Good day!

I am helping my little ones 6 and 7 to develop a robot that can pick up and stack cubes three high as well as gather blocks. They came up with the design that enables them to pick up three cubes at a time when lined up and then pull up the claw, turn, drive to another cube, place the cubes on the stand still cube and release.
Well they got the claw made with two rods connected to gears to motor and the rods reinforced, then they made insect like legs 3 - pairs of two on the rods, with gripper feet pads on the ends of the legs. All of this works as it opens and closes! The problem is that when they try to close the claw on the cubes and pick up all three cubes, the first - closest to motor feet have a nice tight grip, the second - middle feet - have a lighter grip and just barely can lift the block, and third - farthest from motor doesn't even have a grip on the blocks.
I think it's because the second and third set of feet are farther from the motor. But how can they evenly disperse the tension load so the claw can pick up all three blocks? I tried putting elastics on the feet for better grip and unless we put ten on each foot for the third set and maybe five on the second set it wont work. Even though it's a quick fix I would like to help them figure it out the proper way of spreading the load so to speak. We also tried putting a small band on the third set of legs. The robot could still open and close and that worked for the third set but not the second. We tried putting a band on the second and third but the legs wouldn't open anymore. I could use a lighter band but is there another way? We only have one little motor to run it so we can't give all the leg sets it's own motor and even if we did there would be weight issues.
THank you in advance!

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    $\begingroup$ Please attach a photo or your kids' CAD drawings. While your description is good, it is ambiguous in places. $\endgroup$ – James Waldby - jwpat7 Feb 11 '15 at 3:31

Tension distribution is easily accomplished using a simple mechanism called a whiffletree. Classically invented to allow several horses of different strengths to pull evenly on a single load, this also allows a single motor to pull several linkages with the same amount of force. It would also allow the mechanism to self-adjust slightly to construction inaccuracy, and also allow the gripper to better adapt to the shape of what its holding.

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