Glossary of Fastener Terminology

H

Hardenability: In a ferrous alloy, the property that determines the depth and distribution of hardness induced by quenching.

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.

Head: The head of a fastener is the enlarged shape performed on one end of a headed fastener to provide a bearing surface:

Binding Head: The binding head has a rounded top surface, slightly tapered side surface and a flat bearing surface, a portion of which is sometimes undercut adjacent to the shank.

Button Head: A button head as applied to threaded fasteners has a low rounded top surface with a large flat bearing surface.

Fillister Head: The fillister head has a rounded top surface, cylindrical side surface and a flat bearing surface.

Flat Fillister Head: The flat fillister head has a flat top surface, cylindrical side surface and a flat bearing surface.

Head Angle: The head angle is the included angle of the bearing surface of the head.

Head Diameter: The head diameter is the diameter at the largest periphery of the head.

Head Eccentricity: Head eccentricity is the amount that the head of a fastener is eccentric with the fastener body or shank.

Head Height: For a flat bearing surface head, the head height is the overall distance, measured parallel to the fastener axis, from the extreme top to the bearing surface. For a conical bearing surface head, the head height is the overall distance, measured in a line parallel to the fastener axis, from the extreme top to the intersection of the bearing surface with the extended thread major diameter cylinder on a threaded fastener or with the shank on an unthreaded fastener. For flat and oval undercut heads, it is this distance measured to the intersection of the bearing surface with the undercut. For oval heads and undercut oval heads, the overall distance is referred to as total head height.

Head Length: For rectangular or irregular shaped heads, the head length is the distance along the longest axis of the head, measured in a plane perpendicular to the axis of the fastener.

Head Taper: Head taper is the angle formed by the side or sides of the head and the axis of the fastener. This is not applicable to conventional countersunk heads and should not be confused with head angle.

Head Width: The head width is the distance across opposite flats of hexagon, square, or twelve-point heads measured in a plane perpendicular to the fastener axis. For rectangular or irregular shaped heads, the head width is the distance along the narrowest axis of the head measured in a like manner.

Headed Fastener: A headed fastener is a fastener having one end enlarged or pre-formed.

Headless: A headless threaded fastener is a fastener normally having a slot, recess, or socket in one end.

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.

Hexagon Head: The hexagon head has a flat or indented top surface, six flat sides, and a flat bearing surface.

Hexagon Washer Head: The hexagon washer head is a washer head upon which a hexagon head is formed.

Header: A header is a specialized form of press.

Heading: Heading is a manufacturing process involving the use of a header. This process may or may not involve upsetting or extruding. A part made from wire below the recrystallization temperature is said to be cold headed whereas parts made from wire above the recrystallization temperature are said to be hot headed.

Headless Fastener: A headless fastener is a fastener, either threaded or unthreaded which does not have either end enlarged.

High Strength Fastener: A high strength fastener is a fastener having high tensile and shear strengths attained through combinations of materials, work hardening, and heat treatment.

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.

Hot Dip Galvanizing: Hot dip galvanizing is the process of immersing the parts to be coated in a bath of molten zinc.

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.

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