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Bearing Damage? No.1

Bearing Damage? No.1

(OP)
Any ideas?
Two row self aligning bearing removed from a cantilevered mixer, shown is from the lower row, upper row appears undamaged. Seals were intact, cavity was full of clean grease. Mixer blades were nicked, assumed damage from concrete debris striking the blades.
Thanks
Steve

RE: Bearing Damage? No.1

Perhaps the concrete debris rattled around in the drum for a while.
It looks like one or two big spalls fell off, and then the bearing did an imitation of a rock crusher, reducing them to smaller debris.

Mike Halloran
Pembroke Pines, FL, USA

RE: Bearing Damage? No.1

Need a lot more info like brand of bearings, part number, speed, temperature, how long have they been on....

RE: Bearing Damage? No.1

Can gease be introduced remotely, with the bearing assembled?
"cavity was full of clean grease" could have happened AFTER the problem was cascading out of control.

The damage is very advanced. I think bearing manufacturer's analysts would need all the parts to evaluate, and even then might say something very vague at best.

Are you able to makes some "Predictive Maintenance" vibration measurements on the bearing housings when in service?
Even simple "overall" readings made in velocity or acceleration could help a lot keeping things from getting to this sorry state.

Dan T

RE: Bearing Damage? No.1

The cavity may have been full of clean grease, but was the local area at the worn rollers also clean and free of oxidation? The heat in a set of failing rollers can build up so quickly that the surrounding grease does not stay involved. Don't forget to consider if very cold temps were involved, then what was the grease like in those temps?

Also, spherical rollers that take thrust will find the unloaded rollers to coast down and stop rotating at times. A change in thrust (e.g. jarring force from something striking the impeller) can disturb that equilibrium, causing the unloaded rollers to jump into rotation. In some of these cases, they skid into rotation and develop severe damage very quickly. This is why spherical rollers can be used to handle significant thrust, but many designers don't do it.

RE: Bearing Damage? No.1

(OP)
These bearings were in a sewage digester mixer, belt driven by a 10 HP motor. Rotational speed was 300 RPM. The mixer/digester was maintained at 90 deg. F. A typical small town treatment plant operator might notice a earthquake, but only if the building falls down. The mixer/motor assy. was mounted atop a domed tank, accessible by stairs and walking on a grit surfaced walkway placed on insulation on top of the tank. The insulation meant that snow was not melted. As the failed (lower) bearing was submerged 10-feet in the tank, the noise/vibration was not noticed until real damage had taken place. I personally did not see the tear down but am getting second hand info. The concrete debris left in the tank has made this a Contractor issue (maybe?) and he seems bankrupt, so it’s a bonding company issue now. Lots of finger pointing. I am with the plant design group, my group specified the roof replacement and mixer, but not the mechanical design.
Steve

RE: Bearing Damage? No.1

Looking at the damaged roller it is obvious that it failed due to spalling .. fatigue, stress under the bearing surface. Would have caused due to dynamic loads in excess of the designed loads. Manufacture of the bearing with respect to quality standard too contributes to this type of failures.
Malith

RE: Bearing Damage? No.1

Brinelling mark too can be see in the photo 4 indicating excessive sudden impact loading.

RE: Bearing Damage? No.1

looking at the surfaces it looks like it started with fatigue (hairline cracks in the rollers) that subsequently were sometimes opened up under load and or filled with some lubricant and then closed again, causing the cracks to become deeper and wider. that can go on for some time, until stresses get too high and certainly something starts to flake off. and since there is very little room those flakes can start additional processes that run on a continuously speeding up pace, resulting into catastrophic failure within say a few hours or even faster.

although the normal end of any roller bearing is fatigue, it usually does not progress as far as to this stage, because the bearing becomes noisy and someone will investigate. in this case it might either be neglect or not being able to detect or hear additional noise or noise progressing over time, or for some reason mixer blades were jammed leading to a heavy overload starting the initial crack.

that the cavity was full of clean grease is somewhat remarkable. it may be assumed that the failure also caused a substantial rise in temperature that would have lead to oxidation (darkening) of the grease. maybe the maintenance personnel regreased the bearing after they found out about the damage, so that when investigations are carried out the conclusion would be "well greased, no maintenance personnel to blame...."

in this case, based on the information supplied, it is difficult to find out what came first: jammed blades and subsequent bearing damage or bearing damage with subsequent jamming of blades are both possible....

if the bearing load was such that the loads applied were not excessive (within the limits that the bearing manufacturer would advice given the type of application), there is not that much reason to suspect a below standard design. the very fact that the bearing was submerged and thus "noise" can not easily be heart may have contributed to the extend of the damage though - when the bearing had been mounted in such a way that extra noise would have been noted in a earlier stage, the amount of damage might have been more limited.

this to me seems a typical example of damage: most people involved are not that much interested in it's cause but more so in who will pay for the damages.....

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