Eccentric throw

[Anatol]

What is the maximum viable 'throw' of an eccentric?
if crankshaft dia=1, and crankshaft is, for argument;'s sake, at one extreme side of the eccentric, how big a diameter of eccentric is viable. 3?, 4?
thanks!

 

[Charles Lamont]

You have had no other reply to this question because there is no answer.

 

[TonyM]

As Charles said there is no answer to your question. The maximum limit is governed by the crankshaft design and material in order to withstand the loads required when performing the movement and work that it is designed to do.

 

[Anatol]

Thanks guys
I guess I should have been more precise in my question. I'm thinking about pushing a piston/spool type valve directly, so I need significant travel in a pushrod. I've seen eccentrics driving spool valves.

Sealing between ports in a spool valve is - presumably - improved if the solid parts of the valve body are longer. (I was going to ask about dimensioning a spool valve in another thread But then you'd be pushing a valve further, so longer throw on the rod would be needed, so a more 'eccentric' eccentric is needed.

I recognise that, in principle, you could have a 3' dia eccentric on a 3" shaft, if you were willing to use most of the energy of the engine pushing the eccentric. In order to get reciprocating motion off a crankshaft in that orientation, is some other mechanism preferable? I guess an obvious option is a crank, but that requires substantially more machining, Any advice much appreciated.

 

[Richard Carlstedt]

Look at some hot air engine designs and you will see some that have 2 throws or 2 cranks, one for the piston/crankshaft and another for valve or other functions. Eccentrics are bad for long movements due to eccentric size (dia) and friction.

 

[Anatol]

Look at some hot air engine designs and you will see some that have 2 throws or 2 cranks, one for the piston/crankshaft and another for valve or other functions. Eccentrics are bad for long movements due to eccentric size (dia) and friction.

right, I get it, thanks!

 

[Hopper]

Look at the way locomotive valve gear is operated by Walschearts mechanism etc. (Not Stephensons though). Walschearts is operated by a crank mounted to the loco wheel.

That said, most mill engines and the like used eccentrics so there is probably no need for you to reinvent the wheel. Simply follow the dimensions on an established design. After all, eccentrics been working for hundreds of years now.

 

[MachineTom]

Imagine a disk 3 on dia. Now you l
Place a shaft of 1 dia on the disk, with the outer edges aligned. If the 1 shaft is concentric in rotation, the 3 disk is eccentric in rotation. So the center of the shaft has a radius 1/2", and the disk a radius of 1 1/2"-the 1/2=1" that would be the stroke distance as setup.

 

[rlukens]

Imagine a disk 3 on dia. Now you l
Place a shaft of 1 dia on the disk, with the outer edges aligned. If the 1 shaft is concentric in rotation, the 3 disk is eccentric in rotation. So the center of the shaft has a radius 1/2", and the disk a radius of 1 1/2"-the 1/2=1" that would be the stroke distance as setup.

Wouldn't 1" be the throw, and 2" be the stoke?

 

[TonyM]

Wouldn't 1" be the throw, and 2" be the stoke?

Yes. It makes no difference which one is the fixed axis it is the difference between centres that give the stroke and throw.