Pandey's Fasteners: August 2013

Thursday, August 8, 2013

Quality Control in the Tightening of Bolts

A survey was completed in the United States of automobile service managers which indicated that 23% of all service problems were traced to loose fasteners, with even 12% of new cars being found to have fasteners loose. Presented below is information and techniques intended to assist an Engineer in ensuring that threaded fasteners are tightened so that the desired preload is achieved.

Introduction

The drive for improved quality throughout manufacturing industry has had an impact on the assessment of the accurate measurement of assembly line torques. It is no longer sufficient just to run a nut down a bolt until it stops and hope that it is tight enough.

The critical weakness in many products is the region of joints which exist in the design. Bolted joints in particular can be a source of concern for the Engineer. A single bolt, inaccurately or incorrectly tightened, can lead to the failure of the complete product. Too high a tightening torque and the Engineer sustains the risk of a bolt shank or thread stripping failure. Too low a specified torque and the bolt tension can be inadequate to meet functional requirements. Failure to meet the tightening specification can have unfortunate consequences for the reliability of the product. Such failures could occur either during production assembly or during subsequent maintenance on the product after it had entered service. Either is obviously undesirable.

The most prevalent controlled method of tightening threaded fasteners is by tightening so that a specified torque is achieved. This method is generally known as torque control. The major problem related to this method is that the clamp force generated as the result of an applied torque is dependent upon the design of the fastener and the prevailing frictional conditions. Despite these problems, it is still the most popular way of ensuring that an assembled bolt complies with an engineering specification.

Distribution of the Tightening Torque within the Fastener

A major problem with using the torque control method to indirectly control the clamp force in the fastener is that only a small proportion of the torque is actually used to extend the bolt. The majority is absorbed by friction between the nut face and the joint and in the threads. Some fasteners use a nylon insert or have a distorted thread so that a torque is required to run the nut down an unclamped bolt thread. Interestingly, for the same stress in the bolt, the required tightening torque between the prevailing and non-prevailing bolt types varies by less than 4%. The prevailing torque type nut however results in significantly less preload (16%). An example of the distribution of torque within the fastener for a M10 Property Class 10.9 bolt is shown in the piechart. (The chart is from the BOLTCALC program.)

Torque Auditing

Checking that a torque controlled tightened bolt complies to a specified torque is known as torque auditing. Most major manufacturing companies have in-house quality audits on their products which include the checking that threaded fasteners have been tightened so that they conform to the engineering specification.

There are two approaches used to audit installation torque. The first is dynamically, which, by the use of in-line transducers attached to the tightening tool, the installation torque is measured directly. The second approach is by an operator or inspector measuring the torque after the installation has been completed.

Dynamic Torque Auditing

The dynamic method gives results which are independent of operator reading accuracy. Generally powered torque tools are used in conjunction with a computer based data collection system. Because this method allows the automatic storage and retrieval of tightening data it can be an important tool in statistical process control. The disadvantage of the method is that it is generally expensive in terms of capital cost and the technical support skills needed.

Torque Auditing after Assembly

There are three basic methods for the checking of torques applied to bolts after their installation; namely, taking the reading on a torque gauge when:

1. The socket begins to move away from the tightened position in the tightening direction. This method is frequently referred to as the "crack-on" method.

2. The socket begins to move away from the tightened position in the un-tightening direction. This method is frequently referred to as the "crack-off" method.

3. The fastener is re-tightened up to a marked position. With the "marked fastener" method the socket approaches a marked position in the tightening direction. Clear marks are first scribed on the socket and onto the joint surface which will remain stationary when the nut is rotated. (Avoid scribing on washers since these can turn with the nut.) The nut is backed off by about 30 degrees, followed by re-tightening so that the scribed lines coincide.

The torque in all three methods should be applied in a slow and deliberate manner in order that dynamic effects on the gauge reading are minimised. It must always be ensured that the non-rotating member, usually the bolt, is held secure when checking torques. The torque reading should be checked as soon after the tightening operation as possible and before any subsequent process such as painting, heating etc. The torque readings are dependent upon the coefficients of friction present under the nut face and in the threads. If the fasteners are left too long, or subjected to different environmental conditions before checking, friction and consequently the torque values, can vary. Variation can also be caused by embedding (plastic deformation) of the threads and nut face/joint surface which does occur. This embedding results in bolt tension reduction and affects the tightening torque. The torque values can vary by as much as 20% if the bolts are left standing for two days.

