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Balance Weight Question

 
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Shrapnel



Joined: 20 Oct 2005
Posts: 16

PostPosted: Tue Mar 14, 2006 9:04 pm    Post subject: Balance Weight Question Reply with quote

Looking at a diagram of the internals of a rotary engine, I was somewhat surprised to see a fairly substantial balance weight hanging off the main drive shaft. The two eccentic sections of the driveshaft supporting the rotors appear to neatly off-set one-another by 180 degrees. If this is correct, what would still be unbalanced on the driveshaft, thus requiring the balance weight? Some advice from a knowledgeable person would be much appreciated.
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Tony Shrapnel
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Graviman



Joined: 24 Feb 2006
Posts: 6

PostPosted: Thu Mar 16, 2006 5:37 am    Post subject: Reply with quote

Balance weights are to keep moments in balance as well as forces. Each rotary cylinder only has 1st order imbalance, due to rotor, but weights counteract this. Rotor is effectively spinning about it's own mass centroid.

That's why a rotary is smoother than it's equivalent 4 cyl 4 stroke piston...

Mart
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Shrapnel



Joined: 20 Oct 2005
Posts: 16

PostPosted: Fri Mar 17, 2006 5:35 am    Post subject: Reply with quote

Hmmm... sorry, but that didn't get through. Is there a way to explain it more simply? I mean, in the rotary engine, what is unbalanced on that driveshaft with the eccentric rotor journals off-setting one another? That looks like a nicely balanced arrangement to me.
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Graviman



Joined: 24 Feb 2006
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PostPosted: Sat Mar 18, 2006 8:43 am    Post subject: Reply with quote

Quote:
Is there a way to explain it more simply?


Not really, but i'll try. You'll just have to think about it to convince yourself about what is happening.

They rotors offset each other, but are shifted along the crankshaft. The engine is balanced only if you "project" the engine onto a sheet of paper. An actual 3D engine will try to oscillate about it's centroid, like two unbalanced washing machines back to back - with unbalance 180 degrees out.

It's one of those things you'll just have to see, but trust me the counterweights are essential.

Mart
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