CIM Motor Direct Drive or Supporting Axle

Good day,

Over Christmas break, I plan on building an automatic tennis ball thrower for my parents dogs. I have purchased two CIM 12V motors as the two spin wheels that will launch the ball.

It will be very close to this project here.
I’m not sure how I would attach these to the hubs though.


I would say it’s not 100% necessary but very much recommended. You are going to be putting a significant amount of sideloading on those CIM shafts. If they aren’t supported, they will likely bend. Also, even with some load on the motor from the wheel, the CIM will be spinning at a significant percent of its free speed. That means it’s spinning really fast (multiple thousand RPM). When the wheels come in contact with the ball a few times, it will likely shred the coating off the ball and the tread off the wheels. You may want to add a gearbox between the CIM and the flywheels to slow them down and give them more torque to push out the ball.

Thanks for your reply.

I recently found a wheel that can mount to the hub I purchased below…

One obvious solution to me would be CIM Shaft >> Hub >> Wheel
This obviously would not have the other side of the wheel supported.

The shaft of the CIM is 30.3 mm and the hub is 20.2 mm.
In order to have the opposite side of the wheel supported would I have to do something like,
CIM Shaft >> Hub >> Some type of extension/coupler >> Bearing attached to frame??


You would be far better off supporting the shaft at the other end. You may also want to look at robot designs from the 2012 FRC competitions. has hubs like what you show that have 1/2", 3/8, 1/4" and other bores. Thus you can do, CIM Shaft >> 8mm Hub >> weel>> 3/8" hub >> 3/8" shaft >> Bearing attached to frame. Also are you familiar with FIRST Robotics already?

From a mechanical standpoint, two CIMs is overkill unless you’re planning to project a stream of balls. Unless your parents have 101 Dalmatians fetching those balls, you won’t need this much power. If you make your wheel systems heavy enough (that is, with a sufficiently high moment of inertia), you can spin your wheel tips just a bit higher than the desired launch speed.

As ballistic range is simply v2 sin22θ / g where θ is the launch angle, to throw a ball 32 feet through the air, you only need a launch speed of 32 fps at a 45° elevation. For a CIM free spinning at 5310 rpm, that would be about a 1.4" diameter wheel. A 1000 rpm gearmotor on a 7" or 8" wheel would provide this same speed. Motors of this type have an 11Ω resistance when not turning (I have several handy, including one that actually lives here at my computer desk.) This implies a stall current of a bit over 1A. If you use one for each wheel, you’re going to have about 6W of mechanical power assuming a nominal 50% efficiency at high speeds (50-80% of free). A tennis ball weighs 2.1 oz, so throwing it at 32fps requires mv2/2 = 2.1 ft-lb, or 2.84 J. At 12W mechanical power and even assuming another 50% loss due to friction, you could throw at least 1 ball per second.
For tread, I would definitely recommend something much softer than a lawn mower tire. AM solid rubber wheels would not be too bad, but blue textured nitrile would be even grippier, though it might need to be changed out every few months of dog play.
Pololu and other vendors sell decent 4mm D-shaft hubs, though getting this up to an 8" wheel might be a stretch.

I don’t seriously recommend the 25m gear motor, but I wanted to point out that a pair of them was (at least theoretically) adequate to the task. A BAG motor or two with VP gearbox(es) and colsons is probably the easiest way to go with plenty of room for adjustment.

Also, these look like custom side plates and base plates. Do you plan on making them yourself or do you have a plan to buy them somewhere?

While I have heard of FIRST Robotics and the competitions and seen designs online, I have never participated myself.

My above comment was a best guess at how I would support the otherside, however I do not believe I was very clear in describing it.

I would attach the 8mm hub to the CIM shaft, this hub would attach to the wheel previously shown. After the hub I would attach a coupler to what is remaining of the CIM shaft, allowing an additional 8mm shaft to go through the middle of the wheel to a bearing that is mounted to the frame. Maybe something like this?

Or would I be better off getting another hub and put on the other side of the wheel, which then connects the additional shaft (though not connected to the CIM shaft) to the bearing/mounting block?

Thanks for all your help, this stuff is just a little over my head. I plan to continue to do research.


GeeTwo, thank you for a technical break down… I kind of figured the CIM’s were a tad overkill, but it was what the referenced project used with good results so I went with them. However, it is important to note that his wheels were milled from solid MDF and were pretty heavy.

Ari, I do plan on fabricated the frame myself. At my dad’s work he has access to steel tubing and plate stock, figure it wouldn’t be a problem for him and I to go there and throw something together. I’ve been looking for an excuse to learn to weld anyways. :cool:

Jman, your suggestion was what I was considering and trying to spit out. At the beginning of this post and also the prior.

I am going to continue look into possible wheels, however I’m slightly under a budget so the the possibility of lawnmower or custom plywood was taking that into consideration. However, I am still up for suggestions regarding them.


The CIM shaft isn’t that long to begin with, it will never make it through a wheel. A hub on both sides of the wheel would be the best way to do it without some extra engineering. I would look in to a wheel with some give to absorb the ball a little and grip it well. Since you seem to have found, take a look at their 8" pneumatic wheel.

