. DACROMET is a registered
trademark of Metal Coatings International, Inc. of Chardon
- DESIGN FORM OF THREAD
- The design form of an internal or external thread is the
thread form in it's maximum metal condition. It is the same
as the basic thread profile except that the thread roots
are rounded. If either the internal or external thread form
exceeds the design form of the thread profile then a potential
- DIRECT TENSION INDICATORS
- Direct Tension Indicators (DTI's) is a term sometimes
used to describe load indicating washers. Projections on
the face of the washer (usually on the face abuting the
bolt head or nut) that deform under loading as the bolt
is tensioned. An indication of the tension in the bolt can
be made by measuring the gap between the washer face and
the nut or bolt head. The smaller the gap - the greater
the tension in the bolt. Commonly used in civil rather than
mechanical engineering applications.
- DYNAMIC FRICTION
- Resistance to relative movement of two bodies that are
already in motion.
- EFFECTIVE DIAMETER
- This is the diameter of an imaginary cylinder coaxial
with the thread, which has equal metal and space widths.
It is often referred to as pitch diameter. Sometimes referred
to as the simple effective diameter to differentiate from
the virtual effective diameter.
- EFFECTIVE NUT DIAMETER
- Twice the effective nut radius.
- EFFECTIVE NUT RADIUS
- The radius from the centre of the nut to the point where
the contact forces, generated when the nut is turned, can
be considered to act.
- ELECTROLESS NICKEL
- A relatively thin, hard coating that can be applied to
threads and deposited uniformly. Bright metallic in appearance
this coating has excellent resistance to wear and corrosion.
- Localized plastic deformation which occurs in the vicinity
of clamped fasteners or in the fastener threads. . Embedding
is local plastic deformations that occur under the nut face,
in the joint faces and in the threads as a result of plastic
flattening of the surface roughness. This occurs even when
the loading is below the yield point of the bolt or limiting
surface pressure of the joint material and is the result
of the real area of contact between surfaces being less
than the apparent area.
- The lap that can be formed at the thread crest due to thread rolling.
- ENVIRONMENTALLY ASSISTED CRACKING (EAC)
- A process that can occur with the use of high strength
steel fasteners in which crack initiation and growth occurs
in the fastener at a comparatively low stress level as a
result of interactions that occur with the environment.
Hydrogen is suspected of causing EAC in high strength steel
fasteners, the hydrogen being produced as a result of chemical
reactions (galvanic corrosion in a moist environment) or
being present from a plating process that may have been
applied to the fastener.
- EXTERNAL FORCE OR LOAD
- Forces exerted on a fastener as a result of an applied
loading to the joint.
- EXTERNAL THREAD
- A screw thread which is formed on an external cylinder,
such as on bolts, screws, studs etc.
- FLOATING TYPE FLANGE JOINT
- A conventional flanged joint in which a gasket is compressed
by bolts - the gasket is not rigidly located. Calculation
methods such as the ASME code in the USA and the EN1591
code in Europe.
- FLUORO-CARBON THREAD COATING
- A low friction coating applied to threads. This type of
coating is frequently used to prevent thread fouling when
an assembly containing threaded fasteners is painted. Unless
masked in some way before painting, electro deposited primers
can cover the threads. If this occurs assembly difficulties
can result unless the expensive chore of cleaning the threads
is completed. A fluoro-carbon thread coating eliminates
the need for masking or cleaning since paint will not adhere
to the coating. This type of coating can also prevent problems
caused by weld splatter obstructing the threads of weld
nuts during their placement. Such coatings also have the
property of reducing the torque-tension scatter during tightening.
- Mechanical resistance to the relative movement of two
surfaces. There are two main types of friction; STATIC FRICTION
and DYNAMIC FRICTION. Typically static friction is greater
than dynamic friction.
