What is air bearing
Bearing, I believe everyone will not be unfamiliar. Our commonly used rolling bearing should be the most familiar one. It is an important part of contemporary mechanical equipment. What is air bearing?
In 1854, the Frenchman G. A. Ian proposed the idea of using gas as lubricant. Finally, in 1896, the first air bearing came out.
Air bearing: sliding bearing with gas as lubricant. The most commonly used gas lubricant is air. Nitrogen, argon, hydrogen, helium or carbon dioxide can also be used as needed. Working medium is often used as lubricant in gas compressor, expander and circulator.
Air bearing refers to the bearing that supports the load with the help of the pressure air film formed between the sliding pair surfaces of the bearing. During operation, the sliding pair surfaces are completely separated by the air film. Air bearing belongs to fluid sliding bearing in sliding bearing. It is fluid lubrication when working, and its lubricating medium is air.
According to the formation mechanism of pressure air film, air bearings are mainly divided into two categories: aerodynamic bearings and aerostatic bearings.
The pressure air film of the aerodynamic bearing is formed by bringing air into the convergence area between the surfaces of the sliding pair through the mutual movement of the sliding pair. The air film is roughly wedge-shaped, as shown in Fig. 1. Aerodynamic bearings are also called “self acting bearings” because they do not require external air supply.
The pressure air film of aerostatic bearing is formed by introducing the external compressed air into the sliding pair surface through the restrictor, as shown in Figure 2. Aerostatic bearings require a clean external air source.
Fig. 1 working principle of aerodynamic bearing
Fig. 2 working principle of aerostatic bearing
Characteristics of air bearing
① Very low friction
Because the viscosity of gas is much lower than that of liquid, the viscosity of air is only one fifth of that of No. 10 mechanical oil at room temperature, and the friction of bearing is directly proportional to the viscosity, so the friction of gas bearing is lower than that of liquid lubricated bearing.
② Wide applicable speed range
Gas bearing has low friction and low temperature rise. When the speed is up to 50000 rpm, its temperature rise does not exceed 20 ~ 30 ℃, and the speed is even up to 1.3 million rpm. Aerostatic bearings can also be used at very low speeds or even zero speeds.
③ Wide temperature range
The gas can remain gaseous in a great temperature range, and its viscosity is little affected by temperature (the viscosity increases slightly when the temperature increases, such as the temperature rises from 20 ℃ to 100 ℃, and the air viscosity increases by 23%). Therefore, the applicable temperature range of gas bearing can reach – 265 ℃ to 1650 ℃.
④ Low bearing capacity
The bearing capacity of hydrodynamic bearing is directly proportional to viscosity. The bearing capacity of gas hydrodynamic bearing is only a few thousandths of that of liquid hydrodynamic bearing with the same size. Due to the compressibility of gas, the bearing capacity of gas hydrodynamic bearing has a limit value. Generally, the load per unit projected area can only be added to 0.36 MPa.
⑤ High machining accuracy
In order to improve the bearing capacity and gas film stiffness of gas bearing, the bearing clearance (less than 0.015mm) smaller than that of liquid lubricated bearing is usually used, and the part accuracy needs to be improved accordingly.
Application of air bearing
Air bearing is a new type of bearing which uses aeroelastic potential energy to support. The only lubricant used is air; Therefore, air bearing technology is ideal for the workpiece or working environment that must require no pollution.
In air bearings, the ball is replaced by an air cushion. Perhaps one of the most well-known applications of air bearings is hovercraft.
Huge fans blow air under the hovercraft, preventing the escape of air through elastic rubber “skirt”. The high air pressure generated under the hovercraft can support the weight of the hull, so it floats on the air cushion.
Since the 1950s, gas bearing has been more and more widely used, and has been widely and deeply studied. At present, gas bearings can be used in textile machinery, cable machinery, instrument machine tools, gyroscopes, high-speed centrifuges, dental drills, refrigerators operating at low temperature, hydrogen expanders and gas circulators operating at high temperature.