Lubrication: Essential for Bearing Performance
The major functions of lubricants include:
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:
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 ANALYSISFive 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.
FAILURE MODEThe 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 PATTERNSClosely 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:
APPEARANCE of BEARINGWhen 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.
Do not destroy the failed bearing until it has been properly inspected. It is also important to save a sample of the bearing lubricant.
APPLICATION DATAUsually 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:
MAINTENANCE HISTORYAn 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.
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 PrecautionsWhen lubricating motor bearings, be sure to follow these guidelines:
Sidebar: Causes of Bearing FailureThe 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