Synchrony Technology Blog

I am often asked about the tradeoffs between using air bearings and magnetic bearings for high speed applications. After all, both are non-contact, oil-less solutions that can improve the performance of rotating machinery. Air bearings also seem less complicated because they don't require active electronics to operate.

I believe that certain applications can benefit from using air bearings instead of magnetic bearings, especially for smaller machines in contamination-free environments (e.g. laboratories). Indeed, in my career, I have helped to develop highly-specialized, small, high speed turbomachines on gas-lubricated bearings. However, some of the limitations of air bearings that preclude their use from a broader range of applications include:

• Limited damping. The vibrational characteristics of a rotating machine close to or at its critical speeds (resonances) are determined by the amount of damping (dissipation) introduced by the bearings. A magnetic bearing can electronically optimize this damping to reduce the vibration at critical speeds. An air bearing will typically have much less damping, which will restrict the operating speed range of the machine, and may require a more complicated balancing process.
• Manufacturing tolerances. The forces in an air bearing are created due to the small clearance between the rotating and stationary surfaces, often times less than 0.001 in for a journal diameter of about 1 in. Because bearing forces are reduced at larger clearances, it is necessary to maintain this small clearance even for much larger shafts. The net result is that it becomes difficult or impossible to manufacture the bearing for larger machines with the required precision. The clearances in a magnetic bearing are between 10 and 20 times larger, which make them suitable for larger machines.
• Intolerance to contaminants. Because of the very small clearances, the performance of the bearing is adversely affected by contaminants in the air. This precludes use of the air bearing in many industrial environments unless special provisions are made to ensure the environment surrounding the air bearing is clean. The performance of a magnetic bearing is relatively insensitive to these contaminants.
• Load capacity varies with speed. For typical operating conditions, the load capacity of an air bearing is proportional to the speed. This means that when running at reduced speed, the bearing will be more susceptible to overloading and damage. The load capacity of a magnetic bearing is independent of the rotational speed.
• Intolerance to overload. If an air bearing reacts to a force that exceeds its load capacity, there will be rubbing between the moving and stationary surfaces that will likely damage the surfaces and either degrade or destroy the performance of the bearing. A magnetic bearing, on the other hand, has auxiliary bearings that protect the bearing in the event of an overload.
• Stop-start cycle wear. Because the load capacity of an air bearing is proportional to rotational speed, there will be a "lift-off" speed below which the shaft will rub in the bearing. The surface wear induced by this rubbing may limit the expected life of the bearing, especially for applications that require many start-stop cycles. By contrast, a magnetic bearing can achieve its full load capacity even when the shaft is not spinning. There is no wear at low speeds.

I anticipate you will continue to see turbomachines supported on air bearings for machines with shaft diameters less than about 1 in and powers less than about 100 kW. However, barring significant improvements in the design of the air bearings, I do not expect air bearings to be widely applied.