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Fastener Resources

Glossary of Fastener Terms

Acorn Die - A form of threading die for use in screw machines. The cutting portion resembles an acorn nut.

Age Hardening - A process of aging that increases hardness and strength and ordinarily decreases ductility. Age hardening usually follows rapid cooling or cold working.

Aircraft Quality Fasteners - is used to describe highly stressed applications, are to be produced under closely controlled, special and, in most cases restricted methods of manufacture and inspection.

Alkaline - An ionic substance that has a pH value greater than 7. A base that dissolves in water and acts as an electrolyte in the presence of metal.

Allowance - The intentional space manufactured into mating threads. The intentional or prescribed difference between the thread design size and the thread basic size. It is the minimum clearance (positive allowance) or maximum interference (negative allowance) between such parts.

Alloy - A metallic substance composed of 2 or more elements.

Alloy Steel - Steel is considered to be alloy when the maximum range given for manganese exceeds 1.65% or a definite minimum quantity for any of the following elements is specified or required within the limits of the recognized field of constructional alloy steels: chromium, molybdenum, nickel, or any other alloying element added to obtain a desired alloying effect. Defined definite mechanical or physical properties, such as higher strength at elevated temperatures, toughness, etc are common in alloy steel.

Aluminum - An element with atomic number 13. This material is known for its excellent strength to weight ratio and is used widely in metal manufacturing.

American Institute of Steel Construction (AISC) - A technical institute and trade association established to serve the structural steel design community and construction industry in the United States.

American Iron and Steel Institute (AISI) - An association of North American steel producers developed in response to the need for a cooperative agency in the iron and steel industry for collecting and disseminating statistics and information, doing investigations, providing a forum for the discussion of problems and generally advancing the interests of the industry.

American National Standards Institute (ANSI) - Establishes the "Unified Screw Thread Standards" which regulates the fastener thread industry.

AN Specifications - Dimensional standards for aircraft fasteners developed by the Aeronautical Standards Group. All drawings are prefixed by AN.

Anchor Bolt - A steel rod or bar with one end intended to be embedded in concrete and the other end threaded and projected for anchoring material to concrete. The end cast in concrete may be straight, bent hook, a forged head or a welded attachment to resist forced imposed on the anchor bolt.

Angle Controlled Tightening - Method of tightening where the person installing the fastener removes the slack from the assembly, and then tightens the fastener a prescribed amount (e.g. 120°) based on the length of the fastener. This method is often coupled with the use of torque for the initial "snug" tightening.

Angle of head - In countersunk heads, the included angles of the conical under-portion or bearing surface usually 82 to 100 degrees.

Angularity - The angle between the axes of two surfaces of a fastener.

Annealing - A heat treating process in which an iron-based metal is fully austenized and allowed to cool slowly, thereby relieving stress and often decreasing strength properties (opposite of rapid quenching from austenitic temperatures) - general term applied to a variety of thermal treatments applied to fasteners for the purpose of softening or homogenizing material properties.

Anode - The material in a corrosive attack that becomes corroded.

Anodizing - The formation of an oxide film on the surface by means of an anodic treatment. This is commonly used on aluminum.

ASME - American Society of Mechanical Engineers. The organization is known for setting codes and standards for mechanical devices.

ASTM - American Society for Testing and Materials is a leading standards development organization. Best known fastener Committee is F-16. This committee has jurisdiction on over 60 fastener standards.

Austenite - A specific metallic solution of steel that allows for higher carbon absorption. Austenite exists in the temperature range of approximately 1341°F (727°C) to 2719°F (1493°C). It is non-magnetic; has the crystal structure of face-centered cubic.

Austenizing - The process of uniformly heating a metal until the grain structure transforms from Ferrite to Austenite. For carbon steels, transformation begins at approximately 1341°F and becomes completely uniform throughout the metal at a higher temperature (varies by carbon content).

Average Coating Thickness - This is determined as either the value obtained by analytical methods or the mean value of a specified number of local thickness measurements that are evenly distributed over the significant surface.

Baking - The process of heating fasteners for an extended period of time preferably within one hour after electroplating. This is done to reduce the risk of hydrogen embrittlement.

Bar - This describes the material (i.e. stainless, brass, carbon steel, alloy steel) that is produced from hot rolled or cast billets for example with the cross sectional dimensions that have a relatively constant dimension.

