Preload

Preload is the initial tensile force placed in a fastener when it is tightened. It clamps joined parts together and helps keep the joint secure under service loads.

Bolt tightening: The process of applying torque or another controlled method to tension a bolt and clamp joined parts. Tightening stretches the fastener and creates its initial clamping force.
Bolted joint: A connection in which a bolt clamps two or more parts together. Preload is the initial clamping condition that a bolted joint is designed around.
Turn-of-nut method: A fastener-tightening method that controls nut rotation after the joint is snugged. It estimates preload through bolt stretch rather than relying only on measured torque.
Fatigue failure: Failure caused by repeated or fluctuating loading, often after cracks initiate and grow. Adequate preload can reduce the variation in bolt stress caused by cyclic external loads.
Torque: A twisting moment that tends to rotate an object around an axis. Torque is commonly applied to generate preload, though friction makes the relationship uncertain.
Torque wrench: A tool that applies a specified torque to a fastener. Torque wrenches provide a practical, indirect way to control tightening preload.
Bolt load cell: A sensor that measures tensile force in a bolt or fastener. It measures fastener force directly, unlike torque-based estimates of preload.
Self-loosening of threaded fasteners: A loss of tightening in a threaded connection caused by vibration or cyclic motion. Insufficient clamping force makes transverse movement and progressive loosening more likely.
Bolt elongation: The increase in a bolt’s length caused by tensile loading. Elastic elongation is the physical deformation that accompanies tensile preload.
Hydraulic tensioning: A method that stretches a bolt hydraulically before its nut is tightened. Directly stretching the fastener can produce more predictable preload than torque alone.

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