Air bearings (also known as aerostatic or aerodynamic bearings) are bearings that use a thin film of pressurized gas to provide a low friction load-bearing interface between surfaces. The two surfaces do not touch, thus avoiding the traditional bearing-related problems of friction, wear, particulates, and lubricant handling, and offer distinct advantages in precision positioning, such as lacking backlash and static friction, as well as in high-speed applications. Space craft simulators now most often use air bearings and 3-D printers are now used to make air-bearing-based attitude simulators for CubeSat satellites.
A differentiation is made between aerodynamic bearings, which establish the air cushion through the relative motion between static and moving parts, and aerostatic bearings, in which the pressure is being externally inserted.
Gas bearings are mainly used in precision machinery tools (measuring and processing machines) and high-speed machines (spindle, small-scale turbomachinery, precision gyroscopes).
Gas bearing types
Gas-lubricated bearings are classified in two groups, depending on the source of pressurization of the gas film providing the load-carrying capacity: Aerostatic bearings: the gas is externally-pressurized (using a compressor or a pressure tank) and injected in the clearance of the bearing. Consequently, aerostatics bearings can sustain a load even in absence of relative motion but require an external gas compression system, which induces costs in terms of complexity and energy. Aerodynamic bearings: the gas is pressurized by the relative velocity between the static and moving surfaces in the bearing. Such bearings are self-acting and do not require an external input of compressed gas. However, mechanical contact occurs at zero speed, requiring a particular tribological consideration to avoid premature wear.
Hybrid bearings combining the two families also exist. In such cases, a bearing is typically fed with externally-compressed gas at low speed and then relies partially or entirely on the self-pressurizing effect at higher speeds.
Among these two technological categories, gas bearings are classified depending on the kind of linkage they realize:
Linear-motion bearings: Support a translation along 1 or 2 directions between two planes Journal bearings: Support a rotation between two parts Thrust bearings: Block the axial displacement of a rotating part, usually used in combination with journal bearings
The main air bearing types fall under the following categories:
Aerostatic bearings
Pressurized gas acts as a lubricant in the gap between bearing moving parts. The gas cushion carries the load without any contact between the moving parts. Normally, the compressed gas is supplied by a compressor. A key goal of supplying the gas pressure in the gap is that the stiffness and damping of the gas cushion reaches the highest possible level. In addition, gas consumption and uniformity of gas supply into the gap are crucial for the behaviors of aerostatic bearings.
Delivery of gas to the gap
Supplying gas to the interface between moving elements of an aerostatic bearing can be achieved in a few different methods: Porous Surface Partial porous surface Discrete orifice feeding Slot feeding Groove feeding
There is no single best approach to feeding the film. All methods have their advantages and disadvantages specific to each application.
Dead volume
Dead volumes refer in particular to chambers and canals existing in conventional aerostatic bearings in order to distribute the gas and increase the compressed pressure within the gap. The cavity inside porous (sintered) gas bearings are also attributed to dead volume.
Conventional aerostatic bearings
With conventional single nozzle aerostatic bearings, the compressed air flows through a few relatively large nozzles (diameter 0.1 – 0.5 mm) into the bearing gap. The gas consumption thus allows only some flexibility such that the bearing's features (force, moments, bearing surface, bearing gap height, damping) can be adjusted only insufficiently. However, in order to allow a uniform gas pressure even with only some nozzles, aerostatic bearing manufacturers take constructive techniques. In doing so, these bearings cause dead volumes (non-compressible and thus weak air volume). In effect, this dead volume is very harmful for the gas bearing's dynamic and causes self-excited vibrations.