"Crack-On" and "Crack-Off" Methods of Torque Auditing

With the "crack-on" and "crack-off" methods there are two ways in which the breakaway torque can be checked. The first is manually by having the operator "feel" the point of breakaway. The second is by use of electronics and a strain-gauged torque wrench and recording the breakaway torque automatically. The automatic method removes the operator variability which does occur.

Marked Fastener Method of Torque Auditing

The most consistent and accurate method for the checking of applied torques after the installation has been completed is by the "marked fastener" method. From tests carried out under laboratory conditions, the maximum torque checking accuracy possible, if the above guidelines are followed, is approximately ± 5% of the value actually applied to the nut.

The specification of the tightening torque is of crucial importance in determining the reliability of the joint. Different thread and joint surface finishes all have an effect on tightening torque. Special fasteners, such as those having a prevailing torque, flanged heads, or reduced shanks, also require special consideration when determining the tightening torque. Prevailing torque fasteners use a nylon insert or have a distorted thread so that a torque is required to run the nut down an unclamped bolt thread. For free spinning nuts, the prevailing torque is zero.

Threads Locked by Adhesive

It has been shown by extensive test work that locking the male to female thread by means of adhesive gives the fastened assembly excellent resistance to vibration loosening. The adhesive may be applied in liquid form at the assembly stage, or the threads may have been coated previously with an anaerobic adhesive which cures when the parts have been assembled.

Most thread locking adhesives tend to display a thread friction coefficient which is higher than what is normally present in the threads, also, a slight prevailing torque characteristic will be present. By the correct specification of tightening torque both these effects can be accounted for accurately. Full cure of the adhesive is normally achieved within 24 hours but is dependent upon the finish applied to the fastener, the bond gap and ambient temperature. With the liquid applied adhesive an activator can be used to improve curing time.

Measuring the assembly torque, dynamically, at the assembly stage does not present any problem when using adhesive to lock the threads together. Problems are present however when the assembly torque is required to be checked after assembly. The use of any of the three methods for torque auditing of threads which have been bonded by adhesive does present problems. If the cured bond is broken to check the torque, the vibration resistance of the fastener assembly may be impaired. Secondly, the hardened adhesive in the threads increases the thread's friction characteristics so that on re-tightening less torque goes into achieving preload and more in overcoming friction. Due to these reasons any of the three methods for post assembly torque auditing are unsuitable for threads which have been bonded together by adhesive.

A common method of checking that the specified assembly has been achieved is to re-tighten the joint up to the specified torque whilst checking that the male relative to the female member does not move. The effect of the adhesive is to augment the assembly torque so that the breakloose torque is some 10% to 30% above the value of the assembly torque. Hence if the torque specification was correctly achieved, no rotation of the male relative to the female member should occur at the assembly torque value.

Methods of Tightening Threaded Fasteners

One of the major problems with the use of bolted joints is the precision, with regard to achieving an accurate preload, of the bolt tightening method selected. Insufficient preload, caused by an inaccurate tightening method, is a frequent cause of bolted joint failure. It is important for the Designer to appreciate the features and characteristics of the main methods employed to tighten bolts. Presented below is a brief summary of the major bolt tightening methods. Note however that whatever method is used to tighten a bolt, a degree of bolt preload scatter is to be expected.
There are six main methods used to control the preload of a threaded fastener. Specifically:
1. Torque control tightening.
2. Angle control tightening.
3. Yield controlled tightening.
4. Bolt stretch method.
5. Heat tightening.
6. Use of tension indicating methods.
Torque Control Tightening
Controlling the torque which a fastener is tightened to is the most popular means of controlling preload. The nominal torque necessary to tighten the bolt to a given preload can be determined either from tables, or, by calculation using a relationship between torque and the resulting bolt tension.
When a bolt is tightened the shank sustains a direct stress, due to the elongation strain, together with a torsional stress, due to the torque acting on the threads. Most tables of bolt tightening torques ignore the torsional stress and assume a direct stress in the threads of some proportion of the bolts yield stress, usually 75%. For high frictional conditions the magnitude of the torsional stress can be such that when combined with the direct stress, an equivalent stress over yield can result, leading to failure. A more consistent approach is to determine the magnitude of the direct stress which, when combined with the torsional, will give an equivalent stress of some proportion of yield. The proportion commonly used with this approach is 90%.
Torque prevailing fasteners (such as Nyloc, Cleveloc nuts etc.) are often used where there exists a risk of vibration loosening. The prevailing torque has the effect of increasing the torsional stress in the bolt shank during tightening. This affects the conversion of the tightening torque into bolt preload and should be allowed for when determining the correct torque value for this type of fastener.
Torque Distribution

