pivots

 Hi all,

You probably read my blog and know I am working on a design for robot legs inspired by the legs of Theo Jansen's Strandbeest.

Below is a picture of a plywood prototype on its way to become version 6.

The three layers of ply are connected in the pivot points by means of paperclip wire through 1mm holes. This is by far not strong and durable enough for the application I have in mind. This leg can easily carry a 2 kg load. But I need to improve the pivots first. As it is, the holes wear out fast and become very loose.

Any inspiration would be welcome.

tj-work_on_drawing3.jpg

You can see the black glass head of a pin sticking out in the center. This pivot hinges from the robot's chassis. I call it point €. At the far right is the point that is to be driven by the crank shaft. All the other pivots connect to leg parts only.

The pivot design I am looking for must be:
* tight enough to keep the flat parts snugly together
* without causing too much friction between the plates
* positioned with precision
* allow the rear side to be mounted flush to the flat robot body (so no screw heads sticking out)

I am considering some metal reinforcements of the holes through the plywood. Like hollow rivets or washers and bushes. I am also considering using a different material, like plastic or metal. But I am afraid that those materials will also wear out very rapidly.

I appreciate your inputs.


Update 16 feb 2009: For completeness, I include Chris' video on brass cutting. Thanks dude!

 

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My calc shows 3.6 / 25.4 as appr. 1/7th of an inch.

And 1/16th inch is almost 1.6 mm (inner diameter). That's a decent, strong pin I guess. The collar would help tuning the tightness of the fixing (and the pressure between the plates).

I guess I have a visit to a model builder's shop in my future.

8ik

i suggest trying with brass eyelet too. you can find them at hardware store and are very cheap, so not a big problem if they don't work.

btw there are some spare ones in the Tamiya Twin Gearbox set IIRC, you could try with that ones. 

Those metal eyelets used in leather and clothing may be good to reinforce the hole in the plywood if you can get ones for material that thick. You can get them in shops that sell material for clothing. They usually come in a little kit with a special tool and are quite cheap. They are usually brass.

Otherwise you can get brass tube from some hobby stores. It is more expensive but you can cut sleeves to length and even solder brass washers to it.

If you want heavy duty then you can buy porous bronze bush tube from a bearing supplier. This is the stuff used for bushes in small motors. Once again cut to length. Put your cut pieces into hot machine oil and leave them there till they cool. The porous bronze will soak up the oil. Then press fit.

I've experimented with polymorph lately, it could be made into sheets and cut to shape. It is reasonably flexable so it may put a spring in your robots step if used properly.

Having punned that, I must say I like the idea. I used grommets before, in tarp like materials.

20030901_You_Can_Fix_It_page001img004.jpg

But indeed, my plywood would be too thick: 3.6 mm. And I want to fix two layers of them together. So I would need to fit a bolt through two grommets or something.

I can see how this leads to the idea of brass tube with washers soldered onto them. Pretty much a home made rivet. That is know technology (for me). Not too complicated either.

In general: any device that would increase the circumference of the hole through the plywood, would alleviate the wear and tear in the wood. The hole would stretch out less as a result.

The real issue seems to be the tightening. How would I tighten a pivot without a threaded pin (bolt, screw) to the exact tension needed?

 

You get two sizes of tube, one that fits over the bolt you want to use, which you cut slightly longer. One that fits neatly over that that becomes the bush in your plywood. The hobby shops should have them in suitable sizes from about 2mm thru to 16mm in diameter.

The other possibility is "Nyloc" nuts so you can set the tension loose enough to move freely without the nut coming off.

Aluminium Alloy sheets!

from strandbeest.com

More of these amazing images at Theo's website.