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This fastener glossary
provides definitions of fasteners and
fastener terms. It is intended to assist fastener
buyers, fastener purchasers, Original Equipment
Manufacturers (OEM's) and engineers.
Galling:
Galling occurs when the stainless steel oxide
surface film breaks down as a result of direct metal
contact. Solid-phase welding can then take place
(whereby material is transferred from one surface to
another). The symptoms of galling include surface
damage and seizing and freezing up of equipment.
Galling commonly occurs when using stainless steel
nuts and bolts together, where the contact points
are subjected to high tightening torques. See
"Galling and how to prevent it".
Galvanic Corrosion:
An accelerated degree of corrosion occurring when
two different metals are in contact with moisture,
particularly sea water. All metals have what is
termed a specific electric potential, so that low
level electric current flows from one metal to
another. A metal with a higher position in the
galvanic series (see below) will corrode
sacrificially rather than one with a lower position,
meaning stainless, for example, will corrode before
gold. The further apart the metals on the chart, the
more electric current will flow and the more
corrosion will occur. No serious galvanic action
will occur by combining the same metals, only
dissimilar ones. To prevent galvanic corrosion, use
insulation, paint or coatings when separating
dissimilar metals; or put the metal to be protected
next to a metal which is not important in the
assembly, so it can corrode sacrificially.
Galvanize:
To treat with a bath of lead and zinc to prevent
rusting.
Gimlet Point:
A taper-threaded point; applied to wood screws, Type
A tapping screws, lag bolts, etc. Usually having a
point angle of 45-50 degrees..
Grain:
An individual crystal in a polycrystalline metal or
alloy; it may or may not contain twinned regions and
subgrains; a portion of a solid metal (usually a
fraction of an inch in size), in which the atoms are
arranged in an orderly pattern.
Grain Boundary:
A narrow zone in a metal corresponding to the
transition from one crystallographic orientation to
another, thus separating one grain from another; the
atoms in each grain are arranged in an orderly
pattern; the irregular junction of two adjacent
grains is known as a grain boundary.
Graphitic Corrosion:
Deterioration of gray cast iron in which the
metallic constituents are selectively leached or
converted to corrosion products leaving the graphite
intact. The term graphic quotation is commonly used
to identify this form of corrosion, but is not
recommended because of its use in metallurgy for the
decomposition of carbide to graphite; deterioration
of gray cast iron in which the metallic constituents
are selectively leached or converted to corrosion
products leaving the graphite intact.
Grind:
To finish or polish a surface by means of an
abrasive.
Grip:
The unthreaded portion of a bolt or screw.
Grip Length:
Total distance between the underside of the nut to
the bearing face of the bolt head; includes washer,
gasket thickness etc.
Half Dog Point:
The same as a dog point but half as long; used on
short screws for the same purposes as the dog point,
but in a shallower hole or slot.
Halogen:
Any of the elements of the halogen family,
consisting of fluorine, chlorine, bromine, iodine,
and astatine.
Hanger Bolt:
One end is gimlet pointed and has a wood screw
thread. The other end consists of a coarse machine
screw thread. The center section is unthreaded.
Hardenability:
In a ferrous alloy, the property that determines
the depth and distribution of hardness induced by
quenching.
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 used a flange headed
nuts and bolts.
Hardening:
A method of heat treating metals by heating to a
temperature within, or above, the critical range,
holding at that temperature for a given time, and
then cooling rapidly, usually by quenching in oil or
water.
Hardness:
Normally stated in terms of Rockwell or Brinell
scale of measurement, hardness shows resistance of a
fastener to rough marks and abrasions, can indicate
yield strength and brittleness, and has a direct
relationship to tensile strength in alloy steel
fasteners. However, for stainless, brass, and
silicon bronze, the correlation between hardness and
tensile or yield is tenuous with no definite
relationship. Case-hardening uses surface heat
treatment on ferrous material to cause a harder
outside surface than the center. Through-hardening
hardens the entire fastener. Bright hardening calls
for heat treatment without oxygen, so no oxides are
formed on the material surface.
