Gas lubricated bearings are plain bearings that use gas as a lubricant.It makes use of the transmissibility (diffusivity, viscosity and thermal conductivity), absorbability and compressibility of the gas to form a complete gas film between the friction pairs under the action of fluid dynamic pressure effect, static pressure effect and extrusion effect, which has the function of supporting load and reducing friction.Gas lubricated bearings are generally divided into three types: gas dynamic pressure bearings, gas static pressure bearings and gas extrusion bearings.The actual lubrication state of bearing often exists in the form of dynamic, static pressure, dynamic, extrusion, static, extrusion and dynamic, static and extrusion mixed lubrication state.
1.Advantages of gas lubrication
(1) high speed
The rotational speed of the small helium turbine expander has reached 650 000r/min, and the linear speed is 238m/s, which has reached 455x106mm·r/min according to the dn value.
(2) high precision
The gas film has error homogenization, its precision is two orders of magnitude higher than the rolling bearing, and the vibration is small, the noise is low.
(3) small friction loss
The gas viscosity is only one thousandth of the viscosity of the lubricating oil, so the friction torque caused by the friction in the gas film is three orders of magnitude lower than that of the oil film.
(4) applicable to a wide temperature range, can resist atomic radiation
High temperature resistance up to 300 -- 500℃, temperature as low as 10K gas bearings can still work.
(5) no pollution
Lubrication gas is generally air or inert gas, emissions to the atmosphere does not produce pollution.The vibration of bearing is small, the noise is low, does not constitute the pollution to the environment.
(6) long service life
Gas hydrostatic bearing has no direct contact with metal, and its theoretical life is infinite.Consider other conditional restriction, should be above 104h commonly.When the dynamic pressure gas bearing starts and stops, there is inter-metal friction. The service life is affected to some extent, but it is also over 3000h.
2.Defects of gas lubricated bearings
(1) low carrying capacity
The bearing capacity of gas-lubricated bearings is about 1 ~ 2 orders of magnitude lower than that of similar oil-lubricated bearings.According to the bearing capacity per unit area, oil-lubricated bearings are generally (0.5 ~ 2)MPa, while gas-lubricated bearings are only (0.02 ~ 0.04)MPa.
(2) poor reliability
On the one hand, the gas-lubricated bearing is easy to be unstable, and the common vibration and eddy instability phenomena of air hammer appear.On the other hand is the gas lubrication bearings stuck, shaft or bite phenomenon also occur.
(3) the required manufacturing precision is high and the cost is expensive
Because the gas film is very thin, generally in a few microns to dozens of microns, so the parts manufacturing precision requirements higher, increased the cost.In addition, there are special technological requirements for machining spiral grooves, throttling holes and porous materials, which increase the manufacturing difficulty and cost.
(4) demanding working conditions
Gas lubricated bearings require a high degree of cleanliness and strict compliance with operating procedures to ensure that the bearings work properly and do not cause accidents.Therefore, to some extent, it has limited its wide application and commercialization.
Application of gas lubricated bearings
Application example of gas lubricated bearing in machine tool
Gas lubricated bearings are mainly used for spindle support in machine tools, including workpiece shaft, tool shaft, sand wheel shaft.Machine tool spindle bearings are generally pneumatic static pressure bearings and hybrid bearings.Small high speed spindle can also be used wave foil bearings.The aerostatic bearing is most widely used in machine tools, among which orifice orifice and torus orifice orifice bearings are the most common.In recent years, porous aerostatic bearings have been used more and more for machine tool active hydrostatic hybrid lubrication bearings.Floating ring static and static pressure gas bearing is a newly developed high speed gas lubricated bearing in recent years.
At present, the grinding speed of the inner circular grinding head of ultra-high speed air bearing can realize the precision grinding of small holes and micro holes.The precision of machine tool is improved by 2-3 orders of magnitude by using air bearing.