PIC: Looking for feedback on our drivetrain design


Hey, we are team 6000 from Shalhevet Highschool This summer we would like to build our first Westcoast drivetrain since we have only used the KOP drivetrain in the past. It uses HTD belts and pulleys utilizing parts we can salvage from kop drivetrains. We wanted to explore the idea of a Westcoast drive without chain and this is our current design. We are looking for any feedback before we begin fabrication in the coming weeks. It is based on VexPro VersaChassis and is similarly sized to a kop square drivetrain configuration. The gearboxes are Evo Slims from andymark. Any and all feedback would be greatly appreciated. Here is the link to our STEP file: https://drive.google.com/file/d/1iMk-M5696doOvtPa07vsywasQExNiMAQ/view?usp=sharing

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What I think you’ll find about most true WCDs is that their drive wheels are cantilevered. In your case, that would mean removing the redundant drive rails on the outside of the frame and creating a “pulley-driverail-wheel” sandwich. This opens up your chassis quite a bit more, however bumper mounting requires more thought.

In addition, I am not sure of the tolerances you can hold, but looking into VexPro Bearing Blocks could be beneficial to ensure your drive belts are properly tensioned.

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Thank you. We considered leaving the outer rails open but decided to leave them on for bumper mountings. Because of the similar size to our past drivetrains our bumpers would be reusable with the outer rails. We have used bearing blocks in the past but our tolerances should be pretty tight. all segments are short enough to fit on our Shapeoko XXL so ideally the belts would be as tensioned as they would be on a kop chassis. Would idlers work for this application as well?

What are the advantages this offers over the KOP chassis?
I would mount the gearbox off the siderail to put the belts inside the chassis, and remove the outer 2x1 like Varun said above. Also, put the gearbox in the center. Otherwise if a belt breaks (which is very possible because you are double-loading one of them) you will be unable to move.

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Instead of the Evo Slim, you might be interested in the performance gain of 6 NEO’s or Falcons running through this upgrade kit

Aside from that, I’d echo the questions from @asid61 above

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The advantages we see are in robustness since we took a defensive strategy this year and found that your sheetmetal drivetrain would bend on impact. This also allows for ease of mounting when it comes to electronics because a single pan can be mounted to the bottom. we also wanted to use 3 Cim gearboxes for more torque.

We considered mounting the gearboxes in the middle and have an alternate design that does that. Would you mind explaining what you mean by double loading one of the belts? Also we have had wires catch the belts before which is why the idea of a separation between the belts and the internals of the drivetrain is appealing.

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Thank you for the recommendation. We are a bit hesitant to use brushless motors be we have managed to burn out cims due to stalling but we will definitely look into this option.

Were you able to do some root cause problem solving to identify why you were stalling and burning out your motors? It isn’t that easy.

Pushing robots with more toque. We have a very aggressive drive strategy.

You have all of your power to the front wheels and the middle wheels going through your first pair of belts. If you put the motors in the middle, then front and rear are driven independently.

[edit] I concur with pretty much everything R.C. suggested, esp. with using brushless and skipping the belts, and using bearing blocks + cam + chain (we make our own blocks, but versablocks are good too).


You may notice our gearbox, due to the small size of the NEO’s, has them sit above the side rails rather than taking up space inside the main frame

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Sam,

If you are going away from the KOP chassis I would recommend these things. TL;DR this is what we’ve been doing on the MCC/CC for the last couple of years and has made building a custom chassis a breeze.

  • 6wd or 8wd drive
  • Omit the outer rail
  • Inner rail needs to be one piece, would not split this part
  • Move the gearbox center
  • Brushless is a good option, I would personally use this or this with Falcons
  • I would add a bellypan of some sort: wood, metal etc…
  • Skip the belts, use versablocks + cam + chain.

This page has robots that use the versachassis over the years.

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Chiming in from a more local team (1197):

If you guys want some help, drop us a line. We’ve been running a custom WCD for years, and it hasn’t failed us yet.

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We actually ran a very similar drivetrain to this in 2019, not our finest design. I also suggest a WCD design or a kitbot.

I want to throw my thoughts here on custom drivetrains- If you are not able to prototype a new drivetrain style or design before season, I highly recommend sticking with one of your team’s past designs or a kitbot. Kitbot is an extremely reliable option and you should not hesitate to use it. It’s know to perform well and you can assemble it and get it running very quickly. Speaking from my own painful experiences, you don’t want to be fiddling with a brand new drivetrain design for weeks in build season instead of spending time iterating your scoring mechanisms when the kitbot is given to you every year. West Coast drives and other systems have benefits over kitbot, but they are mostly negated if you can’t get it built and running until week 4 of build season, when the kitbot can be done in a weekend.

Thank you for your insight. We are beginning this summer to decide if we will pursue the design in build season. We have had problems assembling the KitBot so we wanted our own design that some of our team members will fully understand and be able to teach. This will ideally be a quick assembly as well allowing for a moving drivetrain in week one of build season. It of course has its own benefits as well as it pertains to torque and robustness.

What problems were you having with assembling the kit bot?

I am sure there are tons of people here that could help you to solve those problems.

I would argue that a WCD isn’t much more robust than a kitbot. The kitbot is extremely robust because of the parallel plate design. Also, if you upgrade the wheels on the kitbot or change the ratios in the Toughbox gearboxes or use brushless motors you can get just as much torque from the motors as you can with other gearboxes and options on the market.

Somebody cut the wrong side of the drivetrain attempting to make a hole in the front for the balls to enter. Thankfully we had another drivetrain that we were able to take apart but replacing that part could’ve taken a long time(taking apart the other drivetrain took time too) but we have a lot of versaframe available if a wrong cut were to be made.

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Thank you for your feedback. Are those rails custom? I have not seen versaframe with holes on that side.

4272 ran a chassis very similar to this in 2018, and we run a chassis somewhat similar in 2019 and 2020.

West coast drives with cantilever wheels can be really nice, but I have always really liked the idea of supporting the wheels from both sides. I feel as though it works better with belt driven drive-trains because generally I don’t like to cantilever those large Andymark pulleys.

We also have driven the back wheel on the drive-train before, without any problems. You would load the belts a little more, but I can say we have never had that issue.

Now a couple things I would suggest:

The way you have the gearbox mounted on the inner rail is certainly interesting, but it seems like a pretty weak point in that rail. I’m not sure that it is worth the extra inch of inside frame you are getting.

You are using hex shaft for all the wheels. I actually like using a 3/8" bolt a lot more in this application (like the kitbot does). It makes it so you don’t have to tap holes inside the hex shaft to hold it in place.

Now tbh, I’m not sure that this chassis is going to preform any better than the kitbot but I imagine you wont have too many problems (assuming you can keep your center distances on the holes right). If you have the time in the off season, I certainly see the education benefit to a simple custom chassis like this, but it totally depends on your team’s goals.

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I’d echo what most people are saying. Cantilever the wheels off of a single tube, or stick with the KOP. By blending the two designs, you’ve got none of the advantages of either, and all of the drawbacks of both.

In particular, you should take out that jog in the inner rail. The stresses in that tube are not going to transfer well across the gussets. Slamming the wheels against the rendezvous zone dividers is going to lift and twist that joint and make you sad.

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