Engineers Develop Fabrication Method For Safer Flywheels

April 15, 1997

University Park, Pa. --- Penn State engineers have developed a practical, easy method for making safer versions of the spinning cylinders or disks known as flywheels that could make them more likely candidates to power cars, stabilize household electrical current -- even run space satellites.

Christopher W. Gabrys, a consultant who earned a Ph.D. at Penn State in 1996, and his adviser, Dr. Charles E. Bakis, associate professor of engineering science and mechanics, write that their new method "should theoretically allow fail-safe or limited failure flywheels to be constructed." The researchers describe their method in the May issue of the Journal of Reinforced Plastics and Composites.

The new technique reduces the possibility of catastrophic failure of the flywheels by changing the area where the flywheel can be expected to fail and by limiting the amount of the rotor that will fly off if the wheel is spun too fast.

Bakis explains that composite flywheel rotors are usually made from high-strength, lightweight, carbon, glass or synthetic fibers formed into a continuous filament that is wound around a spindle or mandrel, like thread on a spool. The filaments are impregnated with epoxy to "glue" them together and hold them in a rigid disk or cylinder shape.

In the new Penn State approach, a rubber-like elastomeric material is substituted for the epoxy. The filament passes through a special elastomer solution "bath" that soaks it thoroughly with the rubber-like material. The elastomer-impregnated filament then solidifies in a matter of minutes after being wound on the mandrel.

Bakis says, "The result is a flywheel in which we predict failure will result in only the outer edge fibers peeling from the wheel. The breakdown process is also self-arresting since the inner material is operating at lower stress levels due to the unique properties of the elastomeric matrix flywheel. This is in contrast with rigid epoxy matrix wheels which usually fail explosively -- all at once."

In their paper, the two researchers note that the use of elastomers in composite flywheels is not a new idea and had been theorized and reported in the technical literature previously. However, Gabrys and Bakis are the first to develop a simple, practical way to make very thick disks and cylinders using elastomer-impregnated filaments.

Bakis says, "Making very thick disks was the specific challenge that we took on. We've achieved a ratio of inside to outside diameter of .1, without wavy fibers."

Limited-failure flywheels have the potential to be safe, low cost, non polluting, replacements for chemical batteries in a host of applications, he adds. High-performance composite flywheels are capable of storing or providing 20 to 40 times more power per kilogram than batteries. Their power comes from the fact that, once set rotating, a flywheel tends to keep rotating unless its kinetic energy is deliberately drawn off or friction slows it down. Heavy, low-tech, metal flywheels have been used for years on auto crankshafts to assist gasoline engines between piston strokes. Currently, GM, Ford, Mercedes Benz and Mitsubishi are all known to be developing flywheel/electric hybrid auto propulsion systems incorporating the new high performance composite rotors.

Less well known are some of the other possible flywheel applications. Bakis says, "Power companies often keep a small plant in reserve which they fire up twice a day for peak needs. Flywheels could be a less costly or dangerous alternative that would enable the companies to run their plants at full use around the clock."

He adds, "On satellites, flywheels could not only replace batteries but could even be used as momentum wheels to orient the spacecraft."

All of these applications depend, at least in part, on developing safer rotors.

**bah**

EDITORS: Dr. Bakis is at (814) 865-3178 or CEB5@psu.edu by e-mail.

Penn State

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