pic: Manually machinable one-speed gearbox

[bkahl]

Originally Posted by Knufire

Actually a COTS product: http://www.team221.com/viewproduct.php?id=133

3.5" wheels on the outside is one way to do it that I overlooked, 1/4" drop turns me away though.

Originally Posted by Knufire

tl;dr, it's not easy.

Tl;Dr I'm not going to be trying it anytime soon...

[BrendanB]

Originally Posted by Knufire

tl;dr, it's not easy.

Oh hey that looks familiar. One correction is that the sprockets were elevator not wheel sprockets. It wasn't the easiest thing we've done but it wasn't the hardest with the hours of CAD work paying off with a relatively painless assembly.

3467 was feeling inspired by our new manual mill in our shop we designed a robot that required our drivebase and elevator gearboxes to take up little to no space.

We packaged it all in 2" x 2" x .125" wall tubing. We could have used 1.5" x 2" but opted for the 2" since it made mounting up easier with the Vexpro tubing and a little more room to work with. We used 18T sprockets for 25 chain and spaced out the bearings to be C-C however something in the math was wrong so the chains had a little more slack compared to our C-C we ran with 35 chain the year before but since it was inside the tube and impossible to come off we didn't bother adjusting it. We may have designed it to be less than C-C I'm not sure. As we found out the slack was a nice feature since the chain was easier to install.

Overall it was a very nice setup and we loved this assembly. Each module was responsible for driving the wheels and providing power to our elevator with a duplicate module on the opposite side and an axle higher up that connected the two sides of the elevator to keep them in sync.

IIRC after these were assembled we only touched them once on the practice bot in Week 6 due to one of the axles getting pushed out while adjusting one of the sprockets on the outside for the elevator. It took a while to get the spacers and sprocket back in line since the superglue had done its job during assembly and had broken away so we had three floating pieces inside the tube on each axle. Not ideal but it worked for what we needed and unless you pulled the axles out the inside was solid. After that minor mistake we never had an issue on two robots.

Some pictures of the assembly are available here. Weight as shown fully assembled minus wheels and encoders was 16.5lbs. We had plans to lighten the large gears but always found the weight in other places.

I would highly recommend you try this out at least once in the pre-season. Our original plan was to machine these on a manual bridgeport but we needed 5 of these made and holding those tolerances on one would have been difficult so I brought them to work and CNCed them. It was our first time trying a lot of things this year for the first time IN season which is typically frowned upon but we worked through a lot of CAD revisions until we were happy. Consider the impact running chain inside your tube has on how you can mount to your frame. Since ours was only running to our elevator it wasn't spanning the full length of the tube so we had an easier time mounting cross members in the back and our vertical assemblies needed to be re-thought a few times.

If you have any questions feel free to PM echin the student who designed it & JamesB3467 did the gearboxes. Huge thank you to 118 for sharing some insight into their design that inspired our students to tackle something harder! Also to Vexpro for their great lineup of products making complicated mechanisms easier for teams to try.

Also for getting around the issue of adding a drop into your drive, Colsons are pretty easy to throw on a lathe and shave down to size with the right cutting tool. We did this in 2014 after we accidentally designed our drivebase with no drop.

To the OP: Looks good nice to see you trying something new, hope it works out for you guys. Something to consider, what is that outer plate doing for your drive that the tube/mounting surface can't provide? For some designs you might need it, others its a nice way to eliminate a part and shave some weight out of the base. Just a thought.

[Abhishek R]

Yup, gonna echo a lot of what Brendan said.

We ran our chain in the 2x1 tubing after being inspired by the local experts 118, and were extremely happy with the results. The drivetrain was probably one of the best we've built yet, and we had exactly 0 issues with it during the competition season. And the best part was we didn't have to do any real maintenance on it.

As for the installation, we just put the chain together (no masterlink), put the sprockets on the chain (not fixed, just loosely held by the chain), then dangled it into the tube as we put the shafts in place.

Worked great, 10/10, will probably run again!

[Munchskull]

Originally Posted by Abhishek R

We ran our chain in the 2x1 tube

So just confirming that teams who have done this use vex pro hardware and sprockets in 2x1 .125" wall tubing?

[Knufire]

Originally Posted by BrendanB

Also for getting around the issue of adding a drop into your drive, Colsons are pretty easy to throw on a lathe and shave down to size with the right cutting tool. We did this in 2014 after we accidentally designed our drivebase with no drop.

Could you elaborate on this? We had a less than stellar time trying to shave down colsons this year.

[Abhishek R]

Originally Posted by Munchskull

So just confirming that teams who have done this use vex pro hardware and sprockets in 2x1 .125" wall tubing?

Yup, we used the 16 tooth sprockets inside 2x1 VersaFrame (note this is .1 inches thick).

[BrendanB]

Originally Posted by Abhishek R

Yup, we used the 16 tooth sprockets inside 2x1 VersaFrame (note this is .1 inches thick).

How many chains/sprockets did you run inside the tube or was it a single strand?

Ours was four sprockets on three axles so it was a slightly smaller version of a 6wd setup which requires putting sprockets side by side which gets trickier for space in the tube.

Here is a picture of a product from Team 221 LLC that shows a double chain inside of 2" x 1" x .125" tubing. Not sure if the sprockets shown are their double 17t sprockets that Rahul linked to earlier.

http://www.team221.com/viewproduct.php?id=130

[Abhishek R]

Originally Posted by BrendanB

How many chains/sprockets did you run inside the tube or was it a single strand?