Hastelloy:
Hastelloy, a corrosion resistant alloy, is a
registered trademark of Haynes International.
Head:
The preformed, enlarged end of a bolt, screw, pin,
or rivet, provided with a bearing surface which is
usually either flat or conical.
Header Point:
A chamfer point, usually of 45 degrees included
angle, forming by a die at the time of heading and
prior to threading.
Head Marking: Used to identify the material used in a fastener
such as a bolt or cap screw. The marking is either
raised or indented to specifications.
Heat Treating:
An operation involving the heating and cooling of a
metal to obtain certain desirable conditions or
properties.
Height of Thread:
The distance, measured perpendicular to the axis,
between the major and minor cylinders or cones,
respectively.
Heli-Coil:
Coil of wire used as an insert to accept a screw or
bolt and adding holding power by forcing itself
between the fastener and the walls of the recess
when the fastener is driven in.
Hexagonal:
A recessed hexagon socket in the headd of a cap or
set screw to add greater tightening and loosening
power. Used with a Hex key wrench.
Hexagon Head:
Flat top surface with hexagonal sides andn with a
flat bearing surface. (Six Sides)
High Strength
Fastener: A high strength fastener is a fastener
having high tensile and shear strengths attained
through combinations of materials, work-hardening,
and heat treatment.
Hook Bolt:
A "bent bolt" having the unthreaded end bent to form
a hook, such as a round bend, square bend,
right-angle bend, or acute-angle bend hook bolt.
Hot Dip Galvanizing:
Hot dip galvanizing is the process of applying a
zinc coating to fabricated iron or steel material by
immersing the material in a bath consisting
primarily of molten zinc.
Hot Forming:
Working operation such as bending and drawing sheet
and plate, forging, pressing, and heading, performed
on metal heated to temperatures above room
temperature.
Hot Forging:
Heating metal to red-hot temperatures or
temperatures above the recrystallization point to
soften it before shaping a fastener. Hot forging is
primarily used when the diameter of the metal is to
large for cold forming or the quantity required is
to small to economically set up a cold-forming
machine.
Hot Working:
Deforming metal plastically at sucha temperature and
strain rate that recrystallization takes place
simultaneously with the deformation, thus avoiding
any strain hardening.
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. However, 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.
IFI:
Stands for Industrial Fasteners Institute.
ISO:
Stands for International Organization for
Standardization.
Immersion Plating:
Depositing a metallic coating on a metal immersed in
a liquid solution, without the aid of an external
electric current. Also called dip plating.
Immunize:
To remove small particles of iron or grit from the
surface of stainless steel by pickling in an acid
solution.
Impact Test:
A test to determine the energy absorbed in
fracturing a test bar at high velocity. The test may
be in tension or in bending, or it may properly be a
notch test if a notch is present, creating multiaxil
stresses.
Included Angle of Thread:
The angle between the flanks or the thread measured
in an axial plane.
Inclusions:
Particles of foreign material in a metallic matrix.
The particles are usually compounds (such as oxides,
sulfides, or silicates), but may be of any substance
that is foreign to (and essentially insoluble in)
the matrix.
Incomplete Thread:
On straight threads, that portion at the end having
roots not fully formed by the leadd or chamfer on
threading tools:
Ingot:
Steel formerly in a molten state, transferred to an
ingot mold to solidify.
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
higher resistant to vibration loosening and/or
removal. Some types have special thread forms.
Interference Fit:
A thread fit having limits of size so prescribed
that an interference always results when mating
parts are assembled.
Intergranular Corrosion:
A technical term describing corrosion at grain
boundaries (various outside portions) of a fastener.
It can occur when fasteners are heated above 800
degrees during use, such as welding, which changes
the chromium-carbon bond in stainless, thus allowing
increased oxidation and corrosion. To prevent
intergranular corrosion, low carbon stainless should
be used, or material should be annealed and quenched
after exposure to elevated temperatures, so the
carbon is put back into a austenitic stainless bond.
