75 hp motor problems..2nd time I've seen this, different causes. need oppinions.

[ndk1911]

To start, I had a problem at my previous, and only, job with 40 hp motors snapping the thru bolts...Determined that it was due to the brake on the drive causing excessive vibration on shut down. Changed braking speed and the problem went away.

Fast forward. Just started a new job 2 months ago. I have an idiot plumber "showing me around"." I notice a broken thru bolt on a 75 hp motor and he informs me that he replaces that bolt every 3 months or so. This drive doesn't have a brake set up, and all other components(belts, bearings, etc) seem to be ok. Can a "soft foot" problem create vibration through a belt driven fan that would cause this??

Any input would be much appreciated!

[coilcleaner]

Step away from the plumber.
Definetly some serious vibrations, maybe the belt is way too tight. Mr. plumber might have changed belt sizes and adjusted wrong. Or serious alighnment problem. Did you check the amp draw on motor?

[ndk1911]

Yes I did, but the thing never runs more than 40hz this time of year(or so i'm told). So the amp draw is obviously low. Belts seem to be just about right. I didn't use a tension gauge or anything, but this isn't my first time around the block either. Brought the issue up to my boss and he said nobody ever told him before. This plumber clown is a master bull **** artist.

[hvacker]

If the bolt head is still there I'd check the class in case your plumber uses cheap bolts. It should be at minimum class 5 (sae j429) They have 3 slash marks. Some people over torque all the time. No finesse. That can stress a low shear load bolt. They have no head markings.
I'd think some other bolts would also fail if vibration was present.
I'd also want to make sure the fan isn't riding above it's fan curve causing rough operation.

[duke of earl]

I would manually run the drive from dead stop up through the frequency spectrum slowly and look for a vibration at some particular frequency. All drives will have a jump frequency that will allow the drive to run past that particular frequency in the program.