Expert systems are commonly used in automated systems for troubleshooting when the expert can not always be there.  Sometimes you can do simple IF - THEN or flow chart types of logic to troubleshoot.  True expert systems allow additional functionality such as probabilities.  For example instead of a diagnosis of "probably  replace bearings" you can add 80% chance bearings, 60% chance out of alignment, 40% chance coupling, 20% mount.