Technical Insight: Why Correct Coupling Bolt Torque Does Not Eliminate Fretting Risk

Page Type: Technical Insight

A coupling flange can develop fretting even when its bolts have been tightened to the specified torque.

The reason is that bolt preload and fitted-bolt geometry may perform different mechanical functions.

This is especially important in precision marine shaft couplings.

Torque Confirms Clamp Load

Bolt torque is primarily used to establish the required preload.

That preload helps maintain contact between coupling faces.

However, some coupling designs also use fitted or reamed bolts to control:

• flange location;
• shear transfer;
• relative movement.

Clearance Can Exist With Correct Torque

A fitted bolt may be tight axially while still having excessive radial clearance in its hole.

During torque reversal:

load direction changes → flange moves microscopically → fitted surfaces contact → fretting develops.

The fastener may remain properly tightened throughout this process.

Ahead/Astern Operation Can Reveal the Fault

Marine vessels that repeatedly reverse propulsion torque can expose marginal coupling geometry faster than equipment operating continuously in one direction.

The trigger is not simply rotational speed.

It is repeated reversal of the torque path.

What to Inspect

Relevant checks may include:

• fitted-bolt diameter;
• reamed-hole condition;
• contact marks;
• fretting debris;
• flange-face condition;
• bolt preload.

Why Increasing Torque Is Not a Repair

More bolt torque does not restore worn hole geometry.

It may also create unnecessary fastener stress.

The diagnostic distinction is:

insufficient clamping

versus

adequate clamping with excessive fitted clearance.

For a practical example, see Marine Coupling Fretting Case Analysis: Why Correct Bolt Torque Does Not Eliminate Fitted-Bolt Clearance.

FAQ

Can correctly torqued bolts still allow fretting?

Yes.

Are all coupling bolts fitted bolts?

No.

Can shaft alignment be correct while coupling-hole fretting occurs?

Yes. These are separate interfaces.