Paper 4: Synchronous Whirling of a Shaft within a Radially Flexible Annulus Having Small Radial Clearance

Abstract

Where it is intended to run the shaft at high speed—that is, above the first critical speed—contact between the shaft and annulus may take place in running up to speed just below the critical speed if the mass eccentricity is sufficient in relation to the damping. It is shown that such contact can radically alter the high-speed behaviour of the shaft over a speed range possibly extending to several times the critical speed. In this range, synchronous whirling can take place at a radius exceeding the annular clearance. The whirl radius in this condition may attain between ten and one hundred times the magnitude expected in normal high-speed running at the same speed. The dependence of magnitude and range of this type of whirling on annulus to shaft stiffness ratio, damping and surface friction is examined: the conditions for stability of equilibrium are theoretically examined. The essential features of the theory have been tested on a laboratory rig; some typical results are given. There is some evidence that this type of whirling can occur in centrifugal pumps, the cumulative effects leading to failure.