Barrier Protection - A method of fighting corrosion in which a layer of inert material (e.g.: paint) is placed over the fastener to prevent rust.

Basic Size - Basic size is that size from which the limits of size are derived by the application of allowances and tolerances.

Bearing Surface - The bearing surface is the supporting or locating surface of a fastener with respect to the part which it fastens. The loading of a fastener is usually through the bearing surface.

Bearing-type Joint - A joint where all the shear load is concentrated on the bolts. A bolted connection that assumes that shear forces are transmitted by the bolt bearing against the sides of the holes in the connected material. Often denoted on drawings with an "X" (i.e. A325-X) which excludes threads from the shear plane or "N" (i.e. A325-N) which includes threads in the shear plane.

Belleville Washer - A spring washer with a conical shape which deforms elastically when tightened. A loss in tension would be observed by the return of the washer to a conical shape.

Bending Force - A force that acts to bend a component putting one side of the part in tension and the opposite side in compression. Bending forces have the same units as torque, but instead of twisting a part in-line with its longitudinal axis, bending forces can be applied in any orientation. Typically, these are described as prying or eccentric forces for fasteners.

Bent Test - This test is meant to determine the toughness or ductility of a fastener. It is usually performed by bending the fastener through its axis or on a round mandrel.

Bevel - A surface not at right angles to the rest of the piece.

Binding Head - Slightly rounded top surface and slightly tapered sides. The bearing surface is flat, with annular undercut optional.

Blank - A bolt that has been headed (in some stage of intermediate manufacturing) but has yet to be cut to length and threaded. By stocking blanks, a fastener manufacturer can meet a wide variety of fasteners requests.

Blunt Start - The removal of the partial thread at the beginning end of the thread.

Body - The body of a threaded fastener, i.e. hex head cap screw, is the unthreaded portion of the shank.

Body-Centered Cubic Structure (BCC) - A common crystal structure of steel. Refers to the arrangement of atoms within the metal. Atomically, it contains one lattice point in the center of the unit cell and eight corner points creating 2 full atoms per unit cell.

Bolt - A headed and externally threaded fastener designed to be assembled with a nut.

Bolt Blank - A bar or rod that has been headed with the intention of subsequent threading later.

Bound Body or Fitted - A body which has a definite interference or extremely small clearance with its mating hole.

Bolt End - A bolt end is a headless rod threaded at one end and assembled with a net; it is designed to be welded or otherwise fastened to a part or structure.

Bow or Camber - The amount that a side of a fastener surface deviates from being straight.

Brass - An alloy composed primarily of copper and secondarily of zinc. Brass fasteners are nonmagnetic and soft in comparison to steel.

Brinell Hardness Scale - A hardness scale which measures the indentation resistance of a metal. l Brinell Hardness Testing - Hardness testing is performed by forcing a steel ball onto a material and measuring the dimensions of the indentation left by the ball. Brinelling refers to surface fatigue caused by repeated impact or overloading. It is a common cause of roller bearing failures; and loss of preload in bolted joints when a hardened washer is not used. Engineers will use the Brinell hardness of materials in their calculations to avoid this mode of failure. Fretting corrosion can cause a similar-looking kind of damage and is called false Brinelling since the mechanism is different. It was the first widely used and standardized hardness test in engineering. Standards for this test are defined under ASTM E-10.

Broaching - Broaching is the process of removing metal by pushing or pulling a cutting toll, called a broach, along the surface.

Burnishing - The process of producing a smooth surface by rubbing or rolling a tool against the surface.

Burr - A burr is a small amount of material extended out from the edge of a hole, a shoulder, a thread, etc. as the result of a machining operation.

Button Die - Small adjustable dies for screw machines and similar work, supported in a ring holder.

Cadmium - An element that has atomic number 48. It is often used as a plating metal because it possesses superior corrosion resistance (compared to zinc) and high lubricity which lowers the frictional coefficient of the surface. Highly toxic; restricted under the RoHS directive.

Cap Screw - Usually referring to Hex Head Cap Screws which are manufactured by cold forming, with tighter dimensional tolerances than a hot forged bolt. Note: washer face visual able under the head.

Carbon Steel - A metal alloy that principle elements are carbon and iron. Carbon steel contains other trace elements in undetectable amounts.

Case Hardened - A fastener of ferrous material having a surface which is harder than the core.