As can been seen by study of the above chart, a fundamental problem with torque tightening is that because the majority of the torque is used to overcome friction (usually between 85% and 95% of the applied torque), slight variations in the frictional conditions can lead to large changes in the bolt preload. This effect can be reduced by the use of so called friction stabilisers. These are substances which are coated onto the fasteners to reduce the frictional scatter. Other ways to improve the accuracy of the method are:

1. Do not use plain washers; their use can result in relative motion to change from the nut to washer, to washer to joint surface, during tightening. This as the effect of changing the friction radius and hence affects the torque-tension relationship. If, because of excessive bearing pressure, a larger bearing face is required, thought should be given to the use of flanged nuts and bolts.
2. Determine the correct tightening torque by the completion of tests. Strain gauges can be attached to the bolt shank and tightening completed on the actual joint. A load cell under the bolt head can be used, however it is not as accurate as strain gauging, since the joint characteristics have been changed.
3. If it is not feasible to establish by testwork the actual tightening torque, determine the tightening torque using the best information available i.e. fastener finish, nut head bearing surface size and prevailing torque characteristics, if applicable. (The computer program TORQUE developed by Bolt Science can allow for all these effects.)

4. Ensure that the tightening torque value is specified on the assembly drawing. Quotation of a plus or minus 5% tolerance is good practice. More unusually, quote that a calibrated torque wrench is to be used to check the torque after installation. The method used to tighten the bolt has a significant influence on the preload scatter (see below).

Angle Controlled Tightening
This method, also known as turn of the nut method, was introduced for manual assembly shortly after the second World War when a certain tightening angle was specified. The method has been applied for use with power wrenches, the bolt being tightened to a predetermined angle beyond the elastic range and results in a small variation in the preload due, in part, to the yield stress tolerance. The main disadvantages of this method lie in the necessity for precise, and, if possible, experimental determination of the angle; also the fastener can only sustain a limited number of re-applications before it fails.

Yield Controlled Tightening
This method, developed by the SPS organisation, is also known under the proprietary name "Joint Control Method". Very accurate preloads can be achieved by this method by minimising the influence of friction and its scatter. The method has its roots in a craftsman's "sense of feel" on the wrench which allowed him to detect the yield point of the fastener with reasonable precision. With the electronic equivalent of this method, a control system is used which is sensitive to the torque gradient of the bolt being tightened. Rapid detection of the change in slope of this gradient indicates the yield point has been reached and stops the tightening process. This is achieved by incorporating sensors to read torque and angle during the tightening process. Since angle of rotation and torque are both measured by the control system, permissible values can be used to detect fasteners which lie outside their specification (having too low a yield for example).
A small degree of preload scatter still results from this method due to the influence of friction. The method detects the yield point of the fastener under the action of combined tension and torsion. The higher the thread friction, the higher the torsional stress, which, for a given yield value, results in a lower preload due to a lower direct stress.
The method has been used in critical applications, such as cylinder head and conn-rod bolts, in order that consistently high preloads can be achieved (which can allow smaller bolts to be used). However, because of the cost of the tools necessary to use this method (a hand wrench incorporating the control circuitry costs many times more than a conventional torque wrench), widespread adoption of this method is unlikely. (Although manufacturers may be able to invest in the equipment, unless service staff have similar equipment, the Designer cannot depend upon high preloads being maintained in the field.)
Bolt Stretch Method
A problem relating to the tightening of large bolts is that very high tightening torques are required. Although this can be partly overcome by the use of hydraulic torque wrenches (the reaction of the torque however can be a problem), the use of hydraulic tensioning devices is commonplace for bolts over 20mm in diameter. The method uses a small hydraulic ram which fits over the nut, the threaded portion of the bolt/stud protrudes well past the nut and a threaded puller is attached. Hydraulic oil from a small pump acts upon the hydraulic ram which in turn acts upon the puller. This is transmitted to the bolt resulting in extension occurring. The nut can then be rotated by hand with the aid of an integral socket aided by a tommy bar.
Control of the hydraulic pressure effectively controls the preload in the bolt. A small amount of preload reduction however does occur when the pressure is removed as the nut elastically deforms under the load. Removal of nuts corroded to the bolts can be a problem with this method.
Heat Tightening
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.

