GLOSSARY OF FASTENER TERMS ALLOWANCE commercial products such as machine screws, bolts, The prescribed difference between the design size nuts, and studs for most interchangeable equipment (maximum material) and the basic size. parts and structural applications. It is estimated that TECHNICAL over 90 percent of inch fastener series in Canada and BASIC PROFILE the United States have class 2A and 2B threads. The cyclical outline, in an axial plane, of the permanently established boundary between the Class 3A and 3B provinces of the external and internal threads. All Class 3A and 3B threads have no specific allowance deviations are with respect to this boundary. and are manufactured to restrictive tolerances. These classes of threads are intended for exceptionally high- BASIC SIZE grade commercial products such as socket cap screws, That size from which the limits of size are derived by set screws, aerospace bolts and nuts, and connecting the application of allowances and tolerances. rod bolts where close or snug fit for precision is essen- BEARING SURFACE tial, as well as in applications where safety is a critical The bearing design feature. surface is the CLEARANCE FIT supporting or The maximum material condition clearance between locating surface mating assembled parts. of a fastener with respect to the part which it fastens (mates). COLD WORKING The loading of a fastener is usually through the bearing Cold working is the plastic deformation of metals at surface. temperatures below that which will cause recrystalliza- tion. This cold working is accompanied by an increase BODY DIAMETER in strength and hardness, called work hardening, and The body diam- a decrease in ductility. The cold working effects of eter is the diam- forming bolt and screw heads, of extruding bolt shanks, eter of the body and of roll threading increase strength values, often of a threaded considerably. fastener. CONE POINT CHAMFER A cone point is The conical surface at the starting end of a thread. a sharp conical CHAMFER POINT point designed A chamfer point to perform per- is a truncated forating or align- cone point, the ing functions at assembly. end of which is COUNTERSINK approximately flat and perpendicular to the fastener Flare or bevel at the hole end. axis. These points on threaded fasteners generally have point included angles of 45 to 90 degrees and a ELEMENT point diameter equal to or slightly less than the minor Elements of a thread are flank angle, root, crest, pitch, diameter of the thread. This point is intended to facili- lead angle, surface finish, major, minor, and pitch tate entry of fasteners into holes at assembly. diameters. CLASS OF THREAD EXTERNAL THREAD An alphanumerical designation to indicate the standard A screw thread formed on the outside of a cylindrical grade of tolerance and allowance specified for a thread. surface. Class 1A and 1B FASTENER Classes 1A and 1B are very loosely toleranced, there- A fastener is a mechanical devise for holding two or more fore, this class produces the loosest fit; that is, the bodies in definite positions with respect to each other. greatest amount of play in assembly. An allowance is applied to the external thread in class 1A and 1B. This FULL OR NOMINAL class is ideally suited where quick and easy assembly DIAMETER BODY is of prime design concern. A full or nominal diameter body is a body the diameter Class 1A and 1B is standard for only coarse and fine of which is generally within the dimensional limits of the threads with sizes of 1/4 inch and larger. Very few major diameter of the thread. Sometimes referred to as fasteners produced in Canada and the United States “full size body”. have this class of fit. GIMLET POINT A gimlet point is Class 2A and 2B a threaded Class 2A and 2B is the most common thread class cone point usu- specified for inch series fasteners. Class 2A for ally having a external threads has an allowance while class 2B for point angle of internal threads does not. Class 2A and 2B threads 45 to 50 degrees. It is used on thread forming screws offer excellent value of fit when considering manufac- such as Type “AB” tapping screws, wood screws, lag turing conveniences and economy, against fastener screws, etc. performance. This class offers a good grade of “PROFESSIONALS SERVING PROFESSIONALS” 1 GLOSSARY OF FASTENER TERMS GRADES OF FASTENERS PHYSICAL PROPERTIES In the SAE system, grades are designated by Physical properties are the properties defining the numbers from 1 through 8. These numbers have no basic characteristics of the material or fastener. quantitative relationship to strength properties, except PITCH that increasing numbers represent increasing tensile The pitch of a thread having uniform spacing is the strengths. Decimals after whole numbers indicate the distance, measured parallel to its axis, between