S... Glossary of Fastener and Fixing Terminology






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SAE Specifications   

Standards developed by the Society of Automotive Engineers, USA


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, also known in the USA as a tap bolt. The term also applies to fastener or fixing items without a parallel thread such as wood screws, self-tapping screws and other types of fixings. 


A ridge of constant section which is manufactured so that a helix is developed on the internal or external surface of a cylinder.  Thread


A screw with a head designed to be driven by a unique driver. Also screws where the driver part of the head breaks off at a predetermined load. see tamper proof  Product


This term as applied to the finish of fasteners, is used to indicate that the fastener has had no supplementary surface treatment, such as plating and coating, other than being oiled. Sometimes referred to as self finish or plain


Self drilling screws tap and drills its own hole during application avoiding the need to do this as a separate operation. Product


Where binding action is self-induced on the body of the bolt, screw or nut to keep it rigid and prevent movement and vibration.  


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.


Tapping screws are normally fully threaded and designed to cut or form their own threads in steel plate on installation.   POINTS

  • Type A   Self thread-forming tapping screw having a gimlet point and a thread of relatively coarse pitch, used in punched, or nested holes in metal sheets or in treated plywood; this is the most popular self tapping screw. Product 
  • Type B  Also called " Z" . Self thread-forming tapping screw, having a blunt point with tapered threads of moderate pitch, used with punched, drilled or nested holes.   
  • Type C  Self thread-forming tapping screw having a blunt point with tapered threads at the end, having UNC or UNF threads and designed for fastening metal sheets.  
  • Type D  Also called " EC"   Self thread-cutting tapping screw having the same thread as type C with a fluted end produced at thread rolling or a milled slots produced after thread rolling. 
  • Type F   Self thread-cutting tapping screw having the same thread form as type C with a multiple flute tapered end to facilitate tapping.   
  • Type FZ  Self thread-cutting tapping screw having the same thread form as type B with a multiple fluted tapered end to facilitate tapping.   
  • Type G   Also called "EC"  Self thread-cutting tapping screw having the same thread form as type C with a slot across the end to facilitate tapping in hard materials or deep holes   
  • Type H   Also called " DB" or "25" . Self tapping screw having the same thread as a type B with a slot in the end to facilitate tapping in plastics.  


Self Threading Nuts have a low height with small exterior dimensions so they are ideal where space is restricted. The thread forming displaces material without making swarf. also called Palnuts®   Product


SEMs nuts capture a washer into the assembly, the washer is free to rotate. Product names are Carp, Combi, SEMs nut.   Product   


A captive 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. 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.  Screws 


Washers designed to go with SEMs screws.   Washers


Flange bolts have a washer flange as an integral part of the head removing the need for a washer and speeding up assembly time. Serrations can be added to the flange to produce a locking element.  Product


Flange nuts have an integral washer flanges, Serrations can be added to the flange to produce a locking element   Product


Washers can be produced with a number of locking elements, serrated locking has been taken to a high level of sophistication.  Products

SET SCREW        

The term is used to describe a full threaded hexagon bolt in the UK described as a tap bolt in the USA  Product

The term is also used to describe 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 body 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 drive 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:


  • SET SCREW CONE POINT   Cone - Pointed end, this type of point generates the highest torsional holding power and is typically used for a permanent connection. Product 
  • SET SCREW CUP POINT    Cup - Hollowed end, is the most commonly used point style. Used when the digging in of the point is not undesirable, leaves a ring mark. Product 
  • SET SCREW DOG POINT    Dog - Flat end with the threads stopping short of the end with the end fitting into a locating hole. Product   Full Dog Point 
  • SET SCREW FLAT POINT   Flat - Cause little damage to the shaft and are used when frequent adjustment is required. Product 
  • SET SCREW KNURLED CUP POINT   Knurled cup point is a cup point with knurls' cut in the outside of the cup for improved locking. Product 
  • SET SCREW OVAL POINT   Oval - Rounded end that is typically used when frequent adjustment is required. The oval end reduces indentation damage. Product  
  • SET SCREW THRUST POINT    Thrust point is a rounded point the allows movement at the base of the set screw. Product 
  • SET SCREW TIP   Tips at the end of the set screw are in various materials and are designed to reduce damage. Product  
  • SET SCREW PLUNGER POINT    Set screw containing a spring and ball bearing  Product 



Similar to a fan disc washer the internal or external teeth lock onto two faces creating a locking action.  Product


The non threaded portion of a bolt between the head and the threaded portion.


A security tamperproof bolt, the drive snaps off when the required torque has been achieved.  Product


Can be a fine threaded Thin, Castle or Slotted nut for use with hand shears. 

Can also be a nut that has a head that shears off at an arranged load to create a security fastener. Product


The stress required to produce fracture in the plane of a cross section, the conditions of loading being such that the directions of force and of resistance are parallel and opposite although their paths are off set a specified minimum amount. 


This coating is intended for medium to long term protection To accommodate these thicknesses on a bolt thread the nut thread must be over-tapped. Details


A thin spacer of sheet metal or washer used for adjusting.  Product


A threaded fastener with a plain, precision machined, shank that is used for location purposes, also called stripper bolts. They are typically used for pulleys and linkages. Product


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.