Alternately you could do something like this. It would give you a reduction and would allow you to use a $5-10 wheel instead of a $40 wheel. The only downside is that the bearings in those wheels are horrific at any kind of speed over a walking pace when mounted on a cart (think terrifying vibration and 2nd degree burn heat [ask me how I know about the burns]).

As a rule: never ever side load a motor. So you should not directly drive the wheel from the CIM Motor. You should add a set of gears/belts and pulleys/sprocket and chain to each wheel

Why to not side load the motor? You will likely destroy it. The bearings will not last as long, break and then your motor will burn out. You could also bend the shaft of it or something else can happen

This is the shooter that my team made for the 2012 FRC game. (sorry for the big image)

The wheels are supported by one shaft, with the a belt from the motors and gearbox. You could place the motors and gearbox in between the two sets of wheels you have pictured, and use two belts; one to the upper wheel and one to the lower wheel. I would not recommend using the pneumatic wheels that were linked to above, as the valve causes the wheel to be unbalanced resulting in horrible vibration.

You are right but in 2013 we made a full court Frisbee shooter by mounting a 8mm hub to a cim and a 8" Andymark pneumatic and it survives to this day after several hours of running time since 2013 (we didn’t know better). You really shouldn’t do it if you can avoid it however and this is such a case.

Beware of using the pneumatic wheel at high RPM. I still remember how that max RPM warning showed up on the Andymark product page midway through the 2013 season. The primary problem was balancing it. The inlet port for air needs to be offset buy something on the other side. Also if you want to save money it’s a less optimal choice. I would think andymark 6" or 8" rubber treaded wheels would do fine here and cost less.

Edit: What tools do you have access to? That can change a lot of things.

Does side loading a motor mean driving the wheel directly, supported or not? I was looking to stay away from a bunch of pulleys, gears, etc. The more of those the more mechanically inclined I need to be. While I believe I could pull it off, I’m not sure I want to deal with the headache and price increase of those extra components.

What tools do you have access to? That can change a lot of things.

I have access to basic power tools. No machine shop or anything extravagant like that. I’ll be utilizing my dads work for some steel stock and welding equip. for the frame. But other than that I’m not looking to do any fancy fabricating.

What are your thoughts on THESE wheels? The downside of the rubber threaded wheels on AM is that they are only an inch or so wide, potentially requiring two (side by side) to attain the required width for a tennis ball. These Vex wheels come up to 2.5", however with a lower diameter.

Can anyone comment on the bearing and housing needed on the other side of the wheel that will attach to the frame? I’m not super familiar with them, and what exactly I need. Will something like in the photo above work?


You could use that, or just drill a hole in the side and stick a ball bearing in with some locktite. That might be cheaper, although using what you show above might work too.
I would also recommend making everything out of wood instead of metal, as it is cheaper and easier to work with.

You will sacrifice speed with smaller diameter wheels and they don’t have to be as wide as the ball. The ball just needs to hit the center of the wheel. Two side by side would give better accuracy but this application doesn’t require it so a physical constant that keeps the ball mostly centered as it hits the wheel would be enough.

My suggestion would be to mount the 8mm hub to this wheel and on the other side use a 3/8" hub withthis bearing at the end of the shaft.

If I were designing an FRC robot this assembly would be totally different but this application should’t require it.

This is kind of unrelated to the other conversations here, but I thought I would offer my own suggestion against using this type of set screw hub. I have had to deal with getting these to fit just right and keeping the setscrew from backing out and it is a pain. Especially with your CIM at 5000+ RPM I think this is going to be a headache for you.

I would recommend using this coupler on your CIM shaft to make it so you can use any wheel with a Hex bore. Then you could use tonsof different wheels as long as they have that Hex hole in the middle. You would also be able to use this hub to connect to wheels without the hex hole in the midle.

It’s totally up to you (and I know you already sunk money into that hub), but I just hate trying to use set screw hubs on a keyed shaft.

We did set screws on our 2012 shooter robot, and the way the set screws go onto the keyed area makes it so that the wheels rotate with the shaft. We also have a 2-sprocket reduction before the output wheels, which are attache with set screws. I think it really depends on how hard you go on it, and what tip the set screw has.

You might want to consider going with something like the SpinBox from AndyMark and doing a single spinning axle/wheel with a hood.

Yeah, assuming the set screw is the right size to fit snugly in the keyway, it isn’t quite as bad. In my experience the set screw was slightly too small and the walls of the keyway beat it up a bit.
Either way, even red locktite couldn’t keep that thing from slowly backing out after an hour or two, and I think his application would require something long lasting. I’m thinking that an ungeared CIM like he is planning would make this problem much worse, but that’s purely anecdotal.

Ours is slightly too large and ends up sitting on top of the keyway, actually. That combined with a soft steel shaft might account for the good seating, not to mention the accumulated grime holding it in.

Tennis Ball shooter Project from our AP physics Class