New Way Brings New Solutions to Optical Manufacturing
New Way Air Bearings® is always proud to support the research and understanding of how Porous Media™ benefits emerging markets and applications. Today we want to share how New Way’s Porous Media air bearings supported research funded by the National Science Foundation (NSF).
This research concerned the development and fabrication of Non Rotationally Symmetric (NRS) optical surfaces. The work is covered in full in Design of a Fast Long Ranger Actuator – FLORA II by Erik Zdanowicz, Jeffrey Eischen and Thomas A Dow of the Precision Engineering Center at North Carolina State University.
The Challenges of FLORA
For the traditional machining of NRS surfaces, low spindle speeds or actuators with small range of motion are typically employed. The Fast LOng Range Actuator (FLORA) was designed to help overcome these limitations, with a goal to machine surfaces to +/- 2mm. In order to match a traditionally machined surface, the system required a form error below 150 nm peak to valley and a surface finish of 5 nm RMS. The first iteration of FLORA was developed with orifice air bearings, which produced upwards of 200 nm of vibration; clearly insufficient for the precision machining this task requires.
FLORA II was built around our own porous media air bushings, with the idea being that the even distribution of air through the porous carbon bearing will reduce vibration through the system and minimize deflection from the linear axis of motion.
How Porous Media Bearings Made FLORA II Possible
In order to identify and predict the motion within the system, the air bearing interfaces were modeled through ANSYS. A “bed of springs” model to approximate and predict the softness of the air bearing support, while physical experiments on a 25mm diameter bushing were used to identify static pitch and radial motion, and provide a point of comparison to the analytical FEA model.
The radial stiffness of the bushing was measured at 25.8 N/m, close to the 27.9 derived from New Way published data. Similarly, pitch stiffness was measured as 5.5 N-m/mili-radian, very close to the 5.3 given by New Way.
For dynamic experimentation, the most important parameters to identify were the natural frequencies and noise. A capacitance gauge was used to obtain the modes, with the fundamental mode exhibiting a rigid body motion at 700 HZ. It is worth noting that the rigid body mode is well damped, and the natural frequency itself does not exhibit a substantial amplitude. The natural frequencies resonated only on the order of 2 to 3 times above the expected operating range, and all sub-nanometer even then.
Motion noise was next identified by allowing the shaft to sit in the bearing without external excitation. The original orifice bearing design was on the order of +/- 100 nm. With porous graphite bearings we see a peak to peak of +/- 4nm, providing the stability and accuracy required for NRS production.
With physical baseline data collected, the research team returned to the analytical models. Simple analytical solutions based on equivalent spring constants were derived, and produced the 27.9N/m value we discussed earlier. Equivalent pitch stiffness was derived similarly, and when provided to ANSYS to identify natural frequencies from an equivalent spring mass system, it produces a rigid body mode at 700 HZ, validating the analytical technique and the derived equations.
Air Bearings provide Stiffness and Stability
This study is another definitive victory for air bearings as the ideal choice for precision manufacturing and rotary motion applications. With their naturally damped operation, air bearings avoid dangerous whirl frequencies, and provide accuracy an order of magnitude above their competition, as the team at North Carolina State University quickly understood.
We’re proud to support continued research using our product, and always welcome the opportunity for independent analysis of our revolutionary motion technologies. Please read the rest of our technical papers to learn how other researchers have discovered the incredible capabilities of New Way Air Bearings.