Glossary

Lubricity in terms of fasteners refers to how easily two surfaces slide over one another when a fastener, such as a bolt, screw, or nut, is tightened. It describes the frictional characteristics of the fastener’s threads and bearing surfaces, which directly affect the relationship between the applied torque and the resulting clamping force (preload) in a joint.

When a fastener is tightened, most of the torque applied—often 80–90%—is consumed in overcoming friction between the threads and under the head or nut. Only the remaining portion is converted into actual bolt tension that clamps the joint. A fastener with high lubricity (low friction) requires less torque to achieve the same clamping force, while a fastener with low lubricity (high friction) requires more torque and may result in inconsistent preload.

Lubricity can be influenced by coatings, platings, or added lubricants. For example, zinc plating, phosphate coatings, PTFE (Teflon), or ceramic coatings may reduce friction, while dry, rusty, or damaged threads increase friction and reduce lubricity. Consistent lubricity is critical in precision applications, such as aerospace, automotive, or pressure vessel fastenings, where preload accuracy determines safety and performance.