Inspired by the smooth running of the gear bearings I thought I'd have a go at improving the bearings on my basic crank slider mechanism to see how well it would work. Turns out - pretty well!

In the original crank slider mechanism a square shaft fits into a circular. Simple but not very precise. Square shafts are handy because they can be joined together easily. This is because they have nice flat surfaces for gluing. 

By gluing a circular tube into the square tube I aim to achieve the best of both works. Accuracy and ease of joining.

A secondary side effect of the switch to circular axles is that the hole through which the axle passes is smaller. In this case 10mm diameter instead of 14mm. This means that the ends of the main push rod and be smaller and neater.

The assembled unit fits nicely into the box.

Turn the handle and the crank turns beautifully smoothly with very little play. A big improvement on the previous design whilst still being easy to make. 

It might make a nice cobra!

I'll put together a download version of the mechanism for members to download tomorrow.

<---- edit ---->

It's really nice to have #1 Daughter is back home from uni. She's been watching 'Chalet Girl' with #truelove and I've been cutting and sticking. I made this snake prototype :-)

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Monday 25th Mar 2013 01:21


One again that genuine simplicity of your paper model! 

Cobra not only looks easy to cut but looks elegant too.

How about making its tongue red?

Monday 25th Mar 2013 18:12

hugo leandro

Nice idea, Rob. What about an accordion body for the snake? It might be curious and tricky...

Saturday 30th Mar 2013 22:53


   its a bird,its a plane,no its super rob

i took the paper rob and attached it to the improved crank. trying to see if i can attach a paper strip to the feet so robs arms will move when the crank moves

Ha ha ha! I like it! - RI

Tuesday 2nd Apr 2013 14:50


Hello Rob,

With this new shaft, a square hole instead of a round hole also works. It is for those who find it a bit easier to cut a square instead of a perfect round hole. 


Good idea Urmish! I hate cutting out circular holes. >:-( - RI