Are there oil leaks ?
Check the oil leakages and fix the leakage soon.
Is dirt or your product pilling up on the outside of the drive?
The outside of the gearbox needs to be clean to allow heat to escape.
Is the unit shaking ?
Look for misalignment, bent shaft, loose anchor bolts or broken teeth.
Listen to the unit.
Is there any noise ?
Don't let any one tell you that “Singing gears are happy gears”.
Howls, Whines and thumps should be invested at the next shutdown.
Touch the drive housing.
High temperature – every 18°F increase in temperature shortens Oil life by half. Check oil level.
Too little or too much oil can both cause high temperature.
Keep the outside clean. Check the vibration.
Find the cause and fix it. It could be gear teeth, couplings, bearings, Misalignment or something in the motor or drive machine. It's not likely to go away by itself, though.
Even if you can't shutdown or spend time to solve the problem right away, make enough notes to remember that something has to be done at the next opportunity. Try to record the severity of the noise or Temperature problem so that you can repeat inspection in a shift or the next day under the same operating condition if the symptoms are next day under the same operating condition if the symptoms are worse Then you'll know that you don't have much time to act. Things like Knocking noises and high temperature need to be corrected right away.
Trust your sense to avoid unplanned shutdown.