Visco-elastic dampers

Wednesday, July 09, 2014

Tags :  crankshafts

In the recent series of RET-Monitor articles on crankshafts, we have looked at various designs of vibration dampers/absorbers that can help protect engines from the effects of torsional vibration. There is a wide variety of basic concepts and many variations on each, while some concepts that have disappeared from engines would be more viable these days thanks to new materials, production methods and the ability to engineer the surfaces of components.

As with many aspects of powertrain design, it is possible to combine the thinking behind two concepts, and so it is with the visco-elastic damper. We have described the basic concept of an inertia damper where an inertia ring reacts against vibrations via elastic elements, normally either springs or elastomeric elements such as O-rings. Elastomers can act very effectively as dampers, but we have to be aware of the rate at which we are putting energy into any system containing them. Beyond a certain wattage (or BTU per hour for those who prefer imperial units) per unit volume, the elastomer will quickly degrade into something unsuited for further use as a damping element, so the critical rate of heat addition to a volume of elastomer depends on the exact type and grade used.

So, if we want some controllable damping but don’t want to rely on an elastomer to provide it, is there an alternative? Well, if we are willing to accept a degree of extra complexity, the answer is yes. A visco-elastic damper incorporates springs that have very little damping, and a liquid that takes care of the damping. It allows us to have independent control over the behaviour of the elastic elements (springs) and damping which we don’t have in an inertia damper with elastomer elements. A viscous damper has damping in abundance but no springing.

In a visco-elastic damper, when there is vibration present, fluid is forced to travel through small orifices between pairs of chambers, and the motion of the inertia ring relative to the crankshaft is also controlled by springs. It can be likened to a rotary version of a very simple suspension unit (shock absorber) that we might find on our car or motorcycle. Owing to its elastic elements, the visco-elastic damper can be tuned to react to vibrations that occur at certain frequencies, corresponding to those at which we know serious vibrations occur.

The extra complexity in manufacture comes from having to machine precise orifices for metering the damping fluid and to incorporate seals that will prevent the damping fluid from escaping. There is also the matter of having to put the assembly together with the correct amount of fluid in there. It is fortunate therefore that there are companies which supply such complex units.

I don’t know if any race engine suppliers are using such units, owing to their comparative bulk, but they are certainly used for controlling torsional vibrations on crankshafts and camshafts in industrial engines. Don’t read anything significant into the fact that these are fitted to crankshafts to infer the size of the industrial engines which are equipped with viscoelastic dampers; I have seen more than one highly optimised race engine fitted with camshaft dampers.

Written by Wayne Ward

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Engine designers have also relied on pendulum style crankshaft dampers, most notably in WWII aircraft engines.  A Ford SAE paper of several years ago attributed significant engine smoothness improvements to the application of pendulum crankshaft dampers in lieu of the production units.  More recently, some car companies are putting pendulum dampers in the flywheel assembly.  Perhaps the stubborn smoothness challenge in three  cylinder engines can be  better met through the use of crankshaft pendulum dampers, along with allied units in the flywheel or torque converter.
Has any research been done on the mercury 'engine balancer' machined into a full circle flywheel?