Tension Indicating Methods
This category includes the use of special load indicating bolts, load indicating washers and the use of methods which determine the length change of the fastener. There are a wide number of ways bolt tension can be indirectly measured and the discussion presented here is not exhaustive.
Special bolts have been designed which will give an indication of the force in the bolt. One such fastener is the Rotabolt which measures bolt extension by the use of a central gauge pin which passes down a centrally drilled hole in the bolt. Underneath the head of the gauge pin, a rota is retained which is free to spin in a very accurately set gap. The fastener stretches elastically, whereas the gauge pin does not move since it experiences no load. As tightening continues, the bolt will stretch sufficiently to eliminate the gap and prevent the rota from being able to be rotated. This is the indication that the bolt is correctly loaded. Another proprietary fastener uses a similar method. The HiBolt uses a pin located centrally down the bolt as does the Rotabolt except the pin is gripped by the slight contraction of the bolt diameter; the pin being locked when the correct preload is reached.
The use of load indicating washers is widespread in structural engineering. Such washers have small raised pips on their surface which plastically deform under load. The correct preload is achieved when a predetermined gap is present between the washer and the underhead of the bolt. This is measured using feeler gauges. Generally they are not used in mechanical engineering, but are, extensively, in civil engineering.
The extension which a bolt experiences can be measured either using a micrometer or by a more sophisticated means such as using ultrasonics. The extension can be related to preload either directly, by calibration, or indirectly, by calculation. If ultrasonic measurement is used then the end of the bolt shank and the head may require surface grinding to give a good acoustic reflector.

Terminology related to nuts and bolts

Presented 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.

ACORN NUT: A nut (so-called because of its shape) that has a domed top so that it prevents contact with the external thread.
AEROTIGHT NUT: 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.
ANTI-FRICTION COATING: 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.
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 an all metal construction. Covered by UK patent 1180842 the nut is marketed by GKN Screws and Fasteners Limited.
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.
ANAEROBIC ADHESIVE: An adhesive which 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 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.
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 colour 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 torque necessary to put into reverse rotation a bolt that has not been tightened.
BREAKLOOSE TORQUE: The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.
BRITISH STANDARD BRASS: A specialist thread form based upon the Whitworth thread and consisting of 26 threads per inch whatever the thread diameter.
BSF: 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.
BSW: 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.
BUMP THREAD: 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.
CADMIUM ELECTROPLATING: Coating of threaded fasteners with cadmium can provide the parts with excellent corrosion resistance. The appearance of the coating is bright silver or yellow if subsequently passivated. The friction values associated with this coating are also comparatively low. A chromate conversion coating is frequently applied to the surface to improve corrosion resistance. Cadmium is not now frequently used because of the environmental and worker health problems associated with the coating process and should not be used in applications above 250C or when contact with food is possible.
CLAMPING FORCE: The compressive force which 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 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.
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 minimise galling.
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 coefficent of friction can be further sub-divided 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. For example, a zinc plated nut on a zinc plated bolt, the thread coefficent 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 joint surface finish.
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.
CONELOC NUT: The Coneloc is an all metal prevailing torque type self-locking nut. The locking action is achieved by localised precision deformation of the thread in the cone section on top of the nut. When the nut is tightened onto the bolt, the thread is gripped on the flanks providing the locking action. Coneloc is a trade mark of NUTS BOLTS Limited
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: 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.

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. 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.
DACROMET: 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
DESIGN FORM OF THREAD: 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: 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.
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 centre 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 that 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 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.
ENVIRONMENTALLY ASSISTED CRACKING (EAC): 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.
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. Calculation methods such as the ASME code in the USA and the EN1591 code in Europe.
FLUORO-CARBON THREAD COATING: 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.
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.
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 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.
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 this hardened washers under the bolt head can be used to distribute the force over a wider area into the clamped material. A more modern alternative is to use a flange headed nuts and bolts.
HEAT TIGHTENING: Heat tightening 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.
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. 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.