Ours was four sprockets on three axles so it was a slightly smaller version of a 6wd setup which requires putting sprockets side by side which gets trickier for space in the tube.

Here is a picture of a product from Team 221 LLC that shows a double chain inside of 2" x 1" x .125" tubing. Not sure if the sprockets shown are their double 17t sprockets that Rahul linked to earlier.

http://www.team221.com/viewproduct.php?id=130

I'll have to check, but I believe our gearbox didn't direct drive one of the wheels, so it was 2 strands. I want to say we had some spacers on the shafts with one sprocket as well, and I seem to faintly remember whittling the sprocket width down a bit.

[Knufire]

The SimpleTube chassis CAD shows their double 17T sprocket in 2x1x0.125.

[Bryce Paputa]

Originally Posted by asid61

That is one way to do it. I talked to a couple teams at champs (can't remember which) and from what I heard from them it was a largely painless process after the initial installation. Could be something to start a thread about.
I'm sure that no matter which way you do initial installation it's going to be harder than traditional WCD, but the upkeep should be simple.

May have been us, we were in your division. We did this with 17t sprockets in 2x1x.125 and after installation we had very few issues. One thing to remember is that if you use 17t sprockets then you either need to have a half link or have a center to center distance that is divisible by .125 but not .25 (I think, you should check the math on this).

[Pretzel]

Team 1619 also used a chain-in-tube drivetrain with great success this year, and we were surprised by how easy it was to manufacture and assemble. It was put together using custom sprockets, bicycle chain, and hex shafts to serve as the axles. Since one of the axles was the output shaft of the gearbox, only three sprockets and two runs of chain per side of the drivetrain were required.

We started with our sprockets, which we machined ourselves to allow the use of bicycle chain, which was both lighter and stronger than the other solutions we had explored in the past. The ability to control the offset between sets of teeth on the sprocket (since we were making them ourselves) and the spacers holding the sprockets in the tubing meant we could control the position of the runs of chain within the tubing to still allow us to through-bolt into the chassis where the chain was located. It was just a matter of designing the sprockets and spacers so that the center of the tubing was unoccupied by chain.

The chain itself was quite simple in that we simply bought bicycle chain and then removed links to make it the correct length before adding the master link to create a loop of chain.

The assembly was the part that nobody was looking forward to at first, since we had to put all of it together inside of 2.5x1.25 aluminum tubing that had solid walls without "windows" removed to see inside other than the slots for the bearings that held the axles. Due to this, we actually put together the runs of chain outside the frame by holding one sprocket, then dangling the first chain from it, then cradling the second sprocket in the first chain, then dangling the second chain with the third sprocket cradled inside. We then took the whole assembly, still held by the first sprocket, and gently lowered it inside of the tubing until the top sprocket was visible in the hole for the first axle. It was then a simple matter of inserting the axle to hold the top sprocket in place, then "capturing" the dangling sprockets by inserting their respective axles. The assembly process is the same one described by Abhishek that 624 used.

Provided that you can reach high enough to insert the assembly into the open end of the tubing on your drivetrain (we had to stand on a stood to accomplish this), it is a method that is both quick and easy for installing a chain drive inside of tubing. The downside to this method is that it does require precision machining capabilities if you want to retain the ability to through-bolt in the center of the tubing (so that you can ensure the proper offset to the chain inside the tubing) for the sprockets and spaces, as well as the ability to machine out the bearing slots for your axles in the tubing in the correct location. In the future I would assume that we're likely to use a similar solution due to the positive experience we had with it this year, provided that it continues to be an advantageous solution for our overall game strategy.

If anyone has any specific questions about our drivetrain, feel free to shoot me a message and I'd be happy to provide more details. A powerpoint presentation with CAD models on a similar drivetrain design (which was our fall project last year) can be found HERE for those who are interested.

[Aren_Hill]

Actually ran Texas Tube this year, and had 2 chain runs per tube, entertainingly for different subsystems.

2 runs of #25 chain on 18T sprocket fits perfectly in 2x1 Versaframe (the 0.1" wall is what makes it work, vs the 17T and 0.125" wall that 118 does).

It was a 4wd, so no center drop.

-Aren

[asid61]

Originally Posted by Aren_Hill

Actually ran Texas Tube this year, and had 2 chain runs per tube, entertainingly for different subsystems.

2 runs of #25 chain on 18T sprocket fits perfectly in 2x1 Versaframe (the 0.1" wall is what makes it work, vs the 17T and 0.125" wall that 118 does).

It was a 4wd, so no center drop.

-Aren

Why all the odd sprockets instead of just the "normal" 16t ones you can buy from Vex or AM?

[Joe G.]

Originally Posted by asid61

Why all the odd sprockets instead of just the "normal" 16t ones you can buy from Vex or AM?

I believe that the larger sprocket makes it geometrically impossible for the chain to ever jump off the sprocket, since the chain is constrained by the wall of the tube. This allows this style of drive to be done without requiring tensioners.

[cxcad]

Originally Posted by asid61

Why all the odd sprockets instead of just the "normal" 16t ones you can buy from Vex or AM?

They got lasers "in-house," so they probably can just laser a few custom sprockets for their needs. I'm assuming Aren was referring to his team 148