... Porous carbon air bearings.

As a spin-off of DuPont, TA Instruments. New Castle, DE, manufactures test and measurement systems. Their DMA dynamic mechanical analyzers measure the modulus, damping properties, and other characteristics of materials as those materials are deformed under oscillatory stress conditions. While searching to increase its market share, the company explored improvements to the DMA's componentry, specifically the motor providing deforming forces, the drive shaft support and guidance system providing measurements, the displacement sensor quantifying the measurement, and the temperature control system. Engineers selected a high-performance voice coil motor with temperature-compensated neodymium magnets, an optical encoder displacement sensor, and a bifilar wound furnace for precise temperature control, leaving the drive shaft support and guidance system selection as the remaining vulnerable link in the chain.

"The motion control system was a wireframe suspension architecture. It wasn't smooth and it lacked stiffness. The motion reproducibility was less than ideal," says Jim Moore, TA's manufacturing engineering manager at the time of the upgrades. "With the temperatures and stresses used in dynamic mechanical analysis, the motion control system was just not stable. The structure would relax: the vibration would impact on the test results. The friction it had to overcome at startup was sometimes greater than the resistance of the material being measured." The system was cited as needing improvement, yet there seemed to be no technology currently available to do so.

In a chance trade show meeting, Moore and the TA R&D manager encountered porous carbon air bearings being demonstrated by Drew Devitt and Nick Sotiropoulos from New Way Precision, Aston, PA. The R&D manager was quite taken with the technology, but Moore remained skeptical, opining that air bearings "were expensive and delicate, strictly a laboratory approach ... highly impractical."

Many engineers think of air bearings as bearing plates drilled with precise orifices and machined with grooves. However, these proprietary air bearing alternatives are comprised of porous carbon "foam" that has millions of pores through which the air or gas bleeds, thereby providing uniform airflow through its body. When set into a metal housing and deployed on bearing surfaces, the desiccated, compressed air pumped through the bearing provides a frictionless pad on the bearing's surface to support the moving loads.

Moore's objections were overcome by demonstrations and tests. Explains founder, Drew Devitt, "They needed precise motion control, and one of the advantages of porous air bearings in general is the lack of friction--that means the motor and controller does not have to deal with the differences between static and dynamic coefficients of friction, that must be overcome at the onset of motion." Adds Terry Kelly, TA's vice president of sales and marketing, "When a machine is testing modulus, that's pretty meaningful. We began to see other advantages to the porous air bearings as an alternative to other motion control bearing choices. New' Way's air bearings are oil-free, so there is no chance of contaminating test materials. The positive air pressure tends to keep dust and other contaminants out of the system."

TA engineers discovered that the new bearing design could support a motor-driven square shaft moving up and down without any rotation. The square shaft delivers higher first order natural frequency and a damping function that the previous wire-frame design lacked.

The space constraints precluded the use of an existing porous air bearing. "We had to design custom housings to fit within the physical size of the machine," says Devitt. "Plus, we had to be able to adjust and align the bearings to give tree movement with good stiffness when the air was flowing. And, with the air off, they had to be tight enough to lock the slide."

"The porous air bearing contributed directly to the overwhelming market success our new model 2980 DMA enjoyed. The air bearings increased our continuous range of motion to 25 mm," explains Russell Ulbrich, TA's product manager. "This allowed true creep testing over a large deformation range. And the air bearing meant no stress head stepper motor was required for static force loading. I believe this also contributes to noise reduction in the measurements," continues Ulbrich.

With a sales increase of 500% in the year of the Model 2980 introduction, TA believes the improvements were meaningful to its customers. "At this point in time, we do not manufacture a DMA or rheometer that does not use air bearings. We believe our customers are keenly aware of their benefits," states Kelly.--SG

Circle 286--TA Instruments, or connect directly at www.rsleads.com/ 502df-286

Circle 287--New Way Precision, or connect directly at www.rsleads.com/502df-287