pic: 775Pro to CIM Gearbox

After seeing all the 775Pro gearboxes people have been posting over the past few months, I’m surprised I haven’t seen anything like this: a basic gearbox that converts a 775Pro to a CIM-like output.

This was the most basic configuration I could think of, it uses a 12t and a 40t 32dp gear from West Coast Products for a ~3.33:1 reduction, which results in:
[]Free Speed - 5619rpm (~5% faster than a CIM, ~4% slower than a MiniCIM)
]Stall Torque - 2.37N-m (~2% less than a CIM, ~69% more than a MiniCIM)
[]Output - 8mm (0.314in) shaft with 2mm keyway
]Weight - ~1.05lbs (a bit lighter than a MiniCIM)
[*]Profile - 2.5in diameter (roughly the same as a CIM) with a small protrusion on one side for the 775Pro motor to mount

Now of course there is bound to be a bit of efficiency loss that isn’t accounted for in the numbers above, and I still don’t think I’d use this as a complete substitute for a CIM/MiniCIM due to the potential for overheating. However, could you use this as an alternative to a MiniCIM something like a 3 CIM gearbox as an alternative to running 2 CIMs and a MiniCIM? I don’t see why not.

Any feedback is appreciated.


Just a note that your pinion is mated backward. That boss is supposed to rest against the inner race of the bearing on the 775pro.

The CIM-ile is geared for a RS775 not a 775Pro, the resulting RPM using the RS775 gearing makes it useless as a CIM replacement when used with a 775Pro motor.

I think I did this initially because I was concerned about spacing and worried I might have to trim the end of the gear off if it protruded out of the gearbox (and I wanted as much tooth contact between the gears as possible), this ended up not being an issue, so yes, I’ll reverse this.

Great idea!

I would recommend removing the portion of the output plate that extends beyond the 2.5" circle, as this version of the output plate would make it more difficult to mount in a Tougbox Mini or other gearbox with a recessed area to center the CIM. You may also want to include alternate output plate mounting holes in case the offset is desired in some direction other than perpendicular to the mount holes.

> From motors.vex.com

> Cimile comes with 9:29 for RS-550

775Pro ; 18730 RPM
Banebots RS-550 ; 19000 RPM

But you’d still need to use the mounting for the RS775. Chris is right that the CIMilie won’t get you perfectly down to 5,310 rpm (or even within that vicinity), but for most applications where you’d need a CIM, the slight boost in rpm with the CIMilie’s reduction won’t hurt much.

3:1 Versa Planetary with a 8mm output shaft and then step down your pinion teeth by one (11t in place of a 12t, etc).

From the looks of the Toughbox Mini housing, there’s only about a 90 degree space that you couldn’t mount this on. Based on pictures at least, if you positioned the motors to the outside of the gearbox or towards the top you shouldn’t have any issues with clearance. You could certainly make the half of the gearbox that interfaces with the toughbox just have a 2.5in diameter mounting plate with no extension (since the motor doesn’t interface with the other plate at all), but I liked having it extended for rigidity.

All that said, having additional mounting holes at different angles for mounting flexibility isn’t a bad idea. The holes currently being used for the standoffs are actually on the same 1in radius as the CIM motor holes, so in theory those could be used interchangeably.

As Andrew pointed out, the mounting holes and gear spacing (as well as the shaft size required to mount the 9 tooth pinion) do not allow the use of the RS-550 reduction, at least without substantial modification (I had actually originally looked into this possibility before coming up with this design after realizing it was not possible).

Using the RS-775 reduction on a 775pro gives you an free speed of ~7750rpm which is much faster than a CIM motor and could cause problems.

I had considered that option as well, but the cost and weight made it less appealing…
$60 vs ~$30
1.36lbs vs ~1.05lbs

Plus I wasn’t sure if the wear characteristics of a planetary gearbox would cause problems long-term.

for most applications I wouldn’t want the extra length a VP adds on.
To OP: The CIM-ile will do it almost as well, but if you don’t like the plastic housing or the slightly higher speed then this is a good design. You can definitely reduce your spacer count though, I wouldn’t put more than 4 on this. Using through-spacers and tapping one plate is also faster to assemble than the way you have it now.
Make sure you add two tapped 10-32 holes for mounting like a CIM has, and add the 0.75" boss as well if you can.

If there isn’t a reason for the plates to be identical, you may want to make the front plate a plain 2.5" diameter circle instead of the oval-ish shape it is now. This will better model the CIM mounting face and allow your gearbox to fit on more COTS gearboxes (e.g. not sure this would work with the TB mini).

Are those plates aluminum or polycarbonate (or something else)? I can’t get a perfect measurement from the renders, but they look to be ~8mm thick. If they’re aluminum, that’s overkill; you can save some weight by making those plates thinner. If this is a plastic, you’ll need some kind of captive nut system to act like the CIM motor mounting holes. I think you might run into some trouble here if you try to tap plastic and use that to hold your gearbox and motor.

Also you may want to work on the air holes in the mounting plates a bit. From what I’ve heard, the holes in the motors aren’t exactly where they’re modeled in CAD, so it’s better to make the plate holes a bit bigger to accommodate. Also, it looks like you don’t have fillets on the hole corners, which will lead to stress concentrations in those corners and could make the plate unmanufactureable.

So, in making a few changes, I took your suggestion to switch to through-spacers and tapping the plate, and in doing so, I was also able to make the spacer screws 10-32s, which will allow for 3 different mounting angles (± 35 degrees) for the gearbox as the spacer screws and the CIM mounting screws are now interchangeable.

The 0.75" boss was a bit trickier due to the bearings I’m using, the only way I could think of was to make a sort of flange bushing to go between the plate and the bearings and also act as the boss. I’m open to other suggestions though, I feel like manufacturing this piece would be a bit of a pain.

Per other suggestions, I also made a second “round” version in addition to the “reinforced” version I started with…

2.5" Round Version:

Reinforced (Original) Version:

The plates are .25in thick polycarbonate. I wanted it on the thicker side both to accommodate the boss on the 775Pro as well as for the bearings. Aluminum would have been way overkill and I’ve had good experiences with .25in polycarbonate gearboxes in the past.

The CIM Mounting holes were intended to be tapped, though locknuts could certainly also be used. I’ve used tapped .25in polycarbonate for this sort of thing a few times without any issues, though albiet, not on a drivetrain gearbox. If the motor/gearbox were heavier I would be more concerned about it, but a little locktite should keep any screws in place fairly well.

I only have the CAD to go on for the air holes so I just used that for simplicity, if I ever actually build this, I would likely measure the actual motor first to be safe. I did add fillets to the air hole corners, though after I took the screenshots above.

Polycarbonate doesn’t like loctite!

Perhaps there is a polycarb-safe variety? In any case, be sure you’re using something safe for the plastic.

See the first comment on that picture (my comment, incidentally). Loctite doesn’t like ABS, but in my experience is fine on polycarbonate when allowed to dry properly. That said, polycarbonate is susceptible to becoming brittle when exposed to moisture (if you ever water-jet it, make sure to dry it properly, we found this out the hard way).

Alternatively you could also just use screws with threadlocker already on them.