Jerry S. Honeycutt from the Tennessee Valley Authority writes in Reliability Web:
One of the most asked questions of grease lubricated motors is, what is the correct interval to re-grease the bearings? This is usually followed by two questions, 1) how much grease should be added? and 2) what is the correct method for grease addition?
Unfortunately, there are many different answers that are given for these questions depending on who or which company you ask. This leaves it up to the individual over the motor re-greasing program to choose a method that he or she thinks will work for their companies motor applications.
Rolling element bearings used in electric motors have many failure causes such as incorrect bearing selection, improper bearing fits (shaft and housing), poor handling during installation, improper installation techniques, excessive thrust loads, loss of lubricant, grease contamination, and over greasing.
Grease volume control in motors has been a long-standing problem for many industries, and simply following OEM recommendations may not be enough to solve this problem.
To address motor re-greasing issues, a motor re-lubrication practice was developed by the Electric Power Research Institute (EPRI) in 1992 (NP-7502)¹ and is widely used by the nuclear power industry today. The program was designed to minimize over greasing motor bearings in-between required bearing replacements. The EPRI recommended re-lubrication program, associated retrofits, and details are discussed in this write-up.
It was the late 1980s when the problem of over-greasing electric motors was first identified in the nuclear power industry. The problem surfaced when several motors and bearings failed because of excessive greasing.
The problem of over greasing electric motors was first identified in the nuclear power industry in 1988². Several motor and/or bearing failures occurred at various nuclear power plants due to excessive grease addition. In 1992, Electric Power Research Institute (EPRI) Nuclear Maintenance Application Center developed an electric motor predictive and preventive maintenance guide. This guide outlined a complete maintenance program for motors which included a re-greasing guide for electric motors based on size and bearing type. This program has helped utilities save money by reducing labor cost for re-greasing and reducing bearing failures due to over greasing.
Since motors are essentially designed the same for nuclear as they are in the commercial industry, the re-greasing program identified in the EPRI guide is considered to be applicable outside the nuclear industry.
There are two basic bearing housing designs used in most motors with re-greasable rolling element bearings: Flow-through design, used only with open face bearings; and same-side fill and drain, which are used with open, single-, shielded, and double-shielded bearings. More motors are made with same-side rather than a flow-through. For both designs, the drain plug is the only external path for the grease to exit the grease cavity.
In grease lubricated motors, there are four basic bearing types:
Open Face Bearing: This bearing consists of the inner and outer race, the balls, and the ball cage. It does not retain grease within shields and requires grease in a grease cavity for lubrication. This bearing is considered fully re-greasable when used with either of the bearing housing designs listed above.
Single-Shielded Bearing: This bearing has a metallic shield on one side only, and is usually installed with the shield facing the motor winding. There is a small air gap between the shield and the inner race which will allow grease transfer to the inner bearing cap. It can be re-greased and typically has the same re-greasing intervals as an open face bearing. This bearing can only be used with a same side housing design as shown in Figure 2.
Double-Shielded Bearing: This type has a metallic shield on both sides of the bearing and is designed to retain grease between the shields. There is a small air gap between the shields and the inner race which allows a certain amount of oil transfer over a long period of time between the grease in the grease cavity and the grease between the shields. There is some debate whether or not this type of bearing can be regreased. Regreasing double shielded bearings has been successful and this paper provides guidance for those that choose to place double shielded bearings in a regreasing program.
Sealed Bearings: These bearings are designed similar to a double-shielded bearing with one exception. The inner race slides against the seals resulting in the absence of an air gap between the seals and the inner race. This type of bearing cannot be re-greased and is totally dependent on the lubrication packed from the bearing manufacturer for the life of the bearing. If this type of bearing is installed in housings with grease fittings it is highly recommended the fittings be removed and plugs installed.
In addition, there are several varieties 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 poly-urea. 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. This requires grease specification when a double shielded bearing is ordered so it will be compatible with the grease that will be added during regreasing activities.
The bearing, the grease cavity around the bearing, and the grease used during regreasing has to be compatible.
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.
Over-pressurization of the bearing shields – When grease is added to a grease cavity, grease volume and cavity pressure increases. Damage can occur to the shield on a single or double shielded bearing during regreasing if the grease is added too fast or if the grease cavity is full with no escape path for the excess grease. 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.
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.
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 grease volume will cause the rolling elements to churn the grease, resulting in parasitic energy losses and high operating temperatures, which in turn increase risk of bearing failure.
Contamination of grease – Greases are the same as oils when it comes to contamination (water, dirt, fiber, gasket sealant, etc). The more contaminants that are in the grease the shorter the grease life and the greater the reduction of its lubrication properties.
I’ll stop here – this is about half the article. Honeycutt goes further into detail regarding how to add grease, how often, how much. He concludes:
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. The program described in this paper has proven to be effective in providing adequate lubrication during the bearing life. It has also minimized bearing failures from over greasing. Many of the nuclear power plants in the U.S. have successfully implemented this program for motor relubrication since the EPRI report was published in 1992.
Please read the whole thing.