Requirements and advantages of air bearings in precision motion
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作者:topphotonics
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发布时间: 2022-10-27
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Another approach is to highly integrate the bearing function into the structural elements of the mechanical design, i.e. the air bearings are deeply integrated into mechanical structure, e.g. a slideway with areas that serve as air bearings, while at the same time this monolithic structure also offers the mounting surfaces for motor, encoder, functional area for the application. In OEM applications, one will often see this approach, where the air bearings are incorporated into the linear stages of the machine. Another example is the air spindle, where the feed channeling and restrictors are implemented into the housing of the spindle which also holds space for a.o. encoder feedback, direct drive motor and liquid or air cooling.
▲ Figure 1: Left: an individual air bearing component for a rotary shaft support. Right: a highly integrated air bearing solution containing radial and axial air bearing, motor and encoder feedback.
What are the requirements for a good air bearing system?
Before switching to the advantages of air bearings over traditional mechanical bearing solutions, we first highlight the requirements which are important for a proper operation of the air bearing system.
CDA delivery
Clean Dry Air (CDA) is a critically important requirement for the optimal performance of any externally pressurized air bearing system. An air bearing system for precision applications has a typical air gap of only a few µm up to around 10 µm. A multi-stage filtration that removes particles and oil out of the air supply is required to clean the air coming from a compressor unit. General-purpose filter cartridges help to remove the bulk particles, while a coalescing filter removes oil and water including particles that passed the general-purpose filter. Activated carbon filters remove oil vapors and hydrocarbons to bring the contamination level to even lower levels. Membrane, adsorption or refrigeration dryers are used to bring the dew point of the supplied air down to a level which guarantees no condensation downstream in the air bearing application.
LAB Motion Systems recommends the ISO 8573-1:2010 Class 1.3.1 standard as to the required air quality. The requirements included in this standard are as follows:
| In each normal cubic meter of compressed air, the particulate count should not exceed 20.000 particles in the 0,1 – 0,5 micron size range, 400 particles in the 0,5 - 1 micron size range and 10
particles in the 1 - 5 micron size range. |
Class 3: moisture content | A pressure dewpoint (PDP) of -20°C or better |
| In each normal cubic meter of compressed air the amount of oil is limited to 0,01 mg. This is a total limit for liquid oil, oil aerosol and oil vapor combined |
▲Figure 2:To achieve these stringent air quality levels, a careful approach to component selection is needed. When using an oil-free air compressor and storage tank with draining point, an air filter unit can be used to reach the required air quality level. The following schematic shows a layout of the general system setup.
