Proper lubrication of anti-friction bearings is critical for successful performance.
The major functions of lubricants include: Providing a lubricant film for the various sliding and rolling contacts existing between the bearing elements; Protecting the surface finish of the raceways and rolling elements from corrosion and rust; Sealing the bearing from foreign materials; and Assisting in heat dissipation out of the bearing elements.
Grease lubrication is the method most commonly used on small- and medium-size electric motors in the range of 1 to 500 hp for horizontal machines. (Note: Sleeve bearings may be used in large horsepower, 2-pole, or high-speed applications. Vertical pump motors start to use oil-lubricated bearings at about 50 hp. These applications are not covered in this article.)
The lubricant used for grease applications is usually a mixture of oil impregnation into a soap base. The purpose of the soap base is to keep the oil in suspension until it is removed by the moving members of the bearing and adheres to the surfaces. It is obvious that the supply of oil is depleted with time, as it gradually breaks down by oxidation. This process is a function of time, temperature, speed, load, and environment. Selecting the proper grease and using the correct relubrication practices are critical for optimum bearing life.
Greases are usually made of a combination of soap or non-soap-thickening materials mixed with mineral oil and additives. Soaps such as sodium, calcium, aluminum, lithium, and barium are most commonly used.
Present research is making it possible to predict bearing life more accurately. The use of elastohydrodynamic lubrication theory (EHL), introduced in the 1960s for calculating film thickness and pressure profiles, has been the key to many investigations and the basis for understanding failures modes. Since the early 1970s, lubrication and film thickness have been recognized as significant factors in the life equations.
The latest efforts have been in the area of particle contamination and lubricant cleanliness. These new studies are tending to reshape the life prediction equations. According to one bearing manufacturer, the true nature of the failure mode mechanism was hidden and not understood until recently for the following reasons: The high loads used to accelerate testing resulted in insufficient time for wear to manifest itself. Surface-initiated cracks from particle indentation, which penetrate into deeper areas of high stress and culminate in flaking, could not be distinguished from flaking caused by cracks formed below the surface.
Bearing life theory has been further refined to use a family of curves to establish an adjustment factor to the unmodified life. Of primary importance is the N factor used to correct for contamination. An accurate assessment of the N factor requires an analysis on a computer with accurate knowledge of the application. Figure 1 is typical of the curves used to determine the life adjustment factor for contamination (see page 26). These refinements, along with similar actions taken by other manufacturers, can only lead to a more precise determination of bearing life.
In addition to new life prediction theories, new lubricants and lubrication methods are being devised which will extend the operating life. Synthetic greases are capable of extending grease life significantly. Although grease life is a function of more than just oxidation life, it is a good indicator of the type of gain that can be made by using synthetic grease.
Synthetic greases can be formulated with a lower sensitivity to temperature variations, and therefore have a larger useful temperature range and the potential for lower losses.
The question frequently asked about greases deals with their compatibility if they are mixed during the relubrication process. Table 1 is a guideline to assist in this process. If in doubt, do not mix greases without checking with the lubricant manufacturer(s).
METHODOLOGY OF ANALYSIS
Five key areas should be considered in relation to one another to accurately diagnose the root cause of bearing failures. They are:
1. Failure mode;
2. Failure pattern;
4. Application; and
5. Maintenance history.
The following is a brief discussion of each of these areas.
The failure modes (which usually result from combined stresses acting on the bearing to the point of damage or failure) can be divided into the following 12 categories.
1. Fatigue spalling, flaking;
6. Abrasive, abnormal wear;
8. Lubrication failure;
9. True or false brinelling;
10. Electric pitting or fluting;
11. Cracks; and
Failure modes do not represent the cause of the bearing problem. Instead, they are the result or way that the problem is manifested.
BEARING FAILURE PATTERNS
Closely associated with the failure mode, yet distinctly different, is the specific pattern that each bearing failure demonstrates. Such patterns can be grouped according to some combination of the following categories:
Temperature levels (discoloration);
Condition of mounting fits;
Mechanical or electrical damage; and
Load paths and patterns (alignment).
APPEARANCE of BEARING
When coupled with the mode and pattern of failure, the motor, bearing, and load appearance usually give a clue as to the possible cause of failure. The following checklist will be useful in the evaluation.
