EXCITE_Designer_Theory

Theory EXCITE Designer v2011 2.5. Torsional Vibration Analysis

2.5.1. Lumped Mass - Spring - System

2.5.1.1. Lumped Masses (Rotational Inertias)

The real parts of the vibration system with distributed masses have to be substituted by

individual inertias. Thereby the basic formula for the inertia (MTM) has to be used:

L d D MTM 4432 [kgmm 2] whereby

= specific mass of the individual element [kg/mm 3]

D

= outside diameter [mm] d

= inside diameter [mm] L = length [mm]

2.5.2. Throw Stiffness

The stiffness of a crankshaft cannot be calculated exactly due to its throws.

The masses are assumed to act in the middle of the crank throw. The pins and journals

twist and the webs bend. An equivalent shaft is found which has the same torsional

stiffness as the crankthrow.

This problem was analyzed by R.GRAMMEL in 1933 and he made the following

distinctions:

1st kind of torsion:

The load comes from two equal torsional moments.

Figure 28: Crankshaft Torsion 1st kind according to Grammel

2nd kind of torsion:

The load happens by two equal forces working in opposite direction and normal to the

throw plane. Both main and secondary torsion occurs.

2-8329-Jul-2011