So this past year we got a CnC sheet metal sponsor, and took some advantage of it this past season. In the off season, we wanted to go a little more hard core, and make a brand new drive base.
As we all know that Chief Delphi is an excellent place to get critique, we thought we’d post some pictures here and get opinions.
Features include:
112" perimeter.
Robust, etc, etc.
Customizable as 8 x 4" wheels (shown) or 6 x 6" wheels.
1/8" drop center wheel(s)–both centers on the 8wd.
2 speed (5.5 fps, 18 fps).
Plenty of space for upper mechanical mounts.
Multiple battery placement options.
Easy to assemble, easy to maintain (rivet nuts used extensively).
A hair over 40 lbs, inclusive.
Easy, robust bumper mounts (for reversible bumpers).
Why did you decide to go with the octagon frame instead of a hexagon?
Any reason the belly pan isn’t incorporated into the front and back rails as one piece? (971 style, also extending the front and back rail flanges and adding an extra bend to them to make them more box like, again 971 style.)
Do you change the gearbox when you switch from 4" to 6" wheels?
-why five shaft??? yes it keeps the mass centers but robots are rarely 100% symmetrical. i see it as an added fail point, justin claims it make the tranny easier to change and gives you a 100% symmetrical start
-atop the bumper mounting surface the hole pattern changes from outer to inner
-how do i change a belt?
-the belly pan should be notched so the tranny can slid back that length of the output shaft TRUST ME I MESSED UP!
I would add more mounting holes atop each “drive rail” so the hole pattern matches the front and back rails.
While a circle maximizes area (and a hexagon better approximates that), we wanted to keep a wider wheel base.
Our sheet metal sponsor is used to building giant buses, not small parts, and gave us a lot of pointers on what would work well for their people. To whit, more, simpler parts with fewer bends where possible. This has resulted in some compromises between what we feel would be ideal and what they want to work with!
Nope! The plan is to have a drive train that’s easy to modify to one or the other, but it’s not something we intend to change “on the fly.”
0.090"
The five shafts are because of the modularity of 6 wheel vs. 8 wheel.
6 wheel would be more traditional WCD, with a direct-driven center wheel.
I’m not sure what you mean. Can you clarify? (I think that’s an artifact of the image conversion process, and the fact that due to belt interference, rivet nuts go from one side to the other as you traverse the main drive shaft.)
Take off the bumpers, pop off the wheels (clip rings), and then with a few loosened bolts the flat panel (with attached 1x3 bumper supports) comes off (leaving the corner brackets in place). Depending on where we mount upper mechanical stuff, we made the top parts all modular and easy to access, just in case we have to add a step to that. At this time, I believe twelve bolts (that we have easy access to) and we can slide the whole side panel off.
I mean, we’ve never had to replace a sprocket or belt in our drive train since switching to belts way back when, but that doesn’t mean it can’t happen!
What’s the advantage to backing up the transmissions further into the robot? As-is, the extra piece of axle won’t touch anything (and we can always trim it if needs be.)
I agree, but that’s actually going to go the other way–the front and back rails are the 1x2" C channel from VexPro, but we just found out they’re discontinued. Our sponsor wants to see fewer tiny holes where possible to minimize time on the machine (from starts and stops), so we may end up with even fewer than are pictured in the front and back, and then end up drilling more on a press using a template.
In a perfect world, during build season we’ll figure out exactly which mount holes we’re going to use and delete the rest of them in favor of larger lightening holes/slots.
As to the bumper shape: that’s an excellent idea. T’would require some non-trivial re-design of what we’ve got here, but that’s the whole idea! I’ll bring it up to the team.
It looks like you have e-clip grooves on the outside of the bearing and also at the end of the shaft to retain the wheel. You will be better served to remove the interior groove because it creates a failure point in the shaft that’s seeing a lot of torque. Use spacers to maintain the gap you like between the rail and the wheel and keep the e-clip on the very ends of the shaft.
Same question, why 5 shafts? If you lose chain you could be unable to drive any of the wheels, whereas if you direct drive a wheel you are guaranteed at least one wheel under power.
Second, Are you using the w pattern traction wheels? http://www.wcproducts.net/versawheel/
Third, Why go with exactly 112" perimeter? This may sound dumb, but why go as close as you can, because a tape measure may be short or you could bend your frame making it bigger… (my team usually goes at 111" to be on the safe side)
Fourth, I have not worked with sheet metal before, but is it strong enough to support the wheel bearings without versa blocks or something to that extent? i have seen it done with thicker metal, but not sheet metal.
A few questions/comments:
How do you remove the transmission?
What type of bearing blocks are you using (if any)?
If you’re not direct driving a wheel, have you considered the 3 CIM ball shifter? You may be able to get the extra reduction stage between the output shaft and the wheel.
I’d be careful with that snap ring groove between the frame and the wheel. You generally want to avoid these where the shaft is transmitting torque.
The five shafts are for modularity–the center shaft becomes a direct-drive shaft in a six wheel application.
I’m not sure how worried I’d be about breaking two belts simultaneously (which is what would have to happen to lose drive completely on one side)–we’ve never broken one!
In this drawing, yes, but it’s sufficiently modular to accommodate any hex-bore wheels we want to put on. Wheel choice will depend on the field.
Not dumb at all. I brought this up multiple times to my team, and they keep poo-pooing me. If it becomes a problem at inspection, I’ll just strangle them a while. Easy-peasy!
Yes, absolutely–but we’ve got plans just in case that confidence is unwarranted! We intend to build it early this fall and then try to destroy it via driving, and adapt as necessary.
This is a really good idea. Different sponsors, whether they specialize in machined parts or sheet, have certain strengths and specialties. It is always a good idea to work with them and play to the sponsor’s strength. Working with shops in this way has great educational value and is one of the underrated benefits to working with sponsors.
Do you have a specific way of tensioning the belts? Or are you planning on getting it right first and then not changing it?
Also, its a little hard to tell from your picture, but you may want to add in some more ribbing, especially at the corners. From the way the brackets are bent and they way you did the hole pattern, it seems like these corner pieces would mount to the bumper? correct me if I’m wrong. The way it is design it seems like you could see bending of those flanges through the rigors of competition. You can take a look at 971’s CAD screenshots to get a better idea. We put a lot of support on the inside of the drivetrain, independent of using the bellypan as an integral part of the drivetrain. It’s a step up on complexity, but your drivetrain will be much more likely to survive high speed impacts that were more common this season.
Otherwise, it’s great your team is spending the offseason trying to improve/design a drivetrain. I’m looking forward to seeing more from your team.
The latter. We’ve got five years’ experience with belts and are confident in our ability to get it right using this.
We’re happy with how it is right now, but are going to abuse the heck out of it this fall and see what needs improvement on all levels of functionality, but with a particular eye toward robustness. We’ll revisit ribbing before sending it to cut.
The bumpers will be mounted to those corner pieces and to the front and back cross braces, adding the strength of the plywood and aluminum angle to the overall strength of the frame. We’re pretty confident that it’s already overkill, but testing will tell.
Thanks for the post pat. We are in developmental stages on some drive train work too. I’ll take some of these notes into consideration for ours as well.