Cathode - The material in a corrosive attack that attracts electrons from the anode.

Cathodic Barrier Protection - Method of fighting corrosion in which a plating metal it attached to the surface of a substrate material (e.g.: fastener). The plating metal is anodic to the substrate (thereby making the substrate cathodic) so that the plating metal will corrode first, thus protecting the substrate material. It is a method used to protect fasteners from galvanic corrosion.

Cementite - This is a carbon rich steel also known as iron carbide (Fe3C). In carbon steel, it either forms from austenite while cooling or from martensite during tempering.

Clamp Force - The force between two components held together in a joint. Clamp force is typically imparted to a joint through threaded fasteners. Maintaining an appropriate amount of clamp force is critical to the functionality of any joint. Differs from preload in that preload is applied during assembly while clamp force may change after relaxation and elastic interactions have taken place.

Class 1A/1B Threads - These threads have a large tolerance and a wider allowance fit. "A" represents the external thread and "B" represents the internal thread.

Class 2A/2B Threads - The most common class of threads. They have a moderate tolerance that balances manufacturing costs and performance. All of Fastenal's hex cap screws are this class and most nuts have a 2B thread fit.

Class 3A/3B Threads - These threads are made to an extremely close tolerance and have no allowance. Fastenal's socket head cap screws are generally this class. Used for higher-strength bolts in safety-critical applications.

Clearance Fit - An internal and external thread design whose combination of allowances and tolerances allow for a free running assembly.

Coarse Thread - This thread type has a greater pitch and is more forgiving of thread nicks. Coarse threads have a greater resistance to thread stripping and are better for assemblies which encounter fatigue loading.

Coating - Coatings can be applied to fasteners and other metal surfaces to provide corrosion protection, lubricity, and aesthetic appeal. There are a variety of different types such as dip-spin, mechanical and hot dip galvanizing, powder coatings, and spray-on coatings.

Cold Working - Altering the geometry of a metal component below the recrystallization temperature by plastic deformation. Typically performed at room temperature which is considered "cold" in the metal working industry. Processes include rolling, drawing, pressing, spinning, extruding and cold heading.

Constant Pitch - Threads which are designed for repeated assembly and disassembly. Regardless of the diameter, the pitch will always be the same for constant pitch threads. The most common version is the 8-thread series.

Copper - Metallic element having atomic number 29. Possesses good corrosion resistance and ductility. Copper is nonmagnetic, malleable and has excellent electrical and thermal conductivity. Comes in a variety of colors. Shortcomings include a low strength-to-weight ratio and a loss of strength at low temperatures.

Corrosion - The wearing away or alteration of a metal by an electro-chemical reaction or by a direct chemical attack within its environment.

Corrosion, Chemical Attack - Occurs when the base material is soluble in the corroding medium.

Corrosion, Cavitation - see Corrosion, Pitting

Corrosion, Concentration-cell - (see also Corrosion, Pitting) Corrosion that begins in small cracks and crevices when two or more areas of a metal surface are in contact with different concentrations of the same solution. It needs only one metal and one electrolyte

Corrosion, Electrochemical - Occurs when an electrical current flows from the metal of higher potential toward the lower. Exclusive to metals. Examples include galvanic corrosion (see Corrosion, Galvanic) and pitting corrosion (see Corrosion, Pitting)

Corrosion, Galvanic - Corrosion involving two dissimilar metals in the presence of an electrolyte. The more active (anodic) of the two metals sacrifices its ions to the less active, thereby causing breakdown in the active metal.

Corrosion, Pitting - (also called Cavitation Corrosion) Localized corrosion in which a small bubble of air gets trapped on the surface of the metal. This bubble deprives the metal's surface of fresh oxygen supply, causing that area to become anodic. The change in the reactivity of the metal causes the formation of small, sometimes deep, pits.

Corrosion Cracking, Stress - A particular type of condition where cracks are induced and propagated in a fastener under combined effects of stress and corrosive environments.

Creep Range - Range of tension and temperature in which a material starts to experience creep.

Creep - Permanent deformation of a material that occurs under stress and is time-dependent. For most materials, creep only becomes important at high temperatures.

Crest - The prominent part of a thread, whether internal or external.

Cut Thread - A thread produced by removing material from the surface with a form-cutting tool.

Deoxidizer - Any item which removes or reduces the amount of oxygen in a part.

