pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

I was under the impression that all wheels had the be geared the same for this style of drop down, are you considering the possibility of the traction wheels and the omni wheels touching the ground at one? Also what method are you using to keep the chain tensioned?

Other than that, it looks pretty good.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Looks great!

How far are you dropping the traction wheel that's on the cylinder? I'm wondering the same thing as Electronica1, about the wheels possibly touching the ground at the same time.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Since it's a form of butterfly, theoretically only the omnis or the tractions should be touching the ground at once, therefore they can use the omnis as a high "gear" and the tractions as a "low" gear. That being said, I'm not exactly sure how the front set of wheels switches from tractions to omnis. Maybe the back traction wheel extends down far enough to pitch the robot up on the front tractions.

Also, the chain tension is a valid point. If you're using an axis to rotate the wheels down that isn't the same axis as the sprockets, you're going to have tension issues, which is why most people use gears between the omnis and tractions and use the interior (or occasionally exterior) wheel axle as the pivot point.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

In this revision, the drop was 1/8" and we ran into scrub issues. In the next revision, which I'll posting in a week or so, we increased this to 3/16" which should work pretty well. The omniwheels are off of the ground in traction mode. We haven't done enough testing to determine if much energy is lost in drag, but the idea is that when we are pushing or accelerating in omni mode, we rock on to the traction wheels and gain torque, but these traction wheels don't have enough traction to do this effectively, the next revision uses roughtop and will be able to test this better. We've been driving this for a while, and even with its issues it's a huge improvement over our past drivetrains. As for chain tension, we did exact c-c with no tensioners, and it's been very loose since we made it, however, we have not thrown any chains. We think that the looseness will be fixed by adding a bit (.018") to the c-c distance and fixing some spacing issues that left the sprockets a little loose on their axles. The shifting wheel moves in an arc that is always the same distance from the gearbox.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

No, when the front traction wheel is up, one of the traction wheels may touch the ground. We're doing this a test to see if the extra torque gained is worth the drag from the omniwheels. With these traction wheels it is not, but with higher traction wheels it might be.

The outer wheel does pivot around the axis of the inner sprocket. It pivots far enough to lift the omni wheels off of the ground.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

The reason I bring this up is that every iteration of the rear drop (my name for this style) hybrid drive I have seen has the wheels all geared the same and has a transmission to switch speeds. I believe that in a pushing match/change of direction the piston would give a little and all the wheels would be on the ground.

Please tell me you aren't using 25 chain for this. From what you described, you might run into the issues the mk1 version of my teams octocanum had. It worked fine in the shop, but in a competition setting, the stresses on the 25 chain from shifting caused our chain to stretch every match, we later found that during defense it could even shred the chain. We tried switching to 35 chain that summer and haven't run into the issue since. If your chain is getting loose now, it won't stand a chance at competition.

(Just an fyi, we had this issue with a drive that only had 4 cims of power, I have a feeling that the 6 cims might make this issue more prevalent)

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

We are using 25 chain, it isn't getting loose, it began loose. The tension on the chain doesn't change when shifting with this setup. Once we get the next iteration of this built with the initial chain tension issues fixed we'll be able to test if the 25 is sufficient or not, but teams have shown that it is (118).

Unrelated, here is the CAD: https://drive.google.com/file/d/0B0s...ew?usp=sharing

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

The chain should not stretch in any kind of tilting transmission like this one if you design for it. If you make the sprocket concentric with the pivot point it should be fine.

Also, any kind of tilting transmission I've seen usually gears traction wheels for a lower speed than the omnis or mecanums. It seems like a huge waste to have a shifting gearbox when you have the gear the two wheels together anyway; why add another stage of gears when you don't have to?

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Didn't 118 have double strands of 25 chain like this?

Also, I would suggest some serious stress testing (the way my team did this for our octocanum systems is setting up one of our 6 wheel drives and spending a couple hours having the 2 drives pound each other, it was the only way we could recreate the failure for our mk1 system)

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

What's the plate on the back of the CIMs for?

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

118 has single 25 chain runs, there wouldn't be enough room inside the tube for doubled chain or 35. The plate on the CIMs was for reinforcement and attached to the belly pan, but we decided to not put it in the physical robot.

