Investigating Some Fundamental Notions of Propulsion Shaft Alignment

Alignment of the shaft connecting driver and load may be done with the shaft regarded as a rigid body, as an elastic body with journal centers in line, or as an elastic body whose elasticity may be exploited by adjusting the heights of bearings. Through several example cases, the paper questions whether the rigid body school might benefit from moving bearings to exploit elasticity, or conversely, whether moving bearings to benefit bearing load distribution might have undesirable consequences such as excessive bending stress. Alternatives to rigid body alignment are explored through the use of a pump-motor combination for which it is found that reasonable adjustments of bearing heights produces large changes in bearing loads and in shaft bending stress; however neither appears to reach dangerous levels. Applied to a long, multi-bearing shaft for which a "fair curve" alignment might be applied, high levels of combined bending and torsional stress are found, but only for extreme cases. The paper lastly applies elastic bending formulas in testing several common-place rules for coping with the elastic deflections of uncoupled shaft sections.