Air bearings use a thin film of pressurized air to support a load. This type of bearing is called a "fluid film" bearing. Fluid film bearings have no solid-to-solid contact under typical running conditions; instead, a film of lubricating fluid (in this case pressurized air) forms a layer between the solid machine elements and serves to transfer forces from one to the other.
The fluid film of the bearing is achieved by supplying a flow of air through the bearing itself to the bearing surface. This air cushion may be achieved in a couple of ways. The most common means of creating an air film is with an orifice. Other designs deliver the air through a porous medium to ensure uniform pressure across the entire bearing area. The design of the air bearings is such that, although the air constantly dissipates from the bearing site, the continual flow of pressurized air through the bearing is sufficient to support the working loads.
Air bearings allow designers leverage for precision and high-speed applications. Unlike contact roller bearings, air bearings utilize a thin film of pressurized air to provide a frictionless load-bearing interface between surfaces that would otherwise be in contact with each other. Since air bearings are non-contact, they avoid the traditional bearing-related problems of friction, wear, and the need for a lubricant. Air bearings offer distinct advantages in precision positioning and high-speed applications.
Advantages to Using Air Bearings
There are many advantages to using air bearings where possible in place of bearings requiring surface contact. Because of zero static friction, infinite resolution and very high repeatability are possible. This also greatly reduced wear seen in other bearing types thus providing consistent machine performance and low particle generation. Air bearings are not affected by surface discontinuities since there is no contact. With no moving parts as in roller bearings, for instance, the noise level is minimized in air bearings. Being fluid film bearings, air bearings have a squeeze film damping effect resulting in higher dynamic stiffness and better controllability. Using air instead of oil, air bearings self lubricate with air and the air pushes away dust rather than attracting it as oil does in traditional bearings. Air bearings offer consistently superior performance in acceleration and speeds since there are no balls to slip at high accelerations.
As a general rule, air bearings are about 60% efficient. Air pressure, times bearing area equals maximum theoretical lift, times 60% equals actual load capacity. Although this is a reliable design guideline for air bearings, lower air pressures, increased damping, increased stability and increased stiffness are all improved with increased surface area.
Guide Surface Choices
Granite and hardcoat aluminum are the most popular guide surface choices. Air bearings can run on steel such as hardened steel bands or inserts, however there is no oil and steels used must be corrosion resistant. Polymers represent an opportunity for large cost savings in the production of precision guide way surfaces. The surface finish should be 16 RMS or better. Local flatness-straightness (the area under a bearing at any one time) should be flat to within 50% of the air gap in air bearings.
Typical applications for air bearings include use in coordinate measuring machines, semiconductor processing machines and other clean room, high speed, and precision environments.
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