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.

[PaulR]

Bit of a necro post but I'm still cogitating on this one!

I'm thinking of adding links to both sides of the bell crank. In this diagram showing a plan view, link 1 is straightforward as the bell crank and the valve are moving in the same plane but what about link 2? Am I right in thinking the holes at each end should be 90 degrees apart ie one horizontal and the other vertical? And would anything be needed to keep the valve rod parallel to the cylinder bore? Any help appreciated 

[steam guy willy]

hi could you use a revolving centre rod  with an off centre pin rather like an eccentric that is free to line up automatically ??!!

[PaulR]

hi could you use a revolving centre rod  with an off centre pin rather like an eccentric that is free to line up automatically ??!!

Umm, sorry I don't understand  Can you explain a bit more?

[crueby]

What you are trying to accomplish is what a ball joint does - move in two axes without having the links collapse in the center. The picture you showed earlier was a ball joint. This page shows another way to construct it, first picture below the table of contents list at the top:
https://www.theengineeringknowledge.com/what-is-a-universal-joint/

[PaulR]

What you are trying to accomplish is what a ball joint does - move in two axes without having the links collapse in the center. The picture you showed earlier was a ball joint. This page shows another way to construct it, first picture below the table of contents list at the top:
https://www.theengineeringknowledge.com/what-is-a-universal-joint/

Thanks Chris, yes I see now that a simple fixed two axis solution won't work, as it needs to allow a combination of movement in those axes. I guessed that the picture showed a similar joint but I can't quite see how that one works. Either way, it's way beyond my ability to make so I'll have to use the original idea with some slack in the joints or come up with something else.

[cnr6400]

Hi Paul, I have used small threaded ball links from DuBro (pic attached) on bell crank steam engines in the past. They allow movement over a wide range of angles and will operate fine in shockingly fast small steam engines. You might be able to get some at an RC model plane hobby shop, they are very inexpensive compared to industrial ball links. Hope this helps.

[PaulR]

Hi Paul, I have used small threaded ball links from DuBro (pic attached) on bell crank steam engines in the past. They allow movement over a wide range of angles and will operate fine in shockingly fast small steam engines. You might be able to get some at an RC model plane hobby shop, they are very inexpensive compared to industrial ball links. Hope this helps.

Interesting, thank you. I'll see how I might be able to make those fit in. 

[crueby]

Agreed - I used those on my Holly pumping engine for the valve cranks, they work very well. They come in a couple sizes, and also some versions have metal ball connections that look just like the ones in the picture you showed near the start of the thread. The metal versions are easier to disassemble, the ones with the plastic housings around the ball must be pried/popped off the ball, and can only take so many removals before distorting. They make all the types/sizes for the RC aircraft market, most hobby shops that cater to RC plane/copters will have them. Very handy connectors.

[cnr6400]

 

[cnr6400]

I just found a pic of an engine I built in 2012 to test a design published on the internet, but with modifications. I added adjustable valve timing and stroke mechanism, a toothed pulley power take off for driving accessories via a toothed rubber belt, and one of the snap on / off type ball joints Chris mentioned, to drive the bellcrank. Pic attached. Just FYI, hope the ideas help.