Bell Cranks

[PaulR]

I drew this fledgling plan quite a few years ago when I was thinking about building a simple engine like the 'McCabe Runner', controlled by a simple sliding valve. All of the examples of that engine I've seen don't seem to take any account of the rotational effects of the bell crank and rely on sloppy fits or very loose joints. I thought about using slots rather than holes - does that look right?

I'm thinking of a piston with wrist pin on one side and a Scotch yoke linking to the bell crank - with one or more guides to keep that rod straight.

[Davyboy]

I like it.

[PaulR]

I like it.

I've found plenty of examples that introduce an additional link to solve the problem but having slots instead of holes seems simpler. I found this example with an extra link on a photo of a marine engine, I like the design - apart from the weird hoop and ball connection on the other arm (there's probably a name for this but I don't know it).

[crueby]

On many full size engines they were able to do without the slots or extra links since the valve travel was very short in relation to the length of the crank arm, so the slight slop in the gland packing let the valve rod take up the tiny offset movements. I've done the same on a few models, they work just fine with no slots on the arm. Worth doing the math on your design to see if you need the slots, if the offset movement is only a couple thou you can skip them. If the crank travel is very large, I'd  go with the link since the slot would bind slightly  when at the ends of the travel.

[PaulR]

On many full size engines they were able to do without the slots or extra links since the valve travel was very short in relation to the length of the crank arm, so the slight slop in the gland packing let the valve rod take up the tiny offset movements. I've done the same on a few models, they work just fine with no slots on the arm. Worth doing the math on your design to see if you need the slots, if the offset movement is only a couple thou you can skip them. If the crank travel is very large, I'd  go with the link since the slot would bind slightly  when at the ends of the travel.

Great advice, much appreciated.