Anti-Seize Compound: Used on the threads of fasteners in some applications to prevent galling of mating surfaces. Frequently used with stainless steel fasteners. Used to improve corrosion resistance to allow the parts to be subsequently disassembled. Can provide a barrier to water penetration since the threads are sealed by use of the compound.
Allowance: 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.
Angle Controlled Tightening: A 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. 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.
Basic Thread Profile: This is the theoretical profile of external and internal threads with no manufacturing tolerance applied.
Bearing Stress: The surface pressure acting on a joint face directly as a result 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 the term used for a threaded fastener, with a head, designed to be used in conjunction with a nut.
Breakaway Torque: The initial force needed to cause an object to rotate around an axis.
Breakloose Torque: The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.
Clamping Force: The compressive force which a fastener exerts on the joint.
Coefficient of Friction: 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. 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 in general a difference in values between the two coefficients due to typically the contact surfaces being different.
Commingling: 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.
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 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: 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.
Decompression Point: 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.
Design Form of Thread: 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.
Design Load: The engineering design load for each support point.
Direct Tension Indicators (DTI’s): load indicating washers. Projections on the face of the washer (usually on the face abutting 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.
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.
Embedment: Localized plastic deformation which occurs in the vicinity of clamped fasteners or in the fastener threads. 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.
External Force or Load: Forces exerted on a fastener as a result of an applied loading to the joint.
External Thread: A screw thread which is formed on an external cylinder, such as on bolts, screws, studs etc.
Floating Type Flange Joint: A conventional flanged joint in which a gasket is compressed by bolts – the gasket is not rigidly located.
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 used on fasteners with the intention of reducing the scatter in the thread and bearing 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 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.
Galling: A severe form of adhesive wear which occurs during sliding contact of one surface relative to another. Clumps of one part stick to the mating part and break away from the surface. (Can frequently occur when both the nut and bolt are made from stainless or high alloy steels, titanium or zinc coated fasteners.)
Grip Length: Total distance between the underside of the nut to the bearing face of the bolt head; includes washer, gasket thickness etc.
Hanger Spring Load: The load being applied to the pipe while in the working travel range.
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.
Heat Tightening: Heat tightening utilizes the thermal expansion characteristics of the bolt. The bolt is heated and expands: the nut is indexed (using the angle of turn method) and the system allowed to cool. As the bolt attempts to contract it is constrained longitudinally by the clamped material and a preload results. Methods of heating include direct flame, sheathed heating coil and carbon resistance elements.
Helical Spring Washer: A split type of spring washer whose purpose is to prevent self loosening of the nut or the bolt.
High Strength Friction Grip Bolts: (HSFG bolts) 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.
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 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.
Hot Bolting: 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.
Hydraulic Tensioner: 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.
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 Center of Rotation: The point in space that an eccentrically shear loaded joint rotates about. The deformation and the load sustained by an individual bolt in a bolt group is dependent upon the distance that the bolt is from the instantaneous center. The direction that the individual bolt force acts is perpendicular to a line joining that bolt to the instantaneous center.
Integral Fastener: A term used to describe types of fasteners which are highly resistant to vibration loosening and/or removal. Some types have special thread forms.
Internal Thread: A screw thread which is formed in holes, such as in nuts.
K-Factor: also known as the nut factor and the torque coefficient. The factor in the torque tightening equation:
T = K x S x A x D
T – TORQUE (FT – LB)
K – AN EXPERIMENTAL FACTOR
S – STRESS APPLIED ON FASTENER (PSI)
A – STRESS AREA OF FASTENER (IN2)
D – DIAMETER OF STUD (IN)
Load Adjustment Percentage: The design load of the hanger can be adjusted generally within (ten) or (twenty) percent depending upon manufacture. This is stated in + or – of design hanger load. Should this range of adjustment be insufficient, then the cause of over/under load should be investigated. It will also identify that the hanger may need replacing.
Lock Nut: two common uses of this term:
1. A nut which provides extra resistance to vibration loosening by either providing some form of prevailing torque, or, in free spinning nuts, by deforming and/or biting into mating parts when fully tightened.
2. The term is sometimes used for thin (or jam) nuts used to lock a thicker nut. When used in this way the thin nut should be adjacent to the joint surface and tightened against the thick nut. If placed on top of the thick nut the thin nut would sustain loads it was not designed to sustain.
Major Diameter: 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 tightening torque values produced by the same tightening tool on hard and soft joints. A hard joint typically gives a higher torque value than a soft joint. Generally speaking, the lower the meanshift of a tightening tool, the better it will be in achieving a specified torque value irrespective of the joint condition.
Metal-to-metal Contact Flange Joint: A flanged joint in which a gasket is compressed by bolts – the gasket being located in a recess within the joint so that it is compressed by the bolt loads until metal to metal contact occurs.
Minor Diameter: This is the diameter of an imaginary cylinder which just touches the roots of an external thread, or the crests of an internal thread.
Molybdenum Disulphide: A solid lubricant that acts as a high pressure resistant film. Can be used by itself as a dry lubricant as well as in with other solid lubricants and in oils and greases. Used in threads, such lubricants act as a separating film to prevent corrosion formation on the thread surface (even under adverse temperature and environmental conditions) ensuring the release of the threaded connection.
Nominal Diameter: The diameter 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 design of the tool is such that attempts are made to ensure that the applied torque is independent of joint stiffness.
Octagon Head: bolt or screw whose head cross section is a regular polygon with 8 sides.
