"slider-crank"-style lift mechanism. What workaround to avoid dead point/singularity?

[thorq]

I need at least a 3rd crank for three main reasons (maybe there are more):
- first is to prevent platform tilt/rotation around the axis defined by the two insertion points of the crank's arm (the blue bearing housings), when there are only two opposite cranks as you suggest;
- second would be to minimize play (due to the small errors in the bearings shafts and in tightening of the nuts in the cranks arms),
- third would be to prevent bowing of the platform due to size or weight.

What sprocket are you referring to? I am using the big blue wheel as sprocket/pulley and if I can't find another simple solution to keep the rotating arm in sync with the rotation of the blue wheel I will have to add gears -one fixed to the blue wheel and one fixed to the end-arm rotation point and put a belt between them.

 

[Jefflucasart]

3 or 4 cranks is fine- 2 on side A and 2 on side C, none on sides B and D. This keeps all your shafts parallel.
As for the sprocket, look closely at the initial linkage movie. Note what is NOT moving. The holes are in that bigger sprocket to make clear the fact that it is not rotating.

I need at least a 3rd crank for three main reasons (maybe there are more):
- first is to prevent platform tilt/rotation around the axis defined by the two insertion points of the crank's arm (the blue bearing housings), when there are only two opposite cranks as you suggest;
- second would be to minimize play (due to the small errors in the bearings shafts and in tightening of the nuts in the cranks arms),
- third would be to prevent bowing of the platform due to size or weight.

What sprocket are you referring to? I am using the big blue wheel as sprocket/pulley and if I can't find another simple solution to keep the rotating arm in sync with the rotation of the blue wheel I will have to add gears -one fixed to the blue wheel and one fixed to the end-arm rotation point and put a belt between them.

 

[thorq]

oh sure i got that (i was referring to exactly that when I was talking about rotating the arm in tandem with the blue wheel). The fixed gear/sprocket makes sure the arm is rotating with the same speed (if gears are the same size) and in a controlled manner.

I just wish I could have avoided somehow having a belt for each crank.

Hmm, having two cranks per side is intriguing indeed. I didn't considered that so far. Here's a link to my current design:

https://www.dropbox.com/s/7gn17i6uousuenq/screenshot.411.jpg?dl=0

With my current design, using a single cable for all cranks, I have to use the four sides to redirect it and even if I would be using 2x2, I would still to run it from side A to side C and back. Is there other advantage to using 2x2?