JDW Grasshopper engine

I looked at the geometry of Elmer's wooden grasshopper right hand pivot as it is clearly constrained by the leg to travel in an arc but horizontally by the linkage. It works out from the original dimensions that the horizontal pivot movement is ±0.032" and since the arc has a radius of 5.125" this results in a vertical error of 0.0001" or ±0.00005" . I don't think I need to worry about this slight error in the motion locking everything up since I am also intending to make the pivot bearings out of some Oilon (Nylon 6 sheet with added oil) I have in my scraps collection which will add a little bit of give into the mechanism.

Martin C

I found similar playing with the #37 grasshopper some more, if I added a couple of microns tolerance to the beam/vertical link joint then it could be fully animated so allowing for the clearance that would be needed in all the pivots there should not be a problem if link is half beam distance.

.

At last … the micron is vindicated !!

MichaelG.

As Jason has found, if you attach the right hand end of the beam to a vertical link and constrain the left hand end to go in a straight vertical line it locks up. If we take the vertical link to be 3 long, and the other dimensions as my previous post, the top of the vertical link moves back and to by 0.1022. If we get a bit clever and move the fixed bottom pivot across to the left by half of this, the vertical link will swing plus and minus asin(0.1022/2/2)=0.98 degrees. The top of the vertical link will now be in the right position at top middle and bottom of the stroke, in between it will be too high by 3*(1-cos(0.98))=0.00044, and the left hand end will describe a very shallow S shape. I could work out how much it deviates from a straight line, but it's a lot more difficult than what has gone before, so I'm not going to. If you make the vertical link too short it might start to become significant, but all the full size grasshoppers I've seen had long vertical links.

As Jason has found, if you attach the right hand end of the beam to a vertical link and constrain the left hand end to go in a straight vertical line it locks up. If we take the vertical link to be 3 long, and the other dimensions as my previous post, the top of the vertical link moves back and to by 0.1022. If we get a bit clever and move the fixed bottom pivot across to the left by half of this, the vertical link will swing plus and minus asin(0.1022/2/2)=0.98 degrees. The top of the vertical link will now be in the right position at top middle and bottom of the stroke, in between it will be too high by 3*(1-cos(0.98))=0.00044, and the left hand end will describe a very shallow S shape. I could work out how much it deviates from a straight line, but it's a lot more difficult than what has gone before, so I'm not going to. If you make the vertical link too short it might start to become significant, but all the full size grasshoppers I've seen had long vertical links.

Have completed the engine and it runs on compressed air. Just need to make proper keep pins for the ends of shafts and fit gaskets, then strip and paint.

Jason, I made the side link hole centres at 85.5mm as advised and this works ok, so thanks again.

Apologies if photos are in wrong orientation.

Geoff

Edited By JasonB on 12/11/2022 15:07:42

Looks to have turned out well and good to know it ran with the changed sizes.