We have a condition on a overhung fan where if we leave one of the motor mounting bolts loose the vibration is considerably reduced. Does anyone know of any reference studies or case histories that would relate to this situation. Our structural guys have nightmares over loose bolts but we (vibe guys) see it as a solution (short term) to a high vibration condition. Any experiences or input ?

Sounds like a classical case of softfoot. Check the motor for softfoot and shim to correct. Exactly what frequency increases when you tighten the bolt? 1x running speed or 2x line frequency?

I have to agree with Don. It sounds like soft foot to me. Could be you have a foot that is not in the same plane as the others or is not parallel with the base. Try taking a mag base dial indicator and check each foot for movement.I've seen soft foot where the shim pack did not get loose when all the bolts were loosened. Required step shimming because of non parallel to base foot.
Regards,

Sounds like a softfoot to me also. Gap the foot with the loose bolt and fill it with the appropriate shim. I've run in to this problem a few times. Good Luck

Sounds like soft foot to me too... but I have also seen a case of foot related resonance. There was no soft foot, but when all four feet were tightened the motor was resonant at running speed. Fixed it by putting a beveled washer under the head of each bolt (changed the stiffness) while letting all four bolts be tightened.

Soft foot is just side effect, real problem is resonance. So by loosing one foot, you are changing stiffness, physical properties of a structure. You should bamp it and try to find what is natural freq or excitation of a structural resonance. Change stiffness or speed.
Personally I don't think it is beg deal leave one foot loose, just remove bolt, because soon or later one of our friends operators will tight it up.
Good luck.
Dragan

Just to add to the other comments, check all 4 corners of the loosened foot with a feeler gauge. It could be an angular soft foot, in which case you'll need a tapered shim, or one of the plastic shims.

quote:

Originally posted by DragonT:
Soft foot is just side effect, real problem is resonance. So by loosing one foot, you are changing stiffness, physical properties of a structure.

Why is it resonance? A soft foot can cause a distortion of the motor itself resulting in a variable gap which can generate high rotor bar pass or high 2x line and have nothing to do with resonance.

quote:

Originally posted by rgf:
We have a condition on a overhung fan where if we leave one of the motor mounting bolts loose the vibration is considerably reduced. Does anyone know of any reference studies or case histories that would relate to this situation. Our structural guys have nightmares over loose bolts but we (vibe guys) see it as a solution (short term) to a high vibration condition. Any experiences or input ?

I didn't make my self very clear in the original posting. I know it is a soft foot condition. What I was looking for is any case histories where the condition was not corrected and the machine was returned to service. The machine in question is a seismic qualified Cat 1 fan in nuclear power service. We are doing an evaluation on why it's perferable to run with 1 of 4 bolts loose with low vibes vice shimming the bolt and running with high vibes.
rgf

quote:

Originally posted by rgf:What I was looking for is any case histories where the condition was not corrected and the machine was returned to service. The machine in question is a seismic qualified Cat 1 fan in nuclear power service. We are doing an evaluation on why it's perferable to run with 1 of 4 bolts loose with low vibes vice shimming the bolt and running with high vibes.rgf

Well, the case history would have to be looking at the reason the facility approves of this type of practice. I'm in a paper making facility and we would never allow this condition, we would properly shim the motor to eliminate soft foot and use all four hold down bolts. I'm sure the motor designer has given us four hold downs and the right bolt size to overcome the torque generated at maximum HP. The soft foot check is the first step in our alignment procedure and we use tapered shims if necessary to get it out. We have even taken the motor, where practical, and machined the feet.

RGF,
Man I'm surprized they are even condisering continued operation under these conditions. A company call Ludeca sell a plastic shim that you can insert under the foot that has the soft foot and then torque. It will then harden. They are awesome, I have used them alot due to the age and condition of the foundations at the Mill I used to work at. If angular soft foot is truely your problem these shims will fix it.

Kelly