- FRICTION STABILIZERS
- Coating materials used on fasteners with the intention
of reducing the scatter in the thread and bearing surface
- FUNDAMENTAL DEVIATION
- An intentional clearance between internal or external
thread and the design form of the thread when the thread
form is on it's maximum metal condition. For metric threads
the fundamental deviation are designated by letters, capitals
for internal threads and small letters for external threads.
Some tolerance classes have a fundamental deviation of zero.
For imperial threads the fundamental deviation is called
- FUNDAMENTAL TRIANGLE HEIGHT
- The fundamental triangle height is normally designated
with the letter H. This is the height of the thread when
the profile is extended to a sharp vee form. For 60 degree
thread forms such as metric and Unified thread series, H
equals 0.866025 times the thread pitch.
- A severe form of adhesive wear which occurs during sliding
contact of one surface relative to another. Clumps of one
part stick to the mating part and break away from the surface.
(Can frequently occur when both the nut and bolt are made
from stainless or high alloy steels, titanium or zinc coated
- GRIP LENGTH
- Total distance between the underside of the nut to the
bearing face of the bolt head; includes washer, gasket thickness
- HARD JOINT
- A joint in which the plates and material between the nut
and bolt bearing surfaces have a high stiffness when subjected
to compression by the bolt load. A joint is usually defined
as hard if the bolt is tightened to its full torque and
it rotates through an angle of 30 degrees or less after
it has been tightened to its snug condition.
- HARDENED WASHERS
- The force under the head of a bolt or nut can exceed,
at high preloads, the compressive yield strength of the
clamped material. If this occurs excessive embedding and
deformation can result in bolt preload loss. To overcome
this hardened washers under the bolt head can be used to
distribute the force over a wider area into the clamped
material. A more modern alternative is to use a flange headed
nuts and bolts.
- HEAT TIGHTENING
- Heat tightening utilises the thermal expansion characteristics
of the bolt. The bolt is heated and expands: the nut is
indexed (using the angle of turn method) and the system
allowed to cool. As the bolt attempts to contract it is
constrained longitudinally by the clamped material and a
preload results. Methods of heating include direct flame,
sheathed heating coil and carbon resistance elements. The
process is slow, especially if the strain in the bolt is
to be measured, since the system must return to ambient
temperature for each measurement. This is not a widely used
method and is generally used only on very large bolts.
- HELICAL SPRING WASHER
- A split type of spring washer whose purpose is to prevent
self loosening of the nut or the bolt. The idea or principle
behind the helical spring washer is for one end of the tang
of the washer to indent into the fastener (the nut or bolt
head) and the other into the joint surface so that any loosening
rotation is prevented. Junker in his paper in 1969 on the
cause of self-loosening of fasteners (reference:Junker,
G., New criteria for self-loosening of fasteners under vibration.
SAE Paper 690055, 1969) concluded that this type of lock
washer has no ability to lock. This type of washer is sometimes
called a spring lock washer or sometimes a standard lock
- HIGH STRENGTH FRICTION GRIP BOLTS
- Sometimes abbreviated to HSFG bolts. Bolts which are of
high tensile strength used in conjunction with high strength
nuts and hardened steel washers in structural steelwork.
The bolts are tightened to a specified minimum shank tension
so that transverse loads are transferred across the joint
by friction between the plates rather than by shear across
the bolt shank.
- HOLD AND DRIVE BOLTS
- Special bolts that have a tang at the threaded end of
the shank. This tang is gripped by the tightening tool during
assembly so that the reaction torque is absorbed whilst
the nut is tightened from the same side. Such bolts allow
what used to have to be done by two men to become a one-man
- HOT BOLTING
- This term is used for the completion of maintenance work
on a bolted joint when the joint is under loading. This
can involve the replacement of individual bolts. There are
risks both to the joint itself and to health and safety
associated with this technique.
- HYDRAULIC TENSIONER
- A hydraulic tool used to tighten a fastener by stretching
it rather than applying a large torque to the nut. After
the fastener has been stretched, the nut is run down the
thread to snug up with the joint, the hydraulically applied
load is then removed resulting in tension being induced
into the fastener.
