Gearing Mini CIMs

Hi guys,
My team’s running a small scale build season in the off season, and we’re going to be using mini CIMS for the pickup. The necessary ration is 20:1, but I haven’t really dealt with gears before. Can any of you guys help me? Or point me in the right direction?

A mini-cim is overkill for picking up and I am saying that from a team who used one for our pickup. That being the case I would recommend checking out and the VersaPlanetary or and the Toughbox Nano.

That last depends on what is being picked up… That said, 310 RPM and 20ft/lb (the reduction result) is a stronger pickup than most, I’d say (and likely slower–a fast intake with a good grip and sufficient torque is better than a slow but way-too-powerful intake). Try the BAG motor (700 RPM and 5 ft/lb, as reduced) or a BaneBots motor, or use a lesser reduction. For an intake, I’d generally go BaneBots with a BB gearbox as having a smaller footprint and lower weight, and it’s not terribly hard to get the right gear ratio for the pickup off of that gearbox if the gearbox isn’t exact.

Gear ratios are quite simple. To gear 20:1 you state for every 20 rotations of the input gear, you want 1 rotation of the output gear.
It is also true that for a single stage reduction (2 gears, one input, one output) the Output gear must have 20 times more teeth than the input. If the input gear has 12 teeth, the output must have 240.

240 teeth is a lot. If VexPro carried a gear that size, it would be a whole foot in diameter.* That is unrealistically big, and not surprisingly, they don’t sell one.

What you will need is a 2 stage reduction. That is, 2 sets of gears that, in total reduces the rotational rpm by a factor of 20.

In essence, instead of having one ratio of 1:20, you have 2 ratios, let us say 1:4, and 1:5. The CIM enters the gearbox (let’s assume 5000rpm for ease), and running through the first gear ratio, is reduced from 5000 by 1/4 to 1250RPM. That gear drives a shaft, and consequentially another gear, into the 5:1 gearbox. That 1250rpm gets a reduction to 250rpm, which, what do you know, is 1/20th of the starting rpm, thus a 20:1 ratio.

There are several combinations that can achieve this, such as 10:1 and 2:1, but the premise is the same.

For your application, another great option is a planetary gearbox. The internal architecture is different and hard to easily explain, but the same purpose is achieved. VexPro sells a very good customizable planetary gearbox set that will easily get you the 20:1 you are looking for, at a reasonable price, but you will have to slightly and permanently modify your motor to work in it.

There aren’t a lot of options when it comes to non-planetary reductions reaching as high as 20:1. But here are 2 links to popular planetary gearboxes used in FRC.

The VexPro option, cheaper but requires some modification.
The Banebots option, more expensive, and doesn’t have exactly 20:1 like you want, but requires no modification

Sorry, I was distracted when I wrote the first post, I mean mini CIM for shooter, we’re using a bag for the pickup. And yeah, I think I will be going for the 4:1 and 5:1 solution, thanks for the help.

What kind of shooter will you be using?

Elastic powered catapult. Plan on pulling back the catapult with the motor, and using surgical tubing to fire.

How are you planning on holding the catapult in the pulled-back ready-to-fire position while you’re waiting to fire?

Excellent question, you can’t let the motor burn up holding it in place. We used a ratchet and pawl attached to the big gear (kinda acted as a hub also) in our reduction.

Actually you can use the motor to hold the surgical tubing pulled back. That’s what we did this year. We used a cim but could have used a mini cim. Used the parts from a Andy Mark shifter and added one more reduction stage. Used a force sensor to measure the pull back force and a PI to hold at about 120 LBS of tension. The holding current was surprisingly low and the motor only gets slightly warm even after extended run times. Only problem is the Andymark dog design has very little contact and the dog and gear wear. They have to be replaced periodically. Benefit of the design is that we can just kill power and the ball is just released with out shooting. At the end of the match the shooter is not in a cocked state. good for safety.

I’m pretty sure Keith knows that, but the OP may not know all the conditions necessary for that approach to work: type of motor, amount of gear reduction, how much force on tubing, and in marginal cases how to take advantage of friction in the system (e.g. the difference between the torque required to pull the catapult back vs the torque required to hold it once it’s there).

I was hoping to get a response from the OP. Based on the response, there probably would have been a follow-up dialog, leading to the discussion above.

Sorry, we were planning on using a ratcheting mechanism, although what Gdeaver said sounds interesting, and I may end up trying that instead.

You still need a clutch to fire the catapult, right? If you time things right (cut the motor power a fraction of a second before releasing the clutch), you may be able to reduce the clutch wear.

Yes, out initial plan was cutting the power to the motor right before we fire. Not quite sure yet what we will use as a clutch though.

Take a look at McMaster’s ratchet and pawl setups, or maybe just some ratchet straps (take the ratcheting mechanism out of the strap, or leave the strap on and re-rig the mechanism to suit your design).

Now, the hard part is disengaging and reengaging exactly when you want to, but that’s for your design crew to figure out. I got no ideas off the top of my head.

This is a link to our web site page with a write up on our surgical tubing shooter with a winch pull back. Some pictures to show what we did. Our shooter is an example of what happens allot in first. It was week 5 with no pull back solution and the pneumatic approach was going no where. Dead end. We took an AM shifter that had been used in a previous year and rigged it up. It’s not perfect but the whole system - Roller intake, shooter, swerve worked very well. Looking back, if we had to design this again and had the time, I would have gone with a ball lock release mechanism.

Perhaps some folks can share their ideas more… but we really liked our launching mechanism…

We used ~100lbf of constant-force springs to provide the power, and used a miniCIM in a modified BallShifter to pull it back, hold, and release. We used a unique gear ratio (somewhere around 10:1, which had good torque but was on the non-optimal side since it was only initially designed for 2/3 the spring force), added a 3/8" ratcheting wrench to provide the stop, packaged it into a basic two-plate with standoffs frame, and put a 1" spool of Spectra cord on the output shaft. This all worked great!

I’m about to leave on vacation for the weekend, but there were some important design constraints in doing the above… for one, the dog shifting mechanism won’t be able to retract under that amount of load… we tried… don’t bother (unless you have a sense of adventure, like MacGyvering things, and/or have some extra cash to spend). :slight_smile:

Curious, what force sensor did you use?

You can search all over but there are not many choices for a amplified and conditioned 5 volt force sensor.

Yes, this was one of the build seasons major crisis. We had both a 3 stage andy mark and a vex pro ball shifter. Looked at both. Choose the andy mark because it could be modified the fastest. Was tested after 1 night’s work and we went for it. Looking back the ball shifter is what we should have done but there was no time to devote to it. Make your choices and live with them. Yes, it does take a 1.5 inch pancake at 40 psi to rip the dog out. We replace the dog gear before each competition. I like balls. We also used a ball clutch on our intake roller to prevent RS motor burn out.

Do you cut motor power before disengaging the dog clutch?