- Buyer's Guide
Rolling element bearings used in electric motors have many failure causes such as incorrect bearing selection, improper bearing fits, poor handling during installation, improper installation techniques, excessive thrust loads, loss of lubricant, contamination and overgreasing.
Grease volume control has been a long-standing problem for industry, and simply following OEM recommendations may not be enough to solve this problem.
A motor relubrication practice was developed by the Electric Power Research Institute (EPRI) in 1992 and is widely used by the nuclear power industry today. The program was designed to minimize overgreasing motor bearings in-between required bearing replacements. The relubrication program, associated retrofits and details are discussed in this article.
The problem of overgreasing electric motors was first identified in the nuclear power industry in 1988. Several motor and/or bearing failures occurred at various nuclear power plants due to excessive grease addition. In 1992, EPRI’s Nuclear Maintenance Application Center developed an electric motor predictive and preventive maintenance guide. This guide, outlined a complete maintenance program for various electric motors based on size and bearing type. Part of this maintenance program offered guidance on regreasing motor bearings. This program has helped utilities save money by reducing labor cost for regreasing and reducing bearing failures due to overgreasing.
Bearing Housing Designs
There are two basic bearing housing designs used in most motors with regreasable rolling element bearings.
More motors are manufactured with a same-side design (Figure 2) than a flow-through design (Figure 1). Figure 2 shows the drain plug is the only external path for the grease to exit the grease cavity.
Figure 1. Flow-through design -
used only with open face bearings.
Figure 2. Same-side fill and drain –
used with open, single-shielded,
and double-shielded bearings.
Four Basic Bearing Types
Grease-related Bearing Failures
There are several types of grease-related bearing failures:
Lubricant starvation – Occurs when the grease cavity is not packed with the proper amount of grease during bearing installation, when the bearing is not regreased at the appropriate interval with the proper amount or when the oil is removed from the base of the grease by bearing overheating.
Grease incompatibility – Greases are made with different base compounds such as lithium or polyurea. Not all greases are compatible with each other; therefore it is important to use the same grease or compatible substitute throughout the life of the bearing.
Wrong grease – It is important to use the correct grease for the correct application. Some bearing designs and applications need only general purpose (GP) grease while others need extreme pressure grease (EP). Selecting or regreasing with the wrong grease can lead to premature bearing failure.
Overpressurization of the bearing shields – When grease is added to a grease cavity, grease volume and cavity pressure increase. Damage can occur to the shield on a single- or double-shielded bearing during regreasing if the grease is added too fast. When the motor is placed into service, the grease will thermally expand. If the grease cavity is full, thermal expansion can create damaging pressure on the bearing shields. In either case, the shields can be dislodged from the bearing or the outside shield can be pushed against the bearing cage by grease pressure, which can lead to a bearing failure (Figures 6 and 7).
Figure 6. Overgreasing Failure
Figure 7. Shield was pressurized by
excessive grease which caused a cage failure.
Inside of motor full of grease – If the grease cavity is full and regreasing continues, the excess grease can find its way between the inner bearing cap and the shaft and flow to the inside of the motor. This allows the grease to cover the end windings of the insulation system and can cause both winding insulation and bearing failures (Figure 8).
Figure 8. Overgreasing caused inside
of motor to fill with grease.
Overheating due to excess grease – The balls of a bearing act as tiny viscosity pumps which roll on a small amount of oil film between the balls and the race. Too much volume will cause the elements to churn the grease, resulting in parasitic energy losses and high operating temperatures, which in turn increase risk of bearing failure.
Hardware to Limit Overgreasing and Overpressurization of Bearing Cavities
One thing that happens with adding grease to motors is that there is a limited path for excess grease to exit a bearing cavity. Two examples of hardware that can assist in limiting overgreasing and overpressurization of a bearing cavity are shown in Figures 9 and 10. The use of these fittings can eliminate the need to remove the drain plug for excess grease and pressure release during the regreasing activity.
