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1 February 2002

Steady on the Go

By Raymond J. Misjan
Lord Corporation

Active balancing and damping technologies provide solutions for a world of motion.

Lord Corp.'s Mechanical Products Division has a mission: to leverage its broad experience in mechanical dynamics to provide customers with solutions to their motion control problems, including those caused by vibration. To this end, Lord's recent purchase of BalaDyne Corp., a leader in active balancing systems and the development of active damping technology, provided a catalyst for new motion control technologies from which several industries will benefit.

Fixing Vibration while in Motion

Lord defines active balancing as "correcting an imbalance in machinery while in operation," which is faster and more effective than conventional "off-line" balancing techniques. Since its inception in 1981, BalaDyne of Ann Arbor, Mich., has received numerous patents and government contracts for its pioneering work in developing active balancing technology. For example, BalaDyne has done the following:

• Created proprietary fluid technology for grinders
• Pioneered the application of active balancing systems for industrial fans and reactor coolant pumps
• Developed breakthrough technology for the machine tool, turbomachinery, and aerospace industries

The BalaDyne purchase enhances Lord's technology for handling vibration at its source, thus eliminating its generation. By controlling vibration at its origin, active balancing systems can eliminate motion control problems before they spread throughout a structure.

Lord vice president and general manager of mechanical products Thomas J. Gibbons said, "This new technology will provide the marketplace with another approach to solving vibration problems. It has potential applications in nearly all machinery markets, especially machining and turning centers, fan balancing, turbo machinery for power generation, propellers and turbines for aerospace, as well as spin stands, dryers, and wafer grinding for semiconductor processing."

How Does It Really Work?

Active balancing corrects any imbalance in a machine while it's in operation. Because it's faster and more effective than customary "off-line" balancing techniques, the technology has proved invaluable in a variety of applications:

• When the item being rotated is asymmetrically and inherently unbalanced or suffers imbalance from a buildup (or loss) of material over time
• When frequent tooling changeovers demand balancing

Active balancers measure (or detect) imbalance through vibration and proximity sensors. A microprocessor controller then determines both the imbalance's magnitude and its location and sends a signal to an imbalance compensator permanently attached to the rotating component. The compensator redistributes weight, solving the problem.

Higher rotating speeds make improved balance even more important because dynamic balance is a function of the square of the rotational speed. Potential applications include any rotating equipment where balance is an issue.

Proper Balancing Vital to Aerospace

In the aerospace industry, properly balancing rotors is crucial. Propellers, turbofan engines, helicopter main rotors, drive shafts, and tail rotors are all very sensitive to the dynamic loading that can result from improper balancing.

The uncertainty of the required time to balance an engine or a propeller is extremely disruptive to aircraft operation. The negative effects of poor balancing vary from passenger discomfort and premature failure of onboard equipment and aircraft structure to catastrophic failure of the rotating equipment. Studies of properly balanced propellers indicate as much as a 30% reduction in the number of maintenance person-hours needed for each hour of flight. This represents hundreds of thousands of dollars in savings every year, not including the nonmonetary benefit of increased safety.

Recognizing balancing's importance, the aerospace industry has adopted a wide range of balancing procedures and related devices to ensure the smooth operation of rotating equipment. While these methods have provided better and smoother operation of engines, propellers, and rotors, they also possess several disadvantages that limit their effectiveness:

• Rotor balancing occurs outside the actual operational environment
• Operating conditions used for balancing aren't representative (low speed, low thrust, etc.)
• Balancing is based on data representative of only part of the flight envelope
• The number of iterations required to achieve acceptable results varies greatly from one rotor to the next

Using active balancing for aircraft is a new application and serves as a good example of how the technology increases equipment reliability and structural integrity. This technology's primary advantage lies in removing balancing procedures from the maintenance cycle. Every mechanic who's been involved with balancing understands it's a repetitive process with an undefined number of iterations. Because operators will experience a significant reduction in logistic complexity with active balancing, specific ground equipment, manuals, and mechanic training will no longer be needed with this "set and forget" system.

A "smart" solution, active balancing technology always knows how much balance correction is applied. An alarm is raised when more balance correction is needed than is available from the system capacity. At that point, the controller not only warns the operator but also indicates, with a high degree of accuracy, the actual correction magnitude and phase angle required to optimally balance the rotor. The operator can then schedule maintenance to bring the rotor balance back to within the active system capacity. Moreover, because the needed correction is already known, this field balance maintenance can be performed in a fraction of the normal time. This high level of predictive maintenance is extremely useful to operators' maintenance crews.