Deoxidization - Process of reducing the oxygen content from steel during steel making process by adding strong oxide forming elements, such as aluminum.

Descaling - The process of removing the flaky oxide film known as scale that forms on the surface of the fastener during production.

Dichromate - In the early days of the plating industry, sodium dichromate, a solid, was used in the manufacture of the yellow chromate conversion coating baths. In modern times, no dichromate is used by platers, but the term "dichromate" is still widely used to describe the yellow zinc plating process. All chromate conversion coatings are true chromates and do not contain dichromate.

DIN - Deutsches Institut fur Normung; (the German Institute for Standardization). DIN produces standards governing the dimensional and mechanical properties of metric fasteners.

Drive Torque - Maximum torque to install a screw prior to contact of the head of the fastener with surface of the test plate.

DTI Washer - Direct Tension Indicating Washer; aka Load Indicating Washer. A washer that has raised bumps which plastically deform to become flat under a specific tensile load.

Ductility - A measure of the ability of a material to deform before it fractures. In steels, increasing levels of carbon decreases ductility. Examples of high ductile materials are silver and gold.

e.g. - Abbr- Latin. Exempli gratia (for example).

Effective Diameter - See Pitch Diameter.

Elastic Range - The range in which a material can be deformed and still return to its original shape.

Electrolyte - An electrically conductive fluid or substance.

Electroplating - The deposition of a metallic coating onto an object by putting a negative charge onto the object and immersing it into a solution which contains a salt of the metal to be deposited. The metallic ions of the salt carry a positive charge and are attracted to the part. When they reach it, the negatively charged part provides the electrons to reduce the positively charged ions to metallic form.

Elongation - The stretching of a material in a tension test or under clamp load. Often it is expressed as a percentage of the original length.

EN - European Organization of Standardization

Environmental Hydrogen Embrittlement - (see also Hydrogen Embrittlement) Hydrogen embrittlement caused by hydrogen introduced to the fastener after it is placed in service.

Face-Centered Cubic Structure (FCC) - A cubic structure found in steel. Refers to the arrangement of atoms within the metals structure. Steel in this state has low magnetic permeability. Atomically, lattice points are positioned on the faces of the cube of which each unit cube gets exactly one half contribution, in addition to the corner lattice points.

Fatigue Strength - The maximum stress a component can withstand for a specified number of repeated load cycles prior to its failure. Differs from yield and tensile strengths as fatigue loads affect metal components differently.

Ferrite - Pure iron that has very low solubility of carbon.

Ferrous - Anything made of or containing iron. Also called ferric. Ferrous metals include carbon and alloys steels as well as stainless steels.

Fine Thread - A type of thread with a smaller pitch and a greater number of threads per inch. Can support greater tensile loads due to the increase in tensile stress area. Fine threads allow for greater adjustment accuracy and are less likely to loosen than coarse threads.

Flank - The straight sides of the thread that connect the crest and the root.

Flank Angle - The angle between the flank and a perpendicular line that intersects the root. Half the angle of a thread when the threads are symmetrical.

Fracture Point - The point at which failure occurs and a part under stress breaks.

Friction Type Joint - A joint that employs friction (created by clamp force) between the bolt and joint members in order to avoid shear and keep the joint members from slipping.

Frictional Coefficient - A unit-less value that is the ratio of the weight of an object to the force acting on the object. In bolted joints, this value deals with the friction developed between the bearing surface and clamp member or the friction between mating threads.

Galling - A cold-welding process by which male and female threads become fused together under high amounts of pressure. Most prevalent with fasteners made of stainless steel, aluminum, or titanium because of the oxide surface film that these materials produce.

GO Gauge - An inspection tool used to check a threaded part against allowed tolerances. There are ring gauges (for external threads) and plug gauges (for internal threads). A GO gauge is designed thread on or into the part being inspected.

Grade - A rating in a scale classifying fasteners according to strength.

Grip Length - The distance from under the head of a fastener to the beginning of the mating internal threads of a tapped hole or nut.

Hardenability - The ability of a steel to form martensite during the quenching process. More simply, a steels ability to become harder and stronger through heat treatment by sacrificing ductility.

Hardness - A measure of a material's ability to resist abrasion and indentation.

Hastelloy - Any one of a group of highly corrosion resistant high strength nickel based alloys produced and trademarked by Haynes International Inc.

