Back in the 1970s, at the Massey-Ferguson tractor transmission plant in Livonia, MI, we ran some very old electric motors. Some of them had been retrofit to line shaft machines. When they would fail, normally on Sunday night, we would check their brass tag dates. It wasn’t unusual for them to be older than I was back then. A 30-year motor life was nothing unusual.
Today motor frames may be lighter, but the bearings are better and the clearances more precise. Sometimes, though, we manage to burn up motors at an astonishing rate, wasting money and trashing OEE and profitability numbers in the process.
With the help of Charlie Peterson of Peterson Predictive Maintenance, who spoke at the UE Systems’ Ultrasound World Conference, May 6-10, I have jotted down a few simple steps that can help us avoid some of the most common failures of electric motors.
The process starts the day you take delivery of a new or rebuilt motor. Place the motor on a flat plate and check the flatness of the mounting surface. If one foot or boss needs to be shimmed, you may elect to shim it before bolting or clamping the motor in place. If you elect to shim the motor, document the fact and leave an appropriate shim pack and instructions with the motor. If you don’t want to shim it, send it back.
|J. Stanton McGroarty, CMfgE, CMRP, is senior technical editor of Plant Services. He was formerly consulting manager for Strategic Asset Management International (SAMI), where he focused on project management and training for manufacturing, maintenance and reliability engineering. He has more than 30 years of manufacturing and maintenance experience in the automotive, defense, consumer products and process manufacturing industries. He holds a bachelor of science degree in mechanical engineering from the Detroit Institute of Technology and a master’s degree in management from Central Michigan University. He can be reached at firstname.lastname@example.org or check out his Google+ profile.|
Next, clamp the motor in place and check the output shaft for runout. In most cases it should be no more than .003” total indicator reading (TIR). New motors with shaft runout greater than .003” should probably be returned to the vendor. Rebuilt units may need some discussion with engineering. Depending on the motor, you may wish to take a few more measurements, but now you know whether the motor housing is twisted and whether the shaft is bent. Two important causes of early failure are preventable, if installation is performed using the information you have gained and documented.
While the motor is still clamped in place, check it for grease. If it has been greased, determine what kind has been used and ensure that it matches the grease that will be used after installation. Remember Moly and Poly don’t get along. They’ll dissolve each other, letting oil run into the insulation on the windings and causing early motor failure.
Charlie says to paint a few grease guns pink and fill them with your plant’s choice of electric motor grease. Never use them for anything but motors, and don’t allow any other kind of motor grease in the plant. Of course you’ll have to modify your motor acceptance criteria if you take this approach. It’s not a bad idea though. Mixed grease is a sure motor killer.
If the motor is not greased, determine whether it has sealed, permanently-lubricated bearings. If it has, don’t grease it, and above all, don’t install grease fittings on a sealed bearing motor. Over-greasing is the greatest killer of electric motors, and any greasing of a sealed bearing motor is over-greasing.
If the motor needs grease, get the pink gun and grease it, preferably while listening to its ultrasound output, preferably with an experienced lubrication tech. When the ballsy rumble stops, it’s greased. Run the motor up to operating speed and, once the grease has spread out, perform a vibration check commensurate with the size and cost of the motor. If vibration seems excessive, review it with someone who knows whether the motor is fit for service. If possible, record baseline vibration and/or ultrasound results for the motor and save them so that they can be used in the future for installation and troubleshooting.
Finally, once the motor is accepted and lubricated, mark the shaft end to signify approval and add a mark that can be read like the hour hand on a clock. Then every month when you exercise all the motors in the inventory, you can park the shafts in the position corresponding to the month they were exercised. December goes straight up for 12, March to the right at 3 o’clock, etc. Give the motor appropriate protective covering and put it into inventory, knowing that it’ll be ready for you when you’re ready for it. It will be a great comfort to have your spare motors tested, exercised and ready to run some wintery Sunday night.
I hope every electric motor owner that doesn't already use these guidelines finds this article. I was called to a fresh water pumping plant for a city in MA that had a new electric motor on a vertical lift pump leaking a lot of purple watery gunk from out of the casing vent. The US Electric motor was greased prior to commission with a half a tube of grease. I found that out by asking the plant engineer, when I asked him what the OEM spec for lubrication was he shrugged and turned a little red in the face. I found the spec sheet, “greased at the factory for two years of continuous duty." The spec sheet also advised to use only a polyurea based grease and the application called for less than an ounce, every two years. The half tube represents 7 oz., of the wrong grease.
Great stuff. Common sense and basic stuff we just don't do.