Analysis of Diesel Engine Crankshaft Torsional Vibrations

The development and application of a technique for the steady-state and transient analyses of diesel engine crankshaft torsional vibrations is presented in this paper. Crankshafts in emergency diesel generators undergo torsional vibrations due to the effect of cylinder firing pressure and the inertia of the reciprocating parts. A diesel engine crankshaft is subjected to steady-state loads during normal engine operation (constant speed and constant load) and to transients during startups, coastdowns, and load changes. Often the transients may result in torsional stresses that far exceed those normally designed for at constant speed and constant load.

A technique has been developed for the analyses of torsional vibrations under steady-state conditions. This technique has been extended for application to transient conditions. Computer programs SHAMS (Shaft Harmonic Analysis by Modal Superposition) and STAMS (Shaft Transient Analysis by Modal Superpostion) have been developed at Failure Analysis Associates (FaAA) for the dynamic torsional analyses of crankshafts. Both programs are based upon a modal superpostion approach, also known as the normal mode method, and provide an efficient means for the solution of forced vibration problems. These programs predict the free-end amplitude of vibration and the stress amplitudes at each shaft section in a crankshaft. The modal superpositition method is briefly discussed, followed by detailed descriptions of SHAMS and STAMS. Comparisons of the analytical results obtained using these codes with test data obtained during testing of diesel generator engines is included.