Heat Treating - Any heating or cooling process used to influence the mechanical properties of a metal. In fastener manufacture, it typically refers to the process of austenizing, quenching, and tempering a product in order to form a martensitic grain structure (i.e. improve strength properties).

Hot Dip Galvanizing - Immersion of fasteners in a bath of molten zinc for a controlled time period to obtain specified coating weight or thickness.

HRB - Rockwell Scale B. B-scale is typically used as a hardness measurement for soft metals (e.g. brass, aluminum, etc.)

HRC - Rockwell Scale C. C-scale is used to measure the hardness on harder metals (e.g. medium carbon alloy steel)

HRV - Hardness Vickers Scale. Developed as an alternative method to measure the hardness of materials.

Hydrogen Embrittlement - A process where atomic hydrogen is absorbed by a metallic component resulting in a decrease of ductility. It can cause premature fastener failure.

i.e. - Abbr- Latin. Id est (that is).

Inconel - Nickel-Chromium-Iron alloy (not to be confused with 18-8 Stainless Steel). Exotic material with excellent corrosion resistance.

Internal Hydrogen Embrittlement - Hydrogen embrittlement that results from the manufacturing process and not an outside source.

ISO - The International Organization of Standardization. It is an international-standard-setting body composed of representatives from a variety of national standards organizations.

JIS - Japanese Industrial Institute. It specifies the standards used for industrial activities in Japan.

K-Factor - In bolt tensioning refers to the friction and thread tolerance factors of an internal and external thread combination that can affect torque values. Also referred to as Nut Factor.

Kip - A kip is a unit of force equal to 1000 pounds. Often times, when expressing stresses or pressures, kips per square inch (ksi) are given.

K-Monel - Nickel-Copper-Aluminum alloy. Very corrosion resistant. The inclusion of aluminum (and occasionally titanium) enhances the mechanical strength in comparison to Monel.

kN - Kilo Newton; an SI (metric) unit of measurement used to describe force. One kilo Newton is equal to 1000 Newtons.

Knoop Hardness - A hardness scale that uses a diamond-shaped indenter.

Length of Engagement - The distance through which the internal and external threads of mating components are engaged.

Lot - Quantity of product of one part number that has been processed under the same conditions from the same heat treatment lot and produced from one mill heat of material.

Low Carbon Steel - Steels containing less than 0.25% carbon; insufficient carbon content to provide a predictable, uniform response to heat treatment. Also possess low tensile strength, but it is cheap and malleable; may be called Mild Steel.

Malleable - Capable of being shaped or formed into thin sheets without cracking. Materials can be rolled, hammered, molded, or stamped to achieve the desired effect.

Major Diameter - The largest diameter of the threaded part of a threaded object that spans from the crests of an external thread or the roots of an internal thread.

Martensite - A microstructure of carbon steel formed by rapidly quenching a steel from austenitic temperatures. Notable for high hardness and increased strength capabilities compared to ferrite or austenite.

Medium Carbon Steel - Steels containing 0.30%-0.60% carbon; have sufficient carbon content to yield a consistent heat treatment response. Poses good ductility, strength and wear resistance; used for large parts, forging and automotive components.

Minor Diameter - The smallest diameter found on the threaded portion of any threaded object that spans between the roots of an external thread or the crests of an internal thread.

Monel - Nickel-Copper alloy. Has excellent corrosion resistance in heat and salt water.

MPa - Mega Pascal. The SI unit of pressure or stress. It is a measure of perpendicular force per unit area. One mega Pascal is equal to 1,000,000 Pascals.

Naval Bronze - Contains slightly more zinc than regular brass. Resists salt water. Good for salt water environments.

N-Designation - Designation used in structural bolting which indicates that the threads of the bolt may be included in the shear plane of the joint. This designation refers to the usage of the bolt in the joint. It does not indicate a special bolt.

Neck Down - The name for the process by which a ductile metal elongates and reduces in cross sectional area once it has been stressed beyond its ultimate tensile strength.

Newton - A metric unit of force. One Newton is equal to the force of gravity on one kilogram of mass.

NO-GO Gauge - An inspection tool used to check a threaded part against allowed tolerances. There are ring gauges (for external threads) and plug gauges (for internal threads). A NO-GO gauge is designed to not thread on or into the part being inspected.

Nonferrous - Any alloy that does not contain iron as its main component. The most common examples are copper, nickel, aluminum, and titanium alloys.

