When you review the Operations & Maintenance manual received with your new equipment, there is usually a section in there regarding preparation and/or maintenance of the unit when put into storage.  90% of the time you are going to disregard that section because you are not going to put the unit is storage, you're hot to get it up and running and in production!  Next time, read that section.  It could be a real eye opener for you!  

Rotating equipment is considered in storage if left out of operation for more than 30 days.  Most manufacturers' of motors, pumps, blowers, & fans, request you do PM to the units if they are not in operation for a given length of time.  The PM could be simply rotating the shaft once or twice by hand, or it could be much more involved.  Read the storage section!

With blowers and motors, it is most important to rotate the shaft regularly.  The bearings can be damaged if the rotor is not turned to spread the grease and/or oil, throughout the bearing.  You also want to prevent the bearing from "taking a set" when left in a static position for a long time.

Oftentimes at WWTP's, they might have 3 centrifugal blowers, while only requiring one to satisfy their air requirements. That means 2 units sit idle all of the time.  Rotating operation on a monthly basis will usually satisfy any storage PM requirements.  But if you only run the unit once or twice a year, and you don't do any "storage" PM to it, you could be shortening the life of your equipment, and contributing to premature bearing failure.

If a ball bearing becomes defective-a small dent in the race, a little rust on a ball, or a space with no lube resulting in steel contacting steel, it can fail in a matter of hours!   I have personally installed bearings and had them fail in less than 30 days.  I like to think it was not a result of my installation procedures, but maybe the stockroom guy dropped the bearing off the shelf, or it fell off a table somewhere.       

So not running equipment, and letting it sit idle for a long time, can be much worse than running it even sporadically.  Don't keep that spare motor for a critical piece of machinery on a shelf without doing the storage PM.   Motors & pumps have a shelf life.  There is not much sense installing a 5 year old "new" motor only to have it fail within a few days of operation.  As you can see, the consequences of inadequate storage PM can be very costly! 

Invest in PM, it will save you money!