Electric motors – more specifically, greased-lubricated electric motors – are the prime movers for most machines in plants and factories. Usually, motors that are larger than 25 horsepower in size require occasional regreasing. Doing so properly is essential to ensuring equipment reliability.
Upon inspecting poorly lubricated motors, we often see signs of overgreasing, undergreasing, caked-up thickener suggesting dry-out, and a “rainbow” of colors indicating that the motor is being greased with whatever product is handy. All of this indicates a lack of control over the motor lubrication process.
This article provides some good reasons for you to take dead aim at improving your motor lubrication policies.
Facts and Observations
1) Overgreasing bearings increases heat and leads to separation of the oil from the thickener, as well as grease passing the labyrinth and getting into the windings. Overheating and separation compromise bearing reliability. Grease in the windings inhibits heat transfer and increases the rate at which motor insulation material degrades. A good rule of thumb is that an increase in temperature of 10 degrees Celsius cuts the life of the insulation in half.
2) Undergreasing the motor reduces film thickness, which increases frictional heat in the bearing. This causes the oil to separate from the thickener, which leads to caking and decreases bearing life.
3) Cross-mixing greases can lead to a range of problems. First, counter to conventional wisdom, “grease is not grease.” Grease is a complex combination of chemicals – base oil, additives and thickener. Motor bearings require the right combination of performance characteristics for the application.
Failure to deliver these properties compromises motor reliability. Also, grease components don’t always play nice with one another. Incompatibility can cause separation of the base oil and thickener, sludge, varnish, and compromised lubrication performance.
4) Wrong grease selection frequently occurs. I commonly encounter plants where electric motors are lubricated with “multi-purpose” grease. First, there is not a particular standard related to the term “multi-purpose grease”, but its base oil is typically higher than what’s required for an electric motor application, which wastes energy and unnecessarily generates heat.
It may or may not utilize a thickener that’s appropriate for the application, and multi-purpose grease frequently contains an extreme-pressure (EP) additive that can cause corrosion of the bearing cage and on windings in the event of overlubrication.
Follow These Principles
Here are some simple, easy-to-follow principles to ensure that your motors are properly lubricated, reliable and provide you with the lowest possible life cost of ownership with respect to maintenance and energy consumption.
A) Create a design standard to define cut-off points for using sealed bearings. This may not be an across-the-board decision. In all cases, clearly mark motors that have sealed bearings and are thus “lubed for life.” Note that lubed for life means lubed for the life of the lube. Once the grease is degraded, bearing degradation soon follows.
B) Create a technical standard for electric motors that describes chemical, physical and performance properties for their lubricants. Typically, we look for a high-quality base oil with a viscosity of ISO VG 100-150, polyurea or lithium-complex thickener, high dropping point and good channeling characteristics. Anti-wear additives are occasionally employed, but it’s rare that a motor would require EP.
C) Calculate the required regrease interval and volume. The volume calculation is straightforward. The interval calculation requires adjustment to reflect shaft orientation, vibration, environmental conditions, etc. Decide if you desire to employ ultrasonic monitoring devices to support greasing, keeping in mind that the ultrasonic signal is very fragile. Therefore, you’ll want to be very specific in your procedures.
D) Document your preferred practice into clear, easy-to-follow procedures that specify the variable aspects of motor greasing (lube type, volume required, interval, etc.).
E) Provide clear instructions to your motor rebuild shop, including the specific lube type. Don’t assume that what they’re using is compatible with what you’re using (note that not all polyurea thickeners are compatible). Also, provide instructions related to initial fill volume, whether or not to prelube the fill tube, etc.
Additional Motor Rebuild Shop Note
When a motor goes to the rebuild shop, require feedback about lubrication, even if that’s not the reason why the motor is being rebuilt. Your inspection checklist should report signs of the following:
- Overgreasing (grease in the windings)
- Cross-mixing of greases (different colors)
- Excessive caking or dry-out (indicating grease separation or excessive lubrication interval)
- Pitting on bearing raceways and rolling elements (indicating contact fatigue)
- Corrosion of the bearing cage (can be caused by EP additives)
- Fluting (due to electrical discharge erosion)
- Water etching lines on raceways
- Abrasion on the bearing cage
- Corrosion of windings
- Varnish or staining (indicating excessive thermal or oxidative degradation of the lubricant)
- Rust (indicating exposure to water)
Electric Motors that are Good as Gold
No, there’s not a pot of gold at the end of every rainbow. Motors move your plant. As such, it’s impossible to run a reliable factory without reliable electric motors. Proper lubrication is your most controllable proactive mechanical maintenance strategy for assuring reliable, trouble-free, energy-efficient motor operation.
