10-07-2016 19:57

Cash4587

Re: pic: ABS-122, shifter-in-tube chassis

It doesn't look like you have any form of chain tensioners on this setup. Although you can do without them, I would still really recommend putting them on even in the case of using large tooth count sprockets. We used a setup similar to this in terms of chain, and really wished we had designed a way to tension the chain during the season.

10-07-2016 20:08

asid61

Re: pic: ABS-122, shifter-in-tube chassis

We used 16t #25 sprockets all season with only a single jump at SVR (and that was after another competition in Utah) and that was after the chain got very loose. It's possible to add tensioners in the form of versablocks or even WCP sliding bearing blocks, but I like the set-and-forget of direct C-C. Are you certain that your sprockets were correctly aligned? I'm guessing that they were and you guys just did much harder driving than us, but it never hurts to check.
With massive sprockets such as these I'm not as worried about the chain coming off as much as I am about the chain picking up tape. I could swap these out for #35 chain sprockets, but the extra weight isn't worth it to me. In-season if it became a problem switching to #35 chain would be my first choice.

10-07-2016 20:31

Cothron Theiss

Re: pic: ABS-122, shifter-in-tube chassis

I'm really impressed with how compact this is. What size are those sprockets, by the way? I think you'll be fine leaving it at exact C-C, but since the chain is outside of your tube, it'd be easy to add a static tensioner after your first or second competition. Just for peace of mind you might want to print something that fits around or on the top of your motors to ensure the chain doesn't rub against the motors in the worst situations.

10-07-2016 20:35

asid61

Re: pic: ABS-122, shifter-in-tube chassis

Thank you!
The sprockets are 38 tooth versasprockets from Vex with a minor diameter of just over 2.8". They are inline with the CIMs, so I'm not too worried about the chain hitting. In the past 115 has used polycarbonate chain guards, so if there was a problem I would stick those in.

11-07-2016 00:24

Gregor

Re: pic: ABS-122, shifter-in-tube chassis

I like this. I think it would also be a neat way to do a single speed drive with a PTO, not super clunky like most PTO setups.

11-07-2016 00:49

asid61

Re: pic: ABS-122, shifter-in-tube chassis

I was thinking about making a PTO option of this. With the Versadog it becomes a lot easier too.

11-07-2016 17:56

Cash4587

Re: pic: ABS-122, shifter-in-tube chassis

We never had issues with it slipping or jumping or breaking or ever coming off. We just find it very hard to write fast, accurate and repeatable autonomous modes with so much slack in our chain. It gives semi un repeatable results in our vision code when only the front 2 wheels move while the back 4 don't having to make up the distance in chain slack. It would have been much better to just run tensionors from the start.

11-07-2016 19:31

asid61

Re: pic: ABS-122, shifter-in-tube chassis

I see, that's a good reason to run proper tensioning.
How did it cause errors/to what extent? If two wheels are moving, theoretically the whole bot should move anyway. Would direct driving the center wheel help?

11-07-2016 21:56

Cash4587

Re: pic: ABS-122, shifter-in-tube chassis

So this year our robot had about 60% of our weight on the front wheels. Direct driving the center wheel or any wheel might have helped. We ran a single gear reduction 12-66t and then a 12-22 #35 chain reduction to the wheel. (We later switched from 12-22t to 12-28t and is much better) In our case, direct driving any wheel may have helped but we really enjoyed this setup. We would get no movement on our robot for about 1/8th of a turn on the back wheel to catch up in chain slack. The whole robot wouldn't move because we had tons of turning scrub also so I guess a few things would have helped. But in any case, the chain slack and slop in the hex shaft really was a pain when trying to tune our code. I guess that's why 971 goes through so much to get all of the slack out of everything and makes custom hex.

11-07-2016 22:51

GeeTwo

Re: pic: ABS-122, shifter-in-tube chassis

If 60% of your weight was on the front wheels, I would recommend (other things equal) that you drive the front wheels directly; this is ultimately the same reason that West Coast drives the center wheels directly - to make the drive to the most dependable wheels invulnerable to a broken chain/belt.

On the other hand, if 60% of my robot's weight were on the front wheels in a static configuration (and presuming I had at least six wheels), I would be VERY worried about the robot falling on its face in a braking maneuver.

12-07-2016 00:18

MichaelBick

Re: pic: ABS-122, shifter-in-tube chassis

That is completely dependent on CG height and, given 4587's short robot, probably not a major risk.

12-07-2016 01:56

Cash4587

Re: pic: ABS-122, shifter-in-tube chassis

We had #35 chain direct driving the first wheels. Given the strength of 35 chain, and the fact that we put the chains in line, as well as calculated the direct c-c we had no issue other than chain stretch which is to be expected. We had all 6 drivetrain motors above those wheels accounting for the rest of the weight minus the catapult and battery.

With this being said. After 2 regionals, Champs, and an offseason already we never replaced 35 chain, or 25 chain. We were never worried about it breaking either. It is just something that doesn't occur that often if chain is used correctly, which we did, just without tensioners. Which in our case worked very, very, very well, and in many cases would be fine for most every other team in FRC not wanting to do some of the precision complex things things we need to get our code really dialed in.