Internal Thread:
A thread on the internal surface of a hollow
cylinder or cone. Slang - The female thread.
International Metric Thread System:
A thread form similar to the American standard,
excepting the depth which is greater. There is a
clearance between the root and mating crest fixed at
a maximum of 1/16 the height of the fundamental
triangle or 0.054 x pitch. A rounder root profile is
recommended. The angle in the plane of the axis is
60 degrees and the crest has a flat like the
American standard equal to 0.125 pitch.
Ion:
An atom, or group of atoms, that has gained or lost
one or more outer electrons and thus carries an
electric charge. Positive ions, or cations, are
deficient in outer electrons. Negative ions, or
anions, have an excess of outer electrons.
Iron:
Primarily the name of a metallic element. In the
steel industry, iron is the name of the product of a
blast furnace containing 92 to 94 percent iron.
Other names for blast furnace are pig iron and hot
metal.
J-Bolt:
A "bent bolt" having the unthreaded end reverse bent
to approximately a semicircle.
Jackson Head:
A machine screw with a small oval head. (Trimmed
oval head).
Jam Nut:
(1.) A second nut forced or jammed against the main
nut to prevent loosening. (2.) A thin nut.
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.
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.
Key:
A small block or wedge inserted between shaft and
hub to prevent circumferential movement.
Knurl:
A roughened surface produced by contact with a wheel
which forces metal above the surface while making
indentations below the surface.
Lag Bolt:
A fastener having a hex or square head, with a
gimlet or cone point, and a thin, sharp,
coarse-pitch thread, designed for insertion in wood
or other resilient materials and producing its own
mating thread.
Lead Error:
A variation in the distance between the threads of a
screw.
Left-Hand Thread:
A thread is a left-hand thread, if, when viewied
axially, it winds in a counter-clockwise and
receding direction. All left-hand threads are
designated LH.
Length of Engagement: The axial distance over which an external thread is
in contact with an internal thread.
Length of Fastener:
The length of a flat bearing surface fastener is the
distance, in a line parallel to the axis, from the
bearing surface to the extreme point. The length of
a countersunk head fastener is the distance, in a
line parallel to the axis, from the largest diameter
of the bearing surface to the extreme point.
Lentil Head Screw: Obsolete term for oval head screw.
Lock Nut:
A nut which, in addition to serving the purposes of
an ordinary nut, has a special means for gripping a
threaded member so that a relative rotation between
the nut and the threaded companion member is
prevented in use.
MS Specifications:
Stands for Military Standards. The overriding
characteristic of MS fasteners compared to
commercial products is the extensive inspection and
lot traceability for MS, guaranteeing the chemical,
physical and dimensional qualities. While commercial
fasteners may look similar and happen to pass many
tests given MS products, the commercial fasteners
lack the pedigree of guaranteed quality for
chemical, physical and dimensional aspects that
users who order MS fasteners rely on.
Machineability:
The condition or property of a metal which allows it
to be cut, turned, broached or formed by machine
tools
Magnaflux:
A magnetic method of determining surface and
subsurface defects in metals.
Magnetism:
As related to stainless fasteners, 300 series
stainless is non-magnetic in its raw material
condition. Cold working can sometimes induce traces
of magnetism in 300 series, depending on the
severity of cold working and chemical composition of
the stainless. A rise in magnetism is related to an
increase in tensile strength and work hardening
caused by the heat and friction of cold forming and
does not reduce corrosion resistance or cause any
molecular change in austentic raw material. A higher
portion of nickel can increase stability in
stainless, thus decreasing work hardening and any
possibilities of magnetism. Brass and silicon bronze
are non-magnetic.
Major Diameter:
On a straight thread, the diameter of the coaxial
cylinder which would pass through the crests of an
external thread or the roots of an internal thread.
