Paper 11: Anti-Vibration Journal Bearings

Abstract

An experimental rig has been constructed to establish the vibration characteristics of plain, dammed groove, and tilting pad bearings having a variety of clearances. The rig is perhaps unusual in not being a small piece of laboratory equipment, but consisting of a naval anti-submarine frigate turbo rotor (15 000 rev/min) running in two journal bearings each supported by an isotropic cantilever system of variable stiffness. Two pairs of induction probes mounted close to the test bearing monitored the shaft movement and provided an oscilloscope display of the movement of the bearing centre. Detailed information about the shaft oscillations was obtained from a wave analyser and vibration meter. The static rig natural frequencies with various bearing housing support stiffnesses were primarily determined. From preliminary running tests a fairly stiff bearing housing support, which gave a strong vibration of approxi-mately half rotational speed, was chosen for all subsequent work. With the rotor wheel balanced the tilting pad bearing tested was found to be free from serious vibration. The dammed groove type of bearing tested was also stable at low speeds but was not capable of stable operation at 15 000 rev/min and above. The plain bearing tested produced serious vibration above 9000 rev/min. The frequencies and amplitudes of vibration which occurred in different bearings with a deliberate rotor unbalance were also measured. Increase of the clearance lowered the onset speed of serious vibration. The locus of the mean position of the journal centre of the tilting pad bearing was almost a vertical straight line whereas all the other bearings showed the usual approximate semi-circle. Instantaneous oil film pressures up to 900 lb/in2 were measured using a pressure transducer in the bearing. As the design particulars of the bearings tested differed somewhat the results were not strictly comparable but nevertheless an approximate design rule has been suggested. Practical data provided about the vibration behaviour of the different journal bearing designs can be used by the many theoretical investigators in this field. This is important in view of the lack of practical attested results to substantiate or otherwise the many theories advanced.