FCAFastener Corporation

Glossary of Nut & Bolt Terms

Below is a glossary of terminology on topics related to nuts and bolts, threaded fasteners and tightening techniques. If you have any questions about any of the content or you think we've missed something why not send us an email on the topic.

A - H | I - R | S - Z

A nut (so-called because of its shape) that has a domed top so that it prevents contact with the external thread.

A torque prevailing nut of all metal construction. The nut is slotted in two places, which, after the nut has been tapped, are bent slightly inwards and downwards. When the nut is screwed onto the bolt thread the two slotted parts are forced back to their original position. Their stiffness causes the nut threads to bind onto the bolt threads and thus provides a prevailing torque. Aerotight is a registered trade mark of The Premier Screw and Repitition Co. Ltd of Woodgate, Leicester, United Kingdom, LE3 5GJ.

AF coatings are dry lubricants consisting of suspensions of solid lubricants, such as graphite, PTFE or molydbenum disulphide of small particle size in a binder. Such coatings can be applied to fastener threads to replace metallic coatings such as zinc and cadmium and offer maintenance free permanent lubrication. By careful selection of the lubricants, AF coatings can be designed to meet specific applications. The coatings are permanently bonded to the metal surface and provide a lubricating film preventing direct metal to metal contact.

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.

A torque prevailing nut of an all-metal construction. Covered by UK patent 1180842 the nut is marketed by GKN Screws and Fasteners Limited.

An intentional clearance between internal or external thread and the design form of the thread when the thread form is on it's maximum metal condition. Not all classes of fit have an allowance. For metric threads the allowance is called the fundamental deviation.

An adhesive, which hardens in the absence of air, such adhesives, are often used as a thread-locking medium.

A tightening procedure in which a fastener is first tightened by a pre-selected torque (called the snug torque) so that the clamped surfaces are pulled together, and then is further tightened by giving the nut an additional measured rotation. Frequently bolts are tightened beyond their yield point by this method in order to ensure that a precise preload is achieved. Bolts of short length can be elongated too much by this method and the bolt material must be sufficiently ductile to cater for the plastic deformation involved. Because of the bolt being tightened beyond yield, its re-use is limited.

This is the theoretical profile of external and internal threads with no manufacturing tolerance applied.

The surface pressure acting on a joint face directly as a result of the force applied by a fastener.

A bolt or screw whose cross section of its head is in the shape of a 12 pointed star.

The word black refers to the comparatively wider tolerances employed and not necessarily to the colour of the surface finish of the fastener.

A bolt is the term used for a threaded fastener, with a head, designed to be used in conjunction with a nut.

The torque necessary to put into reverse rotation a bolt that has not been tightened.

The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.

A specialist thread form based upon the Whitworth thread and consisting of 26 threads per inch whatever the thread diameter.

British Standard Fine. A thread form based upon the British Standard Whitworth form but with a finer thread (more threads per inch for a given diameter). This thread form was first introduced in 1908, the thread form is specified in BS 84: 1956.

British Standard Whitworth. A thread form developed by Sir Joseph Whitworth in 1841. The thread form has rounded roots and crests, the thread form is specified in BS 84: 1956. This thread form was superceded by the Unified thread in 1948 and then the metric thread form.

A modified thread profile patented and trade mark of the Bosco Tool Inc. The thread form has a small projection at the pitch diameter that eliminates the clearance from the thread assembly on both flanks. By doing this it is claimed that resistance to vibration loosening is significantly improved.

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.

The compressive force, which a fastener exerts on the joint.

The Class of Fit is a measure of the degree of fit between mating internal and external threads. Three main Classes of Fit are defined for metric screw threads:

  • FINE: This has a tolerance class of 5H for internal threads and 4h for external threads.
  • MEDIUM: This has a tolerance class of 6H for internal threads and 6g for external threads.
  • COARSE: This has a tolerance class of 7H for internal threads and 8g for external threads.
    For Unified threads, a similar designation as for metric threads is used. The thread classes used are 1A, 2A and 3A for external threads and 1B, 2B and 3B for internal threads.

