When electric motors fail, over half the time the failure is caused by bearings. Bearings fail for a variety of reasons, most of those reasons are related to lubrication. What I wrote a few years ago still rings true:
Regarding bearing lubricating, unfortunately, there are more ways to get it wrong than right. One can over-lubricate as well as under-lubricate. Consider:
- Grease itself introduces contaminants into bearings if careful control is not practiced.
- Mixing greases with different bases may cause grease constituents to separate and run out.
- Different motors pose different requirements for the introduction of lubricant and removal of old lubricant.
- Each individual application dictates the amount, type, and frequency of lubrication required.
Creating and adhering to a bearing lubrication management program greatly reduces bearing damages resulting in motor failure.
Contamination in lubrication for bearings is the leading cause of bearing failure whether that contamination is from a particle, chemical, or both. From our prior article:
Here are five steps for consideration to manage oil cleanliness and contamination levels:
- Receipt and of new oil.
- Storage and conditioning of new oil.
- Dispensing of the oil to the machines.
- Stopping contaminants entering the machine.
- Removing contaminants generated in the machine from wear.
A bearing lubrication maintenance program isn’t free; some investment is necessary. The costs may range from $20,000 to $150,000, however, there will be a return on your investment:
The following are three examples of where the investment payback will come from.
- The eliminating of one failed electric motor, due to lack of lubrication.
- The doubling of life of a number of gearboxes due to cleaner oil.
- The reduction in bearing spending, by a conservative 30% annually.
- The doubling in life of all hydraulic systems and pumps.
Writing at Pumps & Systems, Jim Elsey writes that bearing failure is a common culprit in pumps as well as motors:
Most pump bearings fail long before their design life span. The American Petroleum Institute (API) typically requires a minimum bearing life (L10) of 25,000 hours, and ANSI B73.1 specification for horizontal ANSI pumps specifies a minimum L10 bearing life of 17,000 hours at maximum load and rated speed. Prudent end users frequently request bearings with more than 40,000 hoursL10, but most bearings do not reach that many hours of operation before failure.
More than half of pump bearings fail as a result of contamination, excess heat or both. Preventing this introduction of contaminates is easier and less expensive than removing them. Some studies suggest removing contaminates can be eight to 10 times more expensive than prevention.
The way you lubricate your bearings also has a direct effect on their functional lifespan. Elsey writes that the method of lubrication is determined by the design of the bearing.
A properly selected oil-lubricated ball or roller bearing will work for most applications less than 200 HP, 400 F (fluid temperature) and 3,600 revolutions per minute (rpm). For some smaller and lower temperature applications (less than 320 F), grease-lubricated bearings may also work well. Larger pumps at higher speeds and system temperatures will require line, sleeve or plain journal bearings for radial support (hydrodynamic journal bearings) and tilted shoe (pad) designs for thrust bearings.
Methods and designs that are acceptable for ANSI specification pumps may not be acceptable for API, process, marine and power generation applications where HP can often exceed 70,000 brake horsepower (BHP) with speeds in excess of 6,000 rpm.
Because most end users at the high-end of the HP and speed spectrum are aware of oil types, best practice lubrication techniques and bearing selection, this article will examine the middle and lower range.
There are other factors regarding lubrication of your bearings to consider.
Both of these factors accelerate the oil’s oxidation rate and reduce oil and bearing life. For splash lubrication, the oil level should touch the very bottom of the bottom ball in the bearing. If the oil drops below the ball or the outer race, the bearings could be damaged. If the level is too low, the bearing could experience temperature runaway, which is when the bearing gets hot quickly and is permanently damaged.
Most motor bearings are grease-lubricated, so it is important to know if they are open, shielded, sealed or a combination of these options. If they are shielded on one side, the best practice is to place the shielded side toward the grease fitting. I have seen many end users who order pumps with greased bearings still add oil to the housing simply because the installation, operation and maintenance manual (IOM) did not specifically say not to. Running the pump with greased bearings and splash oil lube at the same time is an incorrect solution.
Changing Oil: Elsey suggests that the first oil change should occur relatively soon.
The first oil change should be conducted at a shorter interval than subsequent changes to eliminate the contamination that occurs from startup and run-in operations. Most ANSI and some API pump manufacturers will state that the first change should come at 200 operating hours and subsequent changes at 2,000 hours or 3 months, whichever comes first. The intervals depend on operating temperatures and how contamination ingress is managed. Some operations will require more frequent oil changes, and others can operate for several years. Oil sample analysis and experience will aid in decisions regarding oil change intervals.
Too much lubrication also leads to problems.
One of the most common problems I see in the field beyond not reading the instructions is the overfilling of the bearing housings with oil. Bearing isolators (labyrinth style) will purge any excess oil from the housing. A common misconception is that the isolator has failed, but it is just doing its job. The oil will continue to be expelled until the level is below the expulsion port.