19-07-2016 23:41

cad321

Re: pic: 25lb Drivetrain

Is there a reason you chose not to use bearing blocks on your drive wheels? Although I haven't built a WCD before, my understanding is that their benefits far outweigh the cons (the only con in this I could see is possibly weight as you're limiting it to 25lbs).

19-07-2016 23:48

Sperkowsky

Re: pic: 25lb Drivetrain

By removing the crossmember 2x1s in favor of the bent sheet metal you are saving a bit of weight. But it is not as strong and you are more limited in mounting points. The few pounds its saves is just not worth it imo.

19-07-2016 23:48

Gregor

Re: pic: 25lb Drivetrain

"Taking a year off"

19-07-2016 23:49

asid61

Re: pic: 25lb Drivetrain

The other cons are cost, size, and time. If I had the gearboxes (and sheet metal brake) I could make this very quickly. It's possible to re-use the bearing blocks for sure, but the "set and forget" method of direct C-C chain is great.
From a machining standpoint, depending on which bearing blocks one is using, this can also be easier to machine than a "regular" WCD with bearing blocks.

19-07-2016 23:54

bkahl

Re: pic: 25lb Drivetrain

A cross section of a C is almost negligibly weaker than a box. In this scenario, a single piece belly ban and front and back members is actually extremely rigid.

On top of that, a single-piece bumper could add even more strength.

By your logic, would not all drives with a similar single-piece belly pan and cross-members be weak, and limited in mounting? IIRC 971 uses a similar style design. What about the kit frame? It uses C-channel cross-members too.

EDIT:
Another reason for this design may be for maintenance.

With the one-piece belly pan and members design, you "cap" the end of your drive tubes, making it hard to change a chain in an (unlikely) emergency. Being able to pull bearings from the tube may allow you to slide a chain in without taking the belly pan off (even though it still sounds like a nightmare).

20-07-2016 00:04

Knufire

Re: pic: 25lb Drivetrain

Yup!

There's two main benefits to using sliding bearing blocks: to better support the bearings and to be able to tension the chain. I'm not a big fan of adding tensioners where they might be unnecessary, and had pretty good luck running exact C-C #25 chain drivetrains in 2015 and 2016.

So the question is then, is putting the bearing straight in the tube sufficient support? This is game dependent; large impact loads on your wheels could cause the bearing holes to start to turn into ovals. However, for most games which have no or relatively small field obstacles, bearings straight in the tube should be fine.

20-07-2016 00:23

Harrison.Smith

Re: pic: 25lb Drivetrain

Past 2 years we have ran WCDs. 2015 we ran C-C #25 and we had no outer support with the bearing going straight in the tube. That worked just fine, although RR wasn't very taxing on drives. This year we had a hybrid C-C #25 in tube and C-C #35 on the outside of the tube. We had 3/16 stiffener plates welded to the tube that had couterbores for the bearing. While the plates were there to stiffen the tube since we had a #35 chain run on the outside of the tube resulting in a large slot on the end of the tube, we had no problems with the bearing holes turning into the ovals that likely would have happened if the plates were not present.

20-07-2016 00:25

bkahl

Re: pic: 25lb Drivetrain

https://www.chiefdelphi.com/forums/s...d.php?t=146519

20-07-2016 00:58

Knufire

Re: pic: 25lb Drivetrain

Lol, that's exactly the picture I was thinking of. Glad to see that people reported in that thread that bearings in .125" tube survived Stronghold.

20-07-2016 09:02

Chris is me

Re: pic: 25lb Drivetrain

Putting bearings straight into 1/8" wall tubing is just fine, it's only 1/16" wall tubing you really have to worry about ovalizing with. You should be fine there.

I'd consider riveting the bearings in instead of screws, just because I hate 4/40 screws and breaking taps and stuff. I guess if you have to change out a bearing there will be some rivet scraps inside your tube and that's kind of nasty.

I agree that the one piece belly pan / C-channel member will make this chassis likely rigid enough to perform well, particularly with stuff on top of it, but I wouldn't say a C-channel is "negligibly weaker" than tubing. It's certainly got substantially less resistance to bending. In this case, it's somewhat offset by the fact that it's got a giant bottom flange connecting it to the other side, and that the C-channel hugs the inside of the drive tube. This style of drive has certainly been battle-tested before.

I think this drive would get some strength benefit from making the top flange longer. Just an inch long is a little wimpy as a C-channel, and it could kink in the middle if hit really hard (and the bumpers aren't backed up with their own frame etc). I would just extend the whole flange to the length that is present at the points where it attaches to the frame. You're already using that material anyway; what's another few ounces per side to resist bending on impact a bit better. Especially if the belly pan is .090.

20-07-2016 09:51

Ty Tremblay

Re: pic: 25lb Drivetrain

I wouldn't even bother with holding the bearings in the tube. If you used screws and washers on the ends of the axles (or properly toleranced snap ring grooves) the bearings will have no where to go any way. 319 tapped the ends of our thunderhex shafts 1/4-20 and ran them all season without issues.

20-07-2016 10:26

JesseK

Re: pic: 25lb Drivetrain

The bellypan design is quite interesting. If it were the basis for a kitbot, it would turn the kitbot frame into a 7-piece rivet operation.
It's like the AMU chassis found a way to become WCD.

