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Technical Info
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Acorn Nut - a nut with a domed top which prevents contact with the external thread. Aerotight Nut - an all metal prevailing torque nut. The nut is slotted in two places and are bent slightly inwards and downwards. When tightened, the two slotted parts are forced back to their original position which causes the nut threads to bind onto the bolt threads to provide a "prevailing torque". ANSI - American National Standards Institute ASTM - American Society of Testing and Materials Alloy Steel - a variety of steel which includes elements in addition to carbon, giving the material the ability to retain its strength at higher temperatures. Additionally, alloy steel has greater tensile and yield strength than medium carbon steel. Annealed - the condition of a fastener when it has been heated, then cooled to make it soft or less brittle.
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.
Carbon Steel - a steel whose only significant alloying material is carbon. Case Hardening - the process performed on quenched, tempered, ferrous fasteners which makes the surface of those parts harder than the inner core. Case hardness is measured in the threaded section of the fastener.
Core Hardness - the resistance a fastener material has to being permanently deformed, measured at a spot deeper than the case depth. Ductility - the ability of a material to be permanently bent or stretched without breaking.
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.
Elongation - the lengthening of a fastener caused by a tensile force. Embedment (minimum) - the minimum depth an anchor must be installed to meet the minimum pullout values. It is the distance measured from the concrete surface to the bottom of the anchor.
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.
Ferrous - that which contains iron:
usually refers to fasteners that contain more iron than any other element.
Fillet - the concavely curved section
at the angle formed at two intersecting surfaces of a fastener.
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.
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 zinc
coated.)
Grip Range - the minimum and maximum thickness of materials a rivet can join together.
Hardened Washer - 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
utilizes 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.
Hydrogen Embrittlement - the
condition of a fastener which has had hydrogen introduced into its steel,
causing it to be substantially less ductile and prone to sudden and
premature failure
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.
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.
Kep Nut - 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.
Left Hand 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
usages for this term as follows:
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 handle.
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.
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.
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.
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.
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.
Passivating - the process performed
on stainless fasteners of removing surface imperfections and producing a
slight film on the surface which enhances the parts' resistance to
corrosion.
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.
Proof Load - the tension applied
force a fastener must support without any permanent change in size or
shape.
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 which commonly occurs as a result of embedment. Can also
be caused by gasket creep, differential temperature expansion or vibration
loosening.
Rockwell Hardness - a test performed
on fasteners to determine the resistance a fastener material has to being
permanently deformed by a specifically shaped object under a specific
amount of weight. The test measures how deep an indentation is made in the
fastener. That measurement is converted to a reading on the Rockwell
hardness scale.
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
SAE - Society of Automotive Engineers
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.
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.
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.
Shear Load (ultimate) - a minimum
amount of force, applied transversely, which a blind rivet must withstand
without failing.
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.
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).
Step-Lock Bolt - he 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.
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.
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.
Tensile Strength - the greatest
longitudinal stress a fastener can withstand prior to, or at the time of
breaking apart.
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 Pitch - The nominal distance
between two adjacent thread roots or crests.
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.
Tolerance - the difference between
the lower and upper limits between which a fastener's size(s) must be
measured.
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.
U-Bolt - A U shaped fastener threaded
at both ends used primarily in suspension and related areas of vehicles.
UNC - Unified National Course threads. UNF - Unified National Fine threads. Washer Face - a circular boss on the bearing surface of a screw, bolt, or nut. Yield Strength - the tension-applied stress at which a fastener, under strain, is deformed a measurable amount.
Zinc Plating - 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.
The grade, in regards to a fastener, refers to the material used to manufacture the part. Different types of fasteners have their own designations for the grading but a Hex Head Cap Screw (HCS), is the most common fastener where a choice of grade must be made. Grade 2 - is a low-carbon steel fastener with a minimum tensile strength of 74,000 psi in diameters through 3/4" and 60,000 psi for diameters over 3/4" through 1-1/2". A fine thread in this grade is hard to find and is not recommended due to inferior resistance to stripping during tightening. Grade 2 is also an alternative to ASTM A307 Grade A. Grade 5 - is a medium-carbon steel, heat treated fastener with a minimum tensile strength of 120,000 psi in diameters through 1" and 105,000 psi for diameters over 1" through 1-1/2". Grade 5 meets other specifications such as ASTM A449. Grade 8 - is an Alloy, heat treated fastener with a minimum tensile strength of 150,000 in diameters through 1". Grade 8 meets other specifications such as ASTM A354 Grade BD.
