Air bearing

Air 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 problems of friction, wear, particulates, and lubricant handling associated with conventional bearings, and air bearings offer distinct advantages in precision positioning, such as lacking backlash and static friction, as well as in high-speed applications. Spacecraft 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 and high-speed machines.

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 and injected in the clearance of the bearing. Consequently, aerostatic bearings can sustain loads even in the 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 translation along one or more axes between two planes.
Journal bearings support a rotation between two parts.
Thrust bearings block the axial displacement of a rotating part. These are usually used in combination with journal bearings.

The main types of air bearing fall under the following categories:

Gas bearing type	Technology	Description
Aerostatic	Porous media	Gas flow is controlled through porous material
Aerostatic	Micro-nozzle	Gas flow is controlled through micro-sized holes
Aerostatic	Orifice type	Gas flow is controlled through holes and grooves
Aerostatic	Air caster	Gas flow is controlled through an air bag
Aerodynamic	Foil bearing	Bearing surface is flexible, allowing large displacement and providing a good stability.
Aerodynamic	Spiral groove bearing	Gas film is pressurized by grooves machined on one of the surfaces, achieving high load capacity and stability. The usual groove patterns are herringbone-shaped, spiral or straight

Aerostatic bearings

Pressurized gas acts as a lubricant in the gap between 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 volume refers to chambers and canals in conventional aerostatic bearings, as well as the cavities within porous materials, that exist to distribute the gas and increase the pressure within the gap.

Conventional aerostatic bearings

With conventional single-nozzle aerostatic bearings, the compressed air flows through a few relatively large nozzles into the bearing gap. The gas consumption thus allows only some flexibility such that the bearing's features can be adjusted only insufficiently. However, in order to allow a uniform gas pressure even with few nozzles, aerostatic bearing manufacturers take constructive techniques. In doing so, these bearings cause dead volumes. In effect, this dead volume is very harmful for the gas bearing's dynamic and causes self-excited vibrations.

Single-nozzle aerostatic bearings

The pre-pressured chamber consists of a chamber around the centralized nozzle. Usually, this chamber's ratio is between 3% and 20% of the bearing's surface. Even with a chamber depth of 1/100 mm, the dead volume is very high. In the worst cases, these air bearings consist of a concave bearing surface instead of a chamber. Disadvantages of these air bearings include a very poor tilt stiffness.

Gas bearings with channels and chambers

Typically, conventional aerostatic bearings are implemented with chambers and canals. This design assumes that with a limited amount of nozzles, the dead volume should decrease while distributing the gas within the gap uniformly. Most constructive ideas refer to special canal structures. Since the late 1980s, aerostatic bearings with micro-canal structures without chambers are manufactured. However, this technique also has to manage problems with dead volume. With an increasing gap height, the micro-canal's load and stiffness decreases. As in the case of high-speed linear drives or high-frequency spindles, this may cause serious disadvantages.

Laser-drilled micro-nozzle aerostatic bearings

Laser-drilled micro-nozzle aerostatic bearings make use of computerized manufacturing and design techniques to optimize performance and efficiency. This technology allows manufacturers more flexibility in manufacturing. In turn this allows a larger design envelope in which to optimize their designs for a given application. In many cases engineers can create air bearings that approach the theoretical limit of performance.
Rather than a few large nozzles, aerostatic bearings with many micro-nozzles avoid dynamically disadvantageous dead volumes. Dead volumes refer to all cavities in which gas cannot be compressed during decrease of the gap. These appear as weak gas pressure stimulates vibration. Examples of the benefits are: linear drives with accelerations of more than, or impact drives with even more than due to high damping in combination with dynamic stiffness; sub-nanometer movements due to lowest noise-induced errors; and seal-free transmission of gas or vacuum for rotary and linear drives via the gap due to guided air supply.
Micro-nozzle aerostatic bearings achieve an effective, nearly perfect pressure distribution within the gap with a large number of micro-nozzles. Their typical diameter is between 0.02 mm and 0.06 mm. The narrowest cross-section of these nozzles lies exactly at the bearing's surface. Thereby the technology avoids a dead volume on the supporting air bearing's surface and within the area of the air supplying nozzles.
The micro-nozzles are automatically drilled with a laser beam that provides top-quality and repeatability. The physical behaviors of the air bearings prove to have a low variation for large as well as for small production volumes. In contrast to conventional bearings, with this technique the air bearings require no manual or costly manufacturing.
The advantages of the micro-nozzle air bearing technology include:

Efficient use of the air cushion through a uniform pressure within the whole gap;
Perfect combination of static and dynamic properties;
Highest-possible flexibility of the air bearing properties: with a particular gap height, it is possible to optimize the air bearing such that it has, for example, a maximum load, stiffness, tilt stiffness, damping, or a minimum air consumption ;
Multi-approved highest precision of all air bearings, e.g. in the measurement technology due to slightest movements through physical, lowest-possible self-excited vibrations;
Considerably higher tilt stiffness than conventional air bearings such that the air within the gap flows through canals from the loaded to the unloaded areas away;
Vibration-free within the entire operating range even with high air pressure supply ;
Highest reliability due to the large number of nozzles: clogging of nozzles by particles is out of question because their diameters are much higher than the gap height;
Possibility to adjust bearing properties for deformation and tolerances of the bearing and opposite surface;
Proven usability for many bearing materials and coatings.

Some of these advantages, such as the high flexibility, the excellent static and dynamic properties in combination, and a low noise excitation, prove to be unique among all other aerostatic bearings.

Various designs

Standard air bearings are offered with various mountings to link them in a system:

Bearings for flexible connection with ball-pins. This design for standard air bearings is usually supplied on the market.
Bearings with a high-stiff joint instead of a conventional ball-pin. Using this version the stiffness of the complete system is significantly higher.
Bearings with integrated piston for preload of statically determined guidances.
In addition, there are also rectangular bearings with a fixed mounting for guidances with highest stiffness for highest accuracy or highest dynamic.
Furthermore, there are also air bearings with integrated vacuum or magnetic preloads, air bearings for high temperatures with more than 400 °C, as well as ones manufactured with alternative materials.