Specific Features of Aerodynamic Journal Bearings with Elastically Supported Pads

Jiří Šimek

doi:10.3390/lubricants5020010

Abstract

Aerodynamic bearings with elastically supported tilting pads have operational properties comparable with widely-used foil journal bearings. They combine the excellent stability of tilting pad bearings, as a result of very small cross-coupling stiffness terms, with the positive properties of foil bearings, namely their ability to adapt to changing operating conditions and presence of additional damping due to friction between elastic members and bearing casings. Air cycle machines (ACMs) are used in the environmental control systems of aircrafts to manage the pressurization of the cabin. An ACM with the abovementioned type of bearings and an operational speed of 60,000 rpm was designed and successfully tested, even under conditions of strong external excitation. Some problems with rotor stability in certain operation regimes were encountered. Rotor relative vibrations measured at both bearing locations increased substantially when excitation frequency was close to the lowest rotor eigenvalues. In spite of that and the 1000 start/stop cycles passed by the end of the test, any traces of wear on the bearing sliding surfaces were negligible. When the bearing distance had to be shortened in order to insert the machine into the defined space, the rotor quickly became unstable at relatively low speeds. Although rotor stability reserve was reduced only slightly, the rotor had to be redesigned in order to achieve stability. Operation characteristics of aerodynamic bearings with elastically supported tilting pads are presented together with rotor dynamic analysis and validated with measured results.

Highlights

Air cycle machines (ACMs) are used in the environmental control systems of aircrafts to manage the pressurization of the cabin
ACM design is similar to that of a turbocharger, i.e., the rotor is driven by a turbine and pressurized air is provided by a blower impeller
With foil bearing working gap geometry, it is not possible to achieve such small values of cross-coupling stiffness terms (e.g., [7]), and this is why tilting pad journal bearings (TPJBs) are a better solution from the standpoint of rotor stability

Summary

Introduction

Air cycle machines (ACMs) are used in the environmental control systems of aircrafts to manage the pressurization of the cabin. To achieve small dimensions and high efficiency, the rotor should operate at a very high speed. The majority of contemporary high-speed rotating machines use aerodynamic foil bearings, which have very good operational properties [1,2,3,4,5,6]. Even better functional properties can be achieved with tilting pad journal bearings (TPJBs), which have excellent dynamic characteristics. With foil bearing working gap geometry, it is not possible to achieve such small values of cross-coupling stiffness terms (e.g., [7]), and this is why TPJBs are a better solution from the standpoint of rotor stability. A new type of aerodynamic bearing with elastically supported pads, which combines the advantages of foil bearings and tilting pad journal bearings, was designed and successfully used for ACM rotor support

Results

Discussion

Conclusion