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.