The benefits are clear

Air-lubricated glass bearings can help improve the characteristics of scanner spindles, boosting the quality of reproduction

Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering in Jena, Germany, are working on air-lubricated glass bearings intended to improve the reproduction quality of equipment such as scanners, printers and photocopiers.

But the printing industry may not be the only industry that will benefit from the results of their research. According to one of the engineers on the project, Stefan Risse, glass is equally well suited as a bearing material in precision measuring and test equipment and photogrammetry. One large German company is also backing the work because it feels the bearing also has potential in flat screen television design.

Air lubricated bearings can be employed in high-speed and high accuracy spindles which has resulted in a scanner assembly being built. Glass was chosen as a bearing material because of its considerable advantages inside the rotors of printers and scanners. In that environment, the rotors must be able to withstand high rotational speeds and heat build up.

Assemblies manufactured in metal face the risk that components will deform, which means blurred copies and prints. Glass, air-lubricated bearings on the other hand are resistant to corrosion, easy to coat, vibration-free and have a low temperature coefficient.

In the design of the glass bearing assembly, principles originally used in the design of metallic aerostatic bearings were found to be transferable to the design of glass and glass ceramic bearings.

Usually, scanners are built so that the drive, the bearing and polygonal mirror are all mounted together but act as separate components. An alternative approach was taken and in this design the polygonal mirror has been integrated into the bearing, thus reducing the sources of error. The material specified is optical glass BK7.

The design consists of two major components: the stator together with air supply and drive, and the rotor with integrated polygonal mirror and drive shaft.

The stator has two bearing surfaces formed by two plane-convex lenses. The rotor has corresponding concave bearing surfaces. Inside the stator are six air nozzles, which are arranged on each side for air supply to the bearing gap.

In the middle of the rotor lens, and running parallel with its optical axis, is a hole for attachment of the drive shaft. The bearing drive is a dc hollow shaft drive and the whole arrangement is free from lateral forces.

A polygonal mirror with 18 facets is attached to the surface of the rotor. The mirror must be oriented exactly parallel to the rotational axis: the positioning and quality of its facets decide the degree of accuracy of the scanner.

The size of the bearing gaps, the volume of the discharge port and the number, size and figure of the nozzles have an important influence on the static and dynamic behaviour of the assembly.

The cushion of compressed air between the rotor and stator acts as a lubricant and prevents contact between the moving parts. Since there is almost no friction, temperature becomes less of an issue.

Both the bearing surfaces of the stator and the rotor are optically polished. To achieve the highest stiffness, it has been calculated that the optimal size of the bearing gap should be 8 m.

The dynamic performance of the assembly was assessed by determining the wobble in the axis of rotation. A laser beam was reflected off a mirror assembly onto the polygonal mirror of the bearing and then to a CCD line located 7m away from the polygonal mirror.

The signal from just one reflecting surface was measured per revolution. After every revolution the laser beam always had the same structure and the same pixels were activated. The wobble was less than 2arcseconds.

The circular interference fringes created when laser light passes through the stator and rotor characterised deviations in position between the two components. The run out error is less than 30nm at 4800rpm.

Full details of the work carried out by the Fraunhofer research team can be obtained by contacting Dipl-Ing Stefan Risse or Volker Guyenot.

Figure 1: Design of the precision air bearing with polygonal mirror

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