same basic properties, with variations in either corresponding points on adjacent thread forms in the material or processing treatment. The ASTM grades same axial plane and on the same side of the axis. are designated by their document number. Some of Pitch is equal to the lead divided by the number of TECHNICAL the ASTM standards describe two or more types or thread starts. grades with the difference being either a variation of POINT material – for example, ASTM A325 Types 1, 2 and 3 The point of a – or modified properties of the same material – ASTM fastener is the A307 Grades A and B. configuration HEAD of the end of The head of a the shank of a fastener is the headed fastener or of each end of a headless fastener. enlarged shape PROOF LOAD preformed on Proof load represents the maximum useable load limit one end of a of the fastener for many design-service applications. headed fastener to provide a bearing surface. Proof load is commonly defined as the tension applied load which the fastener must support without evidence HIGH STRENGTH FASTENER of any deformation. Often, proof load and yield A high strength fastener is a fastener having high strength are interpreted as being the same. tensile and shear strengths attained through Note: Proof load is a force measurement. The units combinations of materials, work-hardening and heat are pounds or newton. Yield strength is a stress treatment. measurement. The units are PSI or MPa. The stress at the proof load is 90-93% of the yield strength. HEAT TREATMENT The strength and ductility of metals can be significant- REFERENCE DIMENSION ly altered by various types of heating operations. Heat A dimension usually without tolerance, used for treatment refers to any of a number of operations information purposes only. It does not govern involving the heating of the parts in appropriate production or inspection operations. A reference furnaces, gas fired or electric, often with controlled dimension is derived from other values shown on the atmosphere, and the subsequent cooling at controlled drawing or on related drawings. rates. In the manufacture of fasteners the strength and ductility of the parts can in this way be adjusted, with- RIGHT-HAND THREAD in limits, to fit the particular application. A thread is right-hand if, when viewed end-on, it winds in a clockwise and receding direction. A thread is INCOMPLETE THREAD considered to be right-hand unless specifically Threads having crests or roots not fully formed. Incom- indicated. plete threads occur at the end of pointed externally ROOT threaded products, at countersinks in the faces of That surface of the thread that joins the flanks of adja- threaded holes or nuts, and at thread runouts where cent thread forms and is immediately adjacent to the the threaded section blends into the unthreaded cylinder from which the thread projects. shank. SHANK THREAD The shank is A thread is a portion of a screw thread encompassed that portion of by one pitch. On a single-start thread it is equal to one a headed fas- turn. See threads per inch and turns per inch. tener which lies between MAJOR CYLINDER the head and An imaginary cylinder that would bound the crests of the extreme point. an external straight thread or the roots of an internal straight thread. SHEAR STRENGTH Shear is transverse rupture. It is caused by a pushing MAJOR DIAMETER or pulling force at 90° from the axis of a part. Thus, a On an internal thread, the diameter at the root and on rivet used as a pulley axle will shear if the load on the an external thread the major diameter is the diameter pulley exceeds the shear value of the rivet. Shear at the thread crest. strengths generally are 60% of the specified minimum tensile strength. MINOR DIAMETER On an internal thread, the diameter at the crests and SHOULDER on an external thread, the diameter at the root. A shoulder is an enlarged NOMINAL SIZE portion of the The designation which is used for the purpose of body of a general identification. The basic major diameter of a threaded fas- threaded fastener is often referred to as “nominal size”. tener or shank of an unthreaded fastener. 2 “PROFESSIONALS SERVING PROFESSIONALS” GLOSSARY OF FASTENER TERMS TENSILE STRENGTH TOLERANCE Tensile strength, or ultimate strength, is that property The total amount of variation permitted for the size of of a material which determines how much load it can a dimension. It is the difference between the maximum TECHNICAL withstand without breaking. It is calculated by limit of size and the minimum limit of size. determining the tensile stress corresponding to the maximum load observed in a tension test. Cold UNDERHEAD FILLET working raises the tensile strength of most metals and An underhead alloys. Heat treatment can often be used to increase or fillet is the fillet reduce the tensile strength.