What is air bearing
Bearing is a concept most people are familiar with. The commonly used bearing is probably the most recognizable. It is a crucial component of modern mechanical equipment. But what exactly is a pneumatic bearing?
The idea of using gas as a lubricant was first proposed by the Frenchman GA Ian in 1854. It was not until 1896 that the first pneumatic bearing was introduced.
A pneumatic bearing is a type of sliding bearing that uses gas as a lubricant. The most common gas used for this purpose is air, but other gases such as nitrogen, argon, hydrogen, helium or carbon dioxide can be used as needed. The gas is often used as a lubricant in compressors, expanders and circulators.
A pneumatic bearing operates by using the pressure of a film of air to support the load between the bearing's sliding surfaces. During operation, the sliding surfaces are completely separated by a film of air. Pneumatic bearings belong to the category of fluid sliding bearings and use fluid lubrication, with air as the lubricating medium.
According to the formation mechanism of air film under pressure, pneumatic bearings are categorized into two types: aerodynamic bearings and aerostatic bearings.
The air film under pressure in the aerodynamic bearings is generated by the movement of the sliding pair, which draws air to the convergence area between the surfaces. This results in a wedge-shaped film of air, as illustrated in Figure 1. Aerodynamic bearings are called “self-acting bearings” because they do not require an external air supply.
In contrast, the pressurized air film in aerostatic bearings is created by introducing compressed air from an external source to the surface of the sliding pair through a restrictor, as shown in Figure 2. It is important to note that aerostatic bearings require a source of clean outside air.
Fig. 1 Working principle of aerodynamic bearing
Fig. 2 Working principle of aerostatic bearing
Air bearing characteristics
① Low friction
Gas has a much lower viscosity compared to liquids, with the viscosity of air being only one-fifth that of No. 10 mechanical oil at room temperature. The friction of a bearing is directly proportional to its viscosity, making the friction of a gas bearing less than that of a liquid-lubricated bearing.
② Wide speed range
Gas bearings have low friction and generate little heat, even at high speeds of up to 50,000 RPM, with a temperature rise of only 20 to 30 degrees Celsius. Aerostatic bearings can also be used at very low speeds or even zero speeds.
③ Wide temperature range
The gas can remain gaseous over a wide range of temperatures and its viscosity is relatively unaffected by temperature changes. For example, when the temperature increases from 20 to 100 degrees Celsius, the viscosity of the air increases by only 23%. As a result, gas bearings can be used in a temperature range of -265 to 1,650 degrees Celsius.
④ Low load capacity
The load capacity of hydrodynamic bearings is directly proportional to their viscosity. The load capacity of a gas hydrodynamic bearing is only a few thousandths of that of a liquid hydrodynamic bearing of the same size. Gas compressibility also limits the load capacity of a gas hydrodynamic bearing, with a typical maximum load per unit design area of 0.36 MPa.
⑤ High machining precision
To increase the load-bearing capacity and gas film rigidity of gas bearings, a smaller clearance (less than 0.015 mm) is generally used compared to liquid-lubricated bearings. This requires a corresponding improvement in component accuracy.
Air bearing application
Pneumatic bearings are a new type of bearing that relies on aeroelastic potential energy for support. Unlike traditional bearings, pneumatic bearings use air as the only lubricant and are ideal for applications where pollution is not permitted.
In pneumatic bearings, the traditional ball bearing is replaced by an air cushion. A well-known example of air bearings in action are hovercrafts. Huge fans blow air under the hovercraft, which is prevented from escaping by an elastic rubber “skirt.” The resulting high air pressure supports the weight of the hull, causing the hovercraft to float on the air cushion.
Gas bearings have been increasingly used and studied since the 1950s. Today, they are used in a variety of applications, including textile machines, cable machines, instrument machine tools, gyroscopes, high-speed centrifuges. speed, dental drills, low temperature refrigerators, hydrogen expanders and high temperature gas circulators.
