pic: Inverted CIM 2-Speed Gearbox

[EricH]

Originally Posted by GeeTwo

If it weren't for the emoji, I'd have to report you for trolling. My right eye is twitching, even though I never heard of FRC until April or May of 2011.

Actually, I'm not entirely sure what the CoF of a Lunacy wheel is on carpet. I'm pretty sure it's more than on that pesky Glassliner FRP (AKA regolith), though.


For those that don't get the reference... 2009 (Lunacy) was played on a field of Glassliner FRP with about a foot of carpet on all sides next to the rail. If you want to know what that looks/feels like, there's probably something similar in your nearest school/park restroom (as an anti-graffiti/easy-clean sort of measure). All robots were required to use certain wheels for their floor-contacting propulsion--the CoF between said wheels and the floor was something just under 1 as I recall, while your typical nitrile wheels are 1.something-or-other. Unlimited quantity... but that was the ONLY type allowed! Low-traction game, low-speed, low-friction...And then there were the trailers, but I'll end there.

[asid61]

Originally Posted by EricH

Actually, I'm not entirely sure what the CoF of a Lunacy wheel is on carpet. I'm pretty sure it's more than on that pesky Glassliner FRP (AKA regolith), though.


For those that don't get the reference... 2009 (Lunacy) was played on a field of Glassliner FRP with about a foot of carpet on all sides next to the rail. If you want to know what that looks/feels like, there's probably something similar in your nearest school/park restroom (as an anti-graffiti/easy-clean sort of measure). All robots were required to use certain wheels for their floor-contacting propulsion--the CoF between said wheels and the floor was something just under 1 as I recall, while your typical nitrile wheels are 1.something-or-other. Unlimited quantity... but that was the ONLY type allowed! Low-traction game, low-speed, low-friction...And then there were the trailers, but I'll end there.

I was far under 1 iirc. The regular tread wheels get around 1.

[Chris is me]

Originally Posted by EricH

For those that don't get the reference... 2009 (Lunacy) was played on a field of Glassliner FRP with about a foot of carpet on all sides next to the rail. If you want to know what that looks/feels like, there's probably something similar in your nearest school/park restroom (as an anti-graffiti/easy-clean sort of measure). All robots were required to use certain wheels for their floor-contacting propulsion--the CoF between said wheels and the floor was something just under 1 as I recall, while your typical nitrile wheels are 1.something-or-other. Unlimited quantity... but that was the ONLY type allowed! Low-traction game, low-speed, low-friction...And then there were the trailers, but I'll end there.

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.

[Andy Brockway]

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.

[asid61]

Originally Posted by Andy Brockway

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.

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 ). 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.

[Andy Brockway]

Originally Posted by asid61

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 ). 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.

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.

[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.