Catapults and pulleys

Our team is doing a catapult launcher and right off the bat we’re having problems. Not with torque or power, but with the materials. Our catapult design uses a modified wheel that we carved a track into that pulls the string around it. When it gets to a certain point it releases so that works fine. However the pressure from the torque caused our wheel to shred around the axle. causing it to be loose. So now we need to:

  1. make another wheel
  2. reduce the load on the wheel
    (I’d also like to clarify that our gears are functioning fine and that our catapult is pulled down from the tip of the bar.)

I think were gonna use pulleys to reduce the load but what I have found is problematic. since the load is pulling itself up we can’t have the pulley above it.

Any help on this would be great. pictures will be posted shortly.

Now that you have rounded out the hub on the wheel you can remove the remnants and use the steel inserts from the high strength gears. You can pull out one side with needle nose pliers and once removed use a section 1/4 HS shaft as a punch to remove the other side. You might also able to use two lock bars to sandwich the hub which may spread the torque forces out across the spokes contacting the screws and off the hub.

Well not exactly, We shredded it to the point where it moves freely around the axle. Also The center of the wheel isn’t removable. I’ve tried to remove it and it doesn’t come out. for reference here is a picture:

It seems like your lever arm is too much. What I mean by that is that torque is basically force times lever arm, so if your axle is shredding out the inside, you need less torque. I’d recommend changing the size of the wheel and using something much smaller. You could also change the gear ratio for pulling it back to make it just as fast.

I think you should replace your pulley wheal with one that can have a high strength axle and use a high strength axle on it. That way, unless you are doing something really really wrong, it shouldn’t strip out.

Surprisingly its really easy to strip the inside of a 84 tooth high strength gear, I would suggest fashioning a lock bar out of steel C-channel to fix that problem.

Maybe VEX will offer a HS lock bar one day. In the meantime an alternate non-machining solution is to “sister” a couple 36T high strength gears to either side with some screws and spacers to add more “hub” strength.

That would be great, although the only tools I used was a file and a hacksaw. The “sister” gears work really well if you have the space, as it also prevents twisting. I’m using that method on a Choo-Choo mechanism.