- HYDROGEN EMBRITTLEMENT
- Steel fasteners exposed to hydrogen can fail prematurely
at a stress level well below the materials yield strength.
Hydrogen embrittlement occurs in fasteners usually as a
result of the part being exposed to hydrogen at some time
during its manufacturing process but it can also occur through
in-service corrosion. Electroplating is generally considered
to be a major cause of hydrogen absorption in steel fasteners
due to the release of hydrogen during this process. Higher
strength steels are more susceptible to hydrogen embrittlement
than lower strength steels, however it is considered that
there is no lower strength limit. As a rule of thumb, steels
below Rockwell C 35 are considered to be far less susceptible.
Tests such as the incremental load hydrogen embrittlement
test can be completed to assess if hydrogen embrittlement
is present in a batch of fasteners.
- IMPACT WRENCH
- A wrench, usually powered by electricity or air, in which
repeated blows from little hammers are used to generate
torque to tighten fasteners. The torque applied to the fastener
depends upon the time and the air pressure applied to the
tool (for pneumatic wrenches). The torque applied by an
impact wrench to a fastener is influenced by the joint stiffness.
- INSTANTANEOUS CENTRE OF ROTATION
- The point in space that an eccentrically shear loaded
joint rotates about. The deformation and the load sustained
by an individual bolt in a bolt group is dependent upon
the distance that the bolt is from the instantaneous centre.
The direction that the individual bolt force acts is perpendicular
to a line joining that bolt to the instantaneous centre.
- INTEGRAL FASTENER
- A term used to describe types of fasteners which are highly
resistant to vibration loosening and/or removal. Some types
have special thread forms.
- INTERNAL THREAD
- A screw thread which is formed in holes, such as in nuts.
- JAM NUTS
- See LOCKNUT
- JOINT CONTROL TIGHTENING
- See YIELD CONTROLLED TIGHTENING
- JOST EFFECT
- The name given to the reduction in the frictional resistance
that occurs in a direction different to that inwhich slip
is occurring. This effect is used in many applications including
the removal of corks from bottles. If the cork is first
rotated the force needed to pull the cork from the bottle
is significantly reduced. It is also the fundamental reason
why threaded fasteners experience self-loosening. Frictional
resistance is first overcome in the transverse direction
by slip occurring on the joint resulting in the frictional
resistance in the circumferential direction reducing to
a small value. The torque acting on the fastener in the
loosening direction (as a result of its preload) that when
coupled with the Jost Effect results in self-loosening occurring.
The term is named after the Institute that completed
research into this effect, the Jost Institute of Tribotechnology
at the University of Central Lancashire in the UK.
- K FACTOR
- The factor in the torque tightening equation: T=KDF where
T is the fastener tightening torque in Newton metres, D
is the fastener diameter in metres, F is the fasteners preload
in Newtons and K is a factor whose value is often taken
as 0.2. The formula gives the approximate tightening torque
for standard fasteners used under normal conditions. The
K factor is also known as the nut factor and the torque
- A pre-assembled nut and washer assembly (the washer is
attached to the nut so that it won't fall off)- a trademark
of ITW Shakeproof. The origin of the word came from ShaKEProof.
The s on the end being acquired due to them being purchased
in quantities usually greater than one.
- LEFTHAND THREAD
- A screw thread that is screwed in by rotating counterclockwise.
- LENGTH OF ENGAGEMENT
- The axial distance over which an external thread is in
contact with an internal thread.
- LOCK NUT
- There are two common usage's of this term:
- 1. A nut which provides extra resistance to vibration
loosening by either providing some form of prevailing torque,
or, in free spinning nuts, by deforming and/or biting into
mating parts when fully tightened.
- 2. The term is sometimes used for thin (or jam) nuts used
to lock a thicker nut. When used in this way the thin nut
should be adjacent to the joint surface and tightened against
the thick nut. If placed on top of the thick nut the thin
nut would sustain loads it was not designed to sustain.