Normalizing - An annealing process that is used to remove internal strain and reduce coarse (e.g.: martensitic) crystal structure.

Nut Factor - see K-Factor


Pascal - An SI unit of pressure or stress. One Pascal is equal to one Newton distributed over a square meter.

Passivation - The spontaneous formation of a hard non-reactive surface layer that inhibits further corrosion. This layer is usually an oxide or nitride that is a few atoms thick. Under normal conditions of pH and oxygen concentration, passivation is seen to occur naturally in such materials as aluminum, magnesium, copper, stainless steel, titanium, and silicon.

Pearlite - A mixture of ferrite and cementite which form distinct layers or bands.

Pitch Diameter - IFI defines this as the diameter of a theoretical cylinder that passes through the threads in such a position that the widths of the thread ridges and thread grooves are equal.

Pitch - See Thread Pitch.

Pitting - Localized corrosion of a metal surface, confined to a point or small area that takes the form of cavities or pits. See also Corrosion, Pitting.

Plastic Range - The point at which steel under a load deforms plastically and where subsequent stress will only result in further deformation and eventually failure.

Plating - The process of depositing an adherent metal onto the surface of a base metal.

Plug Gauge - An inspection tool for checking an internally threaded part against its allowed tolerances. See GO Gauge and NO-GO Gauge

Preload - The force developed in a bolt during tightening that, in a theoretical joint, is equal to the initial clamping force that holds the joint together. Preload and residual clamping force can differ after relaxation, embedment, and other elastic interactions have occurred in the joint.

Prevailing Torque - The torque necessary to rotate the nut on its mating externally threaded fastener. It is measured while the nut is in motion and before any axial load is developed in the externally threaded fastener.

Pretensioned Joint - A joint that resists the shear load by shear bearing on the bolts, but has been preloaded for other reasons beside slip resistance. Considered a Bearing-type Joint.

Proof Load - An applied tensile load that the fastener must support without permanent deformation and represents the usable strength of a certain standard of fasteners.

Property Class - Designation of strength classes among metric fasteners.

PSI - Pounds per square inch; a unit of measurement for pressure or stress. One PSI is equal to one pound of force distributed over one square inch. It is common to specify material strengths in the English system in units of PSI.

PTFE (Polytetrafluorethylene) - A polymer with a low friction coefficient. Sometimes used as a lubricant. Trade named Teflon®.

Quenching - The rapid cooling of a metal from elevated temperatures during the heat treatment process by means of immersing the metal in a quenching medium. Quenching mediums include oil, water, and air.

Ring Gauge - An inspection tool for evaluating an externally threaded part against its allowed tolerances. See GO Gauge and NO-GO Gauge

Rockwell - A hardness scale which measures indentation hardness of metals. The hardness testing machine applies a major and a minor load to the material and then gives a direct indication of hardness.

Rockwell Hardness Scale - Measures the indentation hardness of materials through the depth of penetration of an indenter, loaded on a material sample and compared to the penetration in some reference material. Its chief advantage is its ability to display hardness values directly, thus eliminating tedious calculations involved in other hardness measurement techniques. Also, the relatively simple and inexpensive set-up enables its installation in a variety of laboratories. It is typically used in engineering and metallurgy and is most common in the USA. Its commercial popularity arises from its speed, reliability, robustness, resolution and small area of indentation.

RoHS - Restriction of Hazardous Substances. Sometimes pronounced Row-Hass or Rose in the industry. Restricts the use of certain materials including lead, mercury, cadmium, hexavalent chromium, Polybrominated biphenyl ether (PBB), PBDE, and DecaBDE.

Rolled Thread - A thread produced by the action of a forming tool which presses the material and radially displaces it to form threads. Rolled threads are better than cut threads for fatigue-type applications.

Root - The bottom of the groove between the two flanking thread surfaces whether internal or external.

Sacrificial Anode - see Cathodic Barrier Protection

SAE - Society of Automotive Engineers. A standards development organization for the engineering of powered vehicles of all kinds.

Saline - A solution containing salt. The presence of a saline solution creates a highly corrosive environment for metal since it will act as an electrolyte (e.g. salt water).

Scale - An oxide film that forms after heat treating or welding on a metal when surface contaminants or other debris are present.

