It seemed as though nothing could stop the squealing.
Not that the Monarch Cement Company’s huge ball mill wasn’t already loud. Powered by a 5,000-HP motor, it pulverizes 100 tons of clinker (a burned mixture of limestone and shale) per hour. But the squealing was not what Randy Riebel wanted to hear. As electrical supervisor at Monarch’s plant in Humboldt, Kan., he knew the noise meant the motor’s bearings were going — again.
In fact, the sound of chronic bearing damage was already familiar at the plant, which has the capacity to produce 1.3 million tons of cement a year. Since 2001, when the ball mill was new, its motor bearings had been replaced three times.
“We kept greasing those bearings, but they kept on squealing,” Riebel said. “We knew that if we waited too long, the bearing race walls would become fluted like they had in the past, and we weren’t looking forward to another replacement because of all the expense and downtime. It takes at least 10 days to pull that motor — it’s a major production. Sometimes we have to hire help, rent a hoist to put it on a truck, and take it away to be rebuilt. So this time (summer of 2009) I decided to try something else.”
The “something else” was the AEGIS™ iPRO Bearing Protection Ring, manufactured by Maine-based Electro Static Technology (EST). By safely channeling harmful electrical currents away from bearings to ground, the iPRO extends the lives of medium-voltage motors and generators, improving the reliability of entire systems in which they are used. It is available in a range of sizes to accommodate generator/motor shafts up to 30” in diameter.
Maintenance-free, the AEGIS iPRO is ideal for medium-voltage motors that drive pumps, compressors, mixers, shredders, conveyors and other machinery used in mining, food processing, wastewater treatment, petrochemical refining and many other high-current applications. The iPRO also protects the bearings of generators in both utility and on-site power generation systems.
Riebel had been discussing electrical bearing damage with Scott Wilkins, manager of Motor Shop Operations for Independent Electric Machinery Company (IEMCO), a local motor and equipment repair shop. Wilkins recommended the iPRO, and Riebel had IEMCO install two of them on the ball mill motor. EST usually recommends installing an iPRO in the drive end and insulation on the non-drive end for most large motors, but for some large motors — especially those that do not have insulation designed into them or where insulation cannot be easily installed — EST recommends installing iPRO rings at both the drive end and the non-drive end of the motor.
Riebel and Wilkins chose the iPRO split-ring model, which is designed to facilitate field retrofits. The mating halves of each iPRO were installed around the motor shaft without the need to decouple the motor from the mill.
A family-owned business founded in 1906, IEMCO sells, services, repairs, and tests motors, generators, hoists, welders and electrical distribution switchgear. IEMCO’s main office in Kansas City, Kan., has a fully equipped machine shop. The company also has 5 other service centers in the region. Because they deal with large motors routinely, IEMCO’s personnel are well aware of the severe damage shaft currents can cause to motor bearings.
Mitigating electrical bearing damage
If not diverted, shaft voltages can discharge through bearings, pitting the balls and race walls.
Without long-term bearing protection, concentrated pitting at regular intervals along a race wall can cause washboard-like ridges called fluting, a source of noise and vibration. The eventual result is motor failure.
Ironically, some products designed to protect bearings, such as conventional spring-loaded grounding brushes, require extensive maintenance themselves. Others, such as insulation and ceramic bearings, can shift damage to connected equipment.