Just a CAD of a simple gearbox made using KOP gears, and 3/16 aluminum plate. The reduction is 3.6:1 and the output shaft is 1/2" keyed.
What’s the weight without the motors, but with the pocketed gears?
Inventor is saying ~1.2 lbs, which seems to be a little bit of an over estimation (but with the long drive shaft it has now it might not be).
Why do you have the plates extending past the sides of the CIM casings? You would have to change the pocketing, but if you lop off those spare centimeters, you’ll save some weight, space, and raw material.
In addition, you can probably get away with making 2 identical plates, even with the holes for the CIM screws. I would rotate the CIMs 90°, to make the symmetrical pocketing easier. Having swappable plates would aid greatly in manufacturing and would reduce the number of spares needed (not that these sorts of things break often).
It makes it easier to tighten the screws that hold the gearbox together. If you’re going to do it this way, you have to be able to keep the plates together, and with the gear/motor setup, I don’t think you can just assemble the box and put in the motor afterwards; you have to put the motor on the plate first. I’m not sure you can remove much sideplate length without reducing the ability to remove the screws in a hurry (say, in case of complete failure of a sideplate, requiring a complete teardown).
To save some weight, you could try having the CIM motor mount screws go through both plates with standoffs in between. This would eliminate the need for the four outer screws, and you could shorten the plates as Steve Kaneb suggested.
Might make the gearbox mounting difficult, though, depending on its mounting setup.
Complete failure of a side plate? I don’t think I’ve ever seen that happen in FRC.
Also, those look like 1/4-20 screws to hold everything together. If the point of this design is to save weight, those are way overkill. Use tapped standoffs and #8 or #10 hardware, and you’ll save a lot of weight. All #10/32 bolts are only approximately 58% of the weight of the same length 1/4-20 bolt.
Another possibility, depending on how the gearbox is mounted, is to use the #10/32 CIM mounting screws with four #10 clearance standoffs to hold the gearbox together. CIMs are pretty reliable motors, and generally don’t ever need to be replaced if you do your homework. I wouldn’t suggest this strategy for Fisher Price, Globe, or Banebot motors as they are more prone to smoke, but CIMs will take a lot before they give up the ghost.
As Steve pointed out, there’s no reason to have both plates so big, unless they are specifically designed to mount in a certain way. The smaller and more compact something is, in general the less it weighs, due to the need for less material.
Using the CIM mount holes as the holes that hold the whole gearbox together is bugging me for some reason. It seems like things could get very complicated when it comes time to put the gearbox on the drive train, or arm, or wherever else it needs to be mounted. For a generic offseason gearbox, I think that what Tyler has is a great start.
I second the recommendation for making the sideplates identical. Doing so allows for additional flexibility during manufacturing should something go wrong.
Nice job.
The pockets in the side plates do make it lighter, but they also hurt stiffness. The only thing holding the plates together are the four screws, right? And these screws connect to the center of the plate through a tiny strip of metal in each corner…
Allow for more metal between the screws and the center of the plate.
Since the discussion seems to be focused on weight savings, one thing I havent heard mentioned yet is to drop from 3/16 to 1/8, will eliminate a 1/3 of the weight from the aluminum. Pocketing is great but it really doesnt remove THAT much weight compared to using thinner material. Either way you are using the same amount of material but often by using the thinner material you can eliminate machining operations. For example, I would be willing to bet that by using 1/8" you could not do ANY of those holes in the plates and the weight would be pretty close. On the other hand, it is better to have too much material where you don’t need it than to not have enough where you do.
Also, why does that shaft have to be so long? That is probably a good deal of your weight, 1/2" steel, about 4" long? That is about .88 lbs right there.
Thanks for the comments guys, I probably make a few changes based on some of the recommendations.
Swap the 1/4-20 bolts for 10-24, change the front plate to 1/8" (the rear plate needs to say the same because of bearing mounting), and maybe rework the pocketing and plate sizes a little.
I probably wont use the CIM mounting holes to hold the plates together and the shaft would be cut down to whatever size you needed once you knew the application.
Yay for simplicity!