Presented below is a glossary of terminology
on topics related to nuts and bolts, threaded fasteners and tightening
techniques. If you have any questions about any of the content or
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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.
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 metal contact.
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.
ALLOWANCE
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.
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 limited.
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.
BOLT
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 diameter.
BSF
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: 1956.
BSW
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 improved.
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
tightening torque. Cleveloc is a registered trade name of Forest Fasteners.
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.
COMMINGLING
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.
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
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.
DACROMET
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 Ohio
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 interference exists.
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.
EMBEDMENT
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.
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.
FRICTION
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 friction coefficients.
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
the allowance.
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.
GALLING
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 fasteners.)
GRIP
LENGTH
Total
distance between the underside of the nut to the bearing face of the
bolt head; includes washer, gasket thickness etc.
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 washer.
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 task.
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.
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.
KEPS
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.
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.
MEANSHIFT
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.
OVERTAPPING
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 thread.
PILES
Term
used in structural engineering for the joint plates.
PITCH
The
nominal distance between two adjacent thread roots or crests.
PLY
A
single thickness of steel forming part of a structural joint.
POOCHING
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.
PRELOAD
The
tension created in a fastener when first tightened. Reduces after
a period of time due to embedding and other factors.
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 head.
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
torque.
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 tensile stress.
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 MPa.
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.
PRYING
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).
RELAXATION
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
SCREW
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.
SEMS
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.
SHANK
That
portion of a bolt between the head and the threaded portion.
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 than pressure.
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.
SNUGGING
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 in metric).
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 torque.
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.
STIFFNUT
A
term used to describe a lock nut which has a prevailing torque.
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 a joint.
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.
STUD
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 gasket factors.
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 is improved.
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.
THREADLOCKER
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 shear strength.
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.
TORQUE
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 hand wrench.
TORQUE
WRENCH
A
manual wrench which incorporates a gauge or other method to indicate
the amount of torque transferred to the nut or 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).
UNC
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.
UNEF
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.
UNF
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.
UNR
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.
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.
Gage Crib Worldwide, Inc.
6701 Old 28th St SE, Suite B
Grand Rapids, MI 49546-6937
Phone: 001-616-954-6581 • Fax: 001-616-954-6583 CONTACT FORMS & INFO
