CIM direct drive


I was wondering how I would use a CIM motor in a direct drive application to directly drive a sprocket on the CIM. The CIM is keyed, but it looks a lot smaller than i thought it would be. What size key is on the CIM, and can I buy a sprocket that is already keyed? I don’t have tools to key it.


it’s a 2mm keyway, and the only place that would sell them would be AndyMark, but im pretty sure they only sell gears with 2mm keyways and not sprockets.

The CIM motor shaft has a 2mm keyway.

What size/material sprocket are you looking at? Unless you are using a metric sprocket, you may be a little hard pressed to find a stock imperial sprocket with a metric broach.

Edit: Cory beat me by three minutes…

Mcmaster has a 8mm sprocket (item 2302K1). also they have 2mm keyway (item 90457A100) though the sprocket is unfinished, so youd have to either cut your own keyway into it or send it somewhere to do so. also its a metric sprocket, and I dont know how ISO and ANSI chain play together.
though if you had the abilities to cut out a keyway and bore it, you could just go with a small ANSI sprocket, and custom bore it and key it.

why would you want to direct drive it though?

Well, I am doing this apart from my team, and dont have large funds to spend. I wanted to go with a route that didnt require me to have to purchase a transmission, but I think i may have to now.

NOTE: I am not completely sure this will work. We did something similar and didn’t notice a problem, but you never know. Maybe somebody with more knowledge can help me.

In 2006, we took an ANSI sprocket and wrapped the CIM shaft with some tin to make it close enough to the same size as the ID of the sprocket so the sprocket would spin fairly true when mounted. We then used the setscrew to hold the sprocket on. The sprocket definitely wobbled when we turned the CIM on, but the number 25 chain never fell off. So it works, but is not a very elegant solution. Thinking back on it, another way would be to take a sprocket with a smaller hole, like a 5/16" bore sprocket, and then bore it out to the size of the CIM shaft. Then, run the set screw into the keyway of the CIM. I am pretty certain that the CIM doesn’t produce enough torque to make the setscrew fail. Does someone else know if that is true?

yeah i’m not sure about this i do not see the chain taking the speed of a cim well unless you have perfect tension and even than it still may not work so well

now a belt there would would work very well here they can take that speed very well

we had a heck of a time keeping our pinions in place this year. Due to having 2 gears on one CIM shaft we had no room to use retaining clips like we usually do and we had to rely on a setscrew to keep the gear from moving. Even with 2 set screws 90* from each other the gears would slide out and eventually maul themselves on the adjacent gear. We had to resort to green loctiting them, which didn’t work well either when we didn’t let it set for 24 hours.

Furthermore the only way to release the loctite is heat. I melted a bit off one of our Delrin sideplates the other day trying to break a stubborn one free.

We hate setscrews and figured this was probably one application where they might be OK. Lesson learned-never again will we rely on them.

I dont know what you plan on using it for but my guess is that you wouldnt be able to gear the CIM motor down to a reasonable speed simply with sprockets. You can get a Toughbox from AndyMark for cheap.

i am not sure what you really want to spin at over 5000 rpm’s but I believe we did this on our poof ball shooter from AIM HIGH. It wasn’t all that perfectly successful but we used a three piece jaw coupling I think.
It was some type of flexible disc coupling.

It is hard, running the CIM directly like this, to get much acceleration (read torque) … it took awhile for our shooter to come up to speed and one of its problems was that as we shot the balls it would decrease speed and we would have to wait to come back up for proper trajectory.

If you anticipate doing a direct drive of the CIM for a wheel drive system
That would not be a good idea… I don’t think that is your application though.


Your best solution is to get an ANSI sprocket with roughly the right diameter hole and put a pin through the sprocket and CIM axle. This is fairly easy to do, but drilling a hole through the CIM axle can be tricky.

I would not recomend attaching sprockets, gears, whatever to the CIM with just set-screws. The CIM produces far to much torque for those methods to work.

Sounds like a Lovejoy coupling I’ve used these before and they’re pretty nice if you can’t keep your shafts perfectly aligned.

I think you are right Cory,
We used the aluminum one I believe … the AL…

It was an interesting mechanism. Originally it was geared down…
When we found out it didn’t have enough speed we made the change to direct drive… and forgot that we were using an IGUS bearing on the other end of the shooter drum… it failed… Just like they told us it would at intermittent speeds above 2000 rpm

The IGUS engineers are really good at helping you out when you have “alternative” uses for what their parts are designed.
They are invariably correct when they tell you not to use it for that.

that makes sense… its a German company…
good engineers the Germans…

Keep in mind that 6.35mm pitch roller chain is the same as #25. You may have an easier time finding parts with a metric bore if you use the metric number for pitch.

Did you grind flats on the motor shafts, for the set screws to seat against?

McMaster sells metric broaches and metric drill bits. Start with a sprocket that is undersized and enlarge the bore with the metric drill and then broach with the 2mm broach. Not sure how many sprockets you will find with a bore of 8mm (5/16") or less.

In the past we’ve directly put a sprocket on a CIM, but it went to a larger sprocket that went into our custom 2 speed transmission. We never really had trouble with the setup, but the more chains you get, the more master links that can fall out… (IRI 2006 right before alliance picking). AndyMark products are nice and reliable, but they are a bit expensive ($88 for the toughbox that shipped in the KOP). Banebots are still pretty expensive… the cheapest is $83. I’m not sure of your ability to build your own parts with a mill or CNC mill, but custom is probably the cheapest way to go, but also the most time consuming. Chain’s are probably the easiest way to go, but their reliability becomes the issue. Gears are the hardest way to go, but their reliability is among the best.

Keep in mind that from 2003 to 2006 we used chains in our custom transmissions and had very little problems, but the problems we had were with the chain derailing or the master links falling out. The chain never broke and we used #25 chain. You may also look into regular bicycle chain as it is easy to find and fairly cheap. We’ve been looking into bicycle chain as an option next year because we are stretching the limits of the #25 chain as far as the forces we apply to it. Luckily with our 4 CIM drive this year, we had no drive train issues at all (knocks on wood for IRI)

I think a modification to the motor shafts is illegal dad… gosh, read the manual! :slight_smile:

Actually, modifying the shaft is not illegal.

<R61> So that the maximum power level of every ROBOT is the same, motors used on the ROBOT shall not be modified in any way, except as follows:

  • The mounting brackets and/or output shaft/interface of the motors may be modified to facilitate the physical connection of the motor to the ROBOT and actuated part.
  • The gearboxes for the Fisher-Price and Globe motors are not considered “integral” and may be separated from the motors.
  • The electrical input leads on the motors may be trimmed to length as necessary.

The intent is to allow teams to modify mounting tabs and the like, not to gain a weight reduction by potentially compromising the structural integrity of any motor. The integral mechanical and
electrical system of the motor is not to be modified. Note that FIRST will not provide
replacements for modified parts.

We’ve had problems using M3 setscrews to hold a gear to the CIM shaft (mainly because the small allen key prevented us from tightening the screw properly) but switching to M4 setscrews completely solved our problem (both last year with a high torque, two-speed gearbox and this year with a single, high speed gearbox).