Arm problems

So our design for gateway is a fast claw that raises with an arm. Our robot design has no room for a four bar. Some teams online a few months ago told me about how to keep our claw the same angle while the arm is raising.
They said to chain the arm to the claw so as the arm raises the claw will rotate also to maintain the position.

We tried this suggestion and it isn’t working. (Didn’t attach claw to arm) We just chained the sprocket on our arm to the sprocket that would be attached to our claw. Once chained we raised our arm, but the chain didn’t move and also the sprocket never either.

Why is this happening?

More details:
On our arm we have our 2 motors on separate shafts. These shaft are connect together by 2 36-tooth gears. On one of these shafts theres a chain that connects to a shaft with a small 12 tooth gear. After the next shaft has a 60-tooth gear that has our arm on it. Besides the 60 tooth gear we have the sprocket that would get chained to the claw.

Picture please?

Sorry for that. Won’t happen again…

Did you actually read his post?

Re Using chain for 4-bar;
I think there is a picture in the gallery, but I couldn’t find it with keyword search “bar chain”.
If you don’t have room for a 4bar, you may not have room for this either.

Here is a text description:

  • bolt a HS sprocket size X to the tower, centered on the pivot point of the shoulder joint. The sprocket should not rotate!
  • Change the hand-arm joint from fixed to a pivot, so the hand just flops down by gravity.
  • bolt a HS sprocket size X (same size) to the hand, centered on the hand-arm pivot point. You should be able to change the orientation of the hand by twisting this sprocket with your hand, but otherwise the hand is still floppy and uncontrolled. The sprocket does not rotate freely, but moves the hand when it rotates.
  • put the arm in horizontal position, and run chain from the hand-sprocket to the shoulder sprocket. Notice how the tension in the chain can hold the hand parallel. The chain now acts as the other parallel bar.

If the sprockets are not centered on the pivot points, then then chain loop will bind when the arm moves. If you leave some slack in the lower loop, this might not be too bad, if you can stand not having a perfect parallelogram.

This demonstrates using two parallel tensile chains (upper and lower) to replace one combined tension/compression parallel bar. You can do the same thing with two pieces of rope, just tie them to the tower above and below the shoulder pivot, and above and below the hand pivot.

You probably don’t have a hand pivot now, so that is the first change needed.