HIGH STRENGTH FRICTION GRIP BOLTS: 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.
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.
HYDROGEN EMBRITTLEMENT: 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.
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 is dependent upon the distance that the bolt is from the instantaneous centre. The direction that the individual bolt force acts is perpendicular to a line joining that bolt to the instantaneous centre.
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.
JAM NUTS: See LOCKNUT
JOINT CONTROL TIGHTENING: See YIELD CONTROLLED TIGHTENING
JOST EFFECT: The name given to the reduction in the frictional resistance that occurs in a direction different to that inwhich slip is occurring. This effect is used in many applications including the removal of 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 (as a result of its preload) that when coupled with the Jost Effect results in self-loosening occurring.The term is named after the Institute that completed research into this effect, the Jost Institute of Tribotechnology at the University of Central Lancashire in the UK.
K FACTOR: The factor in the torque tightening equation: T=KDF where T is the fastener tightening torque in Newton metres, D is the fastener diameter in metres, F is the fasteners preload in Newtons and K is a factor whose value is often taken as 0.2. The formula gives the approximate tightening torque for standard fasteners used under normal conditions. The K factor is also known as the nut factor and the torque coefficient.
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 on the end 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 usage's 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: 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 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. Unlike the FLOATING TYPE FLANGE JOINT, for metal to metal type joints there are no standardised gasket factor definitions, test procedures, nor generally acknowledged calculation procedures available.
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.
MODEL ENGINEERS THREAD (M.E.): A thread based upon the Whitworth thread form that was established in 1912. A very fine thread (a 3/32 inch thread having 60 tpi for example).
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. Such films can also act as friction stabilisers.
NICKED THREADS: Nicks or indentations in threads can occur during the manufacturing process and during fastener transportation. In general, nicked thread problems tend to increase as the thread diameter increases and for fine pitches.
There are acceptance tests for nicked threads that involve measuring the maximum torque required to drive a GO gauge down the thread. Examples of acceptance tests are SAE J123 and the Ford Motor specification WA990 1993. Nicks and indentations in threads are sometimes referred to as gouges.
NOMINAL DIAMETER: The diameter equal to the external diameter of the threads.
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.
NYLOC NUT: A torque prevailing nut that uses a nylon patented insert to provide a locking feature. The nylon insert, it is claimed, helps to seal the bolt thread against seepage of water, oil, petrol, paraffin and other liquids. The nut is covered by UK patent 8028437 and European patent 81303450-1. Nyloc is a registered trade name of Forest Fasteners.
OCTAGON HEAD: A 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.
PLY: A single thickness of steel forming part of a structural joint.
POOCHING: Pooching is 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: The tension created in a fastener when first tightened. Reduces after a period of time due to embedding and other factors.
PREVAILING TORQUE: The torque required to run a nut down a thread on certain types of nuts designed to resist vibration loosening. The resistance can be provided by a plastic insert or a noncircular head.
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 ultilise 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.
PROPERTY CLASS: A designation system which 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.; Hence a fastener with a property class of 8.8 has a minimum tensile strength of 800 MPa and a yield stress of 0.8x800=640 MPa.; 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. For example a nut of property class 8 has a minimum tensile strength of 800 MPa. A bolt or screw of a particular property class should be assembled with the equivalent or higher property class of nut to ensure that thread stripping does not occur.
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.
ROOT DIAMETER: Identical to MINOR DIAMETER
SCREW: A headed threaded fastener that is designed to be used in conjunction with a pre formed internal thread or alternatively forming its own thread. Historically, it was a threaded fastener with the thread running up to the head of the fastener that has no plain shank. However this definition has largely been superseded to avoid confusion over the difference between a bolt and a screw.
SCREW THREAD: A ridge of constant section which is manufactured so that a helix is developed on the internal or external surface of a cylinder.
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.
SEMS: A screw and washer assembly. A screw or bolt which has a captive washer. The washer is frequently loose on the plain shank of the fastener, the shank diameter being equal to the effective diameter of the thread; the thread being rolled from this diameter. The origin of the word is a frequent question. In the 1930's E. C. Crowther was a representative for a company that sold both shakeproof washers and screws. He came up with the idea of placing the washer on the screw before it was thread rolled. The major diameter of the screw being larger than the washer hole prevents it from coming off. The Illinois Tool Works made machines that produced these patented pre-asSEMbled washers and screws. The s at the end of SEMs is thought to have been subsequently picked up because they are not usually purchased individually. In spite of the original patents and trademarks the word SEMS is generally recognised as a generic term applicable to screw and washer assemblies.
