More on dampers

Thursday, May 08, 2014

Tags :  crankshafts

In the previous couple of RET-Monitor articles under this keyword, we have looked at ways to control torsional vibrations in crankshafts. It would be possible to expand on this almost indefinitely, as there is a huge number of devices that have been used with success in engines in the past. Many of them are very complex and lend themselves to use on very large engines rather than the much smaller race engines we are used to working with, simply because the components can be made at a sensible scale.

There are some notable designs of damping device though that could be adapted for use in modern race engines, and the ease with which this is achieved will depend on the space in which the damper can be accommodated.

There are a number of variations of pendulum damper, and although each damping element acts as a pendulum, very few will bring to mind that in a traditional clock.

Several designs of pendulum damper are used on individual crank throws. These can be attached to the side of the crank web on either side, close to the main bearing, or can be designed to form part of the counterweight. It is strange to imagine the counterweight not being firmly fixed to the rest of the crankshaft, but it is this freedom of movement that allows the counterweight mass to move and to oppose the action of the torsional vibration. Some designs have been used successfully in marine and aero engines, and there is no reason why they couldn’t be applied to a modern race engine. Clearly, the stresses in the component supporting the counterweights would be considerable at high engine speeds, so such designs might lend themselves best to lower speed engines.

A variation on the pendulum damper idea is one that might find use more easily in a race engine as a crankshaft damper, as it can be mounted in a carrier and fitted to the end of a crankshaft, as per the more usual elastomer, sprung or viscous dampers. In this design, a number of metallic ‘rollers’ sit within a housing that has compartments into which the rollers fit. When these were first introduced, they were known as ‘roll-form absorbers’, referring to the form of the pendulum as a roller. The difference between the diameter of the roller and that the compartment affects the frequency at which the damper works best.

There may be a damping fluid in each compartment holding a roller, but this is not always the case. The damping fluid helps the device dissipate energy, but it is not required in order for the damper to reduce vibration amplitudes. Problems have been encountered with this type of damper where each pendulum has been made too light and therefore oscillates over a large angle about its mean position, or where there has been insufficient or excessive lubrication*.

The principal problem with large roller pendulum amplitudes is that of slipping, as the change from a pure rolling operation to a combination of rolling and slipping makes predicting the damper’s behaviour difficult. 

* Ker Wilson, W., “Practical Solution of Torsional Vibration Problems”, vol. 4, 3rd edition, Chapman and Hall, 1968

Written by Wayne Ward

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