Hello reader! Meet the hansome-st one, .....me! :p
I have some problem at motor vibration and temperature. Here the story:
Our predictive engineer found level C vibration and occasionally hi temp on our 1400kW Fan Motor and issuing recommendation to change the bearing.

Our motor has two bearing on DE (NU238ECM & 6238) and one bearing on DE (NU238ECM).
With this arrangement : [NDE NU238 ROTOR 6238 NU238 DE]

When plant periodic outage came, our maintenance team changed the bearing.
After bearing change, we run no load test and resulting in level B vibration and hi temp bearing (DE +-90 degC and NDE +-82 degC) and vibration 2.4mm/s.
Then we replenish grease, anf try another no load test resulting (DE +-80 degC and NDE +-70 degC) and vibration 3.2mm/s.

Then we try to couple with fan, and had 8.4 mm/s axial vibration.

Why it become worse after we replaced the bearing?

Here some condition to consider:
- We did a bearing change in dusty area (ESP has been drained).
- Our fan is axial hydraulic type, which means it has variable axial thrust force.
- Before bearing change, sometimes DE motor bearing temperature going as high as 80 degC, but could be back to normal about 40-50 degC mysteriously (already trending vs fan load and has no correlation at all). NDE bearing always in stable temperature about 40-45 degC.
- With this arrangement : [NDE NU238 ROTOR 6238 NU238 DE] the ball bearing outer race is not fitted on bearing housing, not as NU bearing). The ball bearing outer diameter has about 0.75mm gap with bearing housing.
Is this condition expected by design?

After did some research, here the possibilities :::

Standard/common answer:
1. bent shaft
2. Misalignment --> realigned with thermal growth of motor frame allowance
3. Bearing housing
others idea:
- Shaft damage due to bearing inner race movement
- Shaft deformation due to extreme heat
- Over greasing
- Shaft alignment
- Shaft motor not level
- Static discharge through bearing especially on VFD usage ->bearing insulation & shaft grounding brush
- out of balance coupling
- Rotor has axial force try to center on its magnetic center
- Small burr on shaft shoulder, where bearing fits up to
- Shim incorrect installation
- Rotor positioning, by locking DE bearing in proper arrangement by the grease caps and the NDE bearing does the floating to allow for thermal growth of the shaft. If the shaft can be moved axially, then the DE has not been reassembled correctly.

And my motors has the answer from above causal.

And the sudden temperature increase analysis :
- Ball bearing is unlikely except it has high axial force imposed on it
- Suspect on radial/cylindrical bearing because they only have to subjected to a small amount of axial thrust. NU bearing in motors, one of them need to be mounted in a way that it can slide axially, otherwise the two of these bearing will thrust against each other because of thermal expansion of shaft.  -> check this sliding arrangement if incorrectly assembled then correct it.



Semoga bermanfaat :)