Bearings are an essential component in modern machinery, and most people are familiar with rolling bearings, which are probably the most widely used type. These bearings play a crucial role in various machines and mechanical devices. But what exactly is an air bearing?
Air Bearing (gas bearing): An air bearing is a type of sliding bearing that uses gas as a lubricant. The most common gas used as a lubricant is air, but depending on the specific needs, other gases such as nitrogen, argon, hydrogen, helium, or carbon dioxide can also be used. In gas compressors, expanders, and circulators, the working medium itself often serves as the lubricant.
An air bearing works by utilizing a pressure air film that is formed between the bearing's sliding surfaces to support the load. During operation, the sliding surfaces are entirely separated by this air film. Air bearings are a type of fluid sliding bearing, where air acts as the lubricant, and the operation is therefore considered fluid lubrication.
Based on the mechanism for forming the pressure air film, air bearings are mainly divided into two types: hydrodynamic air bearings and static air bearings.
Hydrodynamic Air Bearings: In these bearings, the pressure air film is generated by the relative motion of the sliding surfaces. This motion brings air into the converging region between the sliding surfaces, creating a wedge-shaped air film. Hydrodynamic air bearings do not require an external air supply, and thus, they are also known as "self-acting bearings."
Static Air Bearings: In static air bearings, the pressure air film is formed by external compressed air, which is directed into the space between the sliding surfaces through a throttle valve. Static air bearings require an external clean air supply.
Characteristics of Air Bearings
Extremely Low Friction
Air bearings have significantly lower friction compared to traditional liquid-lubricated bearings. This is because gases have much lower viscosity than liquids. At room temperature, the viscosity of air is only one five-thousandth that of No. 10 mechanical oil. Since bearing friction is directly proportional to viscosity, the friction of air bearings is much lower than that of liquid-lubricated bearings.
Wide Operating Speed Range
Air bearings offer a vast operating speed range due to their low friction and low temperature rise. When operating at speeds as high as 50,000 rpm, the temperature rise typically does not exceed 20–30°C. In some cases, air bearings can operate at speeds exceeding 1.3 million rpm. Furthermore, static air bearings can function even at extremely low speeds, including zero speed, which makes them suitable for very specialized applications.
Wide Temperature Range
Gases maintain their gaseous state over a wide temperature range. The viscosity of gases is only slightly affected by temperature; for example, the viscosity of air increases by only 23% when the temperature rises from 20°C to 100°C. Therefore, air bearings are capable of operating in an extremely broad temperature range, from -265°C to 1650°C, making them highly versatile for various extreme temperature environments.
Low Load Capacity
The load capacity of hydrodynamic air bearings is directly proportional to the viscosity of the gas. However, compared to liquid hydrodynamic bearings of the same size, the load capacity of air bearings is much lower, usually only a fraction. Additionally, due to the compressibility of gases, air hydrodynamic bearings have a limited load-bearing capacity. Typically, the load on a unit projected area can only reach up to 0.36 MPa.
High Precision Manufacturing Requirements
To improve the load capacity and stiffness of the air film in air bearings, smaller bearing gaps are typically used compared to liquid-lubricated bearings (often less than 0.015 mm). This requires higher precision in the manufacturing of the components to ensure proper functioning and performance.
Applications of Air Bearings
Air bearings are a new type of bearing that utilizes the elastic potential energy of air to provide support. The only lubricant used is air, which makes air bearing technology ideal for applications requiring contamination-free workpieces or clean operating environments.
Air bearings replace rolling elements with air cushions. One of the most well-known applications of air bearings is in hovercrafts.
In hovercrafts, large fans blow air beneath the craft. This air is contained by the elastic rubber "skirt" that prevents it from escaping. The high-pressure air produced under the hovercraft supports the weight of the vehicle, allowing it to float on the air cushion.
Since the 1950s, the application of air bearings has expanded greatly, and they are now widely studied and used in various industries. Air bearings are used in several fields such as textile machinery, cable machinery, precision machine tools, gyroscopes, high-speed centrifuges, dental drills, refrigeration machines operating at low temperatures, hydrogen expanders, and gas circulators operating at high temperatures.
These bearings are particularly beneficial in clean environments, as they do not require oil or other traditional lubricants, reducing the risk of contamination. This makes them perfect for applications in sensitive industries like semiconductor manufacturing, aerospace, and high-precision engineering.
For example, air bearings are used in high-speed machine tools, where they allow for frictionless movement, leading to minimal wear and tear. This improves the accuracy and longevity of the machines. In gyroscopes, air bearings help ensure stable rotation with minimal friction, providing highly accurate orientation data. In precision measurement systems, air bearings support delicate instruments, ensuring they operate with high precision without being affected by mechanical contact.
In the textile industry, air bearings are used in high-speed spinning machines, allowing for smooth operation without the risk of oil contamination. Similarly, in cable manufacturing, air bearings help maintain smooth and efficient movement of wires and cables through machinery.
In aerospace, air bearings are used in turbine engines, where they support moving parts without causing additional wear and friction. These bearings can withstand the extreme temperatures and pressures experienced in jet engines and provide long-lasting, reliable performance.
The medical field also benefits from air bearing technology. For instance, in dental equipment, air bearings are used in high-speed drills to ensure smooth operation with minimal friction, improving both the precision of the equipment and the comfort of the patient.
Furthermore, air bearings are essential in low-temperature refrigeration systems, where they help maintain the precise movement of components while operating at very low temperatures. In hydrogen expanders, air bearings are used to support the movement of machinery in a highly efficient, clean manner.
Finally, in high-temperature gas circulators, air bearings play a crucial role in ensuring the smooth and stable movement of components in environments with temperatures that would destroy traditional lubricants.
Conclusion
In conclusion, air bearings are a revolutionary technology that has broadened the scope of bearing applications in industries ranging from high-speed machinery to aerospace and medical equipment. Their ability to operate without traditional lubricants, combined with their low friction, wide temperature range, and precise performance, makes them ideal for many specialized applications.
Although they have some limitations, such as lower load capacity compared to liquid lubricated bearings, their advantages in clean, high-speed, and high-precision environments make them indispensable in many modern technological fields. As research continues, it is likely that air bearings will find even more applications, further demonstrating their versatility and importance in the development of advanced machinery and equipment.
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