impact diagnostic of aerostatic spindle unit with aerostatic beaings

Currently, in Russia ultra-precision processing of various details (including details with difficult form) is demanded to manufacture innovative products in different spheres of industry. This processing can be performed using a special high-precision multi-axes machining center. However, such centers are very expensive and their supply to Russia is limited. Therefore, creating a modular principle-based special machine or stand with the minimal number of controlled axes to perform a specific processing is more efficient in many cases. The fewer number of movable components enhances a natural frequency of the machine, and its total stiffness and accuracy as well. The spindle units are used to perform the rotation in machines. The spindle unit characteristics have predominant influence on the machining accuracy and quality.

This paper presents a model of the spindle unit “RTSH 020” (made in JSC «VNIIINSTRUMENT») which has two angular contact spherical aerostatic bearings. Design features of that type of spindle unit in comparison with the other aerostatic spindle unit designs allow creation of aerostatic bearings with a smaller gap and air consumption, as well as with a higher stiffness and damping characteristic.

An impact-test was executed to determine the frequency, stiffness and coefficient of damping by different forms of a spindle oscillation. The impact impulses were applied to the outer flange of the spindle in different directions with its displacements measured by the high-precision capacitive sensors. It was revealed that an increase of feed pressure increased the stiffness and the oscillation frequency, but a damping coefficient and a decrement were decreased. PC with an analog-to-digital converter performed data records. Data calculations were performed using a fast Fourier transformation and a Prony filtration method. Tests were repeated after changing the air feed pressure to show its effect on the spindle unit characteristics.

The test series showed that with increasing feed pressure the spindle stiffness and oscillation frequency increased too, but the damping coefficient and the logarithmic decrement as well decreased. Within the studied range the curves of all these characteristics versus the feed pressure were close to the linear ones. The certain characteristics depending on the feed pressure allow a selection of optimal operation conditions of the spindle unit for various applications. Design features of the spindle unit enable the use of two sensors in total to determine characteristics of radial, axial, and angular motion mode of spindle.