20-07-2016 22:00

Akash Rastogi

Re: pic: 25lb Drivetrain

Meh, I've done it before. You don't miss out on much if the rest of the bot is planned out with enough attachment points. And the rigidity is there with the bellypan.

20-07-2016 23:40

GeeTwo

Re: pic: 25lb Drivetrain

I do like the concept, and the weight economy! It's designs like this that make me want the team to invest in a sheet metal brake, probably even ahead of a lathe. If we just had the space for some real shop machines (32'x24' for storage and workshop and programming and maybe everything else).

While c-channel is not as strong as tube (especially in torsion, and significantly in around the axis perpendicular to the "missing" face), I do not see this as a showstopper - as previously noted, the 2014-2016 kit bots used folded c-channel to rave reviews. (3946 purchased some 2015 frames on closeout that we plan to use in 2017, or in 2018 on the low-percentage chance that we decide to go another route.)

There is plenty of room for attachment points. The top of the front and rear rails, and the portions of the tube which do not have chain behind them can be perforated as needed.

I understand that chain-in-tube is usually "set it and forget it" over the course of an FRC season, but with the capped tube ends, this design does not seem to have a good way to swap out the chains if disaster should strike. As a first mitigation, I would probably NOT perforate the drive tube for mount points, but rather mount a c-channel or thick-wall tube of versa-frame in a location which would allow a side tube, wheels, chains, gearbox, and motors to be removed as a single unit without removing anything else but the bumpers, or design "everything but" the drive chassis and things mounted to the belly pan to be easily removable to allow drive rail removal and repair.

Finally, This was not a design point, but someone brought it up: I don't see how this could work as a kit frame. Kits usually ship in a box roughly 36" x 8" x 8", but this belly pan unit would have to ship as something rather larger. Even more critically, there is no good way to make this so that a team can select "long/square/wide" using a band saw. (And OBTW, the kitbots since at least 2012 have a LOT more options than are listed in the manual, if you are willing to put in a bit of skull work.) If you skipped the belly pan, much more is possible, but that misses the key feature that makes this such a weight saver.

21-07-2016 08:16

notmattlythgoe

Re: pic: 25lb Drivetrain

Man, if only boxes came in other sizes.

21-07-2016 08:33

Cothron Theiss

Re: pic: 25lb Drivetrain

Just because I'm curious and I've never worked with belly pans before, what advantages/disadvantages do they offer? I've heard that they add a whole lot of torsional stiffness to a chassis in a more weight-efficient way than a cross bar, but other than that I'm not sure.

21-07-2016 08:34

notmattlythgoe

Re: pic: 25lb Drivetrain

Also gives you a place to mount electronics.

21-07-2016 09:36

ASD20

Re: pic: 25lb Drivetrain

And in a game like 2016, they protect the bottom of your robot.

21-07-2016 10:45

Deke

Re: pic: 25lb Drivetrain

Think of the belly pan like a bunch of strings or cables attached and tied everywhere at the base. It provides additional stiffness when the frame moves away from the belly pan and creates tension, but is very weak when in compression. Since it is attached all the way around, some part will always be in tension, providing more stiffness in the frame.

It is just one way of many ways to add stiffness to the frame. A disadvantage is that it is time consuming to make if you plan out all your electronics and place the holes for tapping and bolting then down. Also, depending on the rest of the frame members may not be necessary. In a case like this, where the frame members are light, the belly pan can give the stiffness needed to perform like a heavier drive train.

21-07-2016 12:37

notmattlythgoe

Re: pic: 25lb Drivetrain

Using something like this allows you to create a bellypan very quickly and allows for flexibility in regards to electronics layouts.

21-07-2016 12:43

Chris is me

Re: pic: 25lb Drivetrain

In terms of easily laying out electronics, a perforated polycarbonate belly pan is an excellent choice. In terms of structural rigidity, a large thin sheet of polycarbonate is not ideal. Something like Baltic birch plywood would be a better choice for a combination of rigid, lightweight, and flexible for mounting electronics.

21-07-2016 15:50

Cothron Theiss

Re: pic: 25lb Drivetrain

Thanks to everyone for the info! I don't think my team really has the resources to make a bellypan, but it's always good to learn.

21-07-2016 16:14

Knufire

Re: pic: 25lb Drivetrain

5188 used baltic birch plywood last season. It really doesn't get much easier than that. Just cut a square the size of your frame and rivet to the bottom. We got a 4'x8' sheet from the local Menards.

21-07-2016 16:35

Cothron Theiss

Re: pic: 25lb Drivetrain

Oh, you don't use a lightening pattern if you go with a plywood option? Nevermind, we could do that. (Now that I think about it, that should have been obvious.)

Though, my team has very strong aluminum welding capabilities, so we will probably stick with what has worked for us so far. Also, our robots have weighed in at about 100 pounds the past two years, so we should probably work more on increasing our scoring capability than trying to fix problems we've never had.

21-07-2016 16:45

ASD20

Re: pic: 25lb Drivetrain

You could do a little, though it probably won't save you much weight at all.

21-07-2016 18:03

Knufire

Re: pic: 25lb Drivetrain

Nope.

https://scontent-lga3-1.xx.fbcdn.net...b2&oe=5821B827