Help! > Specific Engine Help
Crank operated lever controls rotary valve and fries brain!
PaulR:
Thanks for the input. Here's the thread on HMEM where this particular example came from, the topic includes the original plans and a couple of builds both of which use an eccentric (which I'd prefer not to do) - hope it's ok to post that here?
https://www.homemodelenginemachinist.com/threads/scraps-and-solder-become-an-engine.26180/#post-408806
Looking at it again, I think valve pin (b in my diagram) is describing a circle based on its distance from the crank pin a (ie with radius a-b). Seems I just need to work out the angle subtended by the arc that link pin c travels which is in turn related to the same radius and the length of the connection between c and the centre of the valve.
Or maybe not! :noidea:
PaulR:
--- Quote from: Jasonb on December 23, 2024, 07:37:08 AM ---Easiest thing is to draw it in cad, section and then you can watch what is happening as you make any changes.
I have done a couple of rocking valve engines but used a pocket in the valve rather than just holes but both were worked out in CAD.
As said the arrangement at the crankshaft end is no different to an eccentric.
Vertical movement at the rod end has little effect unless you make the lever very short
You need to watch any images taken from the likes of Grabcad, it is not unknown for errors to creap in
--- End quote ---
Thanks for this - nice models! I'm just relearning some 2D CAD and had a fiddle with Tinkercad to make some quick mock-ups, I'll look into 3D once my brain cools a bit!
PaulR:
I made this cardboard thingy to no particular dimensions just to see what happens to the link between the crank-pin and the valve-pin and what that does to the valve link/lever at the right-hand end. I think the positions shown are correct but I suppose it could behave differently in practice!
Whether or not its it's pure coincidence or I suppose more likely related to the different radii upon which the pins are set on the crank disk, it appears to move the lever a little over 90 degrees, so not unlike the two lots of plans (105 and 115 degrees).
PS Hopefully the GIF will animate properly in your browser.
crueby:
Gif works fine here!
If the arm is slightly longer or the valve crank closer to the crankshaft, the action would be centered better, and it wouldnt risk jamming when the crankshaft end is all the way out.
Gotta love Cardboard Aided Design!
PaulR:
--- Quote from: crueby on December 23, 2024, 02:31:46 PM ---If the arm is slightly longer or the valve crank closer to the crankshaft, the action would be centered better, and it wouldnt risk jamming when the crankshaft end is all the way out.
--- End quote ---
Thanks for the help... I'll make some adjustment, see what happens.
Cardboard Aided Design, I like it! :Lol:
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