Damping vibration

Wednesday, February 19, 2014

Tags :  crankshafts

One of the main points to consider when designing a cranktrain, or making significant modifications to any of the parts of the cranktrain, is that of vibration – and, most important, torsional vibration. There can be significant mechanical damage associated with cranktrain torsional vibrations; when this occurs, stresses are far in excess of those anticipated during normal operation, and it is not uncommon for crankshafts to break, or for problems to be ‘transmitted’ to the valvetrain, which can result in valve-to-piston contact or loss of valvetrain control. While engines with belt-driven cams can suffer less from vibrations being transmitted from the cranktrain to valvetrain compared to those with gear-driven cams, we still need consider crankshaft vibration.

The problems are most serious where engines are driven at a near-constant speed on full throttle. If this coincides with a critical frequency at which the cranktrain resonates, the amount of energy involved means serious damage can very quickly result. To change the torsional characteristics of the cranktrain so that the operating speeds and resonant frequencies are a safe distance apart can mean that components such as crankshafts, con rods and piston assemblies would need to be redesigned to change their stiffness and inertia. Such changes may not be practical, so we need to see what else can be done without compromising the performance of the engine.

This is where dampers come in. There are many different kinds of damping solutions for cranktrains and valvetrains, but they all aim to reduce the amplitude of vibrations in the operating range of the engine and the high stresses associated with these amplitudes. A good reference for anyone interested in this is the fourth volume of the epic treatise on torsional vibration by Ker Wilson*.

One type of damper that is commonly used is the inertia damper. Here, an inertia ring is contained within a housing, and its rotation is constrained by the friction applied to it by pre-loaded elastomer elements. The ‘tuning’ of the damper is effected by changes to the inertia of the ring, the number, dimensions and damping characteristics of the elastomers and the pre-load on them.

During normal operation, the tendency is for the inertia ring in the damper to oscillate very little, perhaps a few tenths of a degree. When torsional vibration begins, the amplitude and energy in the torsional oscillations builds quickly, and where a damper is fitted the inertia of the ring means that its angular displacement from its normal position is increased, straining the elastomeric elements in the system. This exerts a ‘restoring torque’ on the cranktrain, thus controlling the amplitude of vibration. If properly developed and tuned, such devices can practically eliminate damaging resonances.

There are energy losses associated with dampers, but under normal conditions these are absolutely minimal. The only time when significant losses are found is during periods of torsional vibration, when energy is being dissipated in the form of heat. 

* Ker Wilson, W., “Practical Solution of Torsional Vibration Problems: Devices for Controlling Vibration”, Chapman and Hall, 1968, ISBN 0-4120-8580-1

Written by Wayne Ward

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Three mass crankshaft vibration dampers have been tested--by the Chinese I believe, and found to have some advantages.