Slip Gear Questions for Starstruck Catapults

I do know how a slip gear works and is created, so I’m good on that front. I just have some questions about how to best use them in a Starstruck catapult. If you can answer them for me please, that would be appreciated.

  1. What’s the best gear size to use for a slip gear?

  2. In a two-gear gear train containing a slip gear, is it best to have the slip gear be the larger gear, the smaller gear, or does size difference not matter?

  3. When grinding off the teeth of a slip gear, for each gear size, how much teeth should I grind off, and is there a particular pattern that works best?

The gear size will depend on your set up. Last year many teams used a 36 tooth slip gear while others used the 60 tooth variety but typically on a linear rack, not a circular gear on a catapult about an axis.

You most likely want to be going up in gearing on a catapult as it will need more power than straight out of the motor. So you want from small to big on the drive train but maybe not on the slip part. You also want the slip gear to be on the part constantly turning so it engages again. If the slippy part was on the catapult side you would not be able to pull the catapult back again.

On a circular style slip set up you have to think of how many teeth will be needed to drive the catapult the number of degrees back and then release to be picked up again. Given the need for more power to draw the catapult back, the slip gear may be attached to another gear that is being driven and maybe a different size. So a bit different from last year’s linear punchers. I would start with a 36 tooth slip gear against an 60 tooth attached to the catapult.

Last year I saw about 12 teeth on the slip gear removed for a 36 tooth set up. It is up to you but that gave enough time of travel on the linear puncher. That would give you 24 teeth left to move the catapult back. So how many degrees of rotation would 24 teeth be on a 60 tooth gear? How many would it be on an 84 tooth gear? Is that what you desire for a pull back? Do you need more teeth ground off or less? If you grind off less, does the catapult release in the time it takes for the slip gear to travel around?

You can also do this with a bigger gear with multiple slip zones. One gear is easier to think about. It is all about how many teeth are engaging the driven gear for how many degrees it cocks back prior to release.

When grinding the teeth off, you want enough ground off so the teeth on the driving gear do not touch the slip gear. So grinding the teeth all the way down to flat plastic works great. No chance of engaging against the slipped part of gear.

  1. Depends on how much torque you want and how much control you want over pullback.
  2. The answer to this question lies in how much torque you need. Generally in star struck you would want the slip gear to be the larger gear as this would provide more torque. Difference in gear size matters.
    (Learn more about torque and gear trains here:
    3a. The amount of teeth to grind of depends on how much pullback you want. i.e. You need to pull back 2/3 of the gear. You would then cut the remaining 1/3.
    3b. No

No matter what others tell you, you need to make sure that what works for your robot is put onto the robot otherwise no matter how much you try, you are just not gonna get it right.

1. Small motorized gear to larger gear
But for some advice, you generally want to use torque gear ratios. So that means like a small motorized gear connected to big gear (i.e. 60-tooth or 81-tooth). The physics behind it is that using a smaller gear to spin a larger gear will give the smaller gear a longer “lever arm” which would create more torque:

τ=F x d (d= lever arm)
More ‘d’ equals more torque.

2. Cut teeth off the gear connected to the catapult or generally the larger one
So since I think you are going to torque to power the catapult, the gear that should have teeth cut off should be the larger one because your transmission will be: the smaller gear is connected to the motor and the larger gear will be connected to another axle so you obviously can’t have the gear connected to the motor slip (otherwise that would destroy the motor).

3. Cut fewer teeth off for longer and more powerful shot
When considering how much teeth you would like to cut off, you should think about how far you want to launch the projectile (be that cube or star). If you want to shoot farther, you cut off fewer teeth. But I believe there is a minimum of like 5 or 6 teeth you must grind off in order the gear to actually “slip”. The physics behind this is that the more you pull on an elastic material (in this case it’s more of a wind up), the more “elastic” potential energy you create:

Elastic Potential Energy: 0.5 kx^2 (k=how elastic a material; x=distance to wind up).
Ignore k, you don’t to worry about this. Although you should consider using VEX rubber bands because I find them more durable.
More ‘x’ equals more potential energy thus, when all this potential energy is transferred to kinetic energy, it’ll go farther and faster.

a ratio that does work for 32 tooth gear have 6 teeth, three times and shave off the rest

I snapped a 36-tooth high strength gear in 6 pieces running a slip gear. It would have to be much to powerful for any parts to with stand to make it worth building a slip gear launcher. However I have designed another mechanism that works much better fires faster and launches further. We will be disclosing our design around state time

9 months later