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Characteristics and main performance of medium air bearing in precision motion system2020-07-30

The working principle of air bearing is to support the load by using the air film formed between the relative moving parts of compressed gas. The thickness of the gas film is between 1 and 10 microns. Because the viscosity coefficient of the gas is very high, the thickness of the lubricating film can be very small.

There are five types of structure: single pad, opposed pad, journal, rotary thrust and cone journal / thrust bearings, which are very close to hydraulic bearings.

Because there is no pollution problem, compressed air does not need to be recycled, and is generally directly discharged into the atmosphere

Compared with hydraulic bearing, air bearing has a short history. Air bearing is also produced and developed to meet some special needs in the process of modern industrial development. For example, high speed, high stiffness, extremely high or very low ambient temperature and other working conditions promote the development and application of air bearing; the improvement of computer calculation ability makes the design and analysis of air bearing become more and more easy.

At present, air bearing has been widely used in many industrial fields. The common ones are CMM CMM, LVDT displacement sensor, gear detection equipment, precision machine tools, etc.

Compared with hydraulic bearing, air bearing has the following characteristics: 1. Air bearing is suitable for medium load, medium stiffness and high speed; hydraulic bearing is suitable for large load, high stiffness and medium speed occasions; 2. Damping characteristics of hydraulic bearing are much better than air bearing; 3. Because air film of air bearing is very thin, it requires high machining precision of parts, so the cost is relatively high.

Main performance of air bearing:

1. Speed and acceleration limits. The linear speed of 10 m / S is a barrier for air bearing. When the speed is too high, overcoming the viscous friction becomes a problem. Therefore, for the air bearing with high speed, the film thickness should be large enough and stable.

2. Scope of work. Linear motion air bearing can work in any length of travel, as long as the corresponding guide surface is long enough. In practical application, there are air bearings with tens of meters of travel. The rotating air bearing is not limited by the working angle.

3. Bearing capacity. The bearing capacity is related to the area of air film. Because of the large area of air bearing, it can bear a large load. But compared with the hydraulic bearing, it is much smaller, about 1 / 5 of the hydraulic bearing.

4. Accuracy index. The accuracy of air bearing is usually related to the accuracy of mechanical parts. The requirement of surface roughness is less than 1 / 4 of air film thickness. For example, the surface roughness of bearing surface of air bearing with 10um thickness should be less than 2.5um. Since there is no wear, air bearings can usually be made to sub micron / meter accuracy range.

5. Repeat positioning accuracy. If the compressed gas is clean enough, the bearing design is reasonable, there is no air hammer phenomenon in the selected pressure range, and the ambient temperature is stable, the air bearing can generally achieve the repeatability accuracy of submicron or even nanometer.

6. Resolution. If the performance of the control system is good enough, the resolution of air bearing can be infinitesimal in theory.

7. Preload. In order to ensure the bearing stiffness, the air bearing needs preloading. The main methods of preloading are symmetrical installation, vacuum preloading and magnetic preloading.

8. Stiffness coefficient. Generally, the stiffness of air bearing can reach 100N / um. Now the performance of air bearing has been well understood. There are many empirical formulas and charts, which can be easily determined by calculation.

9. Absorption of vibration and shock. Air bearings can better absorb external vibration and shock. Because of the compressibility of the gas, the design and use of air bearing should avoid the occurrence of air hammer. At the same time, in order to avoid the instability caused by pressure change, it is better to add an air storage tank in the gas path to stabilize the system pressure.

10. Damping characteristics. The viscous force of air bearing is very small and the damping characteristic is general.

11. Friction. The air bearing has no static friction and very little dynamic friction. For applications less than 2 m / s, friction is negligible.

12. Temperature characteristics. Because there is no friction force, the heat produced by air bearing is very small, which can ensure the accuracy of the system. However, there is cooling effect when the gas expands from high pressure to low pressure. Therefore, for high-precision system, the smaller the air consumption, the better.

13. Environmental sensitivity. The compressed air flows out from the air film, which can blow away the dirt on the guide rail surface. The air bearing has the ability of self-cleaning and has no pollution residue. But in order to protect the bearing, it is better to have a protective cover.

14. Dimensions and bearing installation. The structure of air bearing is simple, the size and weight are very small; however, due to the thin air film and small stiffness, the installation position and the installation accuracy between them are required to be higher

15. Weight. Air bearing has medium to high performance weight ratio.

16. Annex. The biggest drawback of air bearing is the high requirement for the purity of compressed gas, which generally requires filtering accuracy below 1 μ M.

17. Maintenance. The main maintenance work of air bearing is to check the purity and pressure of gas periodically, and the filter needs to be replaced regularly. The service life of air bearing system is more than 10 years because of no wear and tear in normal use.

18. Materials. Air bearings have no special requirements for materials, but in order to ensure the stability of oil film thickness, it is better to use materials with similar temperature characteristics to manufacture guide rails and bearing pads. Because different materials are used




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