Terms & Definitions
ANTI-FRICTION COATING
AF coatings are dry lubricants consisting of solid lubricant suspensions, such as a small particle size graphite 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 carefully selecting 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 all-metal construction.
ALLOWANCE
An intentional clearance between internal or external thread and the design form of the thread when the thread form is in its 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 that hardens in the absence of air, such adhesives are often used as a thread locking medium.
ANGLE CONTROLLED TIGHTENING
A tightening procedure in which a fastener is first tightened by a pre-selected torque (called the snug torque) so that the clamped surfaces are pulled together and then further tightened by giving the nut an additional measured rotation. This method frequently tightens bolts beyond their yield point to ensure that a precise preload is achieved. This method can elongate bolts of short length by this method, and the bolt material must be sufficiently ductile to cater to 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 acts on a joint face directly because 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 color of the surface finish of the fastener.
BOLT
A bolt is a 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 a bolt that has not been tightened into reverse rotation.
BREAKLOOSE TORQUE
The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.
BUMP THREAD
A modified thread profile patented and trademark of the Bosco Tool Inc. The thread form has a small pitch diameter projection that eliminates the thread assembly’s clearance on both flanks. By doing this it is claimed that resistance to vibration loosening is significantly improved.
CADMIUM ELECTROPLATING
Coating 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.
CLAMPING FORCE
The compressive force that 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 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 and 8g for external threads.
For Unified threads, a similar designation as for metric threads is used. The thread classes are 1A, 2A and 3A for external threads and 1B, 2B and 3B for internal threads.
CLEVELOC NUT
A torque prevailing nut of all metal construction. The collar of the nut is elliptical in cross-section and it is this that provides the flexible locking element. The nut is pre-lubricated to reduce the torque needed when tightening and to minimize galling.
COEFFICIENT OF FRICTION
A dimensionless number represents the ratio of the friction force to the normal force. Typically for threaded connections, it is between 0.10 to 0.18 but can vary significantly depending upon the materials used and whether a lubricant has been used. In relation to threaded fasteners, the coefficient of friction can be further subdivided into the coefficient of friction between the threads and the coefficient of friction under the nut face. There is generally a difference in values between the two coefficients due to the contact surfaces being different. For example, a zinc-plated nut on a zinc-plated bolt, the thread coefficient of friction would be due to zinc plating contacting zinc plating. The nut face coefficient of friction would be due to zinc plating contacting the
COMMINGLING
A term used to describe the undesirable practice of mixing fasteners from different batches of 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 that will assess the potentially detrimental effects of surface discontinuities. This type of test is sometimes applied to nuts that 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 reduce 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 leakage rate can be anticipated.
DECOMPRESSION POINT
The point at which there is zero pressure at the joint interface because of forces applied to the joint. A gap will form at the interface if the applied force is increased beyond the decompression point. Analytically, a criterion 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. This is why it is frequently taken as a failure criterion in analysis work.
DESIGN FORM OF THREAD
The design form of an internal or external thread is the thread form in its 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 (DTIs) are sometimes used to describe load-indicating washers. Projections on the face of the washer (usually on the face abutting the bolt head or nut) 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 center 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 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 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 because 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.
ENFOLATION
The lap can be formed at the thread crest due to thread rolling.
ENVIRONMENTALLY ASSISTED CRACKING (EAC)
A process that can occur with high-strength steel fasteners in which crack initiation and growth occur in the fastener at a comparatively low-stress level because of interactions 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 because of an applied loading to the joint.
EXTERNAL THREAD
A screw thread is formed on an external cylinder, such as on bolts, screws, studs etc.
FLOATING TYPE FLANGE JOINT
In 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.
