I monitor a number of multi-stage Hoffman blowers and flagged one as "bearings going bad" recently. Blower was replaced with a rebuilt "spare" Friday morning and I went and carefully aligned it, completely reshimming the motor and blower, removing a little soft foot, replacing the Omega coupling, etc. About as good as it could be done I think. As I recall, the blower turned fairly freely while I was aligning it. At least I didn't notice any problem. (I am not sure of the actual size of these blowers... about 40 hp I think)

Started it up, and it was smooth, except for the not unusual 1x vibration. The max peak acceleration was 0.8 g's. But the outboard bearing temperature started climbing. When it got to 235F I started getting concerned (these sometimes run as high as 200F). I have an Exergen infrared thermometer with a pencil probe, and I was able to work it into the shaft between the bearing and the blower housing (after checking with a strobe for protrusions). I was shocked to see a shaft temperature of 400F. Soon after I started hearing a noise from what I assume is the seal where the shaft exits the housing (never seen one apart, or a drawing... there is a seal, right?) The bearing temperature finally stabilized at about 235F but the noise seemed to be getting worse. We shut it down.

This morning they put the old blower back into place, and I dutifully aligned it. Started it up and after 45 mins. the bearing temp stabilized at 115F.... of course the bearings are as rough and noisy as before.

I found the "spare" (that had the hot bearing) and it spins freely... no indication there is a problem.

I am certain there was no soft foot on the "hot" blower. I was very meticulous adjusting the coupling hub spacing. After it was aligned I took the coupling bolts out of the blower-side hub... the hub holes were perfectly aligned with the coupling ring bolt holes.

I am totally baffled. Have any of you seen this before?

Certainly not my choice of coupling for this service; but not the issue here.

Your alignment - "Hot Alignment" or rather 'cold offset'? Did you do shaft run-outs? And now for the plumbing - how was that? Were there any operating parameters off spec?

I had an instance in the past where a customer locked both bearings axially, not allowing for thermal growth of the shaft and we had the same kind of problem. Like you, I haven't been inside a Hoffman but I know one bearing is fixed and the other floats axially. I don't remember if the fixed bearing is shimmed (and they shimmed both ends) or if the housings are machined differently (and they used a wrong bearing housing in the rebuild)but it's something you can look for.

Besides checking for DE side fan bearing for locating and NDE for non locating.I think you should inspect the bearing clearnace(suffix c2,c3,N etc) is same as earlier.Was the bearing clearance checked after mounting.If this is a tapered bore bearing ,is the bearing clearance as per recommended values after tightening the check nut.

My thinking is it has to be a seal rub because the shaft was so hot (400F) compared to the bearing temp (250F). The heat in the bearing is coming from the shaft. I think it would violate some law of physics if the 250F bearing was causing the shaft to run 400F. Right?

Tend to agree with you Rusty. For the shaft to be that hot I would think seal rub or the shaft is spinning on the bearing. The only other thing would be ig the bearing was over packed with grease. But this would produce a hot bearing and not a shaft that is running even hotter.

Rusty, wouldn't the FAN housing temperature in the shaft area be also elevated if it was a seal rub? Did you check it at that time?

On another hand, a fan housing seal likely made out of felt won't generate that much heat.

David

This message has been edited. Last edited by: David_G,

I have seen a pump with lip seal rubbing, and yes, the shaft was much hotter than the housing as Rusty said.

But I have also seen many machines without rub where the shaft is hotter. Take a look at the powerpoint attachment to my post dated 07 January 2008 04:56 PM here:

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...071078753#1071078753

These machines all have light duty felt seals for the grease cavity which I don't think can cause heat like spring-loaded lip seals.. I just inspected machine #11 (since these images) and the felt was pristine - no rub. I'm not sure if it's included in the attachment but #11 shaft temperature spiked up to 170F (150F on the housing) and it was definitely coming from the bearing (we tore apart the bearing and found the smoking gun ... heavily spalled inner race and metal particles spread throughout the bearing and cage causing increased friction).

Why is the shaft be hotter? Good question. Here's a theory - perhaps the interference fit to shaft transmits heat much more efficiently than the shaft than the loose fit to the housing.
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Edited to add: For the majority of the motors linked above where shaft temperature is only slightly higher than the housing, you might chalk it up to heat transmitted from heating in the rotor area above which runs hotter.

But here is image of a temperature spike on that motor 11 with the damaged inner race and metal particles in the grease, where we know the heat definitely started in the bearing. Still shows up mostly on the shaft.

This message has been edited. Last edited by: electricpete,

CW11_LOWERTempSpike.ppt (247 Kb, 20 downloads)

The Hoffman's that I monitor are moving very hot gases (near 600F I believe). If yours are moving hot gas, the heat from the gas will heat the wheels and shaft to temperatures that are higher than the bearings. In my case, the discharge end of the blower is much hotter than the suction end (I guess due to the heat of compression as the gas moves through the stages0. The difference is substantial and has to be allowed for during alignment.

Pete, if the bearings showed any indication of being 'bad' I can see how the bearing might heat the shaft. But these were new bearings, running < 1g peak acceleration, with about 40 minutes run time. I also "listened" to them and they sounded pristine to me.