Scatter (Torque) - This term refers to the relative inaccuracy of using torque to preload a joint. For a given target preload, a variety of torque values can be empirically derived that will achieve this preload. The reason is that variations in surface roughness and geometric differences between manufactured parts means different torque values (scatter) can provide the same amount of preload in a joint.

Shank - The shank is the unthreaded portion of the fastener between the threads and the head.

Shear Force - A force that is applied perpendicular to the fastener's longitudinal axis. Generally, a fastener sees a shear force when the clamping members it is securing shift relative to each other.

Shear Plane - The two dimensional plane created by the contact of two surfaces in a shear joint.

Shear Strength - The maximum load that can be supported prior to fracture when applied perpendicular to the fastener's longitudinal axis. Typically given in PSI as the load in pounds to cause fracture divided by the cross sectional area in square inches of the part along the shear plane.

Shear, Double - When a fastener or other component is loaded in such a way that two shear planes act through the part and if failure were to occur, there would be three separate pieces.

Shear, Single - When a fastener or other component is loaded in such a way that one shear plane acts through the part and if failure were to occur, there would be two separate pieces.

Sherardizing - A cementation process where zinc dust is heated to a temperature near its molten point and is brought into intimate contact with the steel surface to form an iron and zinc coating on the steel by diffusion.

SI - The International System of Units. The modern form of the metric system. World's most widely used system of units.

Silicon Bronze - Bronze with silicon added for corrosion resistance.

Slip-Critical Joint - A joint that resists shear loads by faying surface friction. If slip occurs, the integrity of the joint could be compromised. Considered a Friction-type Joint.

Snug-tightened Joint - A joint that resists shear loads by shear bearing only. Considered a Bearing-type Joint.

Spheroidized Steels - A steel that has been heat treated in such a way as to produce a spherodite microstructure (i.e. a microstructure with small spheres of cementite surrounded by a matrix of ferrite as opposed to layers of cementite).

Stainless Steel, Austenitic - Also called 18-8 or 300 series. Most common stainless steel (over 80% of stainless steel fasteners are 300 series). They are highly corrosion resistant due to the presence of approximately 18% chromium and 8% nickel. Austenitic stainless steels have an FCC structure. Non-hardenable by heat treatment. Can pick up magnetic properties through cold working.

Stainless Steel, Duplex - Contains a mixed microstructure of austenite and ferrite (duplex) with an ideal mixture of equal parts austenite and ferrite. These steels have superior strength and corrosion resistance when compared to austenitic stainless steel and contain high amounts of chromium.

Stainless Steel, Ferritic - Corrosion resistant steel which contains 12%-18% chromium, little to no nickel, and less than 0.2% carbon. Magnetic and non-hardenable by heat treatment, these may contain lead. Common Grades: 430

Stainless Steel, Martensitic - Corrosion resistant steel which contains 12%-18% chromium, little to no nickel, and enough carbon to be heat treated. It is magnetic. Common Grades: 410 & 416.

Stainless Steel, Precipitation Hardened - Stainless steel that has the corrosion resistance of austenitic stainless steels and better strength characteristics than some of the martensitic grades. This is accomplished through complex and lengthy form of heat treatment called age-hardening (due to the time required to complete it). Common Grades: 17-4, 17-7, 630, A-286.

Standards - Documented agreements containing technical specifications and other precise criteria to be used as constants and define characteristics that ensure that materials, products, processes, and services are fit to their purpose.

Strain - Deformation produced on a fastener by an external force. Also, the change in length divided by the initial length.

Stress - Force divided by unit area. Typically specified in units of MPa or PSI.

Stress Corrosion - See Corrosion Cracking, Stress

Stress Relaxation - The tendency of a material to release stress when placed under a constant load.

Stress Relieving - A heat treatment performed to remove residual stress in cold worked or heat treated parts.

Stripping - To damage or break the threads of a screw, nut, or bolt or the teeth of a gear.

Substrate - Any material layer under another. In zinc electroplated fasteners, the steel fastener would be the substrate as it is covered by a layer of zinc.

Tempering - The process of heating a fastener after quenching to give it its final tensile strength and hardness. It is done to "toughen" the metal. Tempering is a stress relieving process.

Tensile Force - A force that acts to pull a component apart.

Tensile Strength - The ultimate strength of a given, material, alloy, or product that determines the peak load it can withstand before breaking. It is determined by performing a tension test and observing the maximum load attained. Usually expressed in pounds or Newtons.