FLUORO-CARBON THREAD COATING
A low friction coating is applied to threads. This type of coating is frequently used to prevent thread fouling when an assembly containing threaded fasteners is painted. Electrodeposited primers can cover the threads if masked in some way before painting. If this occurs, assembly difficulties can result unless the expensive chore of cleaning the threads is completed. A fluor-carbon thread coating eliminates the need for masking or cleaning since the 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 are used on fasteners to reduce the scatter in the thread and bear 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 in its maximum metal condition. For metric threads, the fundamental deviation is 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 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
The 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 these, 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 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 is allowed to cool. As the bolt attempts to contract, it is constrained longitudinally by the clamped material and a preload result. Heating methods 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 method is not widely used 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. This type of washer is sometimes called a spring lock washer or standard lock washer.
HIGH-STRENGTH FRICTION GRIP BOLTS
Sometimes abbreviated to HSFG bolts. High tensile strength bolts are 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 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 to absorb the reaction torque 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 completing maintenance work on a bolted joint when the joint is under loading. This can involve the replacement of individual bolts. There are risks to the joint and health and safety associated with this technique.
HYDRAULIC TENSIONER
A hydraulic tool tightens 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 induced into the fastener.
HYDROGEN EMBRITTLEMENT
Steel fasteners exposed to hydrogen can fail prematurely at a stress level well below the material’s yield strength. Hydrogen embrittlement occurs in fasteners usually because the part is exposed to hydrogen during its manufacturing process, but it can also occur through in-service corrosion. Electroplating is generally considered 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. 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 are dependent upon the distance that the bolt is from the instantaneous center. The individual bolt force’s direction is perpendicular to a line joining that bolt to the instantaneous center.
INTEGRAL FASTENER
A term is used to describe fasteners highly resistant to vibration loosening and/or removal. Some types have special thread forms.
INTERNAL THREAD
A screw thread is formed in holes, such as in nuts.
JOST EFFECT
The name was given to the reduction in the frictional resistance that occurs in a direction different to that in which slip is occurring. This effect is used in many applications, including removing corks from bottles. If the cork is first rotated, the force needed to pull the cork from the bottle is significantly reduced. It is also the fundamental reason why threaded fasteners experience self-loosening. Frictional resistance is first overcome in the transverse direction by slip occurring on the joint resulting in the frictional resistance in the circumferential direction reducing to a small value. The torque acting on the fastener in the loosening direction (because of its preload) that, when coupled with the Jost Effect, results in self-loosening.
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 are being acquired due to them being purchased in quantities usually greater than one.
LEFTHAND THREAD
A screw thread that is screwed in by rotating counterclockwise.
LENGTH OF ENGAGEMENT
The axial distance over which an external thread is in contact with an internal thread.
LOCK NUT
There are two common usages of this term:
1. A nut that provides extra resistance to vibration loosening by either providing some form of prevailing torque or, in free spinning nuts, deforming and/or biting into mating parts when fully tightened.
2. The term is sometimes used for thin (or jam) nuts 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 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. The lower the mean shift 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 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, metal-to-metal type joints have no standardized gasket factor definitions, test procedures, or generally acknowledged calculation procedures available.
MINOR DIAMETER
This is the diameter of an imaginary cylinder that touches the roots of an external thread or the crests of an internal thread.
MOLYBDENUM DISULPHIDE
A solid lubricant that acts as a high-pressure resistant film. It can be used by itself as a dry lubricant, in 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 stabilizers.
NICKED THREADS
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.
Acceptance tests for nicked threads 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 gauges.
NOMINAL DIAMETER
The diameter is 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 tool’s design is such that attempts are made to ensure that the applied torque is independent of joint stiffness.
OCTAGON HEAD
A bolt or screw whose head cross-section is a regular polygon with 8 sides.
OVERTAPPING
Tapping a thread following a plating operation so that the thread tolerances comply with specification, allowing the internal and external threads to assemble.
PILES
The 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 forms part of a structural joint.
POOCHING
Pooching is sometimes used to describe the effect of the area immediately surrounding a tapped hole being raised up because 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 is created in a fastener when first tightened. Reduces after a period due to embedding and other factors.
PREVAILING TORQUE
The torque required to run a nut down a thread on certain types of nuts is 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 with a prevailing torque to prevent 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. Nylon insert torque prevailing nuts utilize nylon (or other polymers) 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 a permanent set.
PROPERTY CLASS
A designation system that 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.