We definitely plan on doing some hard testing.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Forgot to say this, but since each sprocket is held on by only a bolt, part of the plan with this is to switch out the chain every week or so if needed.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

It's perfectly possible to use 25 chain for an articulating drive module, provided you keep two things in mind:

1. Be sure that the chain run pivots with the axle. Easiest way to do this is to put a double sprocket coaxial with the pivot axle, running one loop of chain from this to the wheel and the other loop from this to your gearbox. Otherwise you could be stretching and loosening the chain with each articulation.

2. You want to be careful with the modules flexing under load, especially if the traction wheel is on the articulated axle rather than the fixed axle. There's some "fun" bending moments at play if you don't have a rigid, well supported module. To me, the best / coolest way to accomplish this with a WCD frame is to use a U-shaped module which almost hugs the drive tube (plus some delrin sliders at the contact points) - that way at least in theory the forces transfer nicely to the frame.

It looks like 503 is doing both of these things in this prototype. Overall, nice job. Looks sweet. I'd consider pivoting on the omni wheel if you can, but that does make gearing really messy and annoying.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

We thought the same thing when we did our first one two years ago. We them found out the stretch was just too much. We were having it replace every match. This made turnaround times atrocious. Chris gave you some ways of reducing the load on the chain, but I would argue it would be enough. Look at 35 chain ASAP.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

This shows how the chain stays the same length:

The shifting axle pivots around the gearbox axle and the chain doesn't change tension. The triangular plate highlighted in the back has the bearing and slides up and down on bronze bushings in the smaller slots.

This shows the delrin piece it slides on:


In the physical shifting mechanisms we powdercoated the linkage plates which makes the shoulder bolts bind up, in the next iteration we're accounting for this with undersized spacers.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Got any video of it?

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

I'll try to get some tonight

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Design looks great, we have a group of kids that want to prototype one of these chassis as well. Keep posting your results as you go so we can learn with you.

Chain stretches over time, it's going to happen.
You can also use pre-stretched chain like the type from US Tsubaki.
Part Number RS25, it's shiny silver, not black like other chain.
We run this chain exclusively and it stretches much less.
And #25 chain is fine if you keep your sprockets aligned.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

I like this drive train concept a lot. We were working on something similar earlier this year and never really got it finished.

I definitely would like to know how much the omni's are in the contact with the ground when shifted to traction with a fully loaded robot and if they slip during pushing matches.

Can you show a picture of the ground clearances in both omni and traction mode. We played around a lot with getting both omni wheels off the ground and it seemed like we would need to move the traction wheel a lot to get that done.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Here's a sketch of it

In reality, the omniwheel still touches the carpet but should still carry significantly less weight. I'll get a picture tonight.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

That's what I was expecting. That's about what we were looking at doing as well.

Our thinking was that since so little of the weight would be on the omniwheel it would slip pretty reliably and not really interfere with anything during traction.

We were also considering putting the traction wheels on dead axles and having the chain reduction on the outside of the frame form the omniwheels to the traction wheels.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

What's the distance from the axle to axle in the middle of the transition?

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

While not a member of 118, I have had their systems explained to me by their students, so I can provide a little insight into why they do what they do.

They like the chain in tube because of the small space savings (and no bearing blocks to manufacture) but mostly because it requires no tensioning. The chain may stretch, but there is nowhere for it to go, so losing a chain isn't possible. Note that this doesn't mean the chain is necessarily strong enough for whatever load you are applying, it just means it won't derail as long as you stick it in a small enough space.

The double sprockets don't actually run double chain. Remember that the center wheel(s) are connected to the wheel in front of and behind it, so they have two chain runs on their axles. The people designing/building the drive found it simpler to just put double sprockets on all the axles rather than make spacers just for some of them.

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

This info is accurate. We don't run tensioners as they're not needed. We don't run double chains, but there is a double sprocket and two runs easily fit.

118 runs a modified version of the production units available from www.Team221.com. 118 has used them for the past two seasons with excellent results.

Editorial Note:
If you're breaking #25 chains in your drive base, you're doing something wrong.

It is very hard to generate a enough force in your drive base to violate the working load rating. Especially if you avoid master links...

Re: pic: Frog Force 503 Omnitank Drivetrain (rAC)
 

Sorry I didn't get any video, one of our newer drivers hit a forklift sideways in a drift without bumpers on and broke one of the omniwheels :(

We're getting new wheels and I'll get some video then.