FAQs

Common Bolting Questions and Answers

How much torque should I use to tighten a specific bolt?

To answer this question we need to know some facts about the application:

• What is the bolt diameter?
• What is the bolt material?
• What lubricant is being used?
• How much bolt load is required?

You can estimate the torque requirement by using these facts in the "Short Formula": T = K x L x D

Why should I lubricate bolts and nuts?

• Lubrication helps prevent galling or seizing when tightening and especially when loosening after being in service.
• Less torque is required to tighten (create bolt load) when a good lubricant is applied. Less work = same results.
• Lubricants help prevent rust and corrosion.

Is tighter always better?

No, bolts are designed into machines for specific reasons. Too much torque and/or bolt load can be as bad as too little.

Does lubricant cause bolted joints to become loose?

No, if the bolt is tightened to the required preload (greater than the dynamic load experienced in service); the lubricated bolt will not vibrate loose.

Why do bolts loosen in service?

• Gasketed joints may loosen if the gasket compresses after bolt preload is applied.
• Vibration may create dynamic loads greater than the preload. This can allow the bolt or nut to turn and loosen.
• Temperature changes during service can cause bolts to "grow" or "shrink" relative to surrounding parts. Bolt load may lessen and allow loosening or leakage.

What is stress?

Stress is force exerted against each other by two surfaces in contact.

What is the short formula?

T = K x S x A x D

What is preload?

Initial clamping force or tension in a fastener.

What is bolt load or bolt tension?

The act of stretching tight or state of being stretched tight.  Stress of a pulling force on a body.

What is torque?

Torque is the measure of the tendency of a force to rotate the body upon which it acts about an axis.