Air bearings , aerostatic or aerodynamic bearings (gas bearings) are bearings in which the two bearing partners moving towards one another are separated by a thin film of air . The stick-slip-free and friction -free movement ensures great accuracy. The load capacity is lower than with conventional rolling bearings . Air bearings are preferred in precision machines (measuring and processing machines) and high-speed machines (high-speed spindles).
A distinction is made between aerodynamic bearings, which build up the air cushion through the movement itself, and aerostatic bearings, in which compressed air is introduced to build up the pressure cushion. Aerodynamic bearings do not require a compressed air supply, but have the problem that the two bearing partners touch each other below a characteristic relative speed (linear or rotary) and thus exhibit friction that leads to wear .
The ultrasonic air bearing , which is caused by the near field effect in the ultrasonic field , does not require an external air supply. It is currently used less as a warehouse than for contactless gripping and transporting of flat objects. The means of transport and the goods to be transported are separated by an air gap of 50–300 µm.
Air bearing types
Air bearings basically belong to the class of plain bearings . The in the bearing gap, d. H. Compressed air pressed between the sliding surfaces moving towards one another forms the lubricating medium . At the same time, a pressure cushion is built up with it, which carries the load without contact. The compressed air is usually provided by a compressor. The aim is to achieve the highest possible level of pressure, rigidity and cushioning of the air cushion. Air consumption and the even supply of air over the entire storage area play a decisive role.
So-called dead volumes are all cavities in which the air cannot be compressed when the bearing gap changes. They do not contribute to the storage effect because they are a soft gas spring that stimulates oscillation. The dead volumes include, in particular, chambers and / or channels, as they have conventional air bearings in order to distribute the air evenly and to increase the pressure in the bearing gap. These are extremely damaging to the dynamics of the air bearing and stimulate self-excited vibrations.
Technologically, air bearings differ according to their internal structure, how the air supply and distribution in the bearing gap is implemented:
Conventional nozzle air bearings
With conventional nozzle air bearings, the compressed air flows into the bearing gap through a few, but relatively large inlet nozzles (diameter 0.1-0.5 mm). As a result, their air consumption is not very flexible, and the bearing properties can only be insufficiently adapted to the boundary conditions ( forces , moments , bearing surface , bearing gap height, damping ). In order to be able to distribute the air as evenly as possible in the gap with the small number of inlet nozzles, various design measures are taken. However, they all generate dead volumes (non-compressible and therefore soft air volumes).
Conventional single-nozzle air bearings with an antechamber have a chamber around the centrally arranged nozzle. Their area is usually 3 - 20% of the storage area. Even with an antechamber depth of only a few 1/100 mm, the dead volume of these air bearings is very large. In the worst case, these air bearings simply have a concave bearing surface instead of an antechamber. In addition to many other disadvantages, all these air bearings have, in particular, extremely poor tilting rigidity.
Sintered air bearings
So-called sintered air bearings are a powder metallurgical product. Here the porous bearing material should ensure that the air is evenly distributed. Advantages of the sintered air bearing include smooth running, low wear and the fact that it is RoHS compliant. Disadvantages are the large dead volume (voids in the material) and the uneven outflow of air due to the irregular porosity. This is also associated with the high fluctuations in the bearing properties of these air bearings. Due to the system, sintered air bearings can only be used in a very low temperature range between 0 ° C and 50 ° C
Advantages of the air bearing
Wear-free, service life
Air bearings work without contact and have no solid body friction during operation, only air friction in the gap. Their lifespan is therefore almost unlimited in undisturbed operation (with air supply). Rolling or plain bearings have abrasion, especially at higher accelerations, which, as a result of wear, leads to a reduction in the guidance accuracy and thus to their failure.
Guidance, repetition and positioning accuracy
In chip production, repetition accuracies of 1 to 2 µm for wire bonding and 5 µm for die bonding are already required in the back-end for positioning. With roller bearings , the physical limits are reached if the accelerations are not reduced at the same time. Air bearings are already established in the front end (lithography).