- MAJOR DIAMETER
- This is the diameter of an imaginary cylinder parallel
with the crests of the thread; in other words it is the
distance from crest to crest for an external thread, or
root to root for an internal thread.
- The difference in tightening torque values produced by
the same tightening tool on hard and soft joints. A hard
joint typically gives a higher torque value than a soft
joint. Generally speaking, the lower the meanshift of a
tightening tool, the better it will be in achieving a specified
torque value irrespective of the joint condition.
- METAL TO METAL CONTACT FLANGE JOINT
- A flanged joint in which a gasket is compressed by bolts
- the gasket being located in a recess within the joint
so that it is compressed by the bolt loads until metal to
metal contact occurs. Unlike the FLOATING TYPE FLANGE JOINT,
for metal to metal type joints there are no standardised
gasket factor definitions, test procedures, nor generally
acknowledged calculation procedures available.
- MINOR DIAMETER
- This is the diameter of an imaginary cylinder which just
touches the roots of an external thread, or the crests of
an internal thread.
- MODEL ENGINEERS THREAD (M.E.)
- A thread based upon the Whitworth thread form that was
established in 1912. A very fine thread (a 3/32 inch thread
having 60 tpi for example).
- MOLYBDENUM DISULPHIDE
- A solid lubricant that acts as a high pressure resistant
film. Can be used by itself as a dry lubricant as well as
in with other solid lubricants and in oils and greases.
Used in threads, such lubricants act as a separating film
to prevent corrosion formation on the thread surface (even
under adverse temperature and environmental conditions)
ensuring the release of the threaded connection. Such films
can also act as friction stabilisers.
- NICKED THREADS
- Nicks or indentations in threads can occur during the
manufacturing process and during fastener transportation.
In general, nicked thread problems tend to increase as the
thread diameter increases and for fine pitches.
There are acceptance tests for nicked threads that involve
measuring the maximum torque required to drive a GO gauge
down the thread. Examples of acceptance tests are SAE J123
and the Ford Motor specification WA990 1993. Nicks and indentations
in threads are sometimes referred to as gouges.
- NOMINAL DIAMETER
- The diameter equal to the external diameter of the threads.
- NUT DILATION
- Under load, the wedging action of the threads causes dilation
of the nut resulting in an increase in the minor diameter
of the nut, and reducing the effective shear areas of both
the external and internal threads.
- NUT RUNNER
- A torque control fastener tightening tool that is usually
powered by compressed air. The design of the tool is such
that attempts are made to ensure that the applied torque
is independent of joint stiffness.
- NYLOC NUT
- A torque prevailing nut that uses a nylon patented insert
to provide a locking feature. The nylon insert, it is claimed,
helps to seal the bolt thread against seepage of water,
oil, petrol, paraffin and other liquids. The nut is covered
by UK patent 8028437 and European patent 81303450-1. Nyloc
is a registered trade name of Forest Fasteners.
- OCTAGON HEAD
- A bolt or screw whose head cross section is a regular
polygon with 8 sides.
- Tapping of a thread following a plating operation so that
the thread tolerances comply within specification allowing
the internal and external threads to assemble. It is normal
practice to overtap the internal rather than the external
- Term used in structural engineering for the joint plates.
- The nominal distance between two adjacent thread roots
- A single thickness of steel forming part of a structural
- Pooching is a term sometimes used to describe the effect
of the area immediately surrounding a tapped hole being
raised up as a result of the tension from the stud. Tapped
holes are often bored out for the first couple of threads
to eliminate this problem.
- The tension created in a fastener when first tightened.
Reduces after a period of time due to embedding and other
- PREVAILING TORQUE
- The torque required to run a nut down a thread on certain
types of nuts designed to resist vibration loosening. The
resistance can be provided by a plastic insert or a noncircular
- PREVAILING TORQUE NUT
- A type of lock nut which has a prevailing torque to assist
in preventing self loosening. There are two main categories
of prevailing torque nuts, all metal and nylon insert. All
metal torque prevailing nuts generally gain a prevailing
torque by distorting the threads at the top of the nut by
some means. Nylon insert torque prevailing nuts ultilise
a nylon (or other polymer) insert to achieve a prevailing
- PROOF LOAD
- The proof load of a nut is the axially applied load the
nut must withstand without thread stripping or rupture.
