Back to balance

Unbalanced rotors can damage bearings.

By Mike Bacidore, chief editor

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Rotor balancing is an important tactic for reducing vibration stress on bearings. “The reason that unbalanced rotors damage bearings is that the peak load area in an unbalanced rotor is forcing lubrication away from the load zone, the lubricant is being broken down, the bearing is fatiguing, or a combination of these can occur,” says Bob Matthews, reliability manager at Royal Purple (www.royalpurple.com). “Overloading and under-lubricating bearings is going to damage the bearings and in most cases is avoidable by operating the rotors to design. The best approach to avoid bearing failure is reliability maintenance.”

A rotor imbalance occurs when its center of gravity is different from its center of rotation, explains Jarrod Potteiger, product and educational services manager for Des-Case (www.descase.com). “This can be the result of many factors including variations in material density, tolerances in fabrication, unsymmetrical parts, or shifting parts during operation due to thermal, aerodynamic, or other effects,” he says. “Rotor imbalance causes cyclic loading and vibration that directly impact bearing life as load and speed are the determining factors of a bearing’s L10 life. A 1-oz imbalance at a 12-in. radius can reduce bearing life by 50%. Conversely, reducing vibration by 50% can increase bearing life by as much as 700%.”

Precision balance and alignment become increasingly critical as rotational speeds increase, so plants utilizing high-speed machinery should pay even more attentions to this aspect of the reliability program, suggests Potteiger. “As with most aspects of maintenance and reliability, a successful balancing program requires education, experience, and the right tools,” he offers. “Modern dynamic balancing equipment and techniques can drastically reduce vibration, thereby optimizing the life of bearings and other components. For some, the best place to start is by contracting this work for critical applications as service providers that specialize in on-site balancing are widely available. Many of these service providers also offer on-site training, which will allow for the development of in-house expertise.”

Roller bearings take up the strain on the machine, says John Bernet, product and application specialist at Fluke (www.fluke.com). “If the shaft is imbalanced, the forces on the shaft cause the bearing to wear much faster,” he says. “If the motor and pump shafts are misaligned, the flexible coupling will help some, but there are still high forces on each shaft causing the bearings to wear much faster.”

Vibration testers should be used on a routine basis to measure rotating machines and determine if mechanical faults are causing wear on the bearings, explains Bernet. “When faults arise, instead of just replacing bearings, the root cause of the fault needs to be determined,” he says. “This can only be done by tracking all faults on the machine. Once the machine is properly aligned and the shafts balanced, the bearings will have much less strain on them and will last for years.”

Mike Bacidore has been an integral part of the Putman Media editorial team since 2007, when he was managing editor of Control Design magazine. Previously, he was editorial director at Hughes Communications and a portfolio manager of the human resources and labor law areas at Wolters Kluwer. Bacidore holds a BA from the University of Illinois and an MBA from Lake Forest Graduate School of Management. He is an award-winning columnist, earning a Gold Regional Award and a Silver National Award from the American Society of Business Publication Editors. He may be reached at 630-467-1300 ext. 444 or mbacidore@putman.net or check out his .

Balance can be defined as the state of the mass distribution within the rotating assembly about its axis of rotation, explains Jack Zedek, senior mechanical engineer at Baldor Electric (www.baldor.com). “The eccentricities of this mass distribution are referred to as unbalance,” he says. “The amount of unbalance is stated in units of mass times a distance, such as grams-inches, ounce-inches, gram-centimeters or gram-millimeters. The amount of unbalance contributes additional radial load into the machinery bearings. If the bearings are lightly radially or axially loaded, such as a coupled duty motor, then the effects of unbalance might not be as great in a theoretical sense when calculating a bearing L10 life. For example, if the theoretical L10 life calculations drop from 1.5 million hours to 1.4 million hours because of the contributions from additional radial loading, would anyone really care? However, if the machinery is heavily loaded such as a belted duty motor with high radial loads, or a pump or fan motor with high axial thrusts, the addition of the forces into the bearing could have significant L10 life reductions for the bearings.”

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