Hi, we are Team 4099 from Poolesville High School. This year, for the first time in four years, we have decided to build a custom drivetrain. Our design is heavily based on the VexPro VersaChassis. This is our first ever West Coast Drivetrain, so we’re really excited! Any and all feedback is greatly appreciated. Here is the link to our STEP file: Drivetrain v44.step - Google Drive
This looks like a very clean design and is very similar to the basic chassis we used for our recent motor testing.
You might consider adding a cross bar in front of or behind the gearboxes to tie the two sides of the frame together. With the offsets, gearboxes and motors you have a lot of overhung weight that will be trying to flex the two side rails.
With those specific gearboxes, you can eliminate a huge amount of sag by bolting a piece of 1"x1" or something of that nature through the mounting holes on the top from one gearbox to the other.
What are the specs?
–free speed (estimated)
–gearing
–wheel size
–overall weight estimate
Just a few items that will help in giving feedback; it’s kind of a standard practice to state a couple of those to give some idea of how a given drivetrain is likely to perform.
I am designing something similar. Would churro shaft do the job for this?
Sure! You just have to be careful that it is actually possible to assemble. Having the churro constraining the two gearboxes like that may make future repairs difficult.
I can’t see the .step files, just the render.
+1 on keeping side rails aligned (and esp if churros, locate as high as you can manage, because that’s what will fight the problematic torque).
The combination of closed gearbox and open gearbox uses a lot of width, especially for a CIM drivetrain. Where/what are your stages, gearing and shifting? (If you’re not shifting, those gearboxes are far too deep.)
I’m curious why, with a 4-CIM drivetrain, the CIMs aren’t at the same altitude. Not really a problem, just a curious decision.
This is the VexPro 3 Cim Ball shifting gearbox. I am unsure why 2 of the cims are missing.
Probably because the 2-CIM version does not have a West Coast-style third-stage available. The 3CIM shifter is not that much bigger; it’s a reasonable thing to do.
Drive looks good; nice to see another MoCo team going down the WCD route. I’d recommend belts over chain, but that’s just personal preference.
While I don’t know what gearbox you are using in your design, there is an extremely easy way to integrate churro in WCP or similarly designed custom gearboxes in such a way that doesn’t make maintenance any more of a chore than it already is
https://i.imgur.com/kj7NIL2.png
At the bottom of most WCP gearboxes there two rather large standoffs connecting the two gearbox plates. I believe the plates are drilled for 10 bolts (.190"), but they can easily be drilled out for 1/4-20s . Tap the ends of the churro for 1/4-20 and, with a fairly long bolt, run it from the gearbox to as far as you can go on the churro. Do this on both sides. The great thing about the design of the WCP gearboxes is that there’s a countersink in the gearbox plates that the standoff rests in, so with the top standoffs tightened, the bottom one shouldn’t easily fall out regardless of whether that bolt is in. And removing the gearbox, if you’re using generally accepted methods of mounting WCP gearboxes, requires that you would remove that bolt anyway, so adding a churro to it does nothing to change the process of removal. Just remember that on the outside wall of your drivetrain rail, drill are fairly large hole (1/2-3/4") in line with the bolt so you can easily access and remove that bolt whenever necessary.
If you are using a WCP gearbox, I highly recommend do this as long as it doesnt interfere with any mechanisms or electrical. If you are custom-making a gearbox with a similar profile, I would recommend you consider utilizing that standoff design to do something similar.
While I’m sure that’s excellent advice for teams using an entirely different gearbox than in this design, the pictured gearbox is a 3 CIM Ball Shifter. The assembly concerns presented in using churros to connect them together is due to the large counterbore in the mounting points for churros / thunderhex / etc; while they secure the gearboxes very well, these counterbores can make it difficult to add this crossmember after attaching the gearboxes to the frame and even harder to remove one of the gearboxes from the frame. A piece of aluminum angle or something across the top can be easier to remove, despite being less “clean”.
Here is MORT’s 2017 drivetrain being assembled during the build season with the same gearboxs.
https://drive.google.com/file/d/1yDseZysHvzNV5rUu2QndmZo_Z2IrxSay/view?usp=sharing
https://drive.google.com/file/d/0B537Ypyc4lhHcU5qbUp2T1ptQ0xJaUpSSnZqQVJjU3c2Q21n/view?usp=sharing
You can see in the second picture that the bolts are not tightened yet and the gearboxes have significant sag.
