Properly Lubricate Electric Motors
EP Editorial Staff | March 16, 2020
Follow these practices to maximize electric-motor performance and longevity.
By Mark Barnes, PhD, CMRP, Des-Case Corp.
Treated correctly, electric motors can last a long, long time. In many cases, motors fail prematurely not because of any design, manufacturing, or installation issues, but from the most fundamental of root causes: poor lubrication. Studies by the Electric Power Research Institute (EPRI, Palo Alto, CA, epri.com) reveal that 95% of motors fail prematurely. Of those, 53% are due to bearing failures. Put simply, we are killing our motors through poor lubrication practices.
Electric-motor lubrication is not complicated. You simply need to select the correct lubricant, apply the right amount each time, and do so at the required frequency. So why do so many companies get it wrong?
It all starts with selecting the correct lubricant. For the purposes of this article, we’ll only focus on motors equipped with rolling-element bearings, though some larger-horsepower motors use journal bearings, in which lubricant selection is just as important.Three dominant factors—load, speed, and bearing dimensions—determine lubricant selection.
The purpose of any lubricant is to separate moving surfaces with a film of oil. The amount of separation is referred to as film thickness. Too low a film thickness and metal-to-metal contact will occur. Too great a film thickness and internal churning will result in fluid friction and elevated temperatures. Largely, the viscosity of the oil, relative to the load and speed of the bearing, governs film thickness. As viscosity increases, film thickness increases. Conversely as viscosity decreases, fill thickness and the separation between metal surfaces decreases.
Most electric motors are lubricated with grease, not oil, which is where the first mistake often occurs. Many people simply do not understand how grease works. A grease is nothing more than an oil bound together with a thickening agent and appropriate additives.
It is not the “thickness” of the grease, more correctly called consistency—that creates film thickness, but rather the viscosity of the base oil contained with the grease. For motors operating in the 1,200- to 3,600-rpm range, the ideal viscosity is in the range of 90 to 120 cSt at 40 C. This viscosity will typically create sufficient separation of moving parts, without creating churning from excess fluid friction.
The most common motor-grease-selection mistake is to use multipurpose grease. Multipurpose greases are formulated with base oils in the ISO VG 220 to 460 cSt range, far too high for motor bearings at typical operating speeds. To compound the issue, multipurpose greases often contain EP additives which, in some circumstances, can volatilize and corrode the copper windings of a motor. Multipurpose greases are designed for slower-speed bearings or bearings with higher loads such as pillow-block bearings used on conveyors. They should never be used for electric-motor lubrication.
By contrast, well-formulated electric-motor grease typically contains a base oil with a viscosity around 100 cSt at 40 C, a shear- and temperature-stable thickener such as polyurea, and mild antiwear additives for mixed-film lubrication.
The amount of grease to apply to each motor bearing depends on bearing size. To calculate the right amount, we simply need to know the outside diameter of the bearing (D) and the bearing’s width, or thickness (B). Multiplying D and B, together with a constant, as shown in Fig. 1, will yield a good estimate for the correct quantity of grease. When using this approach, pay attention to the units. To obtain grease quantity in ounces, measure the bearing dimensions in inches and multiply by 0.114. For grams, determine D and B in millimeters, then multiply by 0.005.
Since most grease guns dispense in “shots,” rather than ounces or grams, it is often necessary to convert from the calculated quantities in Fig. 1 to shots from each grease gun. This is easily done by dispensing ten shots of grease onto a scale and dividing by ten to calculate the amount of grease dispensed by one shot of the grease gun. Be sure to do this for each grease gun since different styles and models of grease guns dispense different amounts.
Unlike calculating grease volume, determining the optimum re-grease interval is slightly more complicated. There are several methods to do this but perhaps the easiest is to use the method first proposed by Lloyd “Tex” Leugner in his book, Practical Handbook of Machinery Lubrication (Maintenance Technology International Inc). The Leugner equation uses shaft diameter, speed of rotation, and bearing type to estimate the number of hours between re-greasing.
As the formula suggests, as speed increases, the numbers of hours between re-grease decreases since the stress placed on the lubricant will be higher. In addition to speed, size and bearing operating conditions should also be considered. Depending on the operating environment, one or more of the correction factors shown in Fig. 2 below should be applied to the Leugner equation.
Pumping grease into a motor bearing is not complicated. However, a few basic precautions should be taken. First, any tubing that connects the grease fitting to the motor bearing should be pre-charged with grease. Failure to do so could result in no grease getting to the bearings for the first few shots as any grease that is ejected from the gun is simply filling the supply tube. Second, don’t rush. Rapidly filling a bearing with grease can cause sudden shock. Three to five seconds per shot, ensuring that a full shot is dispensed each time, is preferred.
If equipped, it is also good practice to remove the purge port, but do not necessarily expect grease to purge out. All too often, mechanics pump grease into a motor, intent on seeing grease exit the purge port. Never apply more than the calculated quantity of grease. Though not always practical, some motor experts recommend applying grease when the motor is not running. Since many bearing failures are induced by particle contamination, it is also best practice to clean the grease fitting and the end of the grease gun prior to applying grease to the motor.
In the past 10 to 15 years, many companies have used ultrasonic devices to assist in correctly lubricating motors. By measuring ultrasonic energy in the 20- to 50-kHz range, metal-to-metal contact and churning, caused by too much grease, can be measured as an increase in the amount of ultrasonic energy emitted by the bearing. With this technique, the right quantity and frequency of re-grease calculated using the Leugner formula can be refined simply by observing the ultrasonic response of the bearing to the addition of grease.
In most industrial facilities, motors make up a third to a half of all lubrication points. As such, they are critical to uptime and machine reliability. Do not take a shotgun approach to motor lubrication. Add precision by making sure you use the right grease, calculate the correct quantity and frequency of re-grease, and leverage technologies such as ultrasound, shock-pulse monitoring, or high-frequency vibration analysis to ensure your motors are properly lubricated. EP
Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN (descase.com). He has 21 years of experience in the fields of lubrication management, oil analysis, and contamination control and has published more than 150 technical articles and white papers.