# Linkage suggestions



## joker (Sep 25, 2007)

I'm wanting to build a ground breaker for next year, but instead of the static props I had this year I want to give him some movement. I'm trying to come up with some way to move the arms in a digging/pulling motion. Like the gb is trying to pull his way out of the ground.

Anybody done this or similar?

TIA


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## Northern Touch (Nov 8, 2006)

I'm starting 2 props smiler to what your talking about but I'm using wiper motor's...


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## joker (Sep 25, 2007)

I was thinking about using something similar to a bicycle crank and attaching the arms where the pedals would be and then just use a single motor with a chain. I'm just not sure how well it'll work or what it'll look like movement wise. The arms will probably have to have fixed elbows so that it doesnt just flop around.


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## randyaz (May 26, 2006)

look into 4 bar linkage...


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## slimy (Jul 12, 2006)

Forum member kevin242 came up with this a few years ago. I meant to build one, never got around to it, but other members have and have found great success with it.

http://www.fulcrumsites.com/haunt/html/the_grave_grabber.html

Check it out.


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## joker (Sep 25, 2007)

slimy said:


> Forum member kevin242 came up with this a few years ago. I meant to build one, never got around to it, but other members have and have found great success with it.
> 
> http://www.fulcrumsites.com/haunt/html/the_grave_grabber.html
> 
> Check it out.


Slimy thanks for the link I may end up using that method.

In my neighborhood any movement at all is going to impress the ToT's, but I'd really like to create a reaching up and forward motion then dragging back on the ground towards the body to simulate trying to pull himself out of a hole.


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## Revenant (Mar 17, 2007)

Pardon the crappy MS-Paint artwork, but here's a basic linkage for the reach-up&out/slap-down&drag arm motion (the little circle under the wrist with the "F" on it is a fulcrum, just something smooth that the arm just lays across):








That in it's basic form works well if you have a hand reaching out from under a tarp or something. If you want head & shoulders to be seen above the reaching arms, maybe you could cover the mechanism with a lumpy draping or maybe rig some lightweight upper-arm & shoulder attachments that would passively move along with the linkage... dunno... for what it's worth this gives a nice trying-to-climb-out motion.


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## bourno (Jan 21, 2006)

Here is another idea to look at that would give reciprocating motion.

http://photobucket.com/albums/v630/DaveNTracy/Crawling Skeleton/


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## joker (Sep 25, 2007)

Revenant said:


> Pardon the crappy MS-Paint artwork, but here's a basic linkage for the reach-up&out/slap-down&drag arm motion (the little circle under the wrist with the "F" on it is a fulcrum, just something smooth that the arm just lays across):
> 
> 
> 
> ...


Rev thanks for the drawing. So would the "Fulcrom" in the position of the drawing actually be resting on the ground at that point? Then as the motor continues to turn pushes the fore arm across the hole opening forcing it up and then down as the link peaks at the surface and then dragging back across?

Would the wrist need to be hinged as well?


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## Revenant (Mar 17, 2007)

joker said:


> Rev thanks for the drawing. So would the "Fulcrom" in the position of the drawing actually be resting on the ground at that point? Then as the motor continues to turn pushes the fore arm across the hole opening forcing it up and then down as the link peaks at the surface and then dragging back across?
> 
> Would the wrist need to be hinged as well?


Like I said, the drawing is mondo-crude n crappy and has no sense of positioning or scale, but yeah, that's the basic idea. As the crank rotates (CCW in my pic) from 9:00 to 6, the hand would be in a retracted position and tip upward... from 6 to 9, the arm would reach up and out, a little past 9 it would fall forward on the ground, then as the crank travelled back toward 9 it would drag the hand back. In my pic the fulcrum is way too far back, it would make the forearm tower straight up... you'd want it to be at the ground level some distance in front of the joint that connects the arm to the crank so the arm just reaches forward at an angle. And it looks like a hellacious trench in my pic; really it would only have to be a slot cut in the ground deep and wide enough for the flat stock of the crank and some of the arm to fit down into so it didn't leverage the arm to high on the downward stroke. And as far as the wrist goes, the one I saw done just had a straight arm/hand. I guess you could play with little 4-bar linkages or something to make his fingers clutch at the ground, but I think it looks neat just simple. Don't think I've seen it done with both arms before...


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