The largest diameter of a screw thread.
Malleability:
The ability of a metal to be deformed permanently
under compression load without rupture or fracture
(e.g. hammer or rolled into sheets.
Manganese: A non-magnetic metal which improves strength and
hardness.
Martensitic:
Comprising approximately 5% of stainless fasteners,
martensitic refers mainly to stainless types 410,
416, and 420. Named for Robert Martens, a German
metallurgist, martensitic grades have a high carbon
content which reduces corrosion resistance, but
allows a sharp increase in tensile strength after
heat treatment. Because of its high tensile
strength, martensitic stainless is used for highly
stressed parts such as control rod mechanisms,
valves, shafts, pump parts under high stress.
Martensitic stainless is magnetic, contains no
nickel, loses toughness in very cold temperatures,
and may have tendency to become brittle. Its
corrosion resistance is not as good as austentic or
ferritic stainless, so martensitic fasteners are
used in mild atmospheres.
Master Gage:
A thread-plug gage which represents the physical
dimensions of the nominal or basic size of the part.
It clearly establishes the minimum size of the
threaded hole and the maximum size of the screw at
the point at which interference between mating parts
begin.
Maximum Material Limit:
The maximum limit of size of an external dimension
or the minimum limit of size of an internal
dimension.
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.
Mechanical Plating:
Plating wherein fine metal powders are peened onto
the work by tumbling or other means.
Mechanical Properties:
Those properties of a material that reveal the
elastic and inelastic reaction when force is
applied, or that involve the relationship between
stress and strain; for example, the modulus of
elasticity, tensile strength and fatigue limit.
Micro-Inch:
One millionth of an inch. Used in measuring
imperfections of surface finishes.
Milled from bar (machining):
Made on a screw machine or lathe by cutting material
away from the original piece of metal. It is used
for manufacturing very large diameters which cannot
be cold formed and for small quantities where it
would not be economical to set up cold forming
equipment. However, machining can interrupt the
grain of metal causing a lessening in tensile and
fatigue strength.
Milling Process:
A machining process whereby a surface is generated
with a rotating toothed cutter. Each tooth takes an
individual chip.
Minor Diameter:
On a straight thread, the minor diameter is the
diameter of the coaxial cylinder which would pass
through the roots of an external thread.
Molybdenum:
Before Scheele recognized molybdenite as a distinct
ore of a new element in 1778, it was confused with
graphite and lead ore. The metal was prepared as an
impure form in 1782 by Hjelm. Molybdenum does not
occur native, but is obtained principally from
molybdenite. Molybdenum is also recovered as a
by-product of copper and tungsten mining operations.
The metal is prepared from the powder made by the
hydrogen reduction of purified molybdic trioxide or
ammonium molybdate. The metal is silvery white, very
hard, but is softer and more ductile than tungsten.
It has a high elastic modulus, and only tungsten and
tantalum, of the more readily available metals, have
higher melting points. It is a valuable alloying
agent, as it contributes to the hardenability and
toughness of quenched and tempered steels. It also
improves the strength of steel at high temperatures.
It is used in certain nickel-based alloys, such as
the "Hastelloys(R)" which are heat-resistant and
corrosion-resistant to chemical solutions.
Molybdenum oxidizes at elevated temperatures. The
metal has found recent application as electrodes for
electrically heated glass furnaces and foreheaths.
The metal is also used in nuclear energy
applications and for missile and aircraft parts.
Molybdenum is valuable as a catalyst in the refining
of petroleum. It has found applications as a
filament material in electronic and electrical
applications. Molybdenum is an essential trace
element in plant nutrition. Some lands are barren
for lack of this element in the soil. It is the key
ingredient that seperates 316 stainless from the
rest of the austentitic stainless family.
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.
Monel:
Invented by the International Nickel Co. and
composed basically of two-thirds nickel, one-third
copper, monel has good strength, excellent corrosion
resistance against salt water and in high
temperatures, and is very expensive.
