Lubrication Checkup: A Motor Rebuild

Symptom:
“Dear Dr. Lube, we’ve received a motor [for rebuild] that runs in a hot environment. Its bearings are sealed and the motor is encased in a tube with no access to lubricate the bearings periodically. The OEM specifies SHC 100 bearing grease rated at 350 F. Koyo ball bearings come lubricated with Polyrex® EM grease, also rated at 350 F. Is one grease better than the other?”

Diagnosis:
Most motors are now sold with “sealed for life” bearings that require specialized grease to protect against high speeds, water washout, rust, corrosion and extreme temperatures while providing low noise properties.

Both greases mentioned are Mobil products. SHC 100, a synthetic PAO (polyalphaolefin) product with a lithium complex filler, is cherry-red in color. Polyrex® EM grease, a mineral-based product with a polyurea thickener, is blue. Both are NLGI number 2 greases, designed to operate in high-speed, severe and high-temperature applications. Mobil recommends both as premium electric-motor bearing greases.

In a direct comparison of typical grease properties, there’s little difference in their performance values. Polyrex® EM is a slightly higher viscosity at ISO 115, versus ISO 100 for the SHC 100, and has a slightly higher dropping-point temperature of 550 F versus 510 F for the SHC 100.

Results from a Koyo-bearing-provided Mobil ASTM D 3336 test (grease life in hours at 350 F) show the Polyrex® EM product as having over twice the high- temperature life expectancy of typical synthetic lithium complex greases.

Prescription:
When rebuilding motors, bearings are virtually always replaced. On the odd occasions when bearings are deemed suitable for reuse, it’s wise to flush and repack them with the original specified grease. (This is very important in such an application, as polyurea-based greases are incompatible with lithium-based products.)

When new bearings are used, the rebuilder has a choice of lubricants (as evidenced above). On paper, the Polyrex® EM product may look like the best candidate. That said, I urge you to contact ExxonMobil’s engineering department, describe the exact operating condition for the motor and let those lube experts prescribe which of their greases is best suited for your application. MT

Lube questions? Ask Dr. Lube, aka Ken Bannister, author of the book Lubrication for Industry and the Lubrication section of the 28th edition Machinery’s Handbook. He’s also a contributing editor for Maintenance Technology and Lubrication Management & Technology. E-mail: doctorlube@atpnetwork.com.