# Parallel Motion Linkage...



## RichD (Mar 4, 2018)

Hi,
I know walking beam design is pretty straightforward but recently I  worked up a cad drawing to lay out the hole positions and and details  required to incorporate James Watt's 4-bar linkage.

The image shown is sized for a 1" stroke so scaling up/down should be pretty simple. I show the linkage on both ends, but most times only one end would have the links. The site below has all the formulas I used.

 Visit https://en.wikipedia.org/wiki/Parallel_motion


I figured many here might want a copy.
Regards,
Rich


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## MRA (Mar 27, 2018)

I might be missing it, but you haven't shown the length of the tie-beam which goes from the 'free' side of the parallelogram (not the beam itself), to the chassis or the wall of the engine house.  I've never quite worked how how long this rod needs to be relative to the location of the pivots along the main beam, and where it's anchor point needs to be relative to the locations of the main beam through one full stroke.  I used to be able to handle maths, but I'm getting fuzzier, and fuzzier.


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## Hopper (Mar 29, 2018)

MRA, you would think the distance between centres on the tie down rod would need to be the same as teh distance from the centre pivot of the beam to the centre of the parallel motion link where it hangs from the main beam. That way, the curved path described by the link attached to the beam would be equal and opposite to the other end of the link that was attached to the equally long link in the other direction. 

Hard to describe in words, but I know what I mean! 

According to the link in the OP this gives a slight figure 8 motion, but within one thou in four inches of a straight line. So within a few tenths of a thou at model size. 
Interesting stuff. I have been looking at (er daydreaming of) making a freelance beam engine based on the pic on the front cover of K N Harris's book on model stationary steam engines so has taken me a while to figure out this parallel motion stuff! No plans for his engine seem to be extant, other than a very small GA in the book. 

The book's on the net in PDF form somewhere or other, so you could probably scale the linkages off his GA drawing if nothing else.


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## MRA (Mar 29, 2018)

Hey, Hopper

I used the animation on that wikipedia link in the opening post, since it seemed to 'work' - that is, perhaps it is not just a sketch, (and perhaps the links are all rigid in it and not stretchy!).  I put a Vernier on the screen (!) and tried to measure the tie-rod length (~26mm on my screen) and the distance from the centre pivot of the main beam to the first link (~21mm).  

So I like your logic, and I was groping towards something similar, but that animation doesn't seem to work like that.  Hmm.


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## goldstar31 (Mar 29, 2018)

My Logic is to go to the fountain head- or better still, Cadishead.

Google Stephenson's Gear and the Don Ashton website.

Dear Don isn't very well- as I know too well. We, Don, my late wife and myself go back a --long, long way.



Cheers

Norman


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## Hopper (Mar 29, 2018)

Norman, Yes Don Ashtons site is great. I finally got my head around Walschearts gear ( I think) on there. Must dig a bit further. Can't find beam engines there, yet. 
MRA, the site is http://www.donashton.co.uk/index.html


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## Hopper (Mar 29, 2018)

MRA, I just checked in the book Model Stationary and Marine Steam Engines by K N Harris.
His beam engine GA drawing, which I blew up to about 400 per cent, shows the tie-down link the identical length as from the beam's main pivot to the beam's pin that the parallel motion link hangs downwards from. 
If you take a look at page 17 here http://www.fastonline.org/CD3WD_40/JF/424/19-423.pdf you can check the measurement.


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## RichD (Mar 30, 2018)

MRA,
Here's a 3D picture I drew that may help visualize the chassis link. My first post was a 2D drawing so it was hard to show the chassis link since it is exactly the same length as the other horizontal links. ON some museum examples (and JAy LEno's garage) you see they would mount a horizontal bar to the central post at the same elevation as the centerline of the lower end of the vertical links. When the beam is perfectly straight, all the center lines are in the same plane. Kind of like a giant fork with pivot holes provided for the fixed ends of the chassis links to pivot from.

So for the math part...just take the link and beam length numbers from the first picture I posted, and multiply them by the length of stroke you want for your design. My example was for a 1" stroke, so for a .5" stroke multiply by .5...for 1.75" stroke, multiply by 1.75 and so on. Hope this helps. 

I really should have shown the beam "radius" instead of the total beam length in my first picture since 99% of the time you'll only use one end of the beam for parallel motion and the other connected to a crankshaft with it's own stroke length. The 6" in my picture would then become 3" for design purposes.

Rich


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## Mechanicboy (Mar 30, 2018)

The parallel motion is for piston who need linear motion, but the extra parallel motion in other side of beam for crank?


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## MRA (Mar 30, 2018)

This is really interesting.  I know of a few full-scale examples around here where that (what seems to me, sensible and clear) geometry is not adhered to with the chassis link.  I'll try to make some photos and we can talk about what is going on!


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