Tightening technique glossary

Clamping force

Compressive force that the bolt exerts on the joint materials, also called clampload.

Preload

Tension force that is generated in the bolt.

Axial load

Applied force in the same direction as the axis of the bolts.

Transverse load

Applied force to the joint that is perpendicular to the bolts.

Friction

Resistant force to materials moving against each other.

Torque

Resistant force to materials moving against each other.

Prevailing torque

The amount of torque required to run down a screw or bolt, typically associated with a lock-nut or thread forming screw.

Residual torque

The amount of torque remaining in the joint after tightening.

Static breakaway torque

The amount of torque required to turn the fastener after it has been tightened, usually in the tightening direction.

Relaxation

A loss of clampload in a joint after tightening.

Embedment

A cause of relaxation attributed to high surface pressure in hard surfaces, most apparent in hard joint with multiple hard layers. Relaxation associated with embedment typically stabilizes quickly (less than 0.2 seconds).

Creep

A cause of relaxation attributed to soft materials under high pressure that slowly flow material away from the joint. Can take longer periods of time than embedment losses.

Stress

Force per unit area e.g. Newtons per square millimeter or pounds per square inch.

Strain

Change in length compared to original length.

Elastic area

The region of the angle-torque curve in which the material returns to the undeformed state when applied forces are removed.

Plastic area

The area in which the material deforms permanently.

Yield point

The point separating the elastic from the plastic area.

Snug

The point at which the components of a bolted joint are contacted together and torque starts to build.

Hard joint

The plates and material between the nut and bolt bearing surfaces have a high stiffness. 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 snug.

Soft joint

The plates and material between the nut and bolt bearing surfaces have a low stiffness. This usually has to be tightened by two or more complete turns, after it has been torqued to snug.