I was inspired by 610’s recent updates to their chassis and thought it would be fun to design a “chain and wheel in tube” chassis. This uses 2.5x2 tube and the 3x0.875 colson wheels from VexPro. This is not something we will likely build because there really aren’t any benefits over the current iteration of our WCD.
CAD can be found here: A Fond Farewell to GrabCAD Workbench - GrabCAD Blog
Yes there would be a belly pan and no I have not put in gearbox mounting holes.
It looks like you could make the outer wheels dead axles, which would make it easier to manufacture.
I’m curious, what makes a dead axle easier to machine?
Holes for bolts don’t require the same tight tolerances that bearing holes do, would be the main thing I could think of. But if you’re doing a full tube thing like this, it would seem to me to be quite likely you have some sort of CNC capability, which makes that less of a factor.
Bolt sprocket to wheel. Stick big bolt through spacer, sprocket/wheel/bearing setup, apply locknut/jamnut/other locking fastener. Done. (Or, use a length of steel rod from McMaster, thread the ends with a die or drill for a cotterpin. Still a minimum-machining job.)
A live axle requires a keyway/hex feature and some sort of retention for the wheel/sprocket separate fro each other.
Cool. Good to know. Do you not lose those advantages with this being exact C2C chain in tube though? I’d imagine you need those tolerances to be high still so going to a dead axle doesn’t really help in this case does it?
I’m a Software Engineer not a Mechanical Engineer so these are serious questions.
The exact center-to-center can actually be easier–the main thing is that your precision work is all on the tube, and it’s a smaller hole so it can be easier to get right. I’d be willing to bet that it’s possible to place and drill a dead-axle hole by hand–and drill it straight, too, with the right tools!–or drill press by someone who knew what they were doing.
There’s also the option of throwing a tensioner block into the system, probably an off-center mounted delrin cylinder, but that kind of defeats the purpose of “ease of maintenance”.
As the tube constrains the chain, there is a much greater tolerance for chain slack. You may get backlash, but if the tube fit is tight enough, the chain can’t come off the sprocket unless it breaks.
To be quite honest, that doesn’t make the machining any easier… The way my team does it, which is not the best way to do it but is perfectly fine for FRC requires the same amount of machining. In 2014 we were able to do a c-c belt drive train on a manual mill with no issue. It used the same bearing setup as shown in this picture. Either way, it would be a bit on the mill properly spaced the correct distance from the last hole. We use a 1.125" mill bit to drill bearing holes and have had no issue with tolerance thus far, we do not us a CNC.
Sounds like you have a pretty solid way to get the C-C. I’d suggest live axle in that case, since you’ll constrain the bearings better. If you go with a dead axle, one of the design considerations is keeping the chain tight. If you tap the end of a shaft, your c-c won’t be all that accurate because the clearance around the bolt in the frame adds a lot of play and potential for motion under impacts.
We have a mill with a digital read out it’s pretty useful. I think the best thing about c-c drivetrains is the fact that you don’t have to mill slots for bearing blocks. However, with all of the new vex/wcp parts now pretty widely available, adding tensioning cams to the drivetrain only requires one more hole drilled in the tube. the WCP bearing blocks are huge time savers.
PREACH! The slots for a WCD are the worst part, and due to the inexperience of student machinist there are usually errors every year (except this one!) so going C-C also saves time in my opinion.
Inside a 2" tall x 1/8" wall extrusion, this kind of chain retention only happens when using 17t sprockets. 2363 uses 16t sprockets. There are different reasons why our chains don’t come off the sprockets.
My point was not that all chain-in-tube has close-fit sprockets (the one pictured in OP is not), but that this technique could be exploited with a dead axle to make even “tape measure and drill press” tolerances feasible.
Nothing about a dead axle drivetrain lowers the position tolerance requirement for an exact-center drivetrain at all. I have no idea what you’re talking about here. You need more precision than that regardless of whether or not your axle spins.
Matt,
Having worked through a problem with this design with a team at Championship a few years ago (it did not go well), I can only give you a few words of caution. It is essential that you provide for chain stretch. Chain stretches and it only takes a small amount to jump up during operation and lock the drive against the inside of the tube.
The second is a restating a post I made just a few weeks ago. If you want to win on Einstein, don’t design something that can’t be fixed in a few minutes with simple tools in near dark conditions.
Good Luck!
This is exactly one of the reasons that we won’t be switching from our WCD chassis to this.
I encourage those advocating for dead axles to play with this design and see if you can make it work. Matt has already posted the CAD for you to work from.
In FRC context, dead axle design involves transmitting torque directly from the sprocket to the wheel, without passing any torque to the axle. In this case, you would need to package both a bearing and a torque transmission feature into the 16t (or 17t) sprocket. I’m just not sure it’s possible to package all that in such a small space.
BEARING
To save space, I would probably want to bore the sprocket to fit a needle roller bearing. In a high-risk application like a drivetrain, I wouldn’t want to have needle roller bearings running on anything but precision ground steel shafting, which means that I can’t use a bolt as the dead axle. In doing this, I have modified the 16t sprocket out of its COTS condition; this is a big deal for 2363 for various reasons.
TORQUE TRANSMISSION FEATURE
Maybe some custom bolt pattern will do this? You can’t fit the 1.875" BC here. I don’t think you can fit any part of the versakey system here. In making this torque transmission feature, I have probably created a custom wheel hub and may have even modified the wheel out of its COTS condition.
On top of all this, joining the sprockets to the wheels will require a bigger pocket in one of the walls of the tube, weakening the structure further.
TLDR: The precision machining requirements for this live axle design are WAY lighter than any comparable dead axle design that I can think of.