Greasing Electric Motor Bearings

Over the past years, we have been researching the problem of grease and electric motor bearings. The major problems are the type of grease, the proper application of the grease, and the frequency of application. This is the result of our own research involving grease manufacturers, bearing manufacturers, motor manufacturers, our industry's technical association, and our own experience in our facility.

Probably the least understood part of the problem is the grease itself. Without going into great detail, grease is approximately ninety percent oil and ten percent thickener. The oil does the lubricating; the thickener keeps the oil in place.

The problem arises when you mix greases which have different thickeners. The most common thickener, or base, used in today's electric motor bearings has a polyurea base. The most common grease used by maintenance departments has a lithium base. Polyurea and lithium don't like each other. If you mix the highest quality polyurea based grease with the highest quality lithium based grease, the result can be a severe reduction in the effectiveness of the base. The result is that your grease can become pure oil and flow into the motor, leaving you with no bearing lubrication. This explains why we sometimes see motors which are full of oil, the bearings have failed, and the customer says there is no oil anywhere near that motor.

Most bearing manufacturers build their bearings in plants all around the world. Some have codes that define the grease that is in their bearings. But sometimes, these codes are hard to decipher. We specify polyurea based grease in all the bearings we use in motor rebuilding. This brings us in line with the motor manufacturers. The most readily available brands of polyurea grease are Exxon/Mobil Polyrex, Chevron SRI, Shell Dolium BRB, and Rykon Premium #2. Of these, Exxon/Mobil Polyrex is by far the most recommended brand.

Many customers ask when and how to grease their bearings. Our recommendation is to clean the grease fitting and remove the grease relief plug. With the motor running, pump in new grease until clean grease comes out of the relief port. Leave the relief plug out until grease stops coming out; this may take several minutes or more. Reinstall the relief plug. Your bearings will have the proper amount of grease in them. When you see grease coming out of the end bell around the shaft, you have put in far too much grease. The proper fill is about half of the volume of the bearing cavity, allowing for expansion. More motors fail from too much grease than too little. "A couple shots a week" is not a good policy, especially if the relief plug is never removed.

So, how often should you grease your motor bearings? This is where you will find a wide range of answers. We have seen manufacturers that say never grease a shielded bearing, the most common type supplied in new motors. Other manufacturers give varying intervals. Some take duty cycle into consideration, and some don't. Even though this may not be the answer you want to hear, we believe each application has its own timetable. Our suggestion is to develop a schedule based on the condition of the grease that comes out of the relief port. If the first grease that comes out looks exactly like the grease you are putting in, you can extend your greasing interval. If a solid glob of old grease that has to be, like the constipated mathematician, worked out with a pencil, comes out, you need to shorten your interval. This may result in a chart with greasing intervals that vary from motor to motor. As long as you do it properly, you could set your interval based on the "worst case" motor in your plant. This would require more grease, but it would minimize bearing related motor failures.

In today's manufacturing plants, we see a lot more repairing/replacing and a lot less maintaining of electric motors. Hopefully, this approach will help you save down time and cut repair costs.