Glossary of Fastener Terminology

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Scale: Scale is an oxide of iron sometimes formed on the surfaces of hot headed or forged fasteners.

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 However this definition has largely been superseded to avoid confusion over the difference between a bolt and a screw.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 Stock: Screw stock is metal in the form of wire or rod which is used for making screw machine parts. Usually it is of a free machining type of material.

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.

Seams: Inherent discontinuities in a raw material that run longitudinally. They are folds in the material, not fractures at the grain boundary.

Semi-Finished Fastener: A semi-finished fastener is a fastener made to the same basic dimensions as a finished fastener but having greater tolerances on most dimensions and only the bearing surface and threads finished.

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: The shank is that portion of a headed fastener which lies between the head and the extreme point.

Shank Diameter: The shank diameter is the diameter of the shank of an unthreaded fastener. The diameter of the unthreaded portion of a threaded shank is termed the Body Diameter.

Shank Length: Shank length is the length of shank, measured parallel to the axis of the fastener.

Shaving: Shaving is a cutting operation in which thin layers of material are removed from the outer surfaces of the product.

Shear Fastener: A shear fastener is a fastener whose primary function is to resist forces which tend to shear it.

Shoulder: A shoulder is an enlarged portion of the body of a threaded fastener or shank of an unthreaded fastener.

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.

Sliver: A sliver is an irregular shaped piece of metal clinging loosely to the finished fastener.

Slot Depth: The slot depth on a headed fastener is the distance measured parallel to the axis of the fastener from the highest part of the head to the intersection of the bottom of the slot with the head or bearing surface. The slot depth on a nut or headless fastener is the distance measured parallel to the fastener axis from the top surface to the extreme bottom of the slot.

Slot Eccentricity: Slot eccentricity is the amount that a slot in a slotted head is eccentric with the body of the fastener.

Slot Width: The slot width is the distance measured in a plane perpendicular to the axis of the fastener over the intersection of the sides of the slot with the head surface of a headed fastener or top surface of a nut.

Slotting: Slotting is the process of forming or cutting the slot on the head of a fastener during either the primary or secondary operation.

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 Depth: the socket depth is the distance measured parallel to the fastener axis from the intersection of the socket with the head surface to the extreme end of the socket. In socket head screws, the effective socket depth is most often specified as “Key Engagement’ which is the distance from the intersection of the socket with the head surface to that depth to which the key or wrench will penetrate, measured in a like manner.

Socket Diameter: the socket diameter is the diameter measured in a plane. perpendicular to the axis of the fastener over the intersection of the outermost extremities of the socket with the head surface.

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).

Socket Width: The socket width is the distance measured in a plane perpendicular to the axis of the fastener over the intersection of opposite sides of the socket with the head surface.

Soft: Soft describes the condition of a fastener which, though made from a material which can be, and normally is, hardened by heat treatment, has been left in the as-fabricated temper.

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.

Special Fastener: A special fastener is a fastener which differs in any respect from recognized standards.

Stainless steel: Stainless steel is a corrosion resistant type of alloy steel which contains a minimum of 12 percent chromium.

Standard Fastener: A standard fastener is a fastener which conforms in all respects to recognized standards.

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.

Stock Fastener: A stock fastener is a fastener which is commercially stocked in quantity by manufacturers or distributors of fasteners.

Strain Hardening: Strain hardening is the increase in hardness, and hence strength, resulting from plastic deformation at a temperature below the re-crystallization range. Sometimes called “work hardening.”

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)

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 stud is a fastener with no head, which has threads at both ends of the shank. Like a screw, one end is inserted into an internally tapped hole and tension is induced by tightening a nut on the other end. If a stud is threaded its entire length and a nut is used on both ends to create tension, it serves the function of a bolt and is then classified as a stud bolt.

Surface Treatment: Surface treatment is any treatment which changes the chemical, physical, or mechanical properties of a surface.

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

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