Types of torques...

[bucknico]

Could anybody describe to me quasi-static torque, vibratory torque, and bending moments and how they differ? Also, how they could affect a crankshaft?
Thank you.

 

[rob768]

I assume that with quasi-static torque the nominal torque due to power/revs is meant. The vibratory torque (TV) is the amount of torque related to torsional vibrations. In a diesel driven installation, there will always be an amount of non uniformity causing torsional vibrations. Sometimes, there are other sources for non-uniformity as well (poorly alingned cardan shaft, propeller, compressor and more). This vibratory torque is to superimposed on nominal torque and should be of limited magnitude in order to prevent fatigue.
Bendind moments, as the name suggest, relate to bending and not to torque. They are caused by the combustion forces, acting on the pistons.

In a crankshaft, the torsional vibrations are usually dampened by a vibration damper on the front end. furthermore, a crankshaft is designed to have natural frequencies away from possible excitation sources. In a normal installation, TV should not pose a problem. However, in the case of a defective damper, or when a front drive is added to the system, significantly changing the mass-elastic system of the crankshaft, and thus shifting natural frequencies into operating speed range. The likely result: fatigue fractures.
Bending stres in the crankshaft are limited by bearing clearances. in the case too much clearance, the crankshaft delfection may be too high, resulting in too much bending stress and, again, fatigue. There are other sources as well, and in particual pulley forces at the frond end in the case of a PTO.