This year we are looking to see if it would be worth putting the chains for our wheels on the outside of the chassis (WCD).
In past years we have had trouble with trying to replace or fix our chains. Now we want to help alleviate the pain of trying to fix the chain from the inside by placing it on the outside of the 2x1.
Here is an example below of what it could look like:
What are some opinions, advice, or critiques (pros and cons) about the position of the chains and how they could be designed that could help solidify our plan for the season?
We’re considering the same thing, so I’m curious to hear from others’ experience too.
In 2017 we ran one loop of chain outside the drive rails (to avoid a frame cutout). It worked, but having one chain on either side of the rails tended to skew the Versablocks. Having both outside will avoid that problem, and let you use larger sprockets (to lower the forces on the chain).
Apex Robotics 5803 ran chains on the outside in 2017 and 2019 and had no issues, so it definitely can be done. The bumper mounting has to adapt from a standard WCD. We ran a rail above the wheels
Personally, I love the arrangement because it greatly simplifies the gearbox attachment. We weren’t too concerned with vulnerability since it’s well protected by the bumpers. The main “con” is that it does increase your axle cantilever, but having run off HAB 2 all season plus several falls from HAB 3, there wasn’t any noticeable damage. We pressed our bearings directly into 1/8" wall 2x1 with no bearing blocks also, so adding bearing blocks is an easy way to beef that up if there’s a concern.
We’ve always run them on the outside with no maintenance problems. The VexPro bearing blocks and bearing don’t seem to have issues with the cantilevered shafts.
Our scoring section typically bolts to the top of the drive train side rails. Chains on the outside narrows the available width of the scoring section mounts.
I’ve never run chains (or belts) outside the structure for competition robots; sounds like a recipe for disaster. We did use chains outside the [nanotube] structure for our air cannon for a few years, but we weren’t expecting any collisions in that application.
I’m not sure which example you’re referring to, but now that you mention it, none of the [embedded] ones I see above show anything that I would pass as FRAME PERIMETER/bumper supports.
Nuclear is right, our chains and belts were protected by stout bumpers (baltic birch) in 2017. Those who played that game will recall that the full-field gear scoring cycle resulted in some spectacular mid-field collisions, where robots frequently crossed paths at high speed. Our drive train held up well.
OK, the next question for outside mounted chains is: Are you comfortable mounting your wheels that far from the supports? Perhaps you are, perhaps not. If you haven’t done the math, ask yourself, do I feel lucky?
Cantilevered axle loading was a build-season concern for us in 2017 and we were prepared to replace our 7075 aluminum shafts with 4140 steel if competition had shown the concern to be a real issue; fortunately, that game didn’t include heavy shock-loads on the wheels. 2019 would have been a different story, I expect.
I haven’t run this setup myself, but the math (off the top of my head) is that the stress from an cantilevered shaft is relative to the cube of the overhang. So if adding sprockets on the outside between the tube and wheels doubles the overhang, that would increase the shaft stress by 8x. Will this cause a problem? Who knows; it probably depends on the exact setup and game dynamics.
As a side note, it seems to me that you can get around the cantilever problem by putting the sprockets all the way outside both the tube and wheels. That probably exposes the chain to more risk from collisions, but hey engineering is all about tradeoffs.
I believe that actually makes the problem worse, assuming you’re driving a similar wheel 1:1. Running a given torque through a smaller diameter results in a greater radial force, so moving the smaller diameter farther from the bearing results in a greater radial torque.
