pic: Inverted CIM 2-Speed Gearbox

[KohKohPuffs]

I must bring back this discussion to ask some questions regarding this design, mainly because I'm considering making another inverted CIM gearbox with more desirable high/low speeds.

1a. How can I provide enough space in between the CIMs for the gears without using the original idler design, but not go into belts (This is something I'll consider later on) or custom gears?

1b. So what if I decide to stick with my first first stage design? Because I don't think that anything bad will happen if I do it. For one, the gears are made of steel, and even if it does wear out, the design is made so that those gears are easily replaceable.

2. How can I bring down the weight to a minimum? I came across a topic on CD regarding delrin gearbox plates, and I was thinking about polycarbonate plates beforehand, but I'm not sure if either of those are good ideas.

[cbale2000]

Originally Posted by KohKohPuffs

2. How can I bring down the weight to a minimum? I came across a topic on CD regarding delrin gearbox plates, and I was thinking about polycarbonate plates beforehand, but I'm not sure if either of those are good ideas.

We've used 0.25in polycarbonate for gearboxes before with decent success. So long as you have a solid way to mount the gearboxes and watch out for over-tightening screws you should be fine. You're already using VexPro gears from the looks of it so you should save a decent amount of weight just by doing that instead of using steel.

[Chris is me]

Originally Posted by KohKohPuffs

I must bring back this discussion to ask some questions regarding this design, mainly because I'm considering making another inverted CIM gearbox with more desirable high/low speeds.

1a. How can I provide enough space in between the CIMs for the gears without using the original idler design, but not go into belts (This is something I'll consider later on) or custom gears?

1b. So what if I decide to stick with my first first stage design? Because I don't think that anything bad will happen if I do it. For one, the gears are made of steel, and even if it does wear out, the design is made so that those gears are easily replaceable.

The biggest thing is that I would use bigger idlers. I don't think it makes any sense that you're using 14 tooth pinions on your motors and 12 tooth gears as your idlers. 14 tooth pinions are a pain to use with a gearbox because you can't install them in advance - they are bigger than the hole size needed to fit around the CIM boss, and it's best to design for a tight slip fit around this boss rather than a bigger clearance hole. 12 tooth pinions are great for this reason. The downside is that the gears are small so the tooth loads are higher. Using them as idlers is just getting all of the drawbacks and none of the benefits.

I would use something like a 20 tooth idler that you can put small (1/4" ID) bearings into, then mount them on a shoulder bolt or something like that. You really do want a ball bearing in your idler or you're just throwing efficiency away, and you ideally want them mounted to a shoulder bolt instead of just a screw or something so you have a simple robust round shaft for them.

2. How can I bring down the weight to a minimum? I came across a topic on CD regarding delrin gearbox plates, and I was thinking about polycarbonate plates beforehand, but I'm not sure if either of those are good ideas.

I wouldn't do either of those things, especially since you are cantilevering the first stage. You don't want the extra flex that these materials will add to your gearbox. Spend the few ounces of weight where it matters; I guarantee you something else will be on your robot that you would rather cut weight from than your gearbox plate. If you really want to minimize gearbox weight, pocketing the gears is where you could start shaving a few ounces.

Also, don't forget to add some fillets to your lightening pattern - even if you waterjet these plates, fillets avoid the stress risers of sharp corners and also just look better. If you mill these plates, obviously you can't do interior hard corners.

[cbale2000]

Originally Posted by Chris is me

I wouldn't do either of those things, especially since you are cantilevering the first stage. You don't want the extra flex that these materials will add to your gearbox. Spend the few ounces of weight where it matters; I guarantee you something else will be on your robot that you would rather cut weight from than your gearbox plate. If you really want to minimize gearbox weight, pocketing the gears is where you could start shaving a few ounces.

To clarify my previous comment, you CAN use polycarbonate for gearboxes generally speaking, but you would probably not want to use your current design if you went this route.

A quick redesign using solid (not pocketed) plates and eliminating cantilevered shafts (by simply supporting them on both sides) might still be a worthwhile weight savings.