Glossary of Fastener Terminology

A

Acorn Nut: A nut (so-called because of its shape) that has a domed top so that it prevents contact with the external thread.

Actual Fit: The actual fit between two mating parts is the relation existing between them with respect to the amount of clearance or interference which is present when they are assembled.

Actual Size: An actual size is a measured size.

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.

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.

Alloy Steel: An alloy steel is a steel containing elements other than carbon which have been added to obtain definite mechanical or physical properties, such as higher strength at elevated temperatures, toughness, etc.

Anaerobic Adhesive: An adhesive which hardens in the absence of air, such adhesives are often used as a thread locking medium.

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.

Angularity: Angularity is the angle between the axes of two surfaces of a fastener.

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.

Annealed: A fastener is considered in the annealed state when it has been heated and cooled to make it soft-that is, free of hardness caused by working or previous heat treatment.

Anodizing: Anodizing is the formation of an oxide film on the surface by means of an anodic treatment. This is commonly used on aluminum.

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.

 ASME: The designation of a society of writing specifications. American Society of Mechanical Engineers United Engineering Center Three Park Avenue, New York, NY 10016-5990

ANSI: The designation of a society of writing specifications. American National Standards Institute, Inc. 25 West 43rd Street, (between 5th and 6th Avenues), 4 floor, New York, NY 10036

ASTM: The designation of a society for writing specifications American Society for Testing and Materials 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959 USA

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