Re: pic: Inverted CIM 2-Speed Gearbox
 

Just under 1 is the CoF of the 2008 Kit wheels, if memory serves, which were a bit slicker than other years. The CoF of lunacy wheels on FRP was reportedly 0.06, though it was probably 2 or 3 times that on a worn field.

Re: pic: Inverted CIM 2-Speed Gearbox
 

You could eliminate those idlers by making your own gear sets. A quick search in the white papers using my name or "2005 716" will show a similar gearbox that we have been using off and on since 2005. The 12 tooth intermediate low gear has never been a problem for wear. This gearbox can be made on manual machines. One change is the that we use the stock CIM gear instead of the fancy tapered thread version in the plans.

Re: pic: Inverted CIM 2-Speed Gearbox
 

As a student machinist for my team, I would recommend against making gears if you can avoid it at all. Indexing and cutting 2-3 times per tooth takes a very long time, not to mention all the work of taking a vise off the table for an indexing head or rotary table and centering everything. If you have the resources, go for it, but personally I can think of few situations where you would really want to do that (a 26t or 16t gear being those few situations :P). I've designed swerve drives that can be manually machined, but not ones that you would really want to.
Buying spur gear stock or something similar would be good for making many custom gears.

Re: pic: Inverted CIM 2-Speed Gearbox
 

I also do not recommend machining your own gears unless you have the time. I have been using Martin spur gears and modifying the hubs as needed. They come in almost every gear size and are available on line and from McMaster.

Re: pic: Inverted CIM 2-Speed Gearbox
 

I dunno, we run about 3.5 fps in games where we're doing pushing and it works pretty well for us. I've yet to find a robot (or two at once) our 2014 machine couldn't push. :rolleyes:

Re: pic: Inverted CIM 2-Speed Gearbox
 

Bet I can name one... :P

Re: pic: Inverted CIM 2-Speed Gearbox
 

Once your wheels slip under load, gearing any slower doesn't do anything to increase your pushing force, it just decreases your power consumption. You're traction-limited rather than torque-limited.

For a 150 lb robot with roughtop tread, that "magic number" is around 6 feet per second, depending on your efficiency, exact weight, wheel design, etc.

Re: pic: Inverted CIM 2-Speed Gearbox
 

I've been punching some numbers into JVN design calculator, and I managed 16.85fps high, and 6.76fps low.

This will be a new design (but I'm keeping the inverted CIMs), since to do this I had to have the dog gears be the driven gears rather than the driving gears in the second stage (shifting on upper shaft). Also, in order to have the gear space in the gearbox, I'm most likely going to keep the idlers:

1. The idler gears are easily replacable in case they wear out
2. Using my logic, the idler gears are made of steel, so I doubt wearing down is going to be significant

As for a belted gearbox, I think our team wants to move away from belts. It's a bit complicated, but even if it is better than idlers, I'd rather work on the current style I have going.

Thoughts?

Re: pic: Inverted CIM 2-Speed Gearbox
 

Those speeds are much better, you should be able to withstand a pushing match for a least a little while (I'm getting about 50A current draw with 4.2" wheels, 150lbs, and 1.3 CoF).
Removing the idlers can only do you good, regardless of wearing out. Reduces complexity and weight. If you have to keep them, that makes sense, but I would remove them if at all possible.
What are you guys moving away from belts for? The epitome of flipped-cim gearboxes (192 in 2014) used belts for their first stage and it reportedly went swimmingly. Saves a lot of weight and space in these things IME.

Re: pic: Inverted CIM 2-Speed Gearbox
 

I might be completely wrong about my team and belts, but we've been using belts for a while on our DT, and this offseason we're experimenting with chain DTs.

I mean... this only applies to the chassis, so perhaps I would attempt a belt system for the first stage. However, I'm using a large ratio for that stage, so I'm not even sure if there are the right pulleys for my configuration.

-Is there an online store that specializes in pulleys? It's been a while since I last been there, and my memory is fading away... -_-
-Also, is it possible to make your own pulleys? If so, is there a link to how to do that?

Re: pic: Inverted CIM 2-Speed Gearbox
 

SDP-SI has them, but seeing that in this case you have a very large reduction you wouldn't be able to pull it off and still maintain adequate belt wrap without idlers.

Re: pic: Inverted CIM 2-Speed Gearbox
 

We are moving away from belt in drivetrains because the tenioning was too much work for the benefit. However, tensioning similar to 192's 2014 gearbox is feasible, and in fact, 649 has cadded a similar idea already. It might be tough to get such a large first reduction however with belt. SDP-SI is basically the go-to for pulleys, and 649 already does make their own pulleys, either through our laser cutter or through our wire-edm sponsor.

Re: pic: Inverted CIM 2-Speed Gearbox
 

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.

Re: pic: Inverted CIM 2-Speed Gearbox
 

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.

Re: pic: Inverted CIM 2-Speed Gearbox
 

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.

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.