The proof load of a bolt, screw or stud is the specified
load the product must withstand without permanent set.
- PROPERTY CLASS
- A designation system which defines the strength of a bolt
or nut. For metric fasteners, property classes are designated
by numbers where increasing numbers generally represent
increasing tensile strengths. The designation symbol for
bolts consists of two parts:
- 1. The first numeral of a two digit symbol or the first
two numerals of a three digit symbol approximates 1/100
of the minimum tensile strength in MPa.
- 2. The last numeral approximates 1/10 of the ratio expressed
as a percentage between minimum yield stress and minimum
- Hence a fastener with a property class of 8.8 has a minimum
tensile strength of 800 MPa and a yield stress of 0.8x800=640
- The designation system for metric nuts is a single or
double digit symbol. The numerals approximate 1/100 of the
minimum tensile strength in MPa. For example a nut of property
class 8 has a minimum tensile strength of 800 MPa. A bolt
or screw of a particular property class should be assembled
with the equivalent or higher property class of nut to ensure
that thread stripping does not occur.
- The amplification of an external force acting on a bolt
by a lever action which can occur when that force is an
eccentric tensile load.
- REDUCED SHANK BOLT
- A bolt whose shank diameter is smaller than the nominal
diameter of the bolt (normally the shank diameter of such
a bolt is approximately equal to the effective diameter
of the thread).
- The loss of clamping force in a bolt that occurs typically
without any nut rotation occurring. Commonly occurs as a
result of embedment but can also be due to gasket creep,
metal creep (at elevated temperatures), differential thermal
expansion and stress relaxation.
- RIGHTHAND THREAD
- A screw thread that is screwed in by rotating clockwise.
The majority of screw threads are right handed.
- ROLLED THREAD
- A thread formed by plastically deforming a blank rather
than by cutting. The majority of standard fasteners have
their threads formed by rolling. Most threads are rolled
before any heat treatment operation. Significant improvements
in fatigue life can be achieved by rolling the thread after
heat treatment, this improvement is due to compressive stresses
being induced in the roots of the thread. However, because
of the increased hardness of the bolt blank, the die life
can be significantly reduced. Rolling the thread also generally
improves the surface finish which can have a beneficial
effect on fatigue life.
- ROOT DIAMETER
- Identical to MINOR DIAMETER
- A headed threaded fastener that is designed to be used
in conjunction with a pre formed internal thread or alternatively
forming its own thread. Historically, it was a threaded
fastener with the thread running up to the head of the fastener
that has no plain shank. However this definition has largely
been superseded to avoid confusion over the difference between
a bolt and a screw.
- SCREW THREAD
- A ridge of constant section which is manufactured so that
a helix is developed on the internal or external surface
of a cylinder.
- SELF LOOSENING
- Threaded fasteners can come loose on occasions without
human intervention. This loosening can be due to creep,
embedding, stress relaxation or the fastener self-rotating
(which is often called vibration loosening). Creep, embedding
and stress relaxation will generally not completely loosen
a fastener, these loosening mechanisms occur without the
nut rotating relative to the bolt. The term self loosening
is sometimes used for the nut rotating relative to the bolt
without human intervention. It is know that the fastener
can self rotate under the action of transverse joint movement
that can completely loosen a tightened fastener such that
the nut will become detached from the bolt.
- A screw and washer assembly. A screw or bolt which has
a captive washer. The washer is frequently loose on the
plain shank of the fastener, the shank diameter being equal
to the effective diameter of the thread; the thread being
rolled from this diameter. The origin of the word is a frequent
question. In the 1930's E. C. Crowther was a representative
for a company that sold both shakeproof washers and screws.
