pic: Offseason Drive Train Progress

[JesseK]

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[MattC9]

How are you planning to mount the SS?

[AustinSchuh]

How many teeth are on those sprockets? They look a bit small to me.

25 chain has a working strength of 140 lbs. Assuming a 16 tooth sprocket (guessing from your CAD...), that's 90 in-lbs of torque through the sprocket. A 150 lb robot with 4" wheels and a coeffecient of friction of 1.2 will have 150 in-lbs of torque applied to the set of wheels on each side before slipping wheels. That means that you can have 50% of the robot's weight on the front wheels before you exceed the working strength.

Lots of WCD use 22 tooth sprockets. (This is because the OD with chain is slightly under 2") That gives you a torque of 122 in-lbs. That means you can have 70% of the weight on the front wheels before you exceed the working strength.

I'd go with as big a sprocket as you can put on there before other things like clearance become a concern. Smaller will probably still work, but if you don't need them to be that exact size, make them bigger and give yourself a larger margin of safety.

How are you going to tension the chain? 25 chain stretches when used in a drivetrain, especially with small sprockets.

You might also want to add some cross-bracing to the frame, or mount the electronics panel more securely. A solid hit to the corners will put a lot of stress on the front and back 1x1's. At a minimum, add a 1x1 across the middle of the frame. I'd personally pull the electronics panel up to the bottom of the 1x1's and then bolt it securely to everything you can bolt it to. That will make the frame significantly more rigid. I would also think that the two 1x1 rails down the side will flex a little bit under normal loads. It would probably be worth it to put a plate across the middle near the middle wheel so you don't have to deal with the bearing sliding on the shaft, or in the hole.

Otherwise, it looks pretty reasonable.

[Akash Rastogi]

I'm wondering if all of these frame rails and wheel mounts can easily be replaced with 2x1 box tubing? Doesn't look like you'd really need to save too much weight by avoiding the tubing.

[Brandon Holley]

One other comment: It looks like you have quite a bit of space between sprockets on the center shaft. I can't tell from this picture if thats just the hub of the sprocket, or a separate spacer, but either way it seems like you can really shrink that up (by turning down the hub, or eliminating the spacers). That will allow the space between those two rails the wheels sit in to get smaller, which means shorter shafts which makes the load on those more manageable.

Just my $.02.

-Brando

[JesseK]

Thanks for the heads up about the sprockets Austin. They're 16T since their PD creates to a nice round circumference, and I was wondering if 22T is preferred over 16T. Nice round circumferences make nice round distances between axles, so the chains will not need tensioners. If we need to take out slack, we'll use half-links. We ran tensionerless after DC this year (1 regional, 1 champs, 1 offseason event) without a single issue. On the same note, are the AM 7075 Aluminum sprockets strong enough for the drive train? What's in the CAD is steel, with the full hub. They're a nice round 1/2" wide in total, which simplifies the spacers as well; yet if the AM's have strong enough teeth, then I see no reason to stick with the steel.

Cross members will be added when we do manipulators during the build season. We're also going to manufacture 1/8" plates that serve the dual purpose of pneumatic gauge/tank mounts as well as cross-support for the side rails. Until then, I expect to add a 60-lb anvil above the offseason frame that's moveable via 80/20 so our new drivers can see the effects of c.g.

We like this approach because it allows us to continue to move forward even if someone mucks up a hole by 1/8". While precautions will still be taken, we're still doing all of the drilling/cutting manually and errors will inevitibly happen. Using 2x1 wheel blocks allows a live axle while also allowing us to adjust the wheel block placement on the frame in order to account for these errors. This will be the third time we've done 6WD drop-center; in 2007, we had to redo the frame 3 times because of misaligned holes; in 2011 the wheel brackets were way out of alignment and we didn't figure that out until the 2nd regional. So this design is more tailored to allowing us some wiggle room.

[AdamHeard]

If you have a manual mill, why not do the holes on that?

Originally Posted by JesseK

Thanks for the heads up about the sprockets Austin. They're 16T since their PD creates to a nice round circumference, and I was wondering if 22T is preferred over 16T. Nice round circumferences make nice round distances between axles, so the chains will not need tensioners. If we need to take out slack, we'll use half-links. We ran tensionerless after DC this year (1 regional, 1 champs, 1 offseason event) without a single issue. On the same note, are the AM 7075 Aluminum sprockets strong enough for the drive train? What's in the CAD is steel, with the full hub. They're a nice round 1/2" wide in total, which simplifies the spacers as well; yet if the AM's have strong enough teeth, then I see no reason to stick with the steel.

Actually, if you have the same toothcount on both sprockets, it doesn't matter what their circumference is the exact center to center is still a clean number.

There is also a helpful excel called Dr. Joes chain path calculator for all other situations.

We've run AM 22T for seasons without any issue.

[AustinSchuh]

Originally Posted by AdamHeard

We've run AM 22T for seasons without any issue.

971 ran them last year, and had no issues. They are quite a bit lighter. But, they are a lot more expensive than steel. Your call there.

I personally won't put a chain run on one of my robots without some sort of tensioner, but if this has worked for you in the past, then go for it. I know that 233 has run 25 chain without tensioners, and it has worked for them.

[artdutra04]

Originally Posted by AustinSchuh

971 ran them last year, and had no issues. They are quite a bit lighter. But, they are a lot more expensive than steel. Your call there.

I personally won't put a chain run on one of my robots without some sort of tensioner, but if this has worked for you in the past, then go for it. I know that 233 has run 25 chain without tensioners, and it has worked for them.

We also used 22T 25p 7075 aluminum sprockets in our drivetrain this past year without any problems, but we ended up machining them ourselves from 1/2" 7075 plate (which was cheaper than buying steel sprockets). We designed for initial exact chain length, but needed to add tensioners as the chain stretched. For that, we ended up just throwing in some of our Lexan floating idlers (which are quite possibly the lightest possible roller chain tensioner ever), and they worked without any problems.

[JesseK]

Yea, we're considering the price difference between the steel and aluminum sprockets; $7 vs $16 is a big difference for 16 sprockets (prototype+production).

We actually got some help from 233 in DC to make our chains tensionerless this year. They put alot of emphasis on proper spacing for the chain. We may try to pre-stretch to an extent as well. Maybe we'll also have some Teflon ready in case we wind up needing tensioners.