Catapult Gear Skipping

We built our catapult it utilizes a slips gear and a ratchet. When we have 2 doubled up rubber bands it winds back fine. But we want some more power. When we have 4 doubled up rubber bands it cant wind back. The slip gear just rotates and when the gears of the catapult and slip gear come in contact with each other, the catapult doesn’t wind back, and the slip gear just rotates through it. It was working fine with 4 rubberbands a few days ago and no changes were made to the catapult. I did do some driver practice and used a good bit. Any ideas why the gear are skipping with four rubber bands and how I can fix it?

Hello. I am team Caption from 77000X. We had some problems with our slip gear in our puncher. We fixed it by changing your shafts to high strength shafts. We had problems with them bending when we had 4 rubber bands. We swapped the shaft and we haven’t had problems since.

It is very possible that you bent an axle and have not realized it.

We had the same problem, and we found that the source was the slight amount of space that was caused by attaching two c-channel’s together, and we fixed this by attaching them inside each other instead of screwing them together outside to outside. But a warning is that it was VERY tight this way so you may have to cut the gears down more. Sorry if this is confusing it is hard to explain and I can’t show pictures as of the moment.

Could you post pictures? I have a student using a slip gear with a regular axle and quite a bit of force from the rubber bands without an issue. One of things I wonder about is how the axle is mounted. Does it go through one bearing on each side of the gear or something else? How high is it above the rack?

It’s all about how far apart you space your supporting arms for each piece. I’ve used a metal bar with bearing flats, but something tells me a zip tie would do the same thing, if there’s no space.

try making the shaft as short as you can, because it really helps the overall jankyness.

When I first built my catapult, the bearing block didn’t really kept the shaft in place. The shaft was able to chip out the C-chnl because of the tension force upwards all on one end. This resulted the catapult slipping and not meshing with the slip gear .

To solve this problem, I used the shaft lock bars as bearing by drilling the center hole into a circle so a shaft could freely rotate in it.

This may cause a slight increase in energy lost to friction, but it should be negligible .

What gear ratio are you using for the catapult. We are using a gear ratio of 1:1.6 but it looks like the torque is not enough to pull the catapult against the rubber bands, for it to go high enough.

I saw fro 8059A that they were using 1:7 for catapult … I need to make it better. thanks.

My gut feel is that you might be able to get away with 1:5.
But 1:1.6 is definitely too Low to pull enough rubber bands for your catapult :slight_smile: