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 atRoyal 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.”
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.”
As with most things in the life of an engineer or plant maintenance technician, there are trade-offs to everything, including motor rotor grades of balance. “All of these grades trade cost, lead time, and availability with theoretical improved motor bearing life,” says Zedek.
“Bearings, like all materials, have an upper limit to the amount of tensile load that can be applied to them before they fail,” explains Alan Friedman, senior instructor at Mobius Institute (www.mobiusinstitute.com). “This is called their tensile strength. Fatigue loading is when a repetitive or cyclic load is applied to a material. This is the type of load that vibration from an unbalance puts on the bearings. The load is positive, negative, positive, negative and repeats itself every time the shaft goes around. Think of bending a paper clip back and forth.”
The problem with this sort of fatigue loading is that it can cause damage to the bearing, even if the load is well below the upper tensile strength of the material, he says. “Additionally, the damage is cumulative, meaning that the more revolutions of the shaft, the closer the bearing comes to failure,” explains Friedman. “More unbalance increases these loads and hence leads to earlier failure. In addition to being a source of damage to bearings, vibration from unbalance can cause other problems, as well. Pump seals, couplings, shafts, and foundations are also subject to fatigue that can be hastened by increased levels of vibration. Additionally, vibration traveling from the machine into the floor can affect other machines, even causing false brinelling of bearings in standby machines, and perhaps the quality of the product being manufactured. Think of a machine trying to print tiny circuit boards while being subjected to outside vibration.”
With these consequences in mind, machines should be routinely balanced. “Unbalance can be detected as part of a vibration condition monitoring program, and most vibration data collectors include software that will allow technicians to balance the machines,” says Friedman.
Proper balancing of a rotor to required limits will help ensure that the rotor will operate within acceptable vibration limits during operation, says Greg Chatlos, product engineer in the drive technologies division of Siemens Industry (www.usa.siemens.com/industry). “The effect of an unbalanced rotor on bearings is not only dependent on the residual unbalance, but the rotor's operating speed, as well. The main concern here is vibration where the rotor shaft journal moves independent relative to the bearing outer race or Babbitt,” he says. “This creates a pounding force on the bearing, which can lead to a fatigue failure. A rotor with an unbalance that operates at a high speed will have higher vibration levels than the same rotor at a lower speed. Industry standards such as ISO 1940-1 and API 541 for induction motors govern the suggested balance limits. For instance, API 541 requires that a rotor must be balanced to within 4 W/N limits, where W is the static load on each bearing and N is the rotor speed.”
If a rotor is not properly balanced to the limits specified, the vibration levels of the motor during operation may exceed acceptable limits, adds Chatlos. “For sleeve bearings, excess vibration can wear on the bearings by fatiguing the Babbitt material, which can cause it to crack, chip, flake, or wear away leading to failure,” he says. “For anti-friction bearings, excess vibration can cause fatigue on the rolling surfaces, which may lead to cracks, pits, and spalls leading to failure. It needs to be noted that, if a rotor is not thermally symmetrical, it may go out of balance when the rotor gets hot.” This may at times need to be verified.
“Shaft vibration in machines generates tremendous forces,” says Howard Penrose, PhD, CMRP, vice president at Dreisilker Electric Motors (www.dreisilker.com). “As the vibration occurs, even with a machine running, oil, which is an incompressible fluid, is forced out from between the surfaces of the bearing resulting in metal-to-metal contact. As this occurs, deformities in the rolling elements and races occur generating rough surfaces and increased friction. Depending on the severity of the unbalance or vibration, the equipment can fail rapidly.”
The best approach is to ensure all components are balanced, says Penrose. “When possible, they should be balanced on the shaft they will be operating on,” he says. “The reason for this is that different components balanced within tolerance may add up to significant unbalance and vibration when assembled together.”
