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Resonant Frequencies are Well Damped Thanks to Air Bearings!

Resonant Frequencies are Well Damped Thanks to Air Bearings!

The rotor of a steam turbine Here at New Way Air Bearings, we’re always committed to expanding our Porous Media™ air bearings to new applications and industries. Here, we want to discuss one of the unique capabilities of our bearings which make them so versatile: Shock and vibration mitigation. While traditional contact systems might require discrete spring or fluid damping systems, air bearings are naturally damped owing to the squeeze film effect, and as such provide this two fold benefit to your application.

Both linear and rotary applications have their resonant frequencies mitigated when contact is eliminated through Frictionless Motion™. Direct shocks to a dynamic system are similarly damped by the fluid film medium. If vibration reduction is your goal for a new, more accurate system, then air bearings are your best option.

The Squeeze Film Effect

A CNC lathe Because air bearings support a stage or rotating artifact on a microns-thin layer of air, they are able to utilize the squeeze film effect. This forms the foundation for all of the damping and vibration mitigating properties of air bearings. Simply put, the squeeze film effect states that as a fluid medium is compressed, the fluid medium reacts like a spring, pushing back on the body which acts to compress.

This is a positively correlated relationship. As such, the harder you push on a fluid film, the more it pushes back, creating a system wit increased stiffness with smaller air gaps. This also results in a positive relationship between stiffness and damping ratio, which is functionally unheard of in mechanical systems. The squeeze film effect has numerous direct benefits, such as causing rotating artifacts to self-center, all but removing resonant frequencies from dynamic systems, and making air bearing mounted systems highly shock resistant.

Air Gap (Microns)

Mitigation of Whirl Frequencies

Due to the modular nature of our radial air bearings, Porous Media air bearings can support large rotating artifacts, making them ideal for the turbomachinery and rotor balancing markets. This application has already been extensively studied and analyzed, with significant credibility lent by the findings of General Electric’s Research Center. In their works, they discussed the move from contact bearings towards hydrodynamic bearings, which can function off process fluids and gasses.

A cars red shock absorber springs Damping ratio is a common criteria for optimal bearing selection, often seeking both low stiffness and high damping. In air bearings we find an ideal solution, as the air gap can be adjusted to modulate damping capability, either decreasing the fly height for greater damping or increasing it for reduced damping. Soft mounting air bearings is also an option if damping needs to be kept constant while stiffness is further reduced.

The ability of air bearings to mitigate whirl frequencies in these applications has similarly been studied by our own Drew Devitt, and published in the Proceedings of the Society of Tribologists and Lubrication Engineers. By analyzing coast-down speed tests of a rotor mounted on radial air bearings pads, waterfall and cascade plots of the frequency domain were generated.

These plots are actually 3 dimensional, with X-axis as frequency, Y-axis as time or rotor speed, and the Z-axis (the “bump” in each line) representing amplitude of displacement. In these results we find that varying supplied air pressure can change the undamped natural frequency of the system, and that even if we find while motion growing above the first critical speed, the amplitude is so well damped as to be benign.

Mitigation of Linear Resonance

The benefits of squeeze film damping are not limited to rotating artifacts, but linearly traveling ones as well. Michael Chiu of MIT developed, manufactured, and analyzed a high precision stage built on vacuum replicated air bearing pads and compared it against a standard roller bearing system. Dr. Chiu used a drive point impulse method to apply a known impulse and collect the frequency response of the system as it undergoes vibration.

The results practically speak for themselves. The air bearing system is incredibly well damped, presenting an asymptotic behavior free of peaks, while the roller bear system exhibits numerous peaks each capable of excitation and resonance. Dr. Chiu developed this system with an eye towards high precision linear systems, and we believe in the work of our collaborators to such a high extent that we use their research to validate and improve our own products.

Here you can see our dry diamond turning table, capable of handling pieces up to 3 meters in diameter, built on flat air bearings to support the table and radial air bearings for rotation.

Shock Damping

While obviously not ideal for any system, the squeeze film effect allows an air-bearing-supported system to be highly resilient and crash resistant, as if the entire system were supported on a bed of stiff springs. By supporting a system or object with a large fly height, you provide a large amount of room to compress the fluid film, dampening large impulses which may be applied to the system either by accident or anticipated through the environment.

Learn More

If you want to learn more about the vibration and resonance mitigation capabilities of New Way’s Porous Media air bearings or you already have an application in mind, please reach out to us!

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