An experimental study of high-speed rotor supported by air bearings: test RIG and first experimental results Guido Belforte, Terenziano Raparelli * , Vladimir Viktorov, Andrea Trivella, Federico Colombo Department of Mechanics, Politecnic of Turin, 10128 Turin, Italy Available online 10 August 2005 Abstract The paper contains the description of the test rig and first experimental data regarding the mechanical and the thermal behaviour of a 7 kg rotor, 460 mm long and with a diameter of 50 mm, supported by externally-pressurized air-lubricated bearings. The rig allows to load the rotor radially and axially both in static and dynamic conditions and to measure the orbits on the supports. The rotating imbalance response at different speeds has been measured up to 60,000 rpm. The orbits are always synchronous with the rotational speed and their shape changes also with temperature (though this effect is almost negligible). The rotor is stable and does not present whirl instability. q 2005 Elsevier Ltd. All rights reserved. 1. Introduction In high speed machining, production requirements call for a continual increase in removal rates, both to permit the use of smaller drill bits, and to reduce the processing times involved in milling and grinding. Increased cutting speeds are particularly desirable in operations such as drilling printed circuit boards, machining aluminum components for the aeronautics industry, mold and die finishing, and woodworking. Though highly dissimilar in terms of power requirements, rotational speeds and shear forces, these applications share a need for high tangential velocities. The rolling bearings in common use deteriorate rapidly under high rotational speeds. Ceramic bearings are employed at the highest speeds, but even they do not provide particularly long service lives. One way of overcoming these problems is to use air bearings. In PCB drilling, for example, air bearing spindles are already in industrial use (see www.westwind-airbearings.com). As these units are fully pneumatic and feature no rolling elements, they obviously do not require bearing replace- ment, making service life practically infinite. Air bearings are not widely used in other industrial applications. This is due to a number of factors, including the lack of a readily usable design algorithm, instability problems, and the difficulty of predicting dynamic behaviour [1–3]. In addition, appropriate test benches and spindles are not available: while determining spindle stiffness characteristics is readily accomplished under static conditions, it is far more difficult under dynamic conditions, particularly at high speeds. A large number of experimental studies of the behavior of air bearing rotors are presented in the literature. These investigations address a wide range of bearing types, including, among others, foil bearings [4–7], floating journal bearings installed on O-rings [8] and fixed journal bearings [9,10]. Though the literature is extensive, it is quite rare to find benches capable of loading the rotor with radial and axial forces in order to measure static and dynamic stiffness [11]. The Politecnico di Torino Department of Mechanics is currently investigating the behavior of high speed air bearing rotors. As part of this investigation, a test bench for analyzing spindles with this type of bearing has been designed and constructed. The bench is capable of monitoring rotor position in the bushing and subjecting the rotor to radial and axial forces during rotation, as well as under stationary conditions. The orbits described at different rotational speeds can thus be plotted, while it is also possible to evaluate bearing axial and radial stiffnesses, both of which are essential characteristics for determining spindle operating specifications. Tribology International 39 (2006) 839–845 www.elsevier.com/locate/triboint 0301-679X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2005.07.013 * Corresponding author. Tel.: C39 11 5646929; fax: C39 11 564 6999. E-mail addresses: terenziano.raparelli@polito.it (T. Raparelli), vladi- mir.viktorov@polito.it (V. Viktorov).