pic: Sheet Metal with VexPro parts
 

Re: pic: Sheet Metal with VexPro parts
 

EDIT: Nevermind I realized you are not direct driving. If you have the answers though feel free to reply.

Looks really slick. Simple and will get the job done. One question I have as I am also looking into using the vexpro components on our drivetrain:

For the wheel that is direct driven from the gearbox, did you place the inner of the two Hex VersaHub towards the center of the robot from the inward-most sprocket?

The reason I am asking is that I was thinking about the alignment of the sprockets since in a setup like this only directly driven wheel needs the two Hex VersaHubs while the other two need the bearings which do not add to the "stack" of the VersaHubs and Sprockets.

Maybe a render of the direct-driven wheel would be easiest to understand. Also are you planning on machining the output hex shaft to a certain length to fit your design?

Re: pic: Sheet Metal with VexPro parts
 

I'm trying to find an angle that'll show things a bit better. This doesn't actually directly drive any of the wheels; it uses a two-stage ball shifter with two sprockets on the output shaft. The ratios available on the ball shifter don't really work to my liking for directly driving 4" wheels.

Were I to direct drive, I think I'd put the hex hub on the far side of the wheel and stack the sprockets and bearing hubs up on the other, otherwise the thickness of the hex hub's flange would throw off the stack up for the other wheels and your wheels would be slightly out of alignment.

Re: pic: Sheet Metal with VexPro parts
 

I like this design a lot. We had a lot of discussions about how to mount the ball shifter into a drivetrain. This design is very similar to something I sketched on the whiteboard. :)

Face mount the 2-stage, then add another reduction via spur gear (my preference) or chain to one wheel.

-John

Re: pic: Sheet Metal with VexPro parts
 

I started with spur gears, but I'm not confident enough with working in sheet metal on FRC robots to ensure the wheel/axle setup will remain rigid enough relative to the gearbox output to make it work. I switched to chain to give myself some room for error.

Re: pic: Sheet Metal with VexPro parts
 

Super snazzy.

For someone of us that haven't had a chance to take these parts for a spin in Inventor, what's the output shaft length on those gearboxes? What's the distance between those railings?

And, just for general sheet metal design is "safe" (for lack of a better word) to use dead axle bolts directly like that on a sheet so thin?

- Sunny G.

Re: pic: Sheet Metal with VexPro parts
 

Madison,

I'm with everyone else as well on the design. It looks really awesome from an astethic perspective. Utilizing the extra sheet for mounting is what pops out for me. If you guys run an iteration of this guy in season you guys should definitely give us information on how it held up. :D

Re: pic: Sheet Metal with VexPro parts
 

When using JVN's design calculator, I found the low gear to be 9.3 ft/sec and high gear to be 21.2 ft/sec

What speed are you aiming for with the low gear? 6 or 7 ft/sec? Would 9.3 be considered an acceptable low gear speed?

Re: pic: Sheet Metal with VexPro parts
 

Great concept, I've spent a little time with the VEXpro CAD files myself and one my favorite parts is how much ease of maintenance was included in the design. It looks like the rear wheel would block the awesome access holes that VEX designed into the gearboxes for removing the CIMs. That is one of the reasons their mounting bracket has the cutout in the middle.

Also what chain reduction did you use in the design? I've been playing around with a couple and haven't really made a decision yet.

Re: pic: Sheet Metal with VexPro parts
 

21 fps...overkill's definition.

And low gear isn't necessarily about speed as it is about pushing power. Going to 3-4 FPS means nothing if you spin out your wheels in a pushing match.

- Sunny G.

Re: pic: Sheet Metal with VexPro parts
 

21 FPS isn't overkill if you're 254 with a 6 motor drivetrain :)

Re: pic: Sheet Metal with VexPro parts
 

all speeds are not created equal. 21 FPS on 2 motors is probably slower than 13 or 14 FPS (You could do some quick math to figure this out).

Your acceleration would be SUPER sluggish.

Re: pic: Sheet Metal with VexPro parts
 

Its about time for me to crack out the old CAD machine again :D

Love the design.

- Andrew

Re: pic: Sheet Metal with VexPro parts
 

Quick spreadsheet:

2x CIM motors, 150lbs, 100% weight on driven wheels, 0.9 speed loss const
12.5v initial battery voltage, 0.03 ohm battery resistance

Gear ratio resulting in 21.33fps after speed loss:
2.07sec to 20ft
3.18sec to 40ft
1.27sec to 12fps
0.54sec to <160amp total current
3.23sec to 11v battery (battery remains under 11v for 3.23sec)

Gear ratio resulting in 13.29fps after speed loss:
2.00sec to 20ft
3.40sec to 40ft
1.17sec to 12fps
0.18sec to <160amp total current
1.20sec to 11v battery

Looking at the curves, most of the output distance curves for the two gears are relatively close, the velocity is worse than 13.29fps until ~1.5sec, and the higher gear will be under ~1.5x motor load and significantly lower battery voltage the entire time.

Edit:
When you choose a high gear ratio, you don't usually actually care about top speed. You really want to gear for either sprint-distance or time to speed, with the speed and distance adjusted based on game-specific strategy. Being tied to a specific ratio spread will also pull your high gear slightly based on where your low gear wants to be. You usually want low gear to be traction limited, at a current which is determined by your strategy (how much you want to push).

Re: pic: Sheet Metal with VexPro parts
 

I have a section-view at home, but there's some build up around the axle slots to distribute the loading over a larger area and to deal with side-loading on the tensioning system. This is using a sheet-metal variant of the sliding bearing block tensioning method.

The output shaft of the 2-stage ball shifter is a bit more than 1.5" long measured from the face of its housing. The rails are 3" apart -- inside face to inside face.

The chain reduction is 1:2 right now; 16 tooth hex sprocket to 32 tooth sprocket bolted to a versahub. I just tossed those in there for the sake of figuring out the overall width of the assembly; I'd look more closely at the ratios if we were to build something like this. Generally, I'm less concerned with penciling out the math at this stage and more interested in packaging and assembly.

The ball shifter is face mounted to the inner rail and should be pretty easily removed. The slots in the belly pan around the CIM motor and the hole in the rail allow you to remove the entire transmission assembly as a whole. I've never had to replace a CIM motor on an FRC robot, so I'm not very worried about accessing their mounting holes without a bit more disassembly.