Proper Bearing Lubrication 1

[MacMcMacmac]

Good Day folks. I have recently experienced the failure of a drive end bearing on a 1350hp, 1750rpm drive motor, powering the first stage of a 2MW exhauster through a gearbox.

This motor was overhauled in 2019. It has not been run very extensively since we are in a research facility which has highly variable operating hours.

These bearings were replaced at overhaul. I have no idea on how, or how well they were packed at install. There was a short 3/8" pipe nipple, a 90 elbow and a zerk fitting installed at the time of reassembly to ease access to the grease port.

I was operating the exhauster last week when i heard a sudden growling/moan emanate from the machine. The DE vibes spiked and settled, before spiking a second time. I observed the machine and saw a hazy smoke appearing in the machine hall, so I e-stopped the exhauster immediately. Coast down went smoothly. Bearing temperature spiked to 180F. It was obvious from our temperature readout screen that it was the point of failure since it was the only component in the red.

Upon disassembly, I was disappointed to find that the grease inside the fitting had reached the bend in the elbow and no further. I probed the grease port and found almost nothing. Granted, I had probably not reached the bearing per se, but it was definite evidence that these bearings had not been successfully greased since install. I checked the zerk, and while a battery powered gun would not force grease through, a manual gun did successfully operate the fitting. So, possible failure point there. There was a glob of grease on the exterior of the zerk, so an attempt was made to lube it, albeit unsuccessfully.

There is a vent port at the bottom of the bearing housing. My main question is this, is it acceptable to fully grease this bearing after it has been replaced so that there is fresh grease coming out of the vent port at the bottom of the case, or am I at risk of over-greasing the bearing and causing a different set of problems? I have no data on how much to add. The bearing is a 22222E-c3 double row spherical roller bearing on the drive end of the motor.

Unfortunately, there is an identical motor next to it, overhauled at the same time at the same facility that I now have to worry about. There is a very important test campaign coming up in a few months and this is definitely not something we need to worry about for the duration. It is likely both motors will be treated to new bearings, but I still need to know if I can fully flood these bearings with grease. Are automatic grease applicators a good or bad idea? Thank you for any help you can give.

 

[TugboatEng]

That is a very large bearing considering shaft speed. This requires special attention to lubrication. The housing requires a specific volume of grease. Excessive grease will cause overheating.

Secondly, you must consider the base oil viscosity of the grease. At that speed*diameter, you're going to need to use a grease with a base oil viscosity less than ISO 150. Most greases are ISO 220.

For these low load/high speed bearings, polyurea greases typically perform best if the environment doesn't present any confounding factors such as temperature.

For all of these reasons bearing manufacturers often recommend removal of the zirk fittings after the initial fill to prevent excessive or incorrect greases from being added later in the field.

Also consider that false brinelling can occur in bearings that sit stationary for extended periods of time so regular shaft rotation may be required during layup periods.

 

[MacMcMacmac]

Thank you. I agree about the false brinelling. There are always plenty of other machines operating in the vicinity. It is an active and loud environment.

Temperatures of the bearings always seemed a bit low, and there seems to be a good reason for it. After further investigation, it seems the grease fittings were installed in a blind hole. The bearing grease vent is also a blind hole. Don't asked me how that happened. I looked down each with a bore scope and spun the drive hub and saw nothing. Then out of curiosity, I scoped out each end cover bolt hole and saw the rollers passing by at the end of the holes which I had assumed were blind. There was no way any grease was ever going to reach that bearing. I am amazed they lasted 4 years. The bearing temp probe was installed in a gun drilled hole down one of the bolts. Theoretically, if it had been installed with even a slight lack of care it could have been jammed directly into the rollers. As it was, it had no heatsink paste on it, so it was basically sitting in an empty bore measuring the temperature of the air inside the drilled bolt. So if it hit 180-190F at e-stop, you can imagine the bearing was quite a bit hotter than that! I think the blind holes were the actual correct location for the probes.

As it was related to me, these mid 1930s vintage motors originally had babbitt bearings with oil slinger ring lubricators that were replaced by roller bearings in the mid 90s. I am not sure why, lack of oem parts? At any rate, they had served us well for about 20 years before being replaced at the last motor overhaul.

The overhaul company used Mobil Polyrex EM. Looking at the data sheet I read Viscosity Index, ASTM D2270, 95, so it seems like we are well below 150. It is also a polyurea grease. The motor service company is a serious concern which handles all of our electrical maintenance and testing on a yearly basis, so I am confident in their work. I just need to nail down how I can properly care for the new bearings as this is going to be a pricey, time consuming, labor intensive repair we need to avoid in the future.

 

[electricpete]

> The overhaul company used Mobil Polyrex EM. Looking at the data sheet I read Viscosity Index, ASTM D2270, 95, so it seems like we are well below 150. It is also a polyurea grease.

Just for info, viscosity index doesn't tell you the viscosity, it just tells you how the viscosity changes with temperature. (I think when changing temp from 40C to 100C, the VI 95 will change viscosity by 5% more than the reference 100% oil changes ).

But your conclusion is still correct. The viscosity of Mobil Polyrex EM is 115 at 40C which means ISO 115.