The axial tension in a fastener after installation, before service loads act on it. A tightened bolt behaves like a stiff spring holding the parts together, and enough preload is what prevents joint slip, gasket leaks, self-loosening and fatigue. A common target is about 75 percent of proof load for joints that will be taken apart and up to 90 percent for permanent ones, and torque control alone typically scatters actual preload by 25 percent or more either way.
Bolts keep backing off on the shaker table because there's no preload in them, the paint under the heads is eating it.
When bolts keep loosening or breaking, the usual root cause is too little preload, not too weak a bolt.
Also heard as
- bolt tension
- initial tension
- pretension
Do not mix it up with
- Clamp load: Preload is the tension in the bolt; clamp load is the compression on the joint, equal to preload only until an external load acts.
Related terms
Clamp load
The compressive force a tightened fastener puts on the joint members, holding them together in service.
Torque-tension
The relationship between the torque applied to a fastener and the preload it produces, dominated by friction.
Nut factor
The empirical constant K in T = KDF that lumps all the friction and geometry between torque and tension.
Self-loosening
A tightened nut or bolt turning loose by itself, usually because sideways slip in the joint lets it walk off.
Bolt stretch
The elastic elongation of a bolt under preload, measured directly as the most accurate check of tension.