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There two main points of the PF curve:
1. Potential failure is the point where we notice that a tooth is starting to crack or fail
2. Functional failure is the point where the tooth is no longer operating condition
The time between potential failure and functional failure is called the PF interval.
When determining gear inspection task intervals it is important to do these at frequencies much shorter than the P-F interval to enable proper action to take place to either take action slowing the crack propagation or replace the gearing.