In a plant it so happened that a machine with its shaft and pulley assembly was kept idle for little over three years.
Then one day the engineers decided to run the machine. After two months of running, the pulley came loose on the shaft and started rattling – making just enough noise for the operator to notice it and promptly stop the machine thus averting a nasty accident.
This is a case of fretting corrosion. This happens when things are kept in assembled condition for long without running or components are assembled loosely. The asperities at the contact surface that help to hold two components together are lost; thus loosing the vital grip forcing the components to come loose. This wear process is accelerated in presence of low frequency vibration that usually travel to such joints por assemblies from other running machines. The confirmation of fretting corrosion lies in observing reddish coloured powder in between the closely fitting joint interfaces and assemblies.
The pictures of fretting corrosion as seen in this case are the following:
Ways to manage this failure mode:
1. Take care to assemble correctly
2. Don’t leave a machine idle for a long time.
3. Prevent, as far as possible, low frequency vibrations to travel to a machine.
4. If an idle machine is to be commissioned then take care to inspect the joints and interfaces and replace assemblies as found necessary.
5. May be monitored by Wear Debris Analysis for lubricated joints and interfaces and by vibration monitoring for dry joints and interfaces or simply by visual monitoring.