Bell crank alternatives?

[SignalFailure]

Is there any (simple!) alternative to the bell crank for transferring linear motion through 90 degress? It seems to me that a bell crank suffers from the same rotational forces as beam engine connecting rods without 'parallel motion' - I notice some of Elmer Verberg's engines (ahem) 'overcome' the problem by using bell cranks with slots rather than holes at the connecting ends.

Any suggestions please?

PS Ah, I just thought of something; if the rods were not in the same plane one could have a sort of screw thread that the other would turn, ie one winds the other back and forth....probably not a good explanation but I'm thining of something like an ancient bow-drill!

 

[Ed T]

Not sure that I understand the real question, but how about a cable and pulley or a pair of them if it has to push and pull. How about a cable in a tube. How about a stack of small balls in a tube. There was a Bugatti engine that transferred the cam motion from one side of the head to the other using a semicircular rod captive in a matching feature in the head so that the rod slid back and forth in a small arc carrying the motion thru 180degrees. Theres is a two volume set of books called "ingenious mechanisms" that has a lot of this kind of stuff. During the development of the steam engine, lots of ways were developed for this kind of thing

 

[SignalFailure]

Pat: Thanks for the help, a photo would be great if you have one as I'm not 100% sure about the 'sleeve and the slot' configuration! I did think about using intermittent connecting rods but it adds too much bulk

Ed: Now that is an ingenious idea - a ball-filled elbow, ultra simple in theory but I'd been to think about the practicalities!

Thanks again

Paul

 

[tel]

What is the application Paul? Most bell crank set ups in our sort of sizes have the input and output guided, thus eliminating the problem.

 

[SignalFailure]

Hey Tel The application is pretty much as per your photo - even in that doesn't the top linkage to the valve rod want to rotate downwards when the rod moves right? Maybe it's not enough to worry about at this sort of scale?

 

[tel]

I've never found it enough to bother about

 

[SignalFailure]

I'll make a mock up first, see how it works - thanks for the pics, always helps.

 

[tel]

With the Sirius and the Sun, Stuart Turner used a set of bevel gears to turn the rotating motion vertical, then used an eccentric on the top of the vertical shaft - I got as far as starting a second engine to use that system, but haven't progressed with it

 

[tel]

Westinghouse used the bell crank with (apparently) no problems tho'

 

[SignalFailure]

Nice Westinghouse engine!

Don't get me started on bevel gears again - an earlier project was using them with a crank to control the valve but I later found a major design fault so that one still hovering over the scrap bin! (or more likely to be cannibalised wot with the price of brass and cast iron these days).

 

[SignalFailure]

Thanks Pat, it looks like the link at 'K' does the trick. At the scale I'm thinking of a bit of slop in the pin might work just as well...we'll see!

 

[tel]

If you refer back to my first pic you will notice that I have a pair of links at either end of the bell crank

 

[SignalFailure]

If you refer back to my first pic you will notice that I have a pair of links at either end of the bell crank

Yeah I saw that mate, my eyes aren't as bad as yours yet :big:

 

[tel]

Rof} You are closer to the truth there than you realise Glasshopper!

 

[CallMeAL]

This worked for me. ;D

[ame]http://www.youtube.com/watch?v=3k-P1gmDbEw[/ame]

 

[SignalFailure]

This worked for me. ;D

Very, very nice. Certainly a different take on controlling the valve....the vertical rod running to the eccentric looks like it's off centre - what's the arrangement there?