The all-too-familiar sound of chronic bearing damage—squealing heard above the roar of a 5,000-hp motor powering the huge ball mill—prompted Monarch Cement’s Humboldt, Kan., plant to find a new and lasting remedy. While the operation has the capacity to produce 1.3 million tons of cement a year, turning out 100 tons of clinker per hour, its motor bearings had been replaced three times since 2001 when the ball mill was new.
“Despite repeatedly greasing those bearings, they kept on squealing,” recalls Randy Riebel, electrical supervisor at the Humboldt plant. “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 expensive and time-consuming replacement. Pulling that motor is a major production; it takes at least 10 days. Sometimes we have to hire help, rent a hoist to put it on a truck, and take it away to be rebuilt. The time had come to try something else.”
The solution proved to be Electro Static Technology’s (EST) Aegis iPro Bearing Protection Ring. By safely channeling harmful electrical currents away from bearings to ground, the iPro extends the lives of medium-voltage motors and generators, thus improving the reliability of entire systems in which they are used. It is available in a range of sizes to accommodate up to 30-in.-diameter generator/motor shafts.
Discussing electrical-bearing damage with Scott Wilkins, manager of Motor Shop Operations for Independent Electric Machinery Co. (IEMCO), Riebel was advised to use iPro. Accordingly, Riebel had IEMCO install two grounding rings on the ball-mill motor. Although the manufacturer recommends using an iPro ring in the drive end and insulation on the nondrive end for most large motors, those lacking insulation or not lending themselves to its easy installation are best served by rings at both ends of the motor.
Riebel and Wilkins chose the iPro split-ring model, designed to facilitate field retrofits. The mating halves of each grounding ring were installed around the motor shaft without need to decouple the motor from the mill.
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 causes washboard-like ridges called fluting, a source of noise and vibration. The eventual result is motor failure.
Some products designed to protect bearings, e.g., conventional spring-loaded grounding brushes, require extensive maintenance. Other options, such as insulation and ceramic bearings, can shift damage to connected equipment, EST notes.
To boost the electron-transfer rate, iPro’s inner circumference is lined with multiple rows of conductive microfibers. Locked in the ring’s patented FiberLock channel, these microfibers completely surround the motor shaft, providing millions of discharge points for harmful shaft currents and creating a path of least resistance to divert currents away from bearings to ground. The microfibers are engineered for flexibility to prevent breakage and ensure that the ring will last for the motor’s service life. Because it prolongs the life of bearings, motors, and motor-driven systems, iPro qualifies as sustainable technology under the Federal Energy Management Program.
Contractors and retail home-improvement stores in six midwestern states depend on the Monarch plant, which sends cement by truck and train to its terminals in Des Moines, Iowa, and Dodge City, Kan. The terminals distribute the cement to 13 Monarch subsidiaries serving their respective markets—selling powder in bulk, some in bags, and further processing some to produce ready-mixed concrete.
Now a dim memory, mule-drawn carts used by Monarch at the time of its 1908 founding to move 4-ft.-wide chunks of blasted limestone have given way to huge front-end loaders, 50-ton dump trucks, and conveyors transferring raw material to be processed by computer-controlled crushers, kilns, and mills until it is as fine as face powder. Most of the processing machinery is powered by electric motors, and the problem of chronic bearing damage is not limited to the plant’s ball mills. Many of the motors are controlled by variable frequency drives (VFDs), which induce additional high-frequency currents on motor shafts. A fan or pump motor tends to use less power if the input is modulated by a VFD; yet, the benefits of improved efficiency are lost if the motor continues to break down.
At Monarch’s Humboldt plant, where such breakdowns recurred, iPro was installed on nine more motors following the proven success of the two rings protecting the ball-mill motor bearings. A VFD-controlled cooler-vent fan, for example, was crippled by a 300-hp motor that had to be replaced frequently over almost eight years—shutting down the kiln for at least a day every time. After the old motor was removed, the rebuilt spare motor had to be aligned and coupled.
“We’d send the pulled motor out to be rebuilt, but then three to six months later, we’d have to do the same thing all over again,” Riebel reports. “We tried insulation on both bearings for the cooler vent fan motor. With the insulation, the motor lasted two years between breakdowns. The shunted electricity might have hurt the bearings in the fan itself, since insulation just pushes the problem on down the line. The electricity has to go somewhere if it’s not grounded. The iPro has given this fan a fresh start.” It is now Monarch policy to have the iPro ring installed in the shop whenever a VFD-controlled fan motor is overhauled.
Another such installation was completed on the 2,250-hp motor for an ID (induced-draft) fan that pulls kiln-heated air through a roller mill to dry limestone and shale during the raw grinding process. Other iPro-equipped motors now include four at the plant’s kilns, where air is forced in and out: two 2,000-hp ID fan motors and two 1,000-hp baghouse (dust-collecting) fan motors.
In addition, because a cement plant is a dusty environment and many motors operate outdoors, Monarch has begun to specify that some of its new motors come equipped with EST’s Aegis Severe-Duty SGR bearing isolator shaft grounding ring. Besides secured conductive microfibers that completely surround the motor shaft for efficient grounding, as in the case of iPro, the SGR model has a built-in IP56 noncontact isolation seal to provide extra protection from dust, water, and other contaminants.
Monarch Maintenance Manager Mark Pily, after consulting IEMCO’s Wilkins, authorized the purchase of the plant’s first motor with a factory-installed Severe-Duty SGR. To date, a 200-hp air-compressor motor is the only such engine in operation at the facility.
“We want to keep the bearings clean because we push that motor really hard,” Riebel explains. “We usually lose that motor because of winding failure. I think most of that is caused by the bearings starting to fail, which causes the motor to overload.”
AN OUNCE OF PREVENTION
Using a voltage probe and oscilloscope, Riebel periodically takes shaft-voltage readings on all the plant’s motors with grounding rings. Low readings indicate that the rings have reduced potentially damaging shaft voltages.
Time will determine Monarch’s return on investment, but Riebel is confident that thems by now, but we haven’t had any—no squealing.”
This article was adapted from material provided by Electro Static Technology, (+1) 513-417-1161, www.est-aegis.com; John Shepherd, [email protected]