DRIVE TYPES
HEAD STYLES
POINT STYLES
There are two main types of thread pitches in use today. UNC, or Unified National Course thread, where threads are further apart than that of UNF, or Unified National Fine thread. In both cases, the thread pitch is stated as the number of threads that exist in one inch of the fastener's body. Fastener threads provide both load holding when tightened and the ability to move during tightening and loosening. A fastener's pitch and lead, determined by whether it has a course or fine thread, is a factor in fastener performance. A coarse thread will move a larger distance with each rotation than a finer thread of the same diameter. If speed of assembly is a factor, a coarse thread may be preferred as it will assemble faster. If adjustment is required, a fine thread will provide a less sensitive adjustment and a greater ability to control adjustment. Each flank of thread carries a percentage of the clamping load. This load is not distributed equally among the thread pitches in most applications. It is generally thought that a fine pitch thread will have a more uniform load distribution among its engaged pitches when installed. Finally, starting can be a factor. Fine threads can be more susceptible to cross threading if assembly conditions are not controlled. In general, both coarse and fine threads have advantages depending on the fastening output required. Below is a chart showing the basic data on course and fine threads from 1/4" to 1" diameters.
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Phillips - is the most
recommended drive type as it provides good control when driving. Make sure to
use a driver bit of the proper size and which is in good condition to minimize
the chance of stripping.
Slotted - accepts the
standard blade type screwdriver and requires less downward pressure when
driving parts than those with recessed openings. Make sure to use a proper
fitting blade to minimize the chance of slippage.
Combination - accepts
both phillips and standard blade screwdrivers. The most common use for this
type of drive is for applications where a fastener is expected to be driven
and backed-out several times.
Slotted Hex - accepts
both a hex driver and standard blade screwdrivers. The slot is added for easy
removal of the fastener.
Torx - is a
positive-engaging, fast-locating method which transmits drive torque while
requiring less downward pressure. It also provides a pleasant fastening
appearance.
Square - originally a
standard drive type used in Canada, the square drive increases productivity
with excellent torque transmission while resisting cam-out. Also known as a
"Robertson" drive, it offers a distinctive appearance which discourages
tampering.
Pan Head - recommended
for new designs to replace round, truss, and binding heads. Provides a low
large diameter head, but with characteristically high outer edge along the
outer periphery of the head where driving action is most effective for high
tightening torques. Slightly different head contour where supplied with
recessed head.
Flat Head - supplied
to standard dimensions with an 80 degree to 82 degree angle to be used where
finished surfaces require a flush fastening unit. The countersunk portion
offers good centering possibilities.
Truss Head - also
known as oven-head, stove head, and oval-binding head. A low, neat appearing,
large diameter head having excellent design qualities, and as illustrated can
be used to cover larger diameter clearance holes in sheet metal when
additional play in assembly tolerance is required.
Round Head - Not
recommended for new design but was the most universally used design in the
past.
Oval Head - Fully
specified as "oval countersunk", this head is identical to the standard flat
head but possesses in addition, a rounded, neat appearing upper surface for
attractive design.
Fillister Head - The
standard oval fillister head has a smaller diameter than the round head, but
is higher with a correspondingly deeper slot. The smaller diameter head
increases the pressure applied on the smaller area and can be assembled close
to flanges and raised surfaces. Headed in counterbored dies to insure
concentricity, they may be used successfully in counterbored holes.
Binding Head (straight
side) - most generally used in electrical and radio work because of its
identifying undercut beneath the head, which binds and eliminates fraying of
stranded wire. Offers an attractively designed, medium-low head with
ordinarily sufficient bearing surface. Not ordinarily recommended as a
Phillips Recessed head. 
Type A - a thread forming screw
with a course thread and gimlet point. It's primary applications are light
gauge sheet metal from .015 to .050 thick, wood, plywood, asbestos combinations, among
others. Industry standards have made this point type obsolete and a "Type AB"
point has become the replacement.
Type B - a thread forming screw for
use in heavier metal .050 to .200 thick. Larger root diameter with finer
thread pitch for light and heavy sheet metal non-ferrous castings, plastics,
impregnated plywoods, asbestos combinations, and other materials.
Type
AB - as the name indicates, this thread forming screw combines the finer
thread of the Type B with the sharp gimlet point of the Type A making it the
new standard for use in light and heavy gauge sheet metal, wood, plywoods,
plastics, asbestos combinations among many others.
Type 17
- a thread cutting screw with a course thread for use in wood. This screw has
a special flute which allows it to capture the wood chips and minimizes
splitting in thin materials.