Quote:
Originally Posted by MCahoon
Same point Art pointed out - bike chain is probably made from steel. Three pieces of chain the length you describe would probably weigh more than the ball itself.
I would also suggest using polycarbonate (lexan) cut into the same shape as a lighter alternative.
For the servos to hold the ball as the arm whips around to throw, then release the ball, the centripetal force they would have to hold against just for the ball would be in the neighborhood of 35 N (weight of an object of about 4 kg - about 8.8 lb) to throw the ball at 20 foot per second. If you are - the force of the holding fingers being whipped around in the arc would add to that.
My gut feel is there isn't enough holding force of the 3 servos to hold the fingers and the ball. At 5 inch length, they are pressing inwards at about 300 oz for all 3 - about 19 lb. but only a small component of that inwards squeeze is actually holding the ball up - the rest is just squeezing the ball and holding center of mass of the fingers from flying straight out from the bearings.
If you covered the fingers with a really grippy material such as urethane rubber, then the squeezing would provide a the normal force such that friction would keep the ball from flying out.
Hard to tell much from an analysis because of the complex shapes and the curved fingers - it might work - my gut feel says it probably won't. Suggest you prototype it.
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We don't have the funds to prototype and the pneumatic or window motor or regular motor is too heavy. If I use the polycarbonate it might only be slightly lighter, but I think it'll work better than the chain. Anybody know how much urethane friction would help against flying out?