He came up with the idea of placing the washer on the screw
before it was thread rolled. The major diameter of the screw
being larger than the washer hole prevents it from coming
off. The Illinois Tool Works made machines that produced
these patented pre-asSEMbled
washers and screws. The s at the end of SEMs is thought
to have been subsequently picked up because they are not
usually purchased individually. In spite of the original
patents and trademarks the word SEMS is generally recognised
as a generic term applicable to screw and washer assemblies.
- SET SCREW
- A set screw is a threaded fastener that is typically used
to hold a sleeve, collar or gear on a shaft to prevent relative
motion. It is a threaded member that normally does not have
a head. Unlike most other threaded fasteners it is basically
a compression device normally used to generate axial thrust.
Various socket types are provided to allow the set screw
to be rotated. These types include hexagon socket, fluted
socket, screwdriver slot and square head. Various point
designs are available (the part of the set screw that rotates
against the shaft being secured) and include:
Cup - Hollowed end, is the most commonly used point style.
Used when the digging in of the point is not undesirable.
Cone - Pointed end, this type generates the highest torsional
holding power and is typically used for a permanent connection.
Oval - Rounded end that is typically used when frequent
adjustment is required. The oval end prevents/reduces indentation.
Flat - Cause little damage to the shaft and are used when
frequent adjustment is required.
Dog - Flat end with the threads stopping short of the end
with the end fitting into a hole.
- That portion of a bolt between the head and the threaded
- SHOULDER SCREWS
- A threaded fastener with a plain, precision machined,
shank that is used for location purposes. They are typically
used for pulleys and linkages.
- SKIDMORE BOLT TENSION CALIBRATOR
- The Skidmore-Wilhelm bolt tension calibrator is a hydraulic
load cell used to determine the tension in a bolt or other
threaded fastener. The tension in the bolt compresses fluid
in a hydraulic cylinder, a pressure gauge connected to the
cylinder is then calibrated to read in terms of force rather
- SNUG TORQUE
- The torque required to pull plates together so that direct
contact occurs; often used in angle control tightening.
The snug torque ensures that metal to metal contact occurs
at all the interfaces within the joint. It is only at this
point that the required angle of rotation start in order
that the bolt is tightened sufficiently. The snug torque
is usually determined experimentally on the actual joint.
- The process of pulling parts of a joint together, most
of the input turn during this process is absorbed in the
joint with little tension being given to the bolt.
- SOCKET HEAD CAP SCREW
- A screw with a round head, usually with a hexagon indentation
in the head for tightening purposes. Used on machine parts
and is typically made from high strength steel (grade 12.9
- SOFT JOINT
- A joint in which the plates and material between the nut
and bolt bearing surfaces have a low stiffness when subjected
to compression by the bolt load. In such a joint, the bolt
(or nut) typically has to be tightened by two or more complete
turns, after it has been torqued to the snug condition,
before the full tightening torque is achieved. Often the
placement of a gasket in a joint results in a soft joint.
- SOFT TORQUE
- An alternative name, used by some manufacturers, for snug
- SPIRAL WOUND GASKET
- A type of gasket that is made by winding V-section metal
strip and a softer filler material together. Support or
retaining rings, inside and/or outside the spiral, improve
the gasket's handling and fitting. The filler material used
is typically graphite or PTFE. The metal strip and retaining
rings being typically made from stainless steel.
- STATIC FRICTION
- Friction at rest; a force is required to initiate relative
movement between two bodies - static friction is the force
that resists such relative movement. Sometimes referred
to as stiction.
- STEP-LOCK BOLT (SLB)
- The Step-Lock Bolt (SLB) is a thread form that has been
modified to resist vibration loosening. The thread has several
horizontal portions (i.e. no lead angle) whose purpose is
to prevent torsion being developed in the bolt as a result
of the loosening purpose. It is these horizontal portions
that are known as steps. Published literature indicates
that the thread form performs well when tested on a transverse
vibration test machine. However manufacturing difficulties
may prevent its widespread adoption.