Overtapping: Tapping of a thread following a plating operation so that the thread tolerances comply within specification allowing the internal and external threads to assemble. It is normal practice to overtap the internal rather than the external thread.
Piles: Term used in structural engineering for the joint plates.
Pitch: The nominal distance between two adjacent thread roots or crests.
Pipe Weight: The weight of the piping at the clamp support point. (This is achieved by locking the spring mechanism, in this form the hanger is effectively a ridged rod and the weight recorded will reflect the weight of the pipe).
Ply: A single thickness of steel forming part of a structural joint.
Pooching: a term sometimes used to describe the effect of the area immediately surrounding a tapped hole being raised up as a result of the tension from the stud. Tapped holes are often bored out for the first couple of threads to eliminate this problem.
Preload: Initial clamping force or tension in a fastener.
Prevailing Torque: The torque required to run a nut down a thread on certain types of nuts designed to resist vibration loosening. The resistance can be provided by a plastic insert or a noncircular head.
Prevailing Torque Nut: A type of lock nut which has a prevailing torque to assist in preventing 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 by some means. Nylon insert torque prevailing nuts utilize a nylon (or other polymer) 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 permanent set.
Prying: The amplification of an external force acting on a bolt by a lever action which 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 that occurs typically without any nut rotation occurring. Commonly occurs as a result 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. The majority of screw threads are right handed.
Rolled Thread: A thread formed by plastically deforming a blank rather than by cutting. The majority of standard fasteners have their threads formed by rolling. Most threads are rolled before any heat treatment operation. Significant improvements in fatigue life can be achieved by rolling the thread after heat treatment, this improvement is due to compressive stresses being induced in the roots of the thread. However, because of the increased hardness of the bolt blank, the die life can be significantly reduced. Rolling the thread also generally improves the surface finish which can have a beneficial effect on fatigue life.
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 (which is 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 know that the fastener can self rotate under the action of transverse joint movement that can completely loosen a tightened fastener such that the nut will become detached from the bolt.
Shank: That portion of a bolt between the head and the threaded portion.
Snug Torque: The torque required to pull plates together so that direct contact occurs; 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 angle of rotation start in order that the bolt is tightened sufficiently. The snug torque is usually determined experimentally on the actual joint.
Snugging: The process of pulling parts of a joint together, most of the input turn during this process is absorbed in the joint with little tension being given to the bolt.
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 has to 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 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 being 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.
Stiffnut: A term used to describe a lock nut which has a prevailing torque.
Stress: Forces exerted against each other by two surfaces in contact.
Stress Area: The effective cross sectional area of a thread when subjected to a tensile force. It is based upon a diameter which is the mean of the pitch (or effective) and the minor (or root) diameters of the thread.
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 a high stress is present that is relieved over time; the stress is relaxed with a subsequent reduction in the bolt’s preload.
Structural Bolt: A heavy hexagon head bolt having a controlled thread length intended for use in structural connections and assembly of such structures as buildings and bridges. The controlled thread length is to enable the thread to stop before the joint ply interface to improve the fastener’s direct shear performance.This term is used in civil and structural engineering but is not frequently used in mechanical engineering.
Stud: A fastener which is threaded at both ends with an unthreaded shank in between. One end (which often has a thread tolerance which results in more thread interference) is secured into a tapped hole, the other is used with a nut.
Symmetrical Thread: A symmetrical thread is one which has both flanks of the thread profile inclined at the same angle.
Tension: The act of stretching tight or state of being stretched tight. Stress of a pulling force on a body.
Tension Washers: A general name given to spring washers, curved washers, Belleville washers and disc springs. This type of washer provides a relatively low stiffness (compared to the joint stiffness) and can be used to act as a spring take-up with a bolt to prevent movement between parts.
Thread Crest: The top part of the thread. For external threads, the crest is the region of the thread which is on it’s outer surface, for internal threads it is the region which forms the inner diameter.
Thread Flank: The thread flanks join the thread roots to the crest.
Thread Height: This is the distance between the minor and major diameters of the thread measured radially.
Thread Length: Length 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 so that fatigue performance is improved.
Thread Runout: The portion at the end of a threaded shank which is not cut or rolled to full depth, but which provides a transition between full depth threads and the fastener shank or head.
Tolerance Class: A combination of tolerance grade and a fundamental deviation which is given to an internal or external thread. A tolerance class for an internal thread when combined with the tolerance class for an external thread gives the class of fit for the mating threads.
Tolerance Grade: The difference between maximum and minimum metal conditions for a tolerance applied to a screw thread. For metric threads the tolerance grade is given a number.
Torque: The measure of the tendency of a force to rotate the body upon which it acts about an axis.
Torque Multiplier: A gearbox used to increase the torque produced by a small hand wrench.
Torque Wrench: A manual or power assisted wrench that applies rotational force to a nut or bolt.
Ultrasonic Extensometer: An instrument which 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).
Virtual Effect Diameter: The effective diameter of a thread but allowing for errors in pitch and flank angles.
Waister 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.
Weigh Operation: Transferring of load between the support hanger and pipe clamp to hydraulic load cells, where the hydraulic pressure is converted to load in lbs.
Wire Thread Insert: A threaded insert that 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 which allows a fastener to be tightened to yield. The angle of rotation of the fastener is measured relative to the applied torque, yield being assessed when the slope of the relationship changes to below a certain value. Sometimes called joint controlled tightening.