SET SCREW: A set screw is a threaded fastener that is typically used to hold a sleeve, collar or gear on a shaft to prevent relative motion. It is a threaded member that normally does not have a head. Unlike most other threaded fasteners it is basically a compression device normally used to generate axial thrust. Various socket types are provided to allow the set screw to be rotated. These types include hexagon socket, fluted socket, screwdriver slot and square head. Various point designs are available (the part of the set screw that rotates against the shaft being secured) and include:
Cup - Hollowed end, is the most commonly used point style. Used when the digging in of the point is not undesirable.
Cone - Pointed end, this type generates the highest torsional holding power and is typically used for a permanent connection.
Oval - Rounded end that is typically used when frequent adjustment is required. The oval end prevents/reduces indentation.
Flat - Cause little damage to the shaft and are used when frequent adjustment is required.
Dog - Flat end with the threads stopping short of the end with the end fitting into a hole.
SHANK: That portion of a bolt between the head and the threaded portion.
SHOULDER SCREWS: A threaded fastener with a plain, precision machined, shank that is used for location purposes. They are typically used for pulleys and linkages.
SKIDMORE BOLT TENSION CALIBRATOR: The Skidmore-Wilhelm bolt tension calibrator is a hydraulic load cell used to determine the tension in a bolt or other threaded fastener. The tension in the bolt compresses fluid in a hydraulic cylinder, a pressure gauge connected to the cylinder is then calibrated to read in terms of force rather than pressure.
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.
SOCKET HEAD CAP SCREW: A screw with a round head, usually with a hexagon indentation in the head for tightening purposes. Used on machine parts and is typically made from high strength steel (grade 12.9 in metric).
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.
STEP-LOCK BOLT (SLB): The Step-Lock Bolt (SLB) is a thread form that has been modified to resist vibration loosening. The thread has several horizontal portions (i.e. no lead angle) whose purpose is to prevent torsion being developed in the bolt as a result of the loosening purpose. It is these horizontal portions that are known as steps. Published literature indicates that the thread form performs well when tested on a transverse vibration test machine. However manufacturing difficulties may prevent its widespread adoption.
STIFFNUT: A term used to describe a lock nut which has a prevailing torque.
STRENGTH GRADE: See PROPERTY CLASS
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. The use of this diameter stems from the work of E. M. Slaughter in the 1930's. He completed carefully controlled tests using various sizes of standard threads and compared their strength with machined bars made from the same bar of material. He found that this mean diameter gave results that agreed with the tensile test results to within about 3%. The error on the minor and pitch diameters was about 15%. Tests completed subsequent to these by other investigators have also shown that the stress diameter is a reasonable approximation to a thread's tensile strength. (Referance: 'Tests on Thread Sections Show Exact Strengthening Effect of Threads.' by E. M. Slaughter, Metal Progress, vol 23, March 1933 pp. 18-20)
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. The only way to minimise the effects of stress relaxation is to use materials that have an adequate resistance to it at the product’s operating temperature. The effect of bolt stress relaxation is to reduce the clamp force provided by the bolts; this phenomena 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 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.
TAYLOR-FORGE METHOD: A method developed by four engineers of the Taylor-Forge Company in Chicago in the 1930's that subsequently formed the basis of the ASME code for flanged joint design. The assumptions made by the method are now generally regarded as too simplistic. This method gives rise to the m and y gasket factors.
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.
THREADLOCKER: Can be a term used for a number of vibration resistant products but is now usually reserved for threadlocking adhesives. Specifically, a liquid anaerobic adhesive applied to nut or bolt thread, once hardened it fills the inner spaces between the threads to produce a solid plastic of a known shear strength.
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 aluminium or steel.
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: 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 in newton-metres (Nm) if metric units are used or pounds feet (lb-ft) when imperial units are used.
TORQUE MULTIPLIER: A gearbox used to increase the torque produced by a small hand wrench.
TORQUE WRENCH: A manual wrench which incorporates a gauge or other method to indicate the amount of torque transferred to the nut or bolt.
TURN OF THE NUT METHOD: See ANGLE CONTROLLED TIGHTENING
U BOLT: A U shaped fastener threaded at both ends used primarily in suspension and related areas of vehicles.
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).
UNC: Unified National Coarse (UNC) is a thread form with a 60 degree flank angle rounded roots and flat crests. For a given diameter it has a larger thread pitch than an equivalent diameter UNF thread. The unified thread is based on inch sizes and was first standardised in 1948 unifying the Whitworth and American standard thread forms.
UNEF: Unified National Extra Fine (UNEF) is a Unified thread form with a very fine (small) 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. For a given diameter it 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 has a flat, or optionally, a rounded root contour.) The majority of 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 that is typically used for tapped hole repair or to improve the thread stripping strength of softer metals such as zinc and aluminium. 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.
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 similar to zinc electroplating completed in an acid chloride bath - a small amount of cobalt (typically about 1%) is added to increase the plating speed.
ZINC PHOSPHATE CONVERSION COATING: A zinc phosphate conversion coating is frequently added to zinc electroplated parts, such as bolt threads, to improve corrosion resistance. This type of chemical conversion coating provides a protective passivation layer on the zinc improving its corrosion resistance.