What is an Air Bearing?

Bearings are a familiar concept to most people, with rolling bearings being the most recognizable type, as they are crucial components in modern mechanical equipment. But what exactly is an air bearing?

An air bearing is a type of sliding bearing that uses gas as a lubricant. The most commonly used gas lubricant is air, but nitrogen, argon, hydrogen, helium, or carbon dioxide can also be used as needed. In gas compressors, expanders, and circulators, the working medium often serves as the lubricant.

An air bearing is a bearing that supports the load by means of a pressure air film formed between the sliding surfaces of the bearing. During operation, the sliding surfaces are completely separated by the air film. Air bearings belong to the category of fluid film bearings and operate with fluid lubrication, where the lubricating medium is air.

Based on the mechanism of pressure air film formation, air bearings are mainly divided into two types: aerodynamic bearings and aerostatic bearings.

The pressure air film in an aerodynamic bearing is formed by the relative motion of the sliding surfaces, which brings air into the converging area between the surfaces, creating a roughly wedge-shaped air film, as shown in Figure 1. Since aerodynamic bearings do not require an external air source, they are also known as "self-acting bearings."

The pressure air film in an aerostatic bearing is formed by introducing external compressed air through a restrictor into the space between the sliding surfaces, as shown in Figure 2. Aerostatic bearings require a clean external air source.

Figure 1: Principle of Aerodynamic Bearing Operation  Figure 2: Principle of Aerostatic Bearing Operation

Characteristics of Air Bearings

① Extremely Low Friction Due to the significantly lower viscosity of gases compared to liquids, the viscosity of air at room temperature is only one five-thousandth of that of No. 10 machine oil. Since bearing friction is directly proportional to viscosity, gas bearings have much lower friction than liquid-lubricated bearings.

② Wide Range of Suitable Speeds Gas bearings have low friction and low temperature rise. At speeds up to 50,000 rpm, the temperature rise does not exceed 20°C to 30°C, and the speed can even reach 1.3 million rpm. Aerostatic bearings can also be used at very low speeds, even at zero speed.

③ Broad Temperature Range Gas can remain in a gaseous state over a wide temperature range, and its viscosity is slightly affected by temperature (the viscosity increases slightly with temperature, e.g., air viscosity increases by 23% when the temperature rises from 20°C to 100°C). Therefore, the applicable temperature range of gas bearings can reach from -265°C to 1650°C.

④ Low Load Capacity The load capacity of dynamic pressure bearings is directly proportional to viscosity, so the load capacity of gas dynamic pressure bearings is only a few thousandths of that of the same-sized liquid dynamic pressure bearings. Due to the compressibility of gases, there is a limit to the load capacity of gas dynamic pressure bearings, generally only up to 0.36 MPa per unit projected area.

⑤ High Machining Precision Requirements To improve the load capacity and air film stiffness of gas bearings, smaller bearing clearances (less than 0.015 mm) than those of liquid-lubricated bearings are typically used, which requires a corresponding increase in component precision.

Applications of Air Bearings

Air bearings are a new type of bearing that uses the elastic potential energy of air to support loads. The only lubricant used is air; therefore, air bearing technology is ideal for applications that require no contamination or for environments where污染-free operation is essential.

In air bearings, the balls are replaced by an air cushion. One of the most well-known applications of air bearings might be the hovercraft.

Huge fans blow air beneath the hovercraft, and an elastic rubber "skirt" prevents the air from escaping. The high-pressure air generated beneath the hovercraft supports the weight of the vessel, allowing it to float on an air cushion. Since the 1950s, the application of gas bearings has become increasingly widespread and has been the subject of extensive and in-depth research. Currently, gas bearings can be used in textile machinery, cable machinery, precision lathes, gyroscopes, high-speed centrifugal separators, dental drills, low-temperature operating refrigerators, hydrogen expanders, and high-temperature operating gas circulators, among others.