Flexible Bearing Bodies
For a flat air bearing to function without contact, and therefore without wear, it is essential that the air gap varies little across the entire bearing surface. The bearing gap height is mainly in the range of 10-15 µm, therefore fluctuations in distance between the opposing surfaces should not exceed 2-3 µm.
This can be achieved by manufacturing the bearing and mating surfaces with an flatness of less than 2 µm. This presents a challenge for the manufacturing of large bearings (from approx. 200 x 200 mm²). Furthermore, neither the bearing nor mating surface should deform excessively under load. Due to their stable construction, the bearings inevitably become heavy and poorly suited for use in dynamic applications.
Alternatively, a uniform bearing gap height can be achieved with the design of a flexible bearing body that adjusts to the mating surface when in operation. In this way, it is possible to compensate for long-wave unevenness and a consistent bearing gap height is established (Figure 1). For the bearing body to adjust to the mating surface, a preload is necessary, e.g.
vakuum or magnetic preload. Simulations are used to determine the positioning of the vacuum pockets or magnets and micro nozzles for the air bearing in the bearing surface.
Figure 1: Comparison of a rigid, flat bearing with a flexible bearing on a slightly uneven mating surface.
Highly Dynamic Appplications
A machine’s productivity depends heavily on how dynamic the drive system is. Accelerations must be increased to achieve greater productivity. Because the engine power is limited, the weight of the moving parts must be kept as low as possible. In addition, a high tilting stiffness of the bearing is essential for rapid reversing movements, such as those that occur in dynamic travel profiles. Otherwise, the moments that take place due to changes in accelerations while reversing will lead to bearing damage and failure.
The air bearing surface must be as large as possible to ensure a high bearing stiffness. To minimise the weight of the moving bearing components, it is recommendable to keep the bearing body flexible.
Figure 2 shows a complete air bearing x-y-theta stage in which the x-axis and y-axis can be moved at high accelerations. Accelerations of at least 2 g (20 m/s²) are only achieved on the x-axis because the beam, along which the y-axis and rotary table slide was implemented, is thin and therefore flexible in the z-direction. The entire underside of the beam is air-guided in combination with vacuum pockets as preload against the granite. The weight-optimised moving parts also make it easier to achieve specified accuracies (bidirectional repeatability < 0,2 µm) and significantly reduce manufacturing costs.
Figure 2: Highly dynamic x-y theta stage. The beam for movement in the x-direction is flexible and vacuum-preloaded over the entire surface (underside).
Applications under Thermal Stress
Flexible bearing bodies allow air bearings to be used in applications with large temperature variations during operation. For example, it was possible to implement a bearing for an air-guided slide used for the steam treatment of glass panels in a hydrogen sulphide atmosphere (ALD process) at a temperature of 350°C.
The need for a flexible bearing body for thermal applications arises from the prevailing temperature differences in the process furnace. There is no uniform deformation of the air bearing mating surface (stator) when the furnace is heated. This means that the mating surface, which is even at room temperature, generally loses its evenness by a few tenths of a mm when exposed to the process temperature. For this reason, the moving air-guided slide (slider) must be able to compensate for the deformation.
In this example developed for a specific customer, both air bearing bodies (stator and rotor) had to be flexibly designed due to their length (slide approx. 1.6 m, stator approx. 2.2 m) because the rotor and mating surface could not be manufactured with the commonly required flatness of 2 µm. For the rotor to adopt the shape of the mating surface regardless of the process temperature, vacuum pockets were integrated into the bearing surface between the air-guided surface area. The implementation was fully simulated and reached the required specification in stress tests.
Figure 3: Air bearing slide for vapour deposition on glass substrates. Process temperature: 350°C, hydrogen sulphide atmosphere.