Monarch Cement Company’s plant in Humboldt, Kansas.
It seemed as though nothing could stop the squealing. Not that Monarch Cement’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 wasn’t what Randy Riebel wanted to hear. As electrical supervisor at Monarch’s plant in Humboldt, Kansas, he knew the noise meant the motor’s bearings were going again.
In fact, the sound of chronic bearing damage was all too familiar at the plant, which can produce 1.3 million tons of cement a year. Since 2001, when the ball mill was new, its motor bearings had to be replaced three times.
“We kept greasing those bearings, but they kept on squealing,” Riebel recalls. “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, in the summer of 2009, I decided to try something else.”
This is the ball mill on which the first two grounding rings were installed at the Monarch Cement Company plant. The 5,000-hp motor bearings failed regularly until an AEGIS iPRO Bearing Protection Ring (inset) at each end of the motor diverted damaging voltages. These on-site retrofits were accomplished without the need to decouple the motor from the mill, thanks to the use of a split-ring version of the iPRO.
The “something else” was the Aegis iPro bearing protection ring, manufactured by Maine-based Electro Static Technology (EST). By channeling harmful electrical currents away from bearings to ground, the iPro can extend the lives of medium-voltage motors and generators, thus improving the reliability of entire systems in which they are used. The rings are available in a range of sizes to accommodate generator/motor shafts to 30 in. diameter.
Maintenance-free, the Aegis iPro is used for medium-voltage motors that drive pumps, compressors, mixers, shredders, conveyors and other machinery in mining, food processing, wastewater treatment, petrochemical refining and many other high-current applications. The iPro also protects generator bearings 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 (IEMCO), a local motor and equipment repair shop. Wilkins recommended the iPro, and Riebel had IEMCO install two on the ball mill motor. EST recommends installing an iPro in the drive end and insulation on the non-drive end of most large motors. For some large motors, especially those that don’t have insulation designed into them or where insulation can’t be installed easily, EST recommends installing iPro rings at both ends of the rotor shaft.
Riebel and Wilkins chose the iPro split-ring model, which is designed to facilitate field retrofits. The mating halves install 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, Kansas, has a fully equipped machine shop. The company also has five regional service centers. 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
An AEGIS iPRO Bearing Protection Ring the IEMCO shop installed in an internal bearing retainer as part of a motor overhaul for Monarch Cement Company machinery.
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.
To minimize electrical resistance, the iPro’s entire inner circumference is lined with multiple rows of conductive microfibers. Locked securely in the ring’s patented Aegis FiberLock channel, the microfibers completely surround the motor shaft, providing millions of discharge points for shaft currents and forming the path of least resistance that diverts currents away from bearings to ground.
{pb}The microfibers are engineered for exceptional flexibility to prevent breakage and ensure that the ring lasts for the life of the motor. Because it prolongs the life of bearings, motors and motor-driven systems, the iPro qualifies as sustainable technology under the Federal Energy Management Program.
A nagging, widespread problem
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, Kansas. The terminals distribute the cement to 13 Monarch subsidiaries, which sell it in their respective areas. Some of the cement is sold in bulk, some in bags. And some subsidiaries process it further to fabricate building products or produce ready-mix concrete by adding stone and sand.
When Monarch was founded in 1908, chunks of blasted limestone (shot rock) as large as 4 ft across were loaded by hand into mule-drawn carts. Now this limestone is moved by front-end loaders, 50-ton dump trucks and conveyors to be processed by a series of computer-controlled crushers, kilns and mills until it is as fine as face powder.
Electric motors power most of the processing machinery, and the problem of chronic bearing damage is by no means 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 a VFD modulates the input, but the benefits of improved efficiency are lost if the motor keeps breaking down.
Following the success of the AEGIS iPRO Bearing Protection Ring on its ball mill, Monarch Cement Company installed the ring on this baghouse fan motor (blue) that also required frequent bearing replacements.
Such breakdowns were recurring headaches for Riebel, but because the two Aegis iPro rings installed in 2009 appear to be protecting the ball-mill motor bearings, he has since had IEMCO install the iPro on nine more motors that had to be removed from service.
A case in point is a VFD-controlled cooler-vent fan where the 300-hp motor had to be replaced frequently for almost eight years. Every time, the kiln had to be shut down for at least a day. The old motor was removed, and the rebuilt spare motor had to be aligned and coupled. “We’d send the pulled motor to be rebuilt, but then three to six months later we’d have to do the same thing all over again,” says Riebel. “For the cooler vent fan motor, we tried insulation on both bearings. With the insulation, the motor lasted two years between breakdowns. The shunted electricity might have hurt the bearings in the fan itself. 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.”
Riebel didn’t really realize what the problem was. “There wasn’t much information available about electrical bearing damage,” he explains. “We just knew that bearings would fail and the motor would overheat, but we weren’t looking to see why. Again and again, we just sent the motor out, got it rebuilt and put it back in service. We didn’t know the root cause.”
Solved by a green technology
It’s now Monarch policy to have IEMCO add the iPro ring whenever a VFD-controlled Monarch fan motor is overhauled. Another such installation was on the 2,250-hp motor for an induced-draft (ID) fan that pulls kiln-heated air through a roller mill to dry the limestone and shale during the grinding process. Other motors that have iPros 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.
[pullquote]Because a cement plant is a dusty place and many motors are outdoors, Monarch has begun to specify that some of its new motors must be equipped with the Aegis Severe-Duty SGR Bearing Isolator Shaft Grounding Ring, another Electro Static Technology product. As in the iPro, secured conductive microfibers completely surround the motor shaft for efficient grounding, but this model has a built-in IP56 noncontact isolation seal to provide extra protection from dust, water and other contaminants.
Monarch’s Maintenance Manager Mark Pily authorized the purchase of the plant’s first motor with a factory-installed Severe-Duty SGR, after consulting IEMCO’s Wilkins. As of November 2010, a 200-hp air-compressor motor is the only such motor in operation at the plant. “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
Riebel periodically takes shaft voltage readings on all the plant’s motors with grounding rings with a voltage probe and oscilloscope. He’s pleased with the results because the readings are low, indicating that the rings experience less potentially damaging shaft voltages.
Riebel also gives a high grade to the service IEMCO provides. “In my opinion, they’re by far the best shop we’ve worked with, and we’ve worked with most every shop in the area. Scott is really involved. He’s really a get-it-right-the-first-time kind of person.”
Time will tell exactly how much money the rings will save Monarch, but Riebel is convinced the Aegis iPro provides effective, long-term bearing protection that reduces the costs of downtime and motor maintenance. “So far, so good,” he says. “On the 5,000-hp, because the last set of bearings only lasted a year, chances are we would have noticed problems by now, but we haven’t had any — no squealing.”