The electromagnetically controlled balancers, coupled with a small electronic controller and a vibration sensor, will provide optimal balancing during every minute of every flight cycle. With just seconds needed to optimize the balance correction from the most unfavorable initial conditions, active balancing ensures the smoothest continuous operation for the rotors to which it's applied.

Figure 1 illustrates this feature. Vibration levels were recorded at various revolutions-per-minute values for a high-speed machining spindle, both with and without active balancing. As the system nears maximum balancing capacity, the controller triggers a call for maintenance before the rotating equipment actually needs to be repaired. The system delivers less fatigue for both on-engine and onboard equipment, as well as lower failure rates. Therefore, it should dramatically reduce aircraft operators' direct operating costs by preventing the wear and tear that continuous exposure to vibration inflicts on aircraft structures and equipment.

Applying the Technology to New Markets

To date, active balancing technology has been effectively used on large fans, machine tools, and turbomachinery, but other potential applications include rotating semiconductor processing equipment, granulators, centrifuges, and pumps, as well as turbomachinery in petrochemical manufacturing and power generation applications.

One immediate fit is in the machine tool industry. High-speed machine tools incorporate high-speed linear slides with their own set of motion control issues. These slides, however, must also be mated with high-speed spindles to yield the full benefits of high-speed machining.

With machine tool spindles operating at ever-higher speeds, mass imbalance can cause vibration that affects manufacturing quality. Rotating imbalance can create symptoms that are mistaken for machine motion control problems. Imbalanced milling spindles can lead to elliptical error motion of the cutting tool, causing size and shape errors that may be hard to distinguish from slide motion errors. Because active balancing controls rotating machinery vibration while the machine continues operating, the mass actuators shift balance correction rapidly to minimize vibration in mere seconds. Vibration and other sensors provide feedback to an adaptive controller. The controller then uses this sensor information to command the state of the balance rings, which are permanently mounted on the rotating machine.

Eliminating Resonant Response

While active balancing technology eliminates vibration generation by dealing with it at the source, active damping technology also offers solutions to a variety of industries by removing a motion outside the primary motion control system's sphere of influence.

Active damping technology can be used in any application where disturbance forces cause a resonant response (the amplification of a vibration input caused when the disturbance's frequency matches the system's natural frequency) that creates motion problems. Possible applications exist in semiconductor manufacturing equipment, machine tools, and other precision machinery.

Here's an example: A robot used for machining boat molds must be run at a low processing speed to avoid overexciting a resonant response of the robot arm (Figure 2). Otherwise, the machine moves in a manner not controlled by the primary motion control system. By mounting an active inertial damping system on the robot arm near the tool, the robot runs at substantially higher speeds with greater accuracy. Lord removed the resonant response by applying forces that literally cancel those causing the resonant vibration.

Consider this case of eliminating resonant response: Lord applied active damping modules on pneumatic isolation tables used in manufacturing and inspecting semiconductors. These tables provide good vibration isolation at frequencies above 5 hertz but generally suffer from a resonant response between 1 and 4 hertz. Applying active damping technology enabled us to remove resonant response without sacrificing the quality of isolation at higher frequencies. Consequently, the sensitive production and metrology equipment operated with greater accuracy.

Motion Control's Future

Active balancing and damping are yet two more examples of how the information age is truly integrated into everything we touch, see, and do. Such technology, coupled with a growing understanding of dynamic systems and motion, delivers tangible benefits to complex systems and operations. As design engineers continue to question the "acceptable" limits of noise and vibration, expected maintenance and annoyances may become something of the past. Moreover, as motion control systems become more refined, controlling vibration and resonant response will be important factors in a system's overall design and functionality. MC

Make Contact!

Raymond J. Misjan, market development manager for Lord Corp.'s Mechanical Products Division, has been with Lord since 1989. In his current role, Raymond oversees the development and application of new technologies for emerging markets. Prior experience includes serving as marketing manager for Lord's Industrial Products Division, as well as general manager for Industrial Products–Europe. He has a B.S. in Mechanical Engineering and an MBA, both from Case-Western Reserve. Contact him at tel: (800) 472-5849, ext. 6410.