The fittings shown in Figures 9 and 10 are commercially available from Alemite and have been used successfully in the nuclear power industry.
Figure 9. The pressure cut-off-fill-plug does not allow additional grease to be added to a grease cavity when the pressure exceeds 20 psi. (Left)
Figure 10. The plunger drain plug opens the center plunger on 1 to 5 psi to purge excess grease and pressure. (Right)
Grease degradation is a gradual process. Most grease degrading influences are more present only while the motor is running; however degradation can occur while a motor is idle. Grease degradation can be caused by any of the following conditions:
Clearly, several factors must be considered to develop a sound regreasing program for all of the motors in a plant.
How Should the Grease Be Added?
Because the bearing balls act as tiny viscosity pumps, and the grease is less viscous when hot, a bearing should be regreased while the motor is running. If this is not practical, then perform regreasing immediately after the motor is removed from service while the grease is still hot. Although no program eliminates overgreasing an already filled grease cavity, the steps listed below will help minimize overgreasing-related failures.
The following steps should be performed in the sequence listed:
How Often Should Bearings be Regreased? The program presented in this article and listed in EPRI NP-7502 Report is based on the following information about the motor design and operation:
Table 1 was designed for a relatively clean nuclear plant environment. A dirty or contaminated environment may require adjustments to the recommended intervals.
For intermittent duty cycle motors, the greasing intervals should be the same time frame as continuous duty cycle motors measured by their operation time, not calendar days.
For example, if an intermittent duty cycle motor runs 50 percent of the time and meets the same characteristics in the table as a continuous duty cycle motor that has a 24 - to 36-month regreasing interval, then the intermittent duty cycle motor’s regreasing interval will be 48 to 72 months.
Because there is still some debate whether or not a double-shielded bearing can be regreased, the double-shielded bearing column was not included on the table in the EPRI report. However, for double-shielded bearings, it is recommended to double the frequency in the table and halve the amount added shown on the grease fill chart.
It is worth noting that this regreasing program was designed to minimize overgreasing of bearings in between bearing replacement. When a bearing is replaced, not only should the bearing be packed (open and single-shielded bearings), but the grease cavity is filled with grease to around 50 percent fill, leaving additional space for regreasing. The grease should be added to the grease cavity in such a way as to provide grease around 360 degrees of the cavity. It should also be added in such a way as to allow the grease to come in contact with the bearing. If the grease is packed only in the bottom of the grease cavity, no contact will occur between the grease and the bearing.
For bearing configurations that have their open side toward the bearing cavity, no grease contact would allow the bearing to sling out the grease packed around the balls and cause a lubricant starvation issue and possible bearing failure. Once the grease cavity fills, any excess grease must vent through the designated vent port, or will be pushed into the motor. Unfortunately, due to frequent inaccessibility to the vent port after motor installation, this often does not happen. Overgreasing can lead to bearing shield deformation, cage failure, overheated bearing or filling the inside of the motor with excess grease.
How Much Grease Should be Added?
This is another area in which different manufacturers give differing recommendations. However, to provide guidance on the amount of grease to be added for different size motors, a grease weight versus shaft diameter curve was determined to provide the most useful information (Figure 11).
Figure 11. Grease Fill Curve
For ease of plant implementation, the number of ounces of grease should be converted into strokes for each different type grease gun used, or a calibrated grease meter can be attached to the output of a grease gun.
For motors in standby or lay-up mode and double-shielded bearings, the ounces of grease identified by the above curve for any given motor should be divided by two and that value should be used for the amount added.
The development of a regreasing program that will work for all motors requires ownership by someone familiar with motor designs, operating conditions, history of bearing replacements and type of grease used. Once the program is developed, it can be implemented by simply following procedures. This program has proven to be effective in providing adequate lubrication during the bearing life. It has also minimized bearing failures from overgreasing. Many of the nuclear power plants in the U.S. have implemented this program for motor relubrication since the EPRI report was published in 1992.
This article was published in the Lubrication Excellence 2004 Conference Proceedings. April 2004, Noria Corporation.