Drivetrain Bellypan Issues

Team 4099 will likely use something like this drivetrain for 2019 if the game requires traversing rough terrain. I mostly made this CAD because I wanted to see if there would be any odd issues with something like this, since our team used the KOP chassis in 2016.

I based this on 254’s 2016 drivetrain (as described in their technical binder), other discussions on this forum about how to design drivetrains for rough terrain, and our own team’s manufacturing capabilities.

However, I had a problem that I did not hear discussed in other places – with the center drop that 254 used for their 8" pneumatic wheels, the chain seems to interfere with a bellypan:

I made holes for it in the bellypan here, but if the game is like 2016, from what I have heard, that’s not a great idea because things can get stuck in the chain.

Does anyone have a solution to this?

My team is having the same problem! We are using VersaBlocks w/ #35 chain and 12 tooth sprockets on our Fall Test Chassis and the chain is hitting the bellypan, even when the chain has been properly tensioned. Any ideas?

The issue is that you’re using 18t sprockets instead of 16t sprockets. 16t fits nicely within the profile of a tubing extrusion even with a generous center drop.
You can use these:
Vex 217-2642
Andymark am-0749

With #35 chain, I haven’t seen any that fit within the profile of the tubing, so you’ll have to move to #25.

Guessing that 254 is using a custom machined bearing block on the corner wheels which would allow them to place the bearings higher up relative to the box than you can with the versablock design.

A solution which will still work with the stock versablocks would be to simply place 1" shim strips under the box to push the bellypan down a bit.

The issue is that you’re using 18t sprockets instead of 16t sprockets. 16t fits nicely within the profile of a tubing extrusion even with a generous center drop.
You can use these:
Vex 217-2642
Andymark am-0749

With #35 chain, I haven’t seen any that fit within the profile of the tubing, so you’ll have to move to #25.

Do not do this. Especially don’t do the second one. 16 tooth #25 sprockets are not always enough to handle WCD loads reliably, and an 8" wheel drive in a terrain game loads the chains quite a lot more than a 4" wheel, flat field build. I would not personally trust the more robust 22 tooth option either. There is a reason 254 switched to #35 in 2016.

I just read their 2016 technical binder within the last month. They used 0.125" with no bearing blocks and a 12t sprocket with #35 chain. Also they dropped the center 2 wheels and put the front a bit higher in relation to the rear.

That’s interesting – last year, we ran this configuration on our competition robot and I don’t remember having this specific issue, at least when the chains were tensioned. If we did have this problem, we definitely didn’t do anything to address it, other than cutouts in the bellypan for the actual VersaBlock – our bellypan was made of AndyMark perforated polycarb and we had no issues.

The only reason I’m not doing the same thing here is because I wanted more center drop than the VersaBlocks allow (IIRC, VersaBlocks let you have 1/8" of center drop, significantly less than the .3125" that 254 used with their 8" pneumatic wheels).

This is what I had assumed too, but just to clarify, I’m using the WCP milled slot bearing blocks. The issue that I’m having is with the .3125" center drop 254 apparently used, the rectangular bit on the bearing blocks that locates them vertically on the 2x1 would not fit if the wheels were raised any more. Same issue, but a different technical problem.

The bellypan as I CADed it is meant to be made of baltic birch. Would putting shims to push the bellypan down work for this application? Right now, I’d need to push the bellypan down by more than 1/8" on the middle chain run.

I had actually assumed that 254 just always ran #35 chain and it wasn’t a decision specific to this year. On the other hand, 118 ran 17 tooth #25 chain on their 2016 bot (I was looking at their CAD too). It also seemed that 1678 ran #25 chain, though they did use 6" wheels instead of 8". I was planning to use #25H chain instead of #25, if that helps, but if the failure point is the sprocket, this probably wouldn’t help.

My anecdotal experience with chain is that sprockets are extremely rarely a failure point - this usually happens when teeth shear at stall (doubtful you even have enough torque for that) or when teeth skip, gradually wearing the teeth down. Chain instead most often breaks under shock load - say, if you were to drive into a brick wall at top speed. You can circumvent this by using 25H, adding some form of shock absorption (i.e. springs, bumpers, pneumatic wheels) to the system, and by making sure the chain isn’t excessively tightened.