Thanks! This was something we were in fact already considering, especially because depending on the game, we may add an opening to the drivetrain.
We’re using the VexPro 3 CIM ball shifter, with either 2 CIMs or 3 Mini CIMs (we’re leaning towards switching to Mini CIMs).
The estimated free speed in high gear is 22.33 fps (4.17 : 1), and in low gear is 10.32 fps (9.01:1). We are using 4 inch Colson wheels. I am unsure of how to provide you with an estimate of the weight of the drivetrain (I’m not a CAD member).
To make sure I’m understanding what you’re saying – you are talking about the holes on the top face of the gearbox?
Yeah, that’s basically exactly what we were thinking. We picked chain to give us more flexibility, so we could make it an 8WD instead of a 6WD or something without having to buy new belts. Basically, we want to be able to order parts early.
Whoops – I think we accidentally posted a broken link. The correct one is https://drive.google.com/file/d/1x4lqKBTQV_Kil7Ug2ST6dEpHx6oFLgcW/view
Thanks! We were actually already considering doing something like this, depending on the game. We thought that it would be especially helpful to add a crossbar if we wanted to do a frame opening for an intake or something like that. Would you say that we should add a crossbar even if we do not implement a frame opening?
To be clear, are you referring to the holes on the top face of the gearbox, or the ones near the top on the inside face?
Free speed in high gear: 22.33 fps (4.17 : 1 with either 2 CIMs or 3 Mini CIMs)
Free speed in low gear: 10.32 fps (9.01 : 1)
We are using 4in Colson wheels.
Unfortunately, I am unsure of how to provide a weight estimate (I’m not a CAD student and don’t know the first thing about how to get this information from our model).
Yeah, that’s basically exactly what we were thinking. The reason we chose chain over belts is so that we could start putting together a drivetrain quickly even if we decided to go with an 8WD instead of a 6WD or anything that would lengthen or shorten the wheelbase.
I have a few questions that anyone can probably answer. I’m getting my team to do WCD style using vexpro versa chassis. So here are my questions, 1. should you uses rivets for the gussets and if so what size. 2. what are the best size colson wheels to buy. 3. How do you decide between a shifting and single speed gear box. 4. 6 wheel or 8 wheel.
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I would recommend rivets, because the number of fasteners you need will add up quickly, and rivets are much faster/easier to install than bolts. The VexPro gussets are made for 5/32" rivets or 8 screws.
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I think most teams go for the 4" colsons. My team used the 4" x 1.5" ones last year and had no complaints.
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Single speed is fine for most teams. Dual speed comes in handy particularly on flat fields, so you can switch between a low and high gear depending on if you are sprinting across the field (ex. to get a gear) or trying to line up with the field element (ex. place a gear on a peg). It is really up to your team and drivers as to what you are comfortable doing.
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I would say that for most FRC applications, 6 wheel is sufficient, and saves you weight and decreases the number of component on your drive train.
In addition to what Kristin said, if you think your programming team is up to the challenge of implementing autoshifting, it’s very much worth it. You accelerate and decelerate crazy fast, with a high top speed, and win pushing matches because you’ll automatically switch to low gear.
Awesome Ill talk with the programers
Using Churro or ThunderHex to span between the gearboxes is doable. There is a counter bore to consider as Chris mentioned. To combat this in 2014 while on 3467, we cut the Churro a slightly short and used two washers to better integrate with the back face of the gearbox.
It could be a pain to disassemble with two small washers. In our case we didn’t have room above to span across the top using the captive nuts and aluminum angle or box tube as other teams like 1678 has done. You can almost see it in that picture.
Tip from experience on versaframe & riveting - pick up a cheapie harbor freight hydraulic air rivet gun and an air compressor if you don’t already have one. You can assemble an entire frame in 1 night with this. Also it will drive you to use rivets in other parts of your robot. Use 5/32" rivets or 3/16" rivets, whichever you can get easier. Versaframe is designed for 5/32" so if you’re using VEX’s gussets just buy a bunch of those.
As for wheel size and gearbox selection, take a look at the JVN Mechanical Design calculator. There are tabs in Excel for plugging in wheel size, motors, gear ratios, etc. that will give you an idea of how fast your robot will go & how much current you’ll draw - https://www.chiefdelphi.com/media/papers/download/4893