A torque prevailing nut of all metal construction. The collar of the nut is elliptical in cross section and it is this that provides the flexible locking element. The nut is pre-lubricated to reduce the tightening torque. Cleveloc is a registered trade name of Forest Fasteners.

A dimensionless number representing the ratio of the friction force to normal force. Typically for threaded connections it is between 0.10 to 0,18 but can vary significantly depending upon the materials used and whether a lubricant has been used.

A term used to describe the undesirable practice of mixing fasteners from different batches that are the same size and grade in the same container.

This is an axial applied force applied to a nut when it is seated on a cone shaped washer which has an included angle of 120 degrees. Failure in this test is usually due to the nut splitting. The intention of the test is to introduce a nut dilation operation which will assess the potential detrimental effects of surface discontinuities. This type of test is sometimes applied to nuts which are intended for high temperature service.

Creep is deformation with time when a part is subjected to constant stress. Metals creep can occur at elevated temperature however with gasket materials it can occur at normal ambient temperatures. Creep resistance is an important property of gasket materials. Gasket materials are designed to flow under stress to fill any irregularities in the flange surface. The amount of creep sustained tends to increase with temperature. . However once the tightening is completed it is important that no further flow occurs since such deformation will lead to a reduction in bolt extension and subsequently the stress acting on the gasket. If this stress is reduced to below a certain minimum, which depends upon the type and construction of the gasket and the operating temperature, a high rate of leakage can be anticipated to occur.
The point at which there is zero pressure at the joint interface as a result of forces applied to the joint. If the applied force is increased beyond the decompression point, a gap will form at the interface. Analytically, a criteria of joint failure is often taken as when the applied force on the joint reaches the decompression point. This is because forces acting on the bolt(s) can dramatically increase at this point. Loading beyond this point can also result in fretting at the interface that will lead to bolt tension loss that will subsequently lower the decompression point. This process can continue until bolt failure does occur. The failure can be by fatigue or other mechanism but the underlying cause was loading of the joint beyond the decompression point. It is for this reason that it is frequently taken as a failure criteria in analysis work.

A high performance surface coating that can be applied to fasteners. The coating consists of passivated zinc flakes that are stoved onto the metal surface. The coating can be coloured and eliminates the risk of hydrogen embrittlement associated with electroplated metal. DACROMET is a registered trademark of Metal Coatings International, Inc. of Chardon Ohio

The design form of an internal or external thread is the thread form in it's maximum metal condition. It is the same as the basic thread profile except that the thread roots are rounded. If either the internal or external thread form exceeds the design form of the thread profile then a potential interference exists.

Direct Tension Indicators (DTI's) is a term sometimes used to describe load indicating washers. Projections on the face of the washer (usually on the face abuting the bolt head or nut) that deform under loading as the bolt is tensioned. An indication of the tension in the bolt can be made by measuring the gap between the washer face and the nut or bolt head. The smaller the gap - the greater the tension in the bolt. Commonly used in civil rather than mechanical engineering applications.

Resistance to relative movement of two bodies that are already in motion.

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.

Twice the 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.

A relatively thin, hard coating that can be applied to threads and deposited uniformly. Bright metallic in appearance this coating has excellent resistance to wear and corrosion.

Localized plastic deformation which occurs in the vicinity of clamped fasteners or in the fastener threads. . Embedding is local plastic deformations that occur under the nut face, in the joint faces and in the threads as a result of plastic flattening of the surface roughness. This occurs even when the loading is below the yield point of the bolt or limiting surface pressure of the joint material and is the result of the real area of contact between surfaces being less than the apparent area.

A process that can occur with the use of high strength steel fasteners in which crack initiation and growth occurs in the fastener at a comparatively low stress level as a result of interactions that occur with the environment. Hydrogen is suspected of causing EAC in high strength steel fasteners, the hydrogen being produced as a result of chemical reactions (galvanic corrosion in a moist environment) or being present from a plating process that may have been applied to the fastener.

Forces exerted on a fastener as a result of an applied loading to the joint.

A screw thread which is formed on an external cylinder, such as on bolts, screws, studs etc.

