Bearings are often thought of as mechanical rolling or sliding elements, but linear (or rotary) bearings can also use a thin film of pressurized air to support a load. With no mechanical elements to generate friction or heat, air bearings are ideal for applications that require extremely high precision and stiffness.
Pressure generation and air delivery
Depending on how pressure is generated, air bearings are classified as either hydrodynamic or hydrostatic. Hydrodynamic air bearings depend on relative motion between the bearing surfaces to generate pressurized air. In contrast, a hydrostatic air bearing relies on an external supply to deliver pressurized air (or other gas). Because they can maintain an air gap even when there is no relative motion between the bearing surfaces, virtually all air bearings used in industrial applications are hydrostatic.
The gaseous medium used for air bearings is typically compressed air, which is readily available in most industrial plants and processes. However, any moisture in the air supply can develop into condensation as the air transitions from high pressure to atmospheric pressure, resulting in corrosion on the bearing surfaces. When the quality of supplied air is a concern and corrosion would be detrimental, another gas (typically nitrogen) can be used in place of compressed air. This is often the solution for clean room environments.
Adding preload to an air bearing increases stiffness and helps maintain a constant air gap. As air bearings are loaded, the air gap gets smaller and the pressure in the air film rises—both of which contribute to higher stiffness. There are four common methods for preloading air bearings: by adding weight, through magnetic attraction, through vacuum, and by using two opposed air bearings.
The simplest method for creating preload in an air bearing is to use a weight that is heavier than the load to be applied. This makes the air gap smaller, which increases the system stiffness. The drawback of the weight method of preloading is just that—it adds mass to the system. It is also suitable only for horizontal applications—not inclined or vertical orientations. Magnetic attraction between the moving and stationary parts can also induce preload. But most air bearings are made of non-magnetic material, so this method requires that that a magnetic material be added to both bearing surfaces. A third way to induce preload is to add a vacuum to the bearing surface, which creates a pressure differential and causes the external atmospheric pressure to exert force on the bearing. However, this method is only useful if a vacuum source is available and practical to install.
The most common preloading method is to configure two air bearings opposite each other. Because stiffness is additive, an assembly preloaded in this manner will have double the stiffness of a single bearing. Another benefit is that the errors on either bearing will be averaged, resulting in much higher accuracy than other preload methods can achieve. The drawback to using opposing bearings is that the load capacity will be reduced by approximately half. This method also requires additional space and doubles the mass of the bearing components.
Compensation and media types
In addition to preload, another factor that affects an air bearing’s stiffness is compensation, which is the method by which airflow into the air gap is controlled. Compensation works to create a restriction of airflow thru the orifice and into the gap, before the restriction of the gap itself. By creating a reserve pressure at the orifice, pressure can be increased in the gap, which gives the bearing stiffness.
Gas can be delivered through one of two bearing types—those with orifices or those made from porous media. In an orifice bearing, pressurized air is supplied through precisely sized holes placed on the bearing surface. These holes are typically combined with grooves to distribute the pressurized air evenly across the bearing face. The drawback of orifice delivery is that air loses pressure and creates pressure gradients in the air gap as it expands away from the orifice or groove.
Porous media air bearings allow air to be supplied through the entire surface of the bearing, due to millions of holes in the porous material. However, porous media are much more difficult to clear if they become clogged by oil vapor or other contamination.
Theoretically, the load capacity of an air bearing is simply the input pressure times the surface area. However, pressure is not equal over the bearing surface. As mentioned above, as air exits the orifice, it expands and creates pressure gradients. In addition, there is unavoidable leakage of air around the edges of the bearing. As a rule of thumb, the average pressure under the bearing surface is typically 50 percent of the supply pressure. In other words, air bearings are generally abut 50 percent efficient. (Bearings with more area near their edges, relative to their overall surface area, will be less efficient.) Load capacity is determined by the surface area of the bearing, the input pressure, and the bearing’s efficiency.
Load Capacity = Surface Area x Input Pressure x Efficiency
Benefits of air bearings over mechanical bearings
One of the primary benefits of air bearings is the elimination of friction-induced wear and heat generation. Their lack of mechanical contact also eliminates the need for lubrication, reduces particle generation, and produces much less noise than rolling or sliding bearings. With no recirculating elements, air bearings can achieve higher speeds and accelerations than ball or roller bearings.
The fluid film in an air bearing serves to average out small-scale errors, allowing air bearings to provide more accurate motion than mechanical bearings. And since the fluid film fully supports the load, in contrast to the point or line contact of balls and rollers, it also enables higher stiffness.
Air bearings are linear (or rotary) guides, and as such, are simply one part of a complete motion system. For a drive mechanism, the most common technology used in conjunction with air bearings is the ironless linear motor, due to its high precision and stiffness. (Ironcore motors can also be used, but their inherent cogging effect can generate unwanted errors.) Because of the tight tolerances required for air bearing operation, components used in an air bearing system must be extremely accurate. For example, bearings should be mounted on a precision base—preferably granite, for its ability to be precisely machined and its dimensional stability—with recommended flatness of 1 micron per 50 mm. All of this contributes to the extremely high accuracy motion and stiffness that are the main features of an air bearing system
Feature image credit: Aerotech Inc.