pic: 6.25:1 Gearbox (First Attempt) - 2



My first attempt at a custom gearbox.

First reduction: 50:12
Second reduction: 42:28

This is a view with the output plate (no lightening, yet).

At only 6.25:1, it seems like you could go with a single stage of reduction rather than two. You can save yourself some weight and money (and trade off space in one dimension for space in another) by swapping out the 50T gears for a 74T gear, and cutting out the second stages of gearing, axle, and bearings entirely.

Seconded. This is a great application of KISS (keep it simple stupid)

Nice looking gearbox!

I have a few suggestions which might help you improve your design depending on your use case.

Sean’s suggestion on going with a single stage of reduction is a good one, 74t might be a little big with a 4" wheel( you will only have 0.1" of ground clearance and will get a lot of carpet fluff in your gearbox), so you could go with 11t pinion gears and something around 64-70t which should get you more ground clearance.

Obviously required reduction is dependent on your strategy for each particular season and your wheel size among other factors. The advantage of a dual stage gearbox is that it gives you a few more customization options, especially now that VEXpro stocks all even gear sizes from 14-84t.

I would stick with the dual stage but bring the CIM’s closer together with either a 12-40 or 12-42 initial reduction and also add the extra mounting holes to allow the use of 13-14t CIM pinions.

You should try and reduce the height of your gearbox as much as possible to reduce weight and also the amount of space that it takes up. So maybe something around 22-40t for the final reduction would be better than 28-42 t while still giving you plenty of options. ( remember to leave space for an encoder mounting if you want one on the gearbox)

Thanks for the suggestions and kind words everyone. I did go for a dual reduction in order to minimize issues with ground clearance (as pilleya mentioned, it would be 0.1"). However I didn’t think to use an 11T pinion which would make it easier to get the the desired reduction.

I am working on a lightening pattern right now but I’ll also try to reduce the overall size as well.

Great first attempt! I appreciate that you did not lighten the gearbox before finalizing the design. You should continue this practice indefinitely.

The raised boss on the CIM is actually an alignment feature you should use on your gearbox, especially with 12 tooth pinions. If you make the center hole for your CIM mounts close to the diameter of that boss, the motor will be better aligned with the intermediate shaft, resulting in a smoother more robust first reduction. Reduce that hole to nominally .750 (so maybe, .752 or so if you’re CNCing this) and you’ll be a happy camper.

You will also want a little bit more material around the CIM bolts. The bottom CIM bolt is very close to the edge of your plate. In aluminum a good rule of thumb is to leave 2x the hole diameter between the edge of the hole and the edge of the piece.

Speaking of aluminum, that is the material you’re using here, right? If you are using Delrin, don’t lighten it, and maybe add a few more standoffs to try and coax some rigidity out of the system that otherwise isn’t there.

A 2 reduction gearbox is valid here. It gives you more flexibility in ratios if you design that in - you can switch gears around if you plan for it.

Mounting tabs? Or should I say some way to hold or mount it to a frame?

As the resident grease monkey on the team, I can tell you that keeping track of those supports will be a pain. The 3cim4u has good exterior plates that can be modified to fit your design, and will keep grease from going everywhere

I personally would stay with a two stage reduction.

Your current 6.25 ratio would really only be applicable to 4" tires.

Use of 6" or 8" tires typically needs the double reduction gearbox to obtain the desired drive speeds.

If you picked some convenient second stage tooth count of say 72 Teeth, you could use Vex Gears with a entire range of 9 possible ratios from 3 (54/18) to 1.0 (36/36).

With clearance for a half-inch hex shaft (.577), your first stage would be limited to 66 teeth.

This would give you a ratio range from
(66/12) * (54/18)= 16.5
to
(66/12) * (36/36) = 5.5

Other possible second stage ratios are
52 & 20
50 & 22
46 & 26
44 & 28
42 & 30
40 & 32
38 & 34

Thus with 1 gearbox design, you could just substitute the second stage gears for any challenge that comes along.

The concern with the gear diameter vs. the wheel diameter is only applicable if you plan on direct driving the wheel off the gearbox output shaft. This has been all the rage in the past few years, but it’s not the only way to construct a drivetrain.

Methinks you forgot a word.

I’m in the Pacific time zone, we only use west coast drive trains!

I can see using a non-direct drive if you don’t have a lathe to manufacture custom drive shafts. The added belt/chain with a separated gearbox doesn’t seem like it would provide any competitive advantage.

Even then, most gearboxes have an extra long shaft. Just cut the shaft and use a high strength shaft collar.

Thanks again for the feedback! It has been really helpful.

I will definitely fix the diameter of the CIM holes and probably add some options for the 14t sprockets as well. Thanks for the rule of thumb, as I also thought that the clearance between the hole and the edge was a little tight. Especially since those holes are going to be taking a lot of force from the CIM. And yes, most likely this will be aluminum since that is what we have on hand.

I haven’t quite gotten around to that yet. However I will make a mounting solution soon.

Great point. Thanks for the input. I will look into widening the second stage to a 72 tooth spacing in order to get this added flexibility.

This is true, but it’s much cleaner (in my opinion) to have one direct drive and chain or belt to the other wheels. However you do raise a valid point.