Do You Know About Air Bearings? Let’s Learn Relevant Knowledge
What Is an Air Bearing?
Bearings are familiar to everyone, and rolling bearings are the most common type, serving as vital components in modern mechanical equipment. So what exactly is an air bearing?
Air bearing (gas bearing): A plain bearing using gas as lubricant. Air is the most commonly used gaseous lubricant; nitrogen, argon, hydrogen, helium, carbon dioxide and other gases can also be adopted as required. In gas compressors, expanders and circulators, the working medium is often utilized as the lubricant.
An air bearing supports loads by means of a pressurized air film formed between the sliding surfaces of the bearing. During operation, the sliding surfaces are completely separated by the air film. As a type of hydrodynamic plain bearing, air bearings operate under fluid lubrication with air serving as the lubricating medium.
Based on the formation mechanism of the pressurized air film, air bearings are mainly divided into two categories: aerodynamic air bearings and aerostatic air bearings.
The air film of an aerodynamic air bearing is formed by relative motion of sliding surfaces, which draws air into the converging zone between the mating surfaces. The air film generally presents a wedge shape (see Figure 1). Since aerodynamic air bearings require no external air supply, they are also known as self-acting bearings.
The pressurized air film of an aerostatic air bearing is formed when externally supplied compressed air is introduced between sliding surfaces via restrictors (see Figure 2). Aerostatic air bearings demand a clean external air source.
Characteristics of Air Bearings
Extremely Low Friction Resistance
Gas has far lower viscosity than liquid. At room temperature, the viscosity of air is only 1/5000 of that of No.10 machine oil. Bearing friction resistance is proportional to viscosity, so gas bearings exhibit lower friction than liquid-lubricated bearings.
Wide Applicable Speed Range
Benefiting from low friction and low temperature rise, aerostatic bearings generate a temperature increase of no more than 20~30°C even at a rotational speed of 50,000 rpm; some can operate at speeds up to 1.3 million rpm. Aerostatic bearings can also work at extremely low speeds, even zero speed.
Broad Operating Temperature Range
Gas maintains its gaseous state over an extensive temperature range, and its viscosity is barely affected by temperature (viscosity rises slightly with temperature; for instance, air viscosity increases by 23% when temperature rises from 20°C to 100°C). Therefore, gas bearings can operate within a temperature range from -265°C to 1650°C.
Low Load Capacity
The load capacity of hydrodynamic bearings is proportional to viscosity. The load capacity of aerodynamic bearings is only several thousandths of that of hydrodynamic liquid bearings of identical dimensions. Due to gas compressibility, aerodynamic bearings have a load limit; generally, the load on the unit projected area can only reach 0.36 MPa.
High Requirements for Machining Accuracy
To improve the load capacity and air film stiffness of gas bearings, smaller bearing clearances (less than 0.015 mm) are normally adopted compared with liquid-lubricated bearings, which correspondingly requires higher precision of components.
Applications of Air Bearings
Air bearings are innovative bearings that realize support by utilizing the elastic potential energy of air. Air acts as the sole lubricant. For workpieces or operating environments that demand zero contamination, air bearing technology is an ideal solution.
In air bearings, air cushions replace rolling balls. One of the best-known applications of air bearings may be hovercrafts.
Large fans blow air beneath the hovercraft, and flexible rubber skirts prevent air leakage. The high-pressure air generated under the craft supports its weight, enabling the hovercraft to float on the air cushion.
Since the 1950s, gas bearings have been applied more and more widely and subjected to extensive in-depth research. At present, gas bearings can be used in textile machinery, cable machinery, instrument machine tools, gyroscopes, high-speed centrifugal separators, dental handpieces, low-temperature refrigerators, hydrogen expanders, high-temperature gas circulators and other equipment.
Bearings are familiar to everyone, and rolling bearings are the most common type, serving as vital components in modern mechanical equipment. So what exactly is an air bearing?
Air bearing (gas bearing): A plain bearing using gas as lubricant. Air is the most commonly used gaseous lubricant; nitrogen, argon, hydrogen, helium, carbon dioxide and other gases can also be adopted as required. In gas compressors, expanders and circulators, the working medium is often utilized as the lubricant.
An air bearing supports loads by means of a pressurized air film formed between the sliding surfaces of the bearing. During operation, the sliding surfaces are completely separated by the air film. As a type of hydrodynamic plain bearing, air bearings operate under fluid lubrication with air serving as the lubricating medium.
Based on the formation mechanism of the pressurized air film, air bearings are mainly divided into two categories: aerodynamic air bearings and aerostatic air bearings.
The air film of an aerodynamic air bearing is formed by relative motion of sliding surfaces, which draws air into the converging zone between the mating surfaces. The air film generally presents a wedge shape (see Figure 1). Since aerodynamic air bearings require no external air supply, they are also known as self-acting bearings.
The pressurized air film of an aerostatic air bearing is formed when externally supplied compressed air is introduced between sliding surfaces via restrictors (see Figure 2). Aerostatic air bearings demand a clean external air source.
Characteristics of Air Bearings
Extremely Low Friction Resistance
Gas has far lower viscosity than liquid. At room temperature, the viscosity of air is only 1/5000 of that of No.10 machine oil. Bearing friction resistance is proportional to viscosity, so gas bearings exhibit lower friction than liquid-lubricated bearings.
Wide Applicable Speed Range
Benefiting from low friction and low temperature rise, aerostatic bearings generate a temperature increase of no more than 20~30°C even at a rotational speed of 50,000 rpm; some can operate at speeds up to 1.3 million rpm. Aerostatic bearings can also work at extremely low speeds, even zero speed.
Broad Operating Temperature Range
Gas maintains its gaseous state over an extensive temperature range, and its viscosity is barely affected by temperature (viscosity rises slightly with temperature; for instance, air viscosity increases by 23% when temperature rises from 20°C to 100°C). Therefore, gas bearings can operate within a temperature range from -265°C to 1650°C.
Low Load Capacity
The load capacity of hydrodynamic bearings is proportional to viscosity. The load capacity of aerodynamic bearings is only several thousandths of that of hydrodynamic liquid bearings of identical dimensions. Due to gas compressibility, aerodynamic bearings have a load limit; generally, the load on the unit projected area can only reach 0.36 MPa.
High Requirements for Machining Accuracy
To improve the load capacity and air film stiffness of gas bearings, smaller bearing clearances (less than 0.015 mm) are normally adopted compared with liquid-lubricated bearings, which correspondingly requires higher precision of components.
Applications of Air Bearings
Air bearings are innovative bearings that realize support by utilizing the elastic potential energy of air. Air acts as the sole lubricant. For workpieces or operating environments that demand zero contamination, air bearing technology is an ideal solution.
In air bearings, air cushions replace rolling balls. One of the best-known applications of air bearings may be hovercrafts.
Large fans blow air beneath the hovercraft, and flexible rubber skirts prevent air leakage. The high-pressure air generated under the craft supports its weight, enabling the hovercraft to float on the air cushion.
Since the 1950s, gas bearings have been applied more and more widely and subjected to extensive in-depth research. At present, gas bearings can be used in textile machinery, cable machinery, instrument machine tools, gyroscopes, high-speed centrifugal separators, dental handpieces, low-temperature refrigerators, hydrogen expanders, high-temperature gas circulators and other equipment.