When assembling in electronics production, the accuracy requirements are currently increasing rapidly due to smaller components and a change in technology, e.g. B. for chip-on-board, flip-chip technology or wafer-level chip packaging. This means that repeat accuracies of less than 10 µm will also be required here in the future, which roller bearings can no longer be achieved due to the stick-slip and "drawer" effect. Air bearings are the only ones that offer the prerequisites for future productivity requirements with high purity (free from oil and grease).
Cost advantages and reproducibility
Air bearings are characterized by maximum reproducibility, full automation and low costs due to the fast processing. Therefore, the technology can also be used for the first time for large series B. in printing, textile or automotive technology can be used. In series production, air bearings can even have cost advantages over roller bearings: a ball-bearing high-frequency spindle for roller and air-bearing spindles is around 20% more expensive to manufacture than an air-bearing one.
Purity, oil and fat free
Chip production generally takes place in a clean room. Even minor soiling can cause rejects and thus high costs. In electronics production, the cleanliness requirements for the environment increase with smaller components. In contrast to oil or grease, air is an optimal lubricating medium because it is available in the same purity as the surrounding air. Therefore, with air lubrication, there is no need for complex sealing of the bearing.
The elimination of environmentally harmful, mineral oil-based or synthetic lubricants is currently arousing interest in air bearings outside of the traditional areas. The air bearing offers ecological advantages compared to oil or grease-lubricated plain bearings.
Disadvantages of the air bearing
The required high precision of production with 10 μ for the bearing gap means high production costs.
The air bearing can only be used where asymmetrical loads can be excluded. Unbalance of the stored object can lead to the destruction of the bearing if no precautions are taken against it.
When starting and stopping, friction and wear occur if this process takes place without pre- and post-travel for the assembly and dismantling of the storage medium.
A specially tailored FEM software was developed to calculate air bearings . It was developed on the basis of exact theoretical models. This allows calculation tasks to be solved that are far beyond the possibilities of all previous design formulas and numerical calculations.
Calculation examples are presented below:
The calculation of the bearing properties is based on the simulation of the flow processes in the bearing gap and in the micro nozzles. The result is the pressure curve in the bearing gap. All static parameters can be derived from it. The results are based on the description of the actual physical effects and allow a clear idea of the later real conditions.
Flat air bearings that are moved tilt due to aerodynamic effects in the gap. Depending on the tilting rigidity of the air bearing, its load-bearing capacity decreases with increasing speed up to a limit at which it fails. For air bearings, this limit speed is a few dozen m / s, depending on the design. For air bearings with chambers and variants of channel structures, however, speeds of a few m / s are dangerous.
The stiffness of an air bearing body is often overestimated in relation to the stiffness of the air cushion. The diagram shows the characteristics of a flat bearing element (height 20 mm, diameter 80 mm) compared to the theoretical characteristics of a rigid air bearing of the same geometry. On the basis of such calculations, the material of the air bearings is determined and the arrangement and number of nozzles are specifically adapted to the deformation.
The deformation of the bearing surface due to the surface load of the air cushion is calculated using a structural analysis. The result is then used as a parameter in the calculation of the air bearing characteristics. Using an iterative process, the actual deformation and the actual pressure profile can be calculated for each point on the characteristic curve.
Spindles and cylindrical air bearings
The FEM software also allows the calculation of rotationally symmetrical components. Load capacity , rigidity, tilting rigidity and air consumption of a complete spindle bearing can be calculated exactly at standstill and at speed (including dynamic effects). This enables the prediction of maximum speed and natural frequencies ; an indispensable prerequisite for the construction of an optimal spindle.
Cylindrical air bearings can be optimized with the calculation methods with regard to rigidity, air consumption and the effect of mechanical tolerances in production.
The calculation models have been validated many times through measurements. Typically, the results agree with reality to 5% for the load-bearing capacity and 10% for the stiffness. The deviations do not result from errors in the calculations, but from the properties (form deviations, etc.) of real storage areas.
- Air bearing knife drive
- Air bearing turbocharger
The broad field ranges from very precise drives for measurement technology to complex, robust multifunctional systems for electronics and semiconductor production to inexpensive drives for automation technology .
