Air bearings provide several advantages over rolling element bearings, including higher travel accuracy and reduced friction and heat generation. But because air is compressible, air bearings can be less stiff and exhibit more deflection under load than similar rolling element bearings (although the stiffness characteristics of air bearings are very linear). However, the compressible nature of air can be used to create a preloaded air bearing, which increases its stiffness.
Just as preload in a rolling element bearing is a load induced between a rolling element and its raceway, preload in an air bearing is a load induced between the bearing and its guide surface. There are four common methods for inducing preload on an air bearing: by adding mass to the bearing, by applying a magnetic force, by applying vacuum, or by mounting two air bearings facing each other on opposite sides of a guide surface.
Regardless of the preloading method, as the load is applied to the air bearing (whether via added mass, opposing forces, magnetic force, or vacuum), the air film that supports the bearing compresses, the air gap gets smaller, the pressure in the air film increases, and the air film becomes stiffer.
Each preloading method has advantages and drawbacks, but for applications where added mass would significantly compromise acceleration and settling times, where space constraints make it difficult to mount two air bearings in an opposing orientation, or where adding magnetic material to the surfaces would be too costly, vacuum preloading offers a relatively simple solution that doesn’t add mass or require special assembly and mounting considerations.
Although it might seem that air pressure and vacuum would work against each other, the level of vacuum used for a preloaded air bearing is relatively low compared to the bearing pressure, so the bearing is still lifted from the guide surface by the applied air pressure.
To achieve preload, vacuum is applied to a portion of the bearing surface — typically the center portion. This means that the bearing pressure is applied around the perimeter, creating a seal to contain the vacuum area and prevent the vacuum from drawing in any contamination from the environment. A groove between the vacuum area and the bearing area is connected to ambient pressure and prevents flow between the exiting bearing air and the vacuum. The preload force is equal to the size of the vacuum area multiplied by the differential between the bearing pressure and the amount of vacuum.
The pressure at the area where vacuum is applied is independent of the bearing flying height (the distance above the mounting surface), so preload remains constant even if the flying height changes. For example, if the flying height increases, the pressure in the bearing area decreases, so that the bearing experiences a higher pulling force (preload). This interplay between the air pressure and vacuum ensures an equilibrium between the preload force and the pressure for a given flying height.
Not only do vacuum preloaded air bearings have high stiffness, by adjusting the differential between the bearing pressure and the vacuum, the thickness of the air film — and in turn, the flying height — can be adjusted, making vacuum preloaded versions ideal for applications that involve ultra-fine, precise vertical positioning, such as lens focusing.
Filed Under: Linear Motion Tips