Linkage Geometry Queestion.

[Jasonb]

Possibly one for Marv or Willy may have it in an old book but is there a specific formula for this form of piston rod guide linkage.

Muncaster's drawing does not give lengths and plotting it from sizes taken from the scan has the top of the piston rod pulled to the right. Julius drew up a version but slotted the link with the offset pivot but that would allow sideways movement and not keep the rod vertical.

Thanks, J

[Bluechip]

Possibly one for Marv or Willy may have it in an old book but is there a specific formula for this form of piston rod guide linkage.

Muncaster's drawing does not give lengths and plotting it from sizes taken from the scan has the top of the piston rod pulled to the right. Julius drew up a version but slotted the link with the offset pivot but that would allow sideways movement and not keep the rod vertical.

Thanks, J

A derivation of the Peaucellier maybe.

Dave

[Jasonb]

Yes there are a few similar sort of links but have not been able to nail this one down yet. Evans linkage is close.

[Captain Jerry]

This looks like the normal rocking beam linkage.  I will try to draw it up tonight.  I can capture a video of the motion but I don't remember how to post a   ***.avi file on this forum.


Jerry

[Jasonb]

Very similar to a grass hopper type arrangement but on that the right hand linkage is a lot shorter and with it's pivot near the piston rods line of vertical movement.

The cyan link on this is a lot longer on the Muncaster

https://en.wikipedia.org/wiki/Grasshopper_beam_engine

[crueby]

This looks like the normal rocking beam linkage.  I will try to draw it up tonight.  I can capture a video of the motion but I don't remember how to post a   ***.avi file on this forum.


Jerry

Hi Jerry,
If you have an account on YouTube, you can upload the movie file there, and copy/paste the link they give you if you do their 'share' button. Just pasting the link into a post here should work - if you do the 'Preview' button here (next to the 'Post' button) you can see if it works before making the post for real.
Chris

[Zephyrin]

I just did a test of this linkage with Solvespace...as I was playing with it !
here are a solution which give a approximately linear movements of the crosshead, within 0.2mm for 40 mm stroke...not that bad for such an simple linkage !
this could be improved maybe with some tweakings of the levers and pivot positions.

Solvespace allows the tracing of a point while moving the linkage, the pale blue line on the animation below, and also gives a table of the coordinates

[Jasonb]

Thanks for that which seems to confirm what I was able to get by sketching the parts in Alibre and then playing about with sizes to get them to move. I had to set the fit of the piston in it's hole to +/- 0.2mm and could then get 50mm of stroke on a slightly larger scaled version.

[Zephyrin]

I have found the drawing of the Muncaster engine edited by ET Westbury, a great piece of machinery and a impressive model, but dimensions are scarce !
It is very tricky to optimize lever length to minimize curvature of the path of the crosshead between 2 antagonist arcs, too many variables for my old brain, and I don't have access to artificial intelligence...

PS : a slot in a linkage releases a constrain and would permit a linear movement of the crosshead to occur, (if this part is already linearly guided), but would'not in its own guarantees a rectilinear movement, imho.

[Alyn Foundry]

I visited the Anson engine museum today.

   Graham.

[Jasonb]

Thanks Graham, I did go and dig out the image you posted of that engine previously but alas it just has the more common Watt linkage

Probably more common as it actually produces a straight line, this one gets close but not been able to come up with a combination of fixed pivot loactions and link lengths that work "on the computer" it may well work when yhe small clearances in each pivot and the piston & gland fit are all taken into account but would be nice to have a definitive answer.