- Precision measuring table
- Highly accelerated Doppler drive
The highly accelerated Doppler drive has a carbon fiber mirror (area 500 mm × 250 mm), which is guided with high precision with flexible movement profiles at accelerations of up to 300 m / s². The solution is designed as an air-bearing drive: The guide rail (length 900 mm) to which the mirror is attached is also made of CFRP and carries the magnets of the linear motors. The cables or hoses (motor, air bearing, measuring system) are not moved with them, so that no breaks occur as a result of the high load changes. Air storage is very insensitive to geometry fluctuations due to the influence of temperature.
- Drive for production machines
In addition to performance, reliability has the highest priority in production machines. The air storage is statically determined. The preload is carried out directly by the iron-core linear motor or by piston bearings. This makes the drive easy to assemble and insensitive to changes in geometry, e.g. B. by temperature influences or the installation of the machine.
- Air storage for inspection device
The chip for measuring wafers and flat panels must be guided very precisely and without touching the surface. The chip is integrated directly into the storage area. Its maximum change in distance from the surface, i.e. H. the fluctuation in the gap height of the air bearing is less than 0.5 µm. When placing the air bearing and chip must not touch the surface to be measured. A pneumatic piston serves as the actuator for the up and down movement, which is also air-mounted for reasons of reproducibility. The preload force on the air bearing and thus the bearing gap height is set via the air pressure.
- Chuck with integrated lifting drive
The chuck can be raised up to 3 mm for electrical testing of the wafer stick-slip-free. The force on the test tips is constant regardless of the stroke movement until the required contact force is reached. The lifting drive is based on moving coils; its guidance is air-bearing. An adjustable pneumatic piston between the drive and the chuck limits the contact force.
Grease and oil-free drives for breathing apparatus, stick-slip- free movements for scanners or high speeds for large rotors.
- Air bearing computer tomograph
High speed (> 5.5 / s; 330 / min), low manufacturing costs, low noise, large free diameter of the rotor (> 1 m), low weight of the rotor and frame, the full tiltability of the rotor and the high reliability are the Advantages of this instrument. A version with belt drive instead of direct drive is also possible.
- Air bearings for aligning components
With the help of an air-bearing guide body, optical components can be aligned with a common diameter on a turntable. The guide body floats vacuum-preloaded with a constant bearing gap height without contact on the turntable.
- Adjustment slide for optics production
The linear slide is used for highly precise positioning of the object in optics production. Designed as statically determined bearings, the object to be processed can align itself in the axial direction for grinding in the machine without friction or forces. When the linear slide is clamped for machining, this position remains in the sub-µm range.
- High-precision, heavy-duty aerostat guides in machine tools.
Realized axis lengths up to 15 m and loads up to 600 kN. Self-cleaning ability through escaping air, temperature stable, no complex medium return such as B. a hydrostatic table guide.
In the meantime, the wide spectrum of spindles extends from the smallest spindles with the lowest possible friction to spindles with the highest speeds of over 300,000 / min. They are often in use in the form of circuit board drilling spindles, precision scanner spindles and precision grinding spindles.
Short stroke drive
The innovation of the short-stroke drive is based on its air bearing, which is integrated directly into the iron-core linear motor. This means that the moving mass is low and the highest levels of acceleration and accuracy can be achieved. The drive proves its outstanding advantages with short strokes in the µm and mm range with high frequencies up to well over 100 Hz. Thanks to its optimal air lubrication, the service life is virtually unlimited, even with highly dynamic movements. The drive is ideal z. B. suitable for non-circular lathes (piston production), presses or circuit board drilling machines.
The air-bearing rotary drive is directly driven with its integrated synchronous motor. The rotor has a vacuum supply that is wirelessly transmitted from the stator . The rotary drive is ideally suited for measuring tasks thanks to its extremely smooth running and oil / grease-free storage. Its maximum speed is 500 rpm.
- Mirror bearing
In the Large Zenith Telescope , which has a primary mirror made of mercury, the mercury container is air-bearing.
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