Preload
Air bearings are often used when high precision, high dynamics, or a combination of both is required. In both cases it is important that the bearing has the highest possible stiffness. In precision applications, this ensures that interfering forces only cause minor deviations from the required position. Doing so prevents the slider from tilting too much when the bearing experiences a moment load in highly dynamic movements, for example rapid changes in direction.
The supply pressure and size of the bearing surface determine the maximum force that can be applied to an air bearing. The static stiffness in turn depends on the load capacity and height of the air gap; there is a sharp rise as the gap becomes smaller up to its maximum. AeroLas’s air bearings reach the highest possible stiffness by proxy with a gap height between 5-15 µm, depending on the design of the bearing.
The static stiffness decreases sharply as the bearing gap height increases. The air bearing must therefore be loaded with a suitable force. This is also known as preload.
In the example shown, the recommended bearing gap height is 9 µm (red line). At a suitable preload of approximately 1.400 N, the bearing has a stiffness of 200 N/µm. This can be done, for example, by the weight of the guided object by itself. The optimum preload for an air-guided slider with a statically determined support with three bearing pads, however, means that the slide including the object would need to have a mass of 420 kg.
Generally, the mass of the moving object is much smaller, which limits the stiffness that can be achieved. To achieve a high stiffness nonetheless, an additional preload is necessary. Two options are predestined for this:
- vacuum preload
- magnetic preload
Figure 1: Characteristic bearing curves for an air bearing 55 x 110 mm² with a supply pressure of 4 bar. The red line marks the recommended bearing gap height. The sections show the curves for large gap heights.
Vacuum Preload
Pockets, connected with a vacuum pump, are integrated into the bearing surface, preloading the bearing with vacuum. The bearing surface encloses the vacuum area on all sides, which are therefore sealed to the outside via the bearing gap.
AeroLas’ air bearings have a low air consumption, even with large bearing surfaces. There is therefore little leakage of supply pressure into the vacuum, and a small vacuum pump is enough to achieve the required vacuum. The vacuum preload can be integrated into any bearing surface. The size and position of the vacuum area, just like the number of micro nozzles on the bearing surface, are optimised to achieve the maximum stiffness with the lowest air leakage into the vacuum.
The figure shows an example for which a vacuum preload is essential:
The air-guided rotary drive can be moved in the x and y direction at a high dynamic range and precision because it was designed with an optimum weight, thanks to the vacuum preload. The drive, including the turntable, motors and cable, has a total mass of 4,6 kg. This enables continuous accelerations of 2 g (20 m/s²) in the x/y direction at a repeatable accuracy of better than 0,2 µm. The support bearing (bottom) of the rotary driver has a stiffness of 190 N/µm at a preload force of 1.100 N.
Figure 2: Example of two vacuum-preloaded bearing surfaces of an x-y rotary table for highly dynamic movement.
Summary of vacuum preload:
Advantages:
- minimal weight with high bearing rigidity
- works on all air bearing matin surfaces
(granite, steel, glass, etc.) - „upside down“ operation possible
Disadvantages:
- Vakuum pump required
- Additional supply hose for the vacuum
- In the event of a power failure, the air bearing can detach from the mating surface
Magnetic Preload
For a magnetic preload, one or more permanent magnets are integrated into the air bearing body. The mating surface of the air bearing must be magnetic. The attraction force between the magnet and the mating surface depends heavily on the distance; this must therefore be taken into account when designing the bearing.
At AeroLas, a wide variety of magnets were tested for their preload force in extensive trials. Based on this data, it is possible to design a customized air bearing quickly and reliably without the need for time-consuming tests on a prototype in advance.
If the magnetic preload is used for parts that move in relation to each other, eddy currents are generated. In addition, the movement can lead to a change in magnetization in the mating surface.
Both lead to a force that is directed in the opposite direction to the movement, whereby the force due to the remagnetization predominates. The latter can be prevented by a clever arrangement of the magnets. This allows the counterforce to be minimized and higher accelerations can be achieved.
Examples of air bearings with magnetic preload:
Figure 3: Simple air bearing body with a magnet glued into a pocket
Figure 4: Air bearing with integrated magnet under the bearing surface
Figure 5: Air-bearing carriage with standard air bearings and a magnetic preload against the magnetic track of the ironless motor
Summary of magnetic preload:
Advantages:
- greater force per area than with vacuum preload
- Higher rigidity with the same bearing surface
- Fixation of the bearing to the mating surface even in the event of a power failure
- „upside down“ operation possible
Disadvantages:
- Magnetic mating surface required
- Additional weight due to the permanent magnets
- Risk of injury / damage during handling due to the strong magnets