Tension Joint - A joint with forces acting parallel to the fastener's axis.

Thermal Expansion - An increase in the volume of the bolt or joint when heated.

Thread Fit - The set of allowances and tolerances that determines the degree of tightness or looseness between mating components. See section on Classes of thread fit.

Thread Length - The length of a fastener that features its threads.

Thread Pitch - The distance between corresponding points such as root to root or crest to crest on adjacent threads. Used to designate threads in the Metric system. The inverse of the TPI is the pitch.

Thread Profile, Thread Form - The configuration of the thread in an axial plane. A thread has three parts: the crest, root and flanks. Common thread forms: UN, UNR, and UNJ.

Thread Series - The threads per inch for a given diameter of threaded fasteners. Three are common: UNC, UNF, and 8-UN.

Thread - The thread is the inclined plane that wraps around the cylinder of the fastener; a ridge of uniform section in the form of a helix on the external or internal surface of a cylinder.

Thread Tolerance - The slight variation between the theoretically perfect threads and the unit of actual production that is allowed and expected. The total amount by which a specific dimension is permitted to vary.

Torque - A twisting force that causes the rotation of a shaft or will set up a twist in a stationary shaft and is generally expressed in foot pounds or inch pounds.

Torsional Strength - A load expressed in terms of torque, at which the fastener fails by being twisted until it fractures.

Toughness - A material's ability to absorb impact or shock loading.

TPI, Threads Per Inch - The number of threads contained in one inch of a threaded fastener's length. Used to designate threads in the Unified System. The inverse of the pitch is the TPI.

Ultimate Tensile Strength - See Tensile Strength. The ultimate load which a fastener can withstand before breaking.

UNC (Unified Coarse) Threads - The coarse thread series UNC/UNRC is the most commonly used thread system for screws, bolts, and nuts. It is used for producing threads in low strength materials such as cast iron, mild steel, and softer copper alloys, aluminum, etc. The coarse thread is also used for rapid assembly or disassembly.

UNEF (Unified Extra Fine) Threads - This is used when more threads are desired to be engaged than the fine-thread series would allow. Works well in applications where thin materials are used or fine adjustments must be made.

UNF (Unified Fine) Threads - Have a larger minor diameter than UNC thread, which gives UNF fasteners slightly higher load-carrying and better torque-locking capabilities than UNC fasteners of the same identical material and outside diameter. The fine threads have tighter manufacturing tolerances than UNC threads, and the smaller lead angle allows for finer tension adjustment. More susceptible to thread nicking and thread stripping.

Unified Thread Standard - Specifies the standard thread form, series, class, allowance, tolerance, and designation for unified screw threads. This standard is currently controlled by ASME/ANSI in the United States.

UNJ - Unified thread form with a more heavily rounded root than UNR, causing the minor diameter of the external threads to increase. Therefore, UNJ specifies thread form for both internal and external threads. Commonly used in aerospace applications.

UNR - Unified Radius root threads. Most common unified thread form. Not to be confused with the thread series UNC, UNF, UNEF, or 8-UN which designated pitch diameters as they relate to threads per inch rather than the form of the root and crests of the thread.

UNS (Unified Special) Threads - Unified threads of special diameters, pitches, and lengths of engagement.

Vickers - Hardness test and scale commonly used in the metric system. Quantifies a material's resistance to indentation.

Void - Shallow pocket or hollow on the surface of a fastener.


X-Designation - Designation used in structural bolting which indicates that the threads of the bolt must be excluded from the shear plane of the joint.

Yield Strength - The applied tension load at which a specified amount of permanent deformation occurs. Generally, yield strength is defined as the point at which a 0.2% offset line from the elastic region of the material being tested intersects the stress strain curve.

Zinc & Clear - Zinc plating is applied through electrodeposition of zinc onto the substrate metal. For fasteners, this process is typically done through the barrel plating method. A chromate conversion coating is then applied which provides the final layer of corrosion resistance to the plated part.

Zinc & Yellow - Resembles bright brass but has iridescent streaks of color in it. Is a colored chromatic finish over zinc plate and offers good corrosion resistance, though it may contain hexavalent chromium which would render it non-RoHS compliant.

Zinc & Phosphate - Coating used for corrosion protection of finished fasteners. Better for paint ability and a uniform appearance.

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