12-07-2016 21:19

chrisfl

Re: pic: ABS-122, shifter-in-tube chassis

Looking at the size of those tubes, would you be able to put the wheels inside them to maximize space used in the tubes? That would also allow you to move your side rails out more to create more space in the center of the frame.

12-07-2016 21:23

pwnageNick

Re: pic: ABS-122, shifter-in-tube chassis

The two wheels directly driven by the gearbox have to be outside the tube as the gearbox is inside the tube already there. The other wheels could be put inside the tube, but that doesn't really do anything for you.

12-07-2016 21:28

Monochron

Re: pic: ABS-122, shifter-in-tube chassis

You know, now that you have the gearbox AND the wheels in that tube . . . you could probably get the RoboRio and the VRM in there if you get creative . . .

12-07-2016 21:44

Chak

Re: pic: ABS-122, shifter-in-tube chassis

How to Give Electrical a Heart Attack, part 3

12-07-2016 21:56

frcguy

pic: ABS-122, shifter-in-tube chassis

I do electrical, +10000000000

12-07-2016 23:57

GeeTwo

Re: pic: ABS-122, shifter-in-tube chassis

I know it sounds strange, but if this is the case, I would seriously consider not driving the back wheels at all -- save the chain and sprockets and heartache. I might even consider skipping the back wheels and putting some pegs or furniture gliders. Here's why: If 60% of the weight is on the front wheels, the CoG is closer to the front wheels than the middle wheels. If the CoG is low enough that you are "not at major risk" of braking hard enough to tip forward, you are probably "at minimal risk" of ever needing to put weight on the rear wheels, and at "fuggetaboudit" for need to drive those rear wheels.

13-07-2016 00:45

Cothron Theiss

Re: pic: ABS-122, shifter-in-tube chassis

Interesting idea. From what I can tell, unpowered wheels used to be more common on early FRC drivetrains, but have fallen out of style in the past several years. It would probably work well in this case, but I do see one thing that might be a concern. In a six-wheel drop center (which might or might not be what we're talking about, I don't know 4587's robot), doesn't the robot tip backwards onto the back four wheels when driving forwards? Now I know that the position of the center of gravity affects the tilting of the chassis, but this would be on a case by case basis. If the chassis DOES tilt backwards when driving forwards, not powering the back wheels may make acceleration quite slow.

13-07-2016 01:13

MichaelBick

Re: pic: ABS-122, shifter-in-tube chassis

The major issue with this (for my team) is pushing matches: for example, during forward pushing you can easily have 100% weight on the back two wheels.

13-07-2016 01:27

Clayton Summerall

Re: pic: ABS-122, shifter-in-tube chassis

The back wheels are solely used on defenses. Happy we didn't go 4 wheel.

13-07-2016 02:15

Cash4587

Re: pic: ABS-122, shifter-in-tube chassis

Having un-powered wheels in your drive train is generally not a good idea. They create friction and make it really, really hard to turn. Replacing them with a caster or some low friction nylon for example, is also not a great idea because this year our back wheels help us get over defenses, and when you put something in place of a wheel that doesn't roll, and hit defenses as hard as we do, it won't turn out so well. Removing wheels and just replacing them would not be as good as just designing a drive train with 4 wheels only. In cases like this year, we needed 6 wheels to not get stuck on defenses so that's what we went with.

When it comes to drive trains we go with something we are confident with. 6wd and 8wd are types we've worked with before and have given great repeatable results. Adding furniture sliders or caster wheels to our drivetrain is probably something we will never consider doing on our drivetrain.

13-07-2016 02:17

Bryce2471

Re: pic: ABS-122, shifter-in-tube chassis

To OP:

I recognize that this is a mostly theoretical drive train but have you thought about how you would remove a CIM, if it were necessary?

13-07-2016 02:34

Cash4587

Re: pic: ABS-122, shifter-in-tube chassis

It looks like he oriented the motors in a way that you can access the bolts around the gears. But I am just assuming so I could be wrong.

13-07-2016 02:40

asid61

Re: pic: ABS-122, shifter-in-tube chassis

I actually added a lightening pattern later such that a ball end allen key can access the bolts to remove the CIMs. As it stands right now, removing the gear in between the two CIMs would suffice to remove them.

13-07-2016 02:48

Bryce2471

Re: pic: ABS-122, shifter-in-tube chassis

I guess the question I was trying to get at is: How would you remove the idler gear with the bearing flanges to the inside of the tube?

13-07-2016 03:22

asid61

Re: pic: ABS-122, shifter-in-tube chassis

...
Good point. That raises the other question: how would the shaft get in there to begin with?
The lightening holes might let you get away with not removing the gear. But IRL if I made this, all bearing flanges would be on the outside and/or I would use riveted bearing blocks to save space inside the tube. That particular spot is actually easy to put at least one of the bearings on the outside because I have space in there.