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Playing with some improvements of our 2011 drive train. It reintegrates some concepts from 2007 that worked for us then.
In 2011, our frame was very similar to the old IFI frame -- box aluminum with angle aluminum as wheel brackets. This makes all of the axles live so we can avoid an issue with wheel wobble as well as reduce the number of parts during manufacture. We plan to modify the internal gearing of the super shifter for 4" wheels. Another nifty thing we may do is make the upper frame out of 80/20 for the offseason/prototype bot. Using 80/20 would allow us to prototype faster while the main frame is being welded, then integrate hole/new welds into the CAD. This is a huge plus for our build process.
The electronics board is there just to show some other mentors where it'd go; I don't expect us to have that layout.
Discussion
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21-08-2011 23:46
MattC9
Re: pic: Offseason Drive Train Progress
How are you planning to mount the SS?
22-08-2011 01:33
AustinSchuh
Re: pic: Offseason Drive Train Progress
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.
22-08-2011 03:34
Akash Rastogi
Re: pic: Offseason Drive Train Progress
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.
22-08-2011 08:49
Brandon Holley
Re: pic: Offseason Drive Train Progress
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
22-08-2011 11:44
JesseK
Re: pic: Offseason Drive Train Progress
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.
22-08-2011 11:49
AdamHeard
Re: pic: Offseason Drive Train Progress
If you have a manual mill, why not do the holes on that?
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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.
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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.
23-08-2011 00:54
AustinSchuh
Re: pic: Offseason Drive Train Progress
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Originally Posted by AdamHeard
We've run AM 22T for seasons without any issue.
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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.
23-08-2011 11:03
artdutra04
Re: pic: Offseason Drive Train Progress
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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. |
23-08-2011 13:00
JesseK
Re: pic: Offseason Drive Train Progress
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.
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