Specs:
Reduction Ratio 11:64
High: 54:34
Low: 34:54
Weight: 2.2 LBs
High Speed (Adjusted): 20.49 Ft/s
Low Speed (Adjusted): 8.12 Ft/s
This is the first shifting gearbox I’ve ever designed. This is intended for a game like 2014 or 2013 with a mostly open field.
EDIT: PM me for STEP files.
I’m assuming that weight doesn’t include the CIMs. How much of the output shaft does it include?
Includes the ball shifter shaft and the top shaft as well as the pneumatic cylinder and mounting hardware for it. (assuming the shaft for the first reduction actually went the full length to the next bearing.)
Note that the ball shifter shaft is not long enough to be used in a drive train as is, so we’d need to use a shaft coupler.
The gearbox looks fine. Are you planning on building it in the 2 weeks of off-season we have left? :rolleyes:
Is this intended for a WCD? If so, that output shaft would be far too short. I’m not sure if Vex sells a ballshifter with a longer shaft though.
It looks like you can save quite a bit of weight and material if you move the 4 corner standoffs closer to the center and “round out” the corners. You can even just ditch the 4 corner standoffs and use the screws for the CIMs to hold the gearbox together, like the WCP DS does.
The only problem I have with this gearbox is that the CIMs aren’t black:p .
The picture looks like a screenshot anyways; do you know how to render CAD? (I’m asking because you said this is your first gearbox). If you don’t, say what software you use and I’m sure someone can help you.
Why are the speeds high? I understand that maybe for an open field, you want to travel at 20 fps (although I use 16 fps for my gearboxes), but I’m not sure about 8 fps as a low speed. One person pointed out on one of my gearboxes going at that similar low speed that the goal of the low speed is not to go fast, but to win a pushing match.
So is there a way to bring that low speed down to… something like 6 fps? This would mean more torque and less current draw, therefore a longer time that the bot with this gearbox can be in a pushing match.
A somewhat lower low speed would allow for less current draw but would not increase the pushing force because of traction limitations (at least… according to wcp’s calculator).
I currently get about 56 Amps of draw using 2 cims and a mini cim. This is not unreasonable as the 40amp breakers can last about 50 seconds with 60 amps of draw. That said, using 2 cims and a mini cim or 3 cims with the lower brown out voltage of the roboRIO will probably require my team to dedicate some special efforts to reducing current draw.
One idea we’ve had to limit current draw is only running 1 cim in low gear until the driver chooses to use a pushing mode, in which both cims and a mini cim would be engaged.
EDIT: I do not know how to render cad. I use solidworks.
That’s at the very high end of the error margin given by the manufacturer, the low end is 4 seconds.
The standoffs were made as is to make removing cims easier.
As for the shaft, yes the output is too short, despite this being the “long” shaft vexpro sells. We would need to use a shaft coupler to make attach it to another thunderhex in order to use it in a chassis. 
Interesting. I’ve seen quite a few gearboxes that run their low this high, but it is somewhat concerning that it is possible for the breaker to trip that easily at 60Amps. I may end up adjusting the ratios a bit to account for this.
EDIT: If needed the gearbox can mount a 28 tooth gear and a 60 tooth ball gear for the low gear in order to get a low speed of 6.02 Ft/s. This would draw around 42 amps. Vex does not make an 28 tooth gear, but my team is capable of manufacturing one.
There are a LOT of pockets. You can remove a lot of the supports that connect various things together to make fewer larger pockets. This will make it lighter and easier to machine.
The top corners are sharp and don’t really support anything. You may want to look into removing them for aesthetic and weight reasons. I personally like the look that 254, WCP and many others’ gearboxes have where the outside round of the plate matches the profile of the CIM. 254 example: http://www.nickeyre.com/images/frc2011-3.jpg
Your radii look quite small, this will force you to use a smaller and therefore slower and less durable cutting tool than you may want.
Thanks for the advise on pocketing. I originally had larger pockets but was concerned about strength.
The gearbox will be cut using a laser rather than cnc, I may make the radii somewhat larger.