An investigation into the balancing of high speed, flexible shafts by external application of compensating balancing sleeves

The paper investigates the use of compensating balancing sleeves positioned at the shaft’s end for the balancing of high-speed flexible shafts. The balancing sleeve is a new arrangement that creates a pure balancing moment with virtually zero radial reaction forces. For comparison purposes, experimental results from previous research are used to benchmark performance and to demonstrate the benefits newly proposed topology. The new configuration is commensurate with what is required for the Power Turbine (PT) shaft of a twin shaft industrial gas turbine, with an overhung disc. The study is also aimed at bladed shafts, such as those used in high speed gas turbines/compressors, with a view to improving their volumetric efficiency by reducing the formation of relatively large tip leakage gaps caused by shaft deflection/blade wear of abradable seals. It is shown to be practically possible to separate the two main dynamic balancing functions i.e. the control of bearing reaction loads and shaft deflections, thus allowing for their independent adjustment. This enables the required balancing sleeve moment to be determined and set during low-speed commissioning i.e. before any excessive shaft deflection and resulting seal wear occurs, as is typical when final balancing is undertaken at full operational speed

Generalised analysis of compensating balancing sleeves with experimental results from a scaled industrial turbine coupling shaft

The paper furthers the analysis of a recently proposed balancing methodology for high-speed, flexible shafts. This mechanism imparts corrective balancing moments, having the effect of\ud simulating the fixing moments of equivalent double or single encastre mounted shafts. This is shown to theoretically eliminate/nullify the 1st lateral critical speed (LCS), and thereby facilitate safe operation with reduced LCS margins. The paper extends previously reported research to encompass a more generalised case of multiple, concentrated, residual imbalances, thereby facilitating analysis of any imbalance distribution along the shaft. Solutions provide greater insight of the behaviour of the balancing sleeve concept, and the beneficial implications for engineering design. Specifically: 1) a series of concentrated imbalances can be regarded as an equivalent level of uniform eccentricity, and balance sleeve compensation is equally applicable to a generalised unbalanced distribution, 2) compensation depends on the sum of the applied balancing sleeve moments and can therefore be achieved using a single balancing sleeve (thereby simulating a single encastre shaft), 3) compensation of the 2nd critical speed, and to a lesser extent higher orders, is possible by use of two balancing sleeves, positioned at shaft ends, 4) the concept facilitates on-site commissioning of trim balance which requires a means of adjustment at only one end of the shaft, 5) the Reaction Ratio, RR, (simply supported/ encastre), is independent of residual eccentricity, so that the implied benefits resulting from the ratio (possible reductions in the equivalent level of eccentricity) are additional to any balancing procedures undertaken prior to encastre simulation. Analysis shows that equivalent reductions in the order of 1/25th, are possible. Experimental measurements from a scaled model of a typical drive coupling employed on an industrial gas turbine package, loaded asymmetrically with a concentrated point of imbalance, are used to support the analysis and conclusions

Theoretical investigation into balancing high-speed flexible shafts, by the use of a novel compensating balancing sleeve

Traditional techniques for balancing long, flexible, high-speed rotating shafts are inadequate over a full range of shaft speeds. This problem is compounded by limitations within the manufacturing process, which have resulted in increasing problems with lateral vibrations and hence increased the failure rates of bearings in practical applications. There is a need to develop a novel strategy for balancing these coupling shafts that is low cost, robust under typically long-term operating conditions and amenable to on-site remediation. This paper proposes a new method of balancing long, flexible couplings by means of a pair of balancing sleeve arms that are integrally attached to each end of the coupling shaft. Balance corrections are applied to the free ends of the arms in order to apply a corrective centrifugal force to the coupling shaft in order to limit shaft-end reaction forces and to impart a corrective bending moment to the drive shaft that limits shaft deflection. The aim of this paper is to demonstrate the potential of this method, via the mathematical analysis of a plain, simply supported tube with uniform eccentricity and to show that any drive shaft, even with irregular geometry and/or imbalance, can be converted to an equivalent encastre case. This allows for the theoretical possibility of eliminating the first simply supported critical speed, thereby reducing the need for very large lateral critical speed margins, as this requirement constrains design flexibility. Although the analysis is performed on a sub 15 MW gas turbine, it is anticipated that this mechanism would be beneficial on any shaft system with high-flexibility/shaft deflection.The work presented in this paper is supported by Siemens Industrial Turbomachinery Limited, Lincoln, U