A conventional flanged joint in which a gasket is compressed by bolts - the gasket is not rigidly located. Calculation methods such as the ASME code in the USA and the EN1591 code in Europe.

A low friction coating applied to threads. This type of coating is frequently used to prevent thread fouling when an assembly containing threaded fasteners is painted. Unless masked in some way before painting, electro deposited primers can cover the threads. If this occurs assembly difficulties can result unless the expensive chore of cleaning the threads is completed. A fluoro-carbon thread coating eliminates the need for masking or cleaning since paint will not adhere to the coating. This type of coating can also prevent problems caused by weld splatter obstructing the threads of weld nuts during their placement. Such coatings also have the property of reducing the torque-tension scatter during tightening.

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.

Coating materials used on fasteners with the intention of reducing the scatter in the thread and bearing surface friction coefficients.

An intentional clearance between internal or external thread and the design form of the thread when the thread form is on it's maximum metal condition. For metric threads the fundamental deviation are designated by letters, capitals for internal threads and small letters for external threads. Some tolerance classes have a fundamental deviation of zero. For imperial threads the fundamental deviation is called the allowance.

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.

A severe form of adhesive wear which occurs during sliding contact of one surface relative to another. Clumps of one part stick to the mating part and break away from the surface. (Can frequently occur when both the nut and bolt are made from stainless or high alloy steels, titanium or zinc coated fasteners.)

Total distance between the underside of the nut to the bearing face of the bolt head; includes washer, gasket thickness etc.

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.

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 utilises the thermal expansion characteristics of the bolt. The bolt is heated and expands: the nut is indexed (using the angle of turn method) and the system allowed to cool. As the bolt attempts to contract it is constrained longitudinally by the clamped material and a preload results. Methods of heating include direct flame, sheathed heating coil and carbon resistance elements. The process is slow, especially if the strain in the bolt is to be measured, since the system must return to ambient temperature for each measurement. This is not a widely used method and is generally used only on very large bolts.

A split type of spring washer whose purpose is to prevent self loosening of the nut or the bolt. The idea or principle behind the helical spring washer is for one end of the tang of the washer to indent into the fastener (the nut or bolt head) and the other into the joint surface so that any loosening rotation is prevented. Junker in his paper in 1969 on the cause of self-loosening of fasteners (reference:Junker, G., New criteria for self-loosening of fasteners under vibration. SAE Paper 690055, 1969) concluded that this type of lock washer has no ability to lock. This type of washer is sometimes called a spring lock washer or sometimes a standard lock washer.

Sometimes abbreviated to HSFG bolts. Bolts which are of high tensile strength used in conjunction with high strength nuts and hardened steel washers in structural steelwork. The bolts are tightened to a specified minimum shank tension so that transverse loads are transferred across the joint by friction between the plates rather than by shear across the bolt shank.

Special bolts that have a tang at the threaded end of the shank. This tang is gripped by the tightening tool during assembly so that the reaction torque is absorbed whilst the nut is tightened from the same side. Such bolts allow what used to have to be done by two men to become a one-man task.

This term is used for the completion of maintenance work on a bolted joint when the joint is under loading. This can involve the replacement of individual bolts. There are risks both to the joint itself and to health and safety associated with this technique.

A hydraulic tool used to tighten a fastener by stretching it rather than applying a large torque to the nut. After the fastener has been stretched, the nut is run down the thread to snug up with the joint, the hydraulically applied load is then removed resulting in tension being induced into the fastener.

Steel fasteners exposed to hydrogen can fail prematurely at a stress level well below the materials yield strength. Hydrogen embrittlement occurs in fasteners usually as a result of the part being exposed to hydrogen at some time during its manufacturing process but it can also occur through in-service corrosion. Electroplating is generally considered to be a major cause of hydrogen absorption in steel fasteners due to the release of hydrogen during this process. Higher strength steels are more susceptible to hydrogen embrittlement than lower strength steels, however it is considered that there is no lower strength limit. As a rule of thumb, steels below Rockwell C 35 are considered to be far less susceptible. Tests such as the incremental load hydrogen embrittlement test can be completed to assess if hydrogen embrittlement is present in a batch of fasteners.

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