Are there signs of contamination in the area of the bearings?
Are there signs of excessive temperature anywhere in the motor or driven equipment?
What is the quality of the bearing lubricant?
Are there signs of moisture or rust?
What is the condition of the coupling device used to connect the motor and the load?
What levels of noise or vibration were present prior to failure?
Are there any missing parts on the rotating members?
What is the condition of the bearing bore, shaft journal, seals, shaft extension, and bearing cap?
What was the direction of rotation, overhung load, and any axial thrust? Are they supported by the bearing wear patterns?
Does the outer or inner face show signs of fretting?
Is the motor mounted, aligned, and coupled correctly?
Do not destroy the failed bearing until it has been properly inspected. It is also important to save a sample of the bearing lubricant.
Usually it is difficult to reconstruct the actual operating conditions at the time of failure. However, knowledge of the general operating conditions will be helpful. The following items should be considered:
What are the load characteristics of the driven equipment and the loading at time of failure?
Does the load cycle or pulsate?
How many other units are successfully operating?
How often is the unit started?
What type of bearing protection is provided?
Where is the unit located, and what are the normal environmental conditions?
Is the motor enclosure adequate for the application?
What were the environmental conditions at time of failure?
Is the mounting base correct for proper support to the motor?
Is the belting or method of connection to the load correct for the application?
An understanding of the past performance of the motor can give a good indication as to the cause of the problem. Again, a checklist may be helpful.
How long has the motor been in service?
Have any other motor failures been recorded, and if so, what was the nature of these failures?
What failures of the driven equipment have occurred? Was any welding done?
When was the last time any service or maintenance was performed?
What operating levels (temperature, vibration, noise, etc.) were observed prior to the failure? What tripped the motor off the line?
What comments were received from the equipment operator regarding the failure or past failures?
How long was the unit in storage or sitting idle prior to starting?
What were the storage conditions?
How often is the unit started? Were there shutdowns?
Were correct lubrication procedures utilized?
Have there been any changes made to surrounding equipment?
What procedures were used in adjusting belt tensions?
Are the pulleys positioned on the shaft correctly and as close to the motor bearing as possible?
There are numerous ways to go about failure analysis. The above procedure is a simple one that can be easily taught and communicated to employees with a wide range of skills and backgrounds. This type of analysis will usually lead to the quick elimination of those factors that are not contributing to the failure. When the problem is reduced to the one or two most likely culprits, thoughtful analysis will usually lead to the correct conclusion. It is not one’s brilliance that leads to the truth; instead, it is the ability to sort that which is important from among all of the unrelated data available.
For information, contact Electrical Apparatus Service Association (EASA), 1331 Baur Blvd., St. Louis, MO 63132, or call 314-993-2220.
Publication date: 04/02/2001
Sidebar: Lubrication Precautions
When lubricating motor bearings, be sure to follow these guidelines:
All motor housings, shafts, seals, and relubrication paths must be kept thoroughly clean throughout the motor’s life.
Keep any dirt, moisture, or chips of foreign matter from contaminating the grease.
Identify the temperature range for the application and select grease that will perform satisfactorily.
Be sure to properly purge excess grease. Overgreas-ing may cause elevated bearing and/or winding temperatures, which can lead to premature failures.
When regreasing, be sure that the new grease is compatible with the existing grease, and that it has the desired performance characteristics.
Be aware that synthetic grease may not be as suitable as petroleum greases in high-speed applications. Some applications may require extreme pressure (EP) grease.
Be aware that some common greases are not desirable for motor applications. If they are too soft, whipping can occur. If they are too stiff, noise and poor bleeding characteristics can occur.
Sidebar: Causes of Bearing Failure
The following list contains the most common causes of bearing failures:
1. Thermal overloads
2. Inadequate or excessive lubrication
4. Excessive loading (axial/radial combined)
7. Improper shaft and housing fits
8. Machinery defects
9. Shaft-to-ground currents
10. Incorrect handling or mounting
11. Load, life, and fatigue factors
12. Improper application
13. Damage during transportation or storage
The challenge is to learn how to identify each type of failure with a high level of certainty and repeatability.
Publication date: 04/02/2001