The head, of a bolt or screw, which has one or more slots across the top to fit a slotted screw driver.  Product


A hexagon nut having opposed slots at the end opposite to the bearing face which are perpendicular to the axis, designed for insertion of a cotter pin to secure the nut in place when used with a drilled fastener. Slotted hexagon nuts serve the same objective as castle nuts.  Product  


Manufacturer of wavy layered spring washers that act as washers or springs.  Smalley


Spring retaining rings for bore and shaft the rings are without eyes  Product


This all metal locking nut is a prevailing torque nut.   Product


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.



  • METRIC     A screw with a cylinder head, usually with an internal hexagon forged or broached drive. Used where strength is required and is typically made from high strength steel, grade 12.9 in Metric also available in 10.9 and 8.8 grades. Also generically referred to as an Allen screw   Product
  • UNIFIED    American INCH 1960 series produced to ANSI B 18.3 with UNC, UNF and number series threads   Product



  • IMPERIAL  British INCH socket screws are produced with BSF, Whit and BA threads to BS2470   Product



Internal hexagon drive button head screw generally in grade 10.9 also available with a flange    Button 


Internal Hexagon drive countersunk head screw generally in grades 10.9, 8.8 and branded 12.9; call a flat head in the USA. Available in all thread types   Product


Similar to a normal socket cap screw but with a smaller head, available in all thread types. Metric are produced to two standards  DIN7984  DIN6912

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.


An alternative name, used by some manufacturers, for snug torque.


Pieces designed to prevent crushing of sandwich-type and other lightweight structural materials at fastening points.


A range of specialist retaining rings for shafts and bore.  Product


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 concluded that this type of lock washer has limited ability to lock. This type of washer is sometimes called a spring washer. spring lock washer or sometimes a split lock washer. Product


Also described as a split cotter pin and gets confused with a spring tension pin that is also described as split pin. Split pins are used with castle nuts to lock the nut through a hole in the shank.

Used as a locking device in clevis pins, castellated nuts and slotted nuts with drilled bolts. Product


A small rivet having a split end for securing by spreading the ends also called a Bifurcated rivet  Product


A hardened coil wire having a slightly trapezoidal section after forming which functions as a spring take-up device to compensate for developed looseness and the loss of tension between the parts of an assembly and to prevent the nut from turning.  

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 concluded that this type of lock washer has limited ability to lock. This type of washer is sometimes called a spring washer. spring lock washer or sometimes a split lock washer. Product 


Coiled spring dowels are called spring pins, tension pins, slotted spring pins or roll pins. Spring tension pins are single coil with a single slot, Roll or Spiral ® pins are multi coiled spring tension pins   Product


Truly an “engineered-fastener”, the Coiled Pin is available in three “duties” to enable the designer to choose the optimum combination of strength, flexibility and diameter to suit different host materials and application requirements. The Coiled Pin distributes static and dynamic loads equally throughout its cross section without a specific point of stress concentration. Further, its flexibility and shear strength are unaffected by the direction of the applied load, and therefore, the pin does not require orientation in the hole during assembly to maximize performance.

In dynamic assemblies, impact loading and wear often lead to failure. Coiled Pins are designed to remain flexible after installation and are an active component within the assembly. The Coiled Pin’s ability to dampen shock/impact loads and vibration prevents hole damage and ultimately prolongs the useful life of an assembly. Product


Spring sheet metal clips used to put a thread on sheet plate, used in conjunction with self tapping screws and machine screws   Product


A plain, truss or oval head bolt with a square neck to prevent rotation  Product


In the fastener world it is described as A2, A4, 18/8, 18/10, 304 and 316 for more detail and corrosion information


Push nut is a generic term for Starlock®  and similar brands of push on fastener for plain shafts. Push nuts eliminate costly threading and grooving operations, facilitate rapid assembly, and once fixed in position, are so secure that they cannot be removed without destruction.  Product

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.


An all metal locking nut using a stainless steel deflecting locking insert   Product


The Step-Lock Bolt 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. The thread form performs well when tested on a transverse vibration test machine. However manufacturing difficulties may prevent its widespread adoption.


A term used to describe a locking nut which has a prevailing torque. The nylon insert nut family fit into this category both metric and Inch.

Also the Prevailing torque all metal nuts of Aerotight, Anco, Binx, Cleveloc, Centre lock, Flexloc, Philidas, Snep, Staytite, Stover, Toplock and Vargal are all considered stiff nuts.


The Stover® and Coneloc® are 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. Product


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:   

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.   

The last numeral approximates 1/10 of the ratio expressed as a percentage between minimum yield stress and minimum tensile stress.   

Fasteners with a property class of 8.8 have 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.  More detail


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. 


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.


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. Following the introduction of HSFG less structural bolts are used  


A fastener which is threaded at both ends with an unthreaded shank in between. One end called the metal end, often has a thread tolerance which results in more thread interference and is secured into a tapped hole, the other is used with a nut.  Product 


The Pozidriv® was developed by Phillips to give improved engagement and driver grip to that available from the Phillips® drive (four radial head marks). GKN in the UK subsequently developed the Pozidriv into the Supadriv® (two radial head marks) to provide even more driver engagement, Supadriv® did not, become widely accepted in the market.


A symmetrical thread is one which has both flanks of the thread profile inclined at the same angle.





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