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Old 11-12-2013, 05:01 PM
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Stretch Stretch is offline
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Join Date: Sep 2009
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Quote:
Originally Posted by Adriel View Post

Why put the heaviest to the flywheel?
In the simple - over simple case - the crank is like a long beam with a big weight on the end (flywheel). Although the beam is supported at several points (main bearings) it will have a tendency to bend like a banana along its whole length. This is an exaggerated example of behaviour but there is usually a detectable vibration that is closely related to the length of the crank.

If you're adding four "bouncing" weights (pistons going up and down) at four different points along the beam you are better off putting the heaviest of these bouncing weights at the closest position to the big weight at the end of the beam (flywheel). This is because the effect of the heaviest piston is the least on the whole banana bending effect (that you might be able to imagine) of the beam when it is closest to the big dominating weight at the end (flywheel).

If you do the opposite and stick the heaviest piston furthest away from the flywheel then you might find that you exaggerate the banana bending effect and excite this vibration that is proportional to the length of the crank.

Generally speaking for most "normal" situation vibrations related to large parts of a machine are more of a problem because they often happen within "normal" operating speeds. So you want to avoid exciting / influencing / provoking the big stuff!

This whole model / approximation of bouncing weights on a beam with a big weight attached to the end of it supported at various points along its length, however, gets really complicated when you include the effects of combustion and sudden changes in engine speed...
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