6. Air bearing platforms will be unstable / aren’t stiff enough.
Poorly installed or overloaded air bearing systems can be unstable. However, if sized properly and mounted correctly, a well-tuned air bearing system will be extremely stable. Since there is no friction, there will always be dithering in the direction of travel, usually +/- a few encoder counts. This can be overcome by locking down the air bearing if needed.
Air bearings may be less stiff than a mechanical bearing, but the stiffness of an air bearing is more linear and much better understood than that of mechanical bearings. Mechanical bearings suffer from many non-linear effects and many factors tied to how they are installed and manufactured that cannot always be predicted.
7. Air bearings are great for horizontal applications, but cannot be used vertically.
Air bearings can be used vertically by counterbalancing the payload with an air cylinder. The PI air bearing engineering group has extensive experience with these applications, having designed and built 1,000s of such units.
The need to counterbalance in a vertical application is not unique to air bearings. Any linear stage driven by a voice-coil or linear motor will require counterbalance of the payload to prevent overheating of the motor coil.