I would second the issues with the 16t sprockets but have used the 22t sprokets without problems. In my experience(only 4" and 6" wheels) 25 chain is plenty strong. I do run tensioners, but they are not sticky necessary. IIRC 254 has run 25 chain without tensioners the two years.

If it’s just the slack chain (and not the chain as it rides on the sprocket) then just let it ride…

We’ve done this with wood and plastic bellypans without issue as the contact force is so low. Anyone running chain in tube is likely rubbing along a good length of the tube wall as well.

We ran a versachassis frame with 122 tooth #25 sprockets in 2017. We cut out a square around the sprockets, but left the belly pan intact between axles. The chain rubbed on the (lexan) belly pan a bit, but not so much that it caused any serious friction.

If you’re really concerned, run some smallaluminum angle parallel and inboard of the chains from front to back of the frame and attach your belly pan to that instead of the drive rails.

(Emphasis mine)

To the OP:
The sprocket/chain size combo above is the same per 254’s technical binder.* Your setup uses 18t #25 sprockets which have a slightly larger diameter.

I can think of a few ways to get the sprockets to clear, each of which raise the wheels relative to the tubing as much as possible (list in no particular order):

  1. When using the WCP Side Bearing Blocks, your limiting factor will be the 1.375" boss running into the 1/8" tubing wall. If you’re willing to machine ~.045" off the inner walls, you can get ~.027" of clearance between the chain and the belly pan. I did a quick OnShape Sketch to verify this concept here. For this sketch, I assumed VEXPro’s effective OD with chain values were correct.

  2. Use the WCP bearing blocks + 1/8" wall tubing and simply shim/space the belly pan from the tubing to get appropriate clearance.

  3. CNC machine/laser cut/waterjet plate “bearing blocks” similar to what 973 and 1836 have been running in recent years.

  4. Machine bearing holes directly into the tubing. This method requires experimenting to determine the correct center-to-center distance offset for proper chain tensioning.

*Linking here for newer folks who may not know where to find the binder. I see you’ve already used the 2016 254 binder as a reference.

Thanks for your help! This is the method I ended up going with. I had CADed a drivetrain like this immediately after 1836 posted their design on CD (as a test) but I was worried that we wouldn’t be able to immediately machine 1/4" plate on our router, since this would be the first year we have a CNC router. I don’t think that’s as big of a concern as I had initially thought.

I am somewhat concerned about not being able to tension the chain. Last year, we used VersaBlocks and had to retension chain very frequently with our #35 chain drivetrain. We had initially calculated the center to center and placed the VersaBlocks accordingly, but the chain “stretched” relatively quickly.

With this method, would we have to use a center add? If so, how do teams determine how much to add? Also, were we doing something very wrong last year? Do teams usually have to retension chain regularly?

While 118 did use #25 chain with 17 tooth sprockets in 2016 we had a lot of trouble with it. During the build season we were breaking chain or sprockets almost daily on our practice robot. The biggest contributor to this issue was the large difference in diameters of the sprockets and the 8" wheels we were running.

At our first event we replaced the 6061 AL sprockets originally in the drivetrain with custom 7075 AL versions. We also replaced the #25 chain with #25H. Lastly, we added a second chain run connecting the center wheels inside the gearbox, this additional run isn’t shown in the CAD model. This allowed us to go the entire competition season without any drivetrain failures.

Disclaimer: I have not made a WCD with plate bearing blocks. I’m just reasoning through the problem based on what I know.

Yes, you will most likely need some kind of center distance offset. I recommend keeping your mounting hole pattern nominal and adding the offset to the bearing plates. You can determine the offset value needed by making and testing with a range of bearing plates with different offsets. I have seen values on the order of ~.020" for #25 chain floating around. I’m sure if you look search hard enough you’ll find someone’s magic number for #35, but there’s no substitute for doing your own testing.

Be mindful of your fastener fits and tolerance stack-up. The actual center-to-center distance achieved is dependent on the tolerances of all the components in the system, and this setup has two extra components relative to a bearing block-less WCD with exact center distances.

Did you use the VersaBlocks in conjuction with the WCP Cam? It’s possible that your chain wasn’t actually stretching appreciably, but rather the blocks themselves were sliding back due to the lack of cam.