Most electric motors are equipped with anti-friction, grease-lubricated rolling-element bearings. These bearings are predisposed to fail along with your equipment if they are not lubricated properly—in fact, bearing troubles account for 50 to 65 percent of all-electric motor failures. The most common lubricant-related failures include improper lubricant selection and over-and under-greasing. With the right regreasing strategy, you can actively work to prevent these failures and prolong the life of your equipment.
Know Your Failures
The first step to avoiding failures is knowing your equipment’s common failure modes and root causes.
Lubricant Starvation
Lubricant starvation has several causes including insufficient grease during installation, infrequent lubrication intervals and oil separation from excess heat. The answer to this issue might appear simple: continuously overfill the bearing with grease so it never runs out, right? Not quite. Bearings require a very precise amount of grease to achieve their set range of motion—any amount over or under this affects their ability to function optimally.
The solution is to adopt a condition-based regreasing schedule. With the right tools, a condition-based regreasing method allows you to take an intuitive approach to bearing health, allowing for greater accuracy. Interval-based (or time-based) regreasing schedules often do a disservice to your bearings because there’s a tendency to dispense the wrong amount of grease at the wrong time.
Excessive Grease
Under-lubricating presents a host of issues; however, over-lubricating results in just as many (if not more) harsh outcomes. Firstly, the motor can overheat when the rotating bearing elements churn the excess grease, leading to high energy consumption and oil separation. In addition, the motor casing can overflow with grease and allow lubricant to end up in the wrong places (commonly in the windings) when the pressure from a grease gun is applied.
To solve this issue, we must rethink the grease gun. Grease guns make it extremely difficult for the user to give the bearing the correct amount of grease. Very rarely would a bearing require exactly 1 or 2 full pumps of grease—realistically, it will be somewhere in the middle, which is easy to guess and hard to get right.
Consider using a single-point lubricator to dispense small amounts of grease over time, ensuring precise and consistent lubrication. Think of it this way—if you require 2,000 calories per day, would you rather consume your weekly total of 14,000 all at once or over the span of 7 days? It is similar to your bearings in that they need small amounts of grease over time rather than a total overload.
Wrong Lubricant
Lastly, it’s important to choose the right lubricant—not all greases are the same and using the wrong type can lead to premature bearing failure. Ask your supplier about a grease that is specifically designed for electric motors, which is different from multipurpose extreme purpose (EP) grease.
Furthermore, be cautious when switching greases as not all are compatible with each other due to variances in thickeners such as polyurea, lithium, or calcium. Try to use the same grease or compatible substitute throughout the life of the bearing.
Developing a Preventive Maintenance Plan
Now that you’re aware of common bearing failure root causes, it’s time to put together a comprehensive preventive maintenance plan. By considering the following, you can actively reduce bearing failure and uphold the integrity of your equipment.
Determine Grease Volume
Grease volume control has been a long-standing problem for the industry, and simply following OEM recommendations may not be enough to solve this problem. If you are using a grease gun, there is a simple equation to determine the maximum volume of grease to be added. The formula is:
G = 0.114 x D x B
Where G = the amount of grease in ounces, D = the bearing outside diameter in inches and B = the bearing width in inches.
Once the volume is found, it must be converted into shots, or pumps of the grease gun. The average value is approximately 18 shots per ounce for most manual guns, but grease gun output can vary by a factor of 10, so be sure to calibrate each gun. Noria’s online grease calculator can help in determining this number.
With that said, it’s still difficult to achieve the right volume with a manual method in comparison to an automatic single-point lubricator. One like the OnTrak system uses bearing feedback to get grease volume right, every time. Not to mention this device operates automatically, saving you time and resources.
Establish Regrease Intervals
There are several ways to determine the optimal regrease interval. The best thing to do is to choose an interval that is dependent less on time and more on the condition of the bearing. This is going to require condition monitoring equipment. By measuring the bearing’s grease levels and condition, you can accurately pinpoint the moment you need to regrease the bearing (or have the monitoring equipment regrease your bearings automatically.)
Ultrasonic instrumentation is one of the best tools available to dial in the optimum relubrication frequency. A conservative approach is to use a frequency generation method as a starting point and continuously refine that value based on feedback from the ultrasonic equipment. For greater efficiency (and time-saving), invest in an ultrasonic device equipped with a single-point lubricator for automated lubrication.
Find the Right Grease
Establishing the correct interval and volume means nothing if you don’t have the right grease. When searching for a lube type and manufacturer or supplier, there are several things to consider. The following is a list of qualities of a good electric motor grease:
- Good channeling characteristics
- Excellent resistance to high-temperature oxidation
- Good low-temperature torque characteristics
- Good anti-wear performance (but not EP)
- NLGI Grades 2 to 3
- Base oil viscosity of an ISO VG 100 to 150 or more specifically 90 to 120 cSt at 40°C
- High dropping point, 400°F minimum
- Low oil bleed characteristics, per D1742 or D6184
Polyurea grease is popular with many bearing and motor manufacturers. A significant proportion of equipment manufacturers also specify some type of polyurea grease in their electric-powered machinery. A polyurea-based grease is an excellent grease for electric motors; however, this thickener is incompatible with most other thickeners. Some manufacturers don’t recommend mixing one brand of polyurea with another.
Bottom Line
These considerations are just the foundation of a proper preventive maintenance plan for your electric motors. There are several other steps to take such as developing a task list for upkeep and maintenance and conducting training on the various condition monitoring technologies. With all these factors in mind, you can effectively prolong the life of your electric motors, saving time and money along the way.
