# Offset eccentric



## SignalFailure (Dec 30, 2010)

I've got brain-fade today :-\ Will an arrangement such as this work and will the eccentricity affect the valve travel as normal?

(The valve rod is travelling up/down as indicated but the crankshaft is offset The eccentric and strap would be as normal - the circle at the connecting point is supposed to represent a fork). I guess some form of guide would be needed for the rod...

TIA

Paul


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## Mainer (Dec 30, 2010)

The travel will be cosine(angle) of what it would be if everything was in-line. I think.


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## steamer (Dec 30, 2010)

YES


Dave


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## SignalFailure (Dec 30, 2010)

Thanks both...

Mainer: thanks, but what angle are we talking about?

Dave: :big: YES what? Cos, will work, will require support, all three?


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## arnoldb (Dec 30, 2010)

Hi Paul

To quote Dave, "YES" - all three.

I stole your picture and added in some line drawings.

The angle is x. COS(X) = A/B. With an eccentric offset of B, you will get a valve travel A = B x COS(X)

Of course, when setting your eccentric, and presuming you want it set at 90 degrees to piston travel, you'll also have to compensate, and set it at 90 degrees - X.

Hope this made sense!

Regards, Arnold


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## SignalFailure (Dec 30, 2010)

Thanks Pat, with a total valve travel of about 8mm and a short(ish) rod there might not be space for a guide....

Thanks Arnold, yeah where 'B' is the total length (of the hypoteneuse)...I said I had brain freeze today


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## hammers-n-nails (Dec 30, 2010)

i think this would be a good place for a bellcrank. should transfer motion really close and would result in alot less side load on the vertical shaft.


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## ttrikalin (Dec 30, 2010)

that's the principle behind the desaxe design, no? small offset. 

tom in MA


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## steamer (Dec 31, 2010)

YES 

you will have created essentially a quick return mechanism

The piston speed on 1/2 the stroke will be faster than the other half

The side thrust will vary as the cosine of the rod angle......basically from nothing to a lot.

.....sorry I didn't get back to you sooner....my son's RC car had its own cosine problems...mostly as a result of immovable objects..leaping out in front of it......like brick walls. :big:

Dave


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