Assembly Torque (in-lb) - relates assembly torque to clamp load where:K = torque coefficient (0.15 for clean & oiled threads but can range between .05 and 0.35 based on thread material and condition)D = nominal bolt diameter (in)W = target bolt tension or induced bolt clamp load The range is calculated as plus or minus 20% of the range midpoint. The range midpoint is calculated as 75% of the tensile strength (proof load) times the tensile stress area (thread engagement) in square inches.

Bearing Surface: Supporting or locating surface of a fastener which seats against the part it is fastening

Body: (also known as Grip) The unthreaded portion of the shank

Bolt: An externally threaded fastener which requires a nut to secure fastened joint

Case Hardened: Heat treated fastener in which the surface is harder than the core.

Chamfer: A beveled edge on a fastener. The chamfer angle is measured from the normal to the axis of the fastener.

Clamp Load: The total load across the joint interface in service. This may vary during service life.

Cracks: A clean, crystalline fracture which passes through or across the grain boundaries without inclusion of foreign elements.

Elongation: The stretching of a fastener along its axis.

Fatigue strength: The ability of a fastener to resist fracture when subjected to cyclic variations in stress.

Fillet: Concave junction at two intersecting surfaces of a fastener.

Fit: The resulting range of tightness which may occur due to the application of tolerances and allowances in the mating joint members.

Grip: See Body

Flank Angle: see Thread Profile

Induction Hardened: A heat-treated fastener that has undergone a selective hardening process, using induction coils, to further strengthen a part of the fastener

Lead Thread: The thread length from where it starts to where it becomes full-sized. This distance is usually one-half the fastener diameter.

Length of Engagement: The length of full-sized fastener threads that engage in the nut material. The length of the lead thread is not counted in the length of engagement, since its reduced size minimizes any performance benefits. The length of engagement is usually expressed in relationship to the nominal diameter of the screw (e.g. 2 to 2-1/2 diameters of engagement).

Load Flank: The top portion of each thread which applies the preload or load pressure to the mating threads.

Major Diameter: For an external thread, it is the thread's outside or largest diameter.

Minor Diameter: For an external thread, it is the thread's inside or smallest diameter.

Nominal Diameter: The major diameter of a screw, or, in tri-round fasteners, the "C" dimension.

Pitch Cylinder: A volume parallel to the fastener axis whose diameter is equidistant between the major and minor diameters

Pitch Diameter: On straight threads, the diameter of the pitch cylinder.

Preload: The initial load put on a fastener once assembly is complete.

Proof Load: Quality control check to verify that the yield point is above a specified load.

Radial Stress: Forces that propagate from the fastener towards the outside diameter of the joint.

Rockwell Hardness Test: Test designed to measure the hardness of the fastener, based on an alphabetical-numerical scale. The higher the number, the harder the fastener. Rockwell tests are utilized to test for decarburization and carburization and to determine the amount of resistance to permanent deformation during the testing procedure. They also assure that heat treating was performed to specification. ASTM standards require socket screw products meet specific Rockwell Hardness Standards. Socket screws are typically in the "C" scale, which is the hardest Rockwell designation, but the scale designation is dependent on the size of the socket screw.

Root: The base of the V thread. This is the weakest point on a fastener because it has the smallest cross sectional area.

Runout Threads: The thread section that is between the last scratch of thread and the fillet or body.

Screw: An externally threaded fastener which does not require a nut to secure the fastened joint.

Seams: A narrow, non-crystalline discontinuity, which is usually inherent in the raw material. Seams are usually straight or smooth-curved line discontinuities running parallel to the product axis.

Shank: That portion of a headed fastener that lies between the head and the point.

Shear: Force that tends to divide an object along a plane parallel to the opposing stresses.

Shear Strength: Resistance to transverse loading. Transverse loads should only be applied to a dowel pin or to the unthreaded section of a screw; otherwise, deformation will occur. Shear strength is measured in terms of pounds or kilonewtons.

Socket Depth: Distance measured parallel to fastener axis from the top of the head to the extreme end of the recess

Tensile Strength: The amount of load a fastener can withstand without failure. Expressed in pounds per square inch or megapascals (MPa).

Tensile Stress Area: Selected area or areas used to calculate the tensile strength of an externally threaded fastener, so the fastener strength is consistent with the material strength. It corrects for the notch and helix effect of the threads and is a function of the pitch and minor diameters.

Thread Engagement: The amount of thread tooth that is filled by the application material. This measurement is usually expressed as a percentage and is used to determine optimal hole size.

Thread Profile: The angle between the flank of the thread and a line perpendicular to the axis.

Through Hardened: Heat treated fastener with uniform hardness from the surface to the core.

Torsion: Twisting force applied to a fastener

Yield Strength: Measures the resistance of material to plastic deformation. When a fastener is stretched, yield strength is the point where the fastener will not return to its original length following testing. It is measured in terms of pounds per square inch (psi) or megapascals (MPa). Yield strength is often determined by the offset method illustrated to the left.