- A term used to describe a lock nut which has a prevailing
- STRENGTH GRADE
- See PROPERTY CLASS
- STRESS AREA
- The effective cross sectional area of a thread when subjected
to a tensile force. It is based upon a diameter which is
the mean of the pitch (or effective) and the minor (or root)
diameters of the thread. The use of this diameter stems
from the work of E. M. Slaughter in the 1930's. He completed
carefully controlled tests using various sizes of standard
threads and compared their strength with machined bars made
from the same bar of material. He found that this mean diameter
gave results that agreed with the tensile test results to
within about 3%. The error on the minor and pitch diameters
was about 15%. Tests completed subsequent to these by other
investigators have also shown that the stress diameter is
a reasonable approximation to a thread's tensile strength.
(Referance: 'Tests on Thread Sections Show Exact Strengthening
Effect of Threads.' by E. M. Slaughter, Metal Progress,
vol 23, March 1933 pp. 18-20)
- STRESS RELAXATION
- A significant problem with bolting at high temperatures
is a phenomenon known as stress relaxation. Creep occurs
when a material is subjected to high temperature and a constant
load. Stress relaxation occurs when a high stress is present
that is relieved over time; the stress is relaxed with a
subsequent reduction in the bolt’s preload. The only
way to minimise the effects of stress relaxation is to use
materials that have an adequate resistance to it at the
product’s operating temperature. The effect of bolt
stress relaxation is to reduce the clamp force provided
by the bolts; this phenomena alone will not fully loosen
- STRUCTURAL BOLT
- A structural bolt is a heavy hexagon head bolt having
a controlled thread length intended for use in structural
connections and assembly of such structures as buildings
and bridges. The controlled thread length is to enable the
thread to stop before the joint ply interface to improve
the fastener's direct shear performance.This term is used
in civil and structural engineering but is not frequently
used in mechanical engineering.
- A fastener which is threaded at both ends with an unthreaded
shank in between. One end (which often has a thread tolerance
which results in more thread interference) is secured into
a tapped hole, the other is used with a nut.
- SYMMETRICAL THREAD
- A symmetrical thread is one which has both flanks of the
thread profile inclined at the same angle.
- TAYLOR-FORGE METHOD
- A method developed by four engineers of the Taylor-Forge
Company in Chicago in the 1930's that subsequently formed
the basis of the ASME code for flanged joint design. The
assumptions made by the method are now generally regarded
as too simplistic. This method gives rise to the m and y
- TENSION WASHERS
- A general name given to spring washers, curved washers,
Belleville washers and disc springs. This type of washer
provides a relatively low stiffness (compared to the joint
stiffness) and can be used to act as a spring take-up with
a bolt to prevent movement between parts.
- THREAD CREST
- The top part of the thread. For external threads, the
crest is the region of the thread which is on it's outer
surface, for internal threads it is the region which forms
the inner diameter.
- THREAD FLANK
- The thread flanks join the thread roots to the crest.
- THREAD HEIGHT
- This is the distance between the minor and major diameters
of the thread measured radially.
- THREAD LENGTH
- Length the portion of the fastener with threads.
- THREAD ROOT
- The thread root is the bottom of the thread, on external
threads the roots are usually rounded so that fatigue performance
- THREAD RUNOUT
- The portion at the end of a threaded shank which is not
cut or rolled to full depth, but which provides a transition
between full depth threads and the fastener shank or head.
- Can be a term used for a number of vibration resistant
products but is now usually reserved for threadlocking adhesives.
Specifically, a liquid anaerobic adhesive applied to nut
or bolt thread, once hardened it fills the inner spaces
between the threads to produce a solid plastic of a known
- TIN/ZINC ALLOY ELECTROPLATING
- Tin/zinc alloy coatings (typically 70% tin and 30% zinc)
are applied to threaded fasteners to provide a corrosion
resistant coating. One of the advantages of such coatings
is that bimetallic corrosion will not occur when placed
into contact with such metals as aluminium or steel.