NAS Drawings and Specifications:
Dimensional and material standards for aircraft
fasteners developed by the National Aerospace
Standards Committee. All drawings and specifications
are prefixed by "NAS".
Nail Point:
A sharp pyramidal point of approximately 30 degrees
or 45 degrees included angle.
NAS:
Stands for National Aerospace Standards.
Naval Brass (Naval Bronze):
A corrosion resisant metal containing 60 per cent
copper, 39 1/4 per cent zinc and 3/4 per cent tin.
Neck:
A portion reduced in diamter between the ends of a
shaft.
Nickel: A silver-white metal added to 300 series stainless
to provide corrosion resistance, increased strength
in both high and low temperatures, and increased
toughness in low temperatures. Nickel lowers the
effects of work hardening, thus reducing traces of
magnetism caused by cold forming and making material
flow more freely in manufacturing. Discovered by Cronstedt in 1751 in kupfernickel (niccolite).
Nickel is found as a constituent in most meteorites
and often serves as one of the criteria for
distinguishing a meteorite from other minerals. Iron
meteorites, or siderites, may contain iron alloyed
with from 5 to nearly 20% nickel. Nickel is obtained
commercially from pentlandite and pyrrhotite of the
Sudbury region of Ontario, a district that produces
about 30% of the nickel for the Free World. Other
deposits are found in New Caledonia, Australia,
Cuba, Indonesia, and elsewhere. Nickel is silvery
white and takes on a high polish. It is hard,
malleable, ductile, somewhat ferromagnetic, and a
fair conductor of heat and electricity. It belongs
to the iron-cobalt group of metals and is chiefly
valuable for the alloys it forms. It is extensively
used for making stainless steel and other
corrosion-resistant alloys such as Invar(R), Monel(R),
Inconel(R), and the Hastelloys(R).
Nipple:
A short piece of threaded pipe used to connect two
fittings.
Noble:
The positive direction of electrode potential, thus
resembling noble metals such as gold and platinum.
Noble Metal:
(1) A metal whose potential is highly positive
relative to the hydrogen electrode. (2) A metal with
marked resistance to chemical reaction, particularly
to oxidation and to evolution by inorganic acids.
The term as often used is synonymous with precious
metal.
Noble Potential:
A potential more cathodic (positive) than the
standard hydrogen potential.
Nominal Diameter:
The diameter equal to the external diameter of the
threads.
Non-Ferrous Metal:
Metals or alloys without an appreciable amount of
iron. Examples are aluminum, brass, copper, etc.
Stainless Steel is comonly referred to as
non-ferrous, but in fact, it is not.
Non-Magnetic:
Steels with sufficient quantities of manganese or
nickel to render the steel non-magentic. 18-8 (300
series chrome-nickel steel) is non-magnetic when
annealed. Type 316 is non-magnetic in all
conditions. (See pg. 156 of the Data Book).
Non-standard
Fastener: A nonstandard fastener is a fastener
which differs in size, length, material, or finish
from established and published standards.
Normal Stress:
The stress component perpendicular to a plane on
which forces act. Normal stress may be either
tensile or compresssive.
Normalize:
To remove internal stresses by heatinig a metal
piece to its critical temperature and allowing it to
cool very slowly.
Nut:
A metal block (solid nut) or sleeve having an
internal thread made to assemble with the external
thread on a bolt, screw, or other threaded part. It
may be a fastening means, an adjusting means, a
means for transmitting motion, or a means for
transmitting power with large mechanical advantage
and nonreversible motion.
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.
Nylon Fasteners:
Fasteners made of a material that has a low
dielectric constant and relatively high tensile
strength, enabling it to resist high voltage at
commercial frequencies. It can also operate at
continuous temperatures as high as 250 degrees F.
Any temperatures above has an effect to oxidize
material. Electric and electronic equipment
manufacturers are finding many corrosion-resistant
applicables for this type of fastener.
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