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.
PRYING
The amplification of an external force acting on a bolt by a lever action 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 typically occurs without any nut rotation. It commonly occurs because 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. Most screw threads are right-handed.
ROLLED THREAD
A thread is formed by plastically deforming a blank rather than by cutting. Most 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, the die life can be significantly reduced because of the increased hardness of the bolt blank. Rolling the thread also generally improves the surface finish, which can benefit fatigue life.
SCREW
A headed threaded fastener designed to be used in conjunction with a preformed 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 with 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 the constant section 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 (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 known that the fastener can self-rotate under the action of transverse joint movement, which can completely loosen a tightened fastener so 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 is equal to the effective diameter of the thread; the thread is rolled from this diameter.
SET SCREW
A set screw is a threaded fastener 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 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, fluted socket, screwdriver slot, and square head.
SHANK
That portion of a bolt between the head and the threaded portion.
SHOULDER SCREWS
A threaded fastener with a plain, precision machined shank is used for location purposes. They are typically used for pulleys and linkages.
SNUG TORQUE
The torque required to pull plates together so that direct contact occurs; is 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 rotation angle starts so that the bolt is tightened sufficiently. The snug torque is usually determined experimentally on the actual joint.
SNUGGING
In 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.
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 must 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 a 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 are 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 modified to resist vibration loosening. The thread has several horizontal portions (i.e., no lead angle) that prevent torsion from being developed in the bolt because of the loosening purpose. It is these horizontal portions that are known as steps.
STIFFNUT
A term used to describe a lock nut that has a prevailing torque.
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 pitch’s mean and the thread’s minor diameters.
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 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 minimize the effects of stress relaxation is to use materials with adequate resistance to it at the product’s operating temperature. The bolt stress relaxation effect reduces the clamping force provided by the bolts; this phenomenon 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 enables 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 that 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
An asymmetrical thread is one that has both flanks of the thread profile inclined at the same angle.
TENSION WASHERS
A general name was 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 its outer surface; for internal threads, it is the region that 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
The length of 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 to improve fatigue performance.
THREAD RUNOUT
The portion at the end of a threaded shank that is not cut or rolled to full depth but provides a transition between full depth threads and the fastener shank or head.
THREADLOCKER
It can be used for several vibration-resistant products but is now usually reserved for thread-locking adhesives. Specifically, a liquid anaerobic adhesive is applied to nut or bolt thread; once hardened, it fills the inner spaces between the threads to produce a solid plastic of 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 aluminum or steel.
TOLERANCE CLASS
A combination of tolerance grade and a fundamental deviation is given to an internal or external thread. When combined with the tolerance class for an external thread, a tolerance class for an internal thread gives the class of fit for the mating threads.
TOLERANCE GRADE
The difference between the 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 if metric units are used or pounds-feet when imperial units are used.
U BOLT
A U-shaped fastener threaded at both ends is used primarily in suspension and related areas of vehicles.
ULTRASONIC EXTENSOMETER
An instrument that 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. A given diameter has a larger thread pitch than an equivalent diameter UNF thread.
UNEF
Unified National Extra Fine (UNEF) is a Unified thread form with a very fine 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. A given diameter 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 optionally has a flat or rounded root contour.) Most 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 is typically used for tapped hole repair or to improve the thread stripping strength of softer metals such as zinc and aluminum. The inserts are assembled into a previously tapped hole using a special driving tool. A thread locking compound is frequently used to secure the insert if the assembly is subject to vibration.
YIELD CONTROLLED TIGHTENING
A fastener tightening method that allows a fastener to be tightened to yield. The angle of rotation of the fastener is measured relative to the applied torque, the 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 like zinc electroplating completed in an acid chloride bath – a small amount of cobalt is added to increase the plating speed.
ZINC PHOSPHATE CONVERSION COATING
To improve corrosion resistance, a zinc phosphate conversion coating is frequently added to zinc electroplated parts, such as bolt threads. This chemical conversion coating provides a protective passivation layer on the zinc, improving its corrosion resistance.