- TOLERANCE CLASS
- A combination of tolerance grade and a fundamental deviation
which is given to an internal or external thread. A tolerance
class for an internal thread when combined with the tolerance
class for an external thread gives the class of fit for
the mating threads.
- TOLERANCE GRADE
- The difference between maximum and minimum metal conditions
for a tolerance applied to a screw thread. For metric threads
the tolerance grade is given a number.
- A rotational moment; it is a measure of how much twisting
is applied to a fastener. The units used to measure torque
are in the form of force times length. Usually measured
in newton-metres (Nm) if metric units are used or pounds
feet (lb-ft) when imperial units are used.
- TORQUE MULTIPLIER
- A gearbox used to increase the torque produced by a small
- TORQUE WRENCH
- A manual wrench which incorporates a gauge or other method
to indicate the amount of torque transferred to the nut
- TURN OF THE NUT METHOD
- See ANGLE CONTROLLED TIGHTENING
- U BOLT
- A U shaped fastener threaded at both ends used primarily
in suspension and related areas of vehicles.
- ULTRASONIC EXTENSOMETER
- An instrument which can measure the change in length of
a fastener ultrasonically as the fastener is tightened or
measure the length before and after it is tightened).
- Unified National Coarse (UNC) is a thread form with a
60 degree flank angle rounded roots and flat crests. For
a given diameter it has a larger thread pitch than an equivalent
diameter UNF thread. The unified thread is based on inch
sizes and was first standardised in 1948 unifying the Whitworth
and American standard thread forms.
- Unified National Extra Fine (UNEF) is a Unified thread
form with a very fine (small) pitch that are typically used
on instruments and parts requiring a fine adjustment.
- Unified National Fine (UNF) is a thread form with a 60
degree flank angle rounded roots and flat crests. For a
given diameter it has a smaller thread pitch than an equivalent
diameter UNC thread.
- Unified National (UN) thread form with a rounded root
contour, applies only to external threads. (The UN thread
form has a flat, or optionally, a rounded root contour.)
The majority of fasteners with a Unified thread form have
a rounded root contour i.e. are UNR threads.
- VIRTUAL EFFECTIVE DIAMETER
- The effective diameter of a thread but allowing for errors
in pitch and flank angles.
- WAISTED SHANK BOLT
- A bolt whose diameter is less than the minor diameter
of the thread. Frequently the shank of the bolt is 0.9 times
the root diameter.
- WIRE THREAD INSERT
- A threaded insert that is typically used for tapped hole
repair or to improve the thread stripping strength of softer
metals such as zinc and aluminium. The inserts are assembled
into a previously tapped hole using a special driving tool.
A thread locking compound is frequently used to secure the
insert if the assembly is subject to vibration.
- YIELD CONTROLLED TIGHTENING
- A fastener tightening method which allows a fastener to
be tightened to yield. The angle of rotation of the fastener
is measured relative to the applied torque, yield being
assessed when the slope of the relationship changes to below
a certain value. Sometimes called joint controlled tightening.
- ZINC ELECTROPLATING
- Zinc electroplating is a common way to protect threaded
fasteners from the effects of corrosion. Zinc electroplating
can be completed in acid chloride, alkaline or cyanide baths.
Supplemental coatings are frequently applied to zinc electroplating.
These coatings, such as zinc phosphate or chromate conversion,
provide a protective passivation layer on the zinc which
assists in reducing the corrosion rate.
- ZINC/COBALT ALLOY ELECTROPLATING
- This coating is similar to zinc electroplating completed
in an acid chloride bath - a small amount of cobalt (typically
about 1%) is added to increase the plating speed.
- ZINC PHOSPHATE CONVERSION COATING
- A zinc phosphate conversion coating is frequently added
to zinc electroplated parts, such as bolt threads, to improve
corrosion resistance. This type of chemical conversion coating
provides a protective passivation layer on the zinc improving
its corrosion resistance.
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