pic: Manually machinable one-speed gearbox
 

pic: Manually machinable one-speed gearbox
 

Re: pic: Manually machinable one-speed gearbox
 

Looks solid. What are the two small holes in the center for? How is the gearbox mounted?

Re: pic: Manually machinable one-speed gearbox
 

The two center holes are for the encoder plate, which mounts on the motor side. This is mounted to the frame by using the bolts that go through the spacers.

Re: pic: Manually machinable one-speed gearbox
 

Thsoe bolts look like they're more than 2" apart vertically, so it's difficult to mount to 2x1 with that.
How is the encoder attached to the shaft?

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Almost impossible to do chain in tube in 2x1 anyway. It's probably designed for 3x1 or larger.

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I meant the horizontal bolts. The encoders are mounted to the GB by an adaptor plate. The shaft is mounted to the encoder by either a shaft coupler or the surgical tubing method

Re: pic: Manually machinable one-speed gearbox
 

I think it's possible. We were planning on converting our competition bot to it in the offseason.
Using something like versablocks as bearing blocks allows for more than ample space in the 2x1 for 2 16t #25 sprockets side-by-side.

So this is mounted using the bolts along the bottom? I feel like that could cause the gearbox to flex back, but I could be wrong (something like that happened to our 2012 robot, although it was a slightly different situation).

Re: pic: Manually machinable one-speed gearbox
 

The bottom bolts. I don't flex should be an issue, because motor load isn't that cantilevered

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It looks like you're trying to direct drive a wheel with this. 11:64 is far too fast unless your wheel diameter is significantly smaller than 4".

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~16fps free speed according to JVN calc. We ran that last year.

Re: pic: Manually machinable one-speed gearbox
 

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

Re: pic: Manually machinable one-speed gearbox
 

Could some one explain how you get the chain in the tub. To me it seems like it would be a complex process.

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tl;dr, it's not easy.

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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.

Re: pic: Manually machinable one-speed gearbox
 

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

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

Re: pic: Manually machinable one-speed gearbox
 

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.

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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!

Re: pic: Manually machinable one-speed gearbox
 

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

Re: pic: Manually machinable one-speed gearbox
 

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

Re: pic: Manually machinable one-speed gearbox
 

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

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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

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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.

Re: pic: Manually machinable one-speed gearbox
 

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

Re: pic: Manually machinable one-speed gearbox
 

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).

Re: pic: Manually machinable one-speed gearbox
 

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.

Re: pic: Manually machinable one-speed gearbox
 

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

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

Re: pic: Manually machinable one-speed gearbox
 

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.

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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

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Vexpro does sell 18t sprockets for 25 chain which I assume is what Aren used. We used them too and loved them.

http://www.vexrobotics.com/vexpro/mo...sprockets.html

Re: pic: Manually machinable one-speed gearbox
 

So where does every one get #25 17t sprockets?

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Re: pic: Manually machinable one-speed gearbox
 

At risk of seeming ignorant, I would like to ask two more questions on in tube sprockets. First, now that I know that 221 has #25 17t double sprockets, where could I purchase single version of them, preferably in hex profile?

And second, why use 17t vs 16th or 18t?

Re: pic: Manually machinable one-speed gearbox
 

I think it's because it's a double sprocket, and 16t double sprockets are unknown to me.

Re: pic: Manually machinable one-speed gearbox
 

Presuming you're talking about #25 chain, yes. #25 chain has a 1/4" chain pitch, and #35 has a 3/8" chain pitch. A chain with a regular master link (or made with a high-end tool that needs no master link) between two sprockets with the same even number of teeth should have a separation that is an integer multiple of the chain pitch. Between two sprockets with same odd number of teeth, this becomes an odd multiple of half the chain pitch, or a multiple of the chain pitch with a half link. Well, for equal even-toothed sprockets you could use an odd multiple of a quarter of the chain pitch and a half link, but why would you want to make your robot more complicated twice?
And you aren't going to fit #35 chain inside of 2" Versaframe using any sprockets I've seen; they would require a very tight bend of the chain.

Re: pic: Manually machinable one-speed gearbox
 

I'm actually working with a different team nowadays, and we just used the 18t 1/2" hex VEXpro sprockets, nothing special.

-Aren

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With two bearings directly pressed into 2x1x.125 tube, you have 0.5" of shaft space left. The 221 17T double sprockets are .43" wide, while two 16T single sprockets are .7725" wide. If you want to use the 16T sprockets, there are a bunch of easy solutions, such as adding some 1/8" plates on the outside of the tube to space the bearings out, etc. However, IMO, the convenience of having less pieces to pull the shaft through during assembly is worth it.

Re: pic: Manually machinable one-speed gearbox
 

Vexpro would have a HUGE market for a reasonably priced #25 chain, 16T, 1/2" wide double sprocket.

(HINT, HINT)

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We really like the 17T double sprocket approach to Texas Tube.

When we put two pieces of chain side by side on 16T double sprockets we found the width of the chain interfered with the bearings pressed into the side of the tube.

When we put a piece of chain on an 18T sprocket it crashed with the top and bottom walls of the extrusion.

The magic of the 17T is that the chain rides over the OD of the bearing that protrudes through the extrusion wall and the chain doesn't hit the top or bottom of the inner extrusion walls. Its such a perfect fit and the chain doesn't have anywhere to go even if it wanted to jump off of a sprocket tooth. As Aren mentioned, with the thinner walled VEX extrusion, you can squeeze an 18T sprocket in.

Prior to 2013 we talked about and CADed Chain-In-Tube (now Texas Tube), and then the smaller robot size in 2013 convinced us to try it. We machined 4 bearing holes in a 2x1 extrusion, waterjet a 17T tooth profile out of 3/8" aluminum, some post sprocket lathe work, and we had a working drive rail prototype in one night. We've run the 2013, 2014, and 2015 seasons with only one drivetrain failure, diagnosed as a piece of improperly built chain... but otherwise its extremely reliable.

Its a simple, robust, easy, light, and small way to build a drivetrain. This year we bought sprockets from 221, and machined our chassis rails on a manual mill. Ain't nobody got time to be reinventing a drivetrain every year.

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Has anybody run an 1/8" drop with 17t or larger sprockets in .1 wall 2x1?

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They don't. To achieve the center drop, teams lathe down the wheel diameter for the outer wheels. Basically, all the axles are in line and only the wheel diameter changes

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Yeah, I know. But 118 specifies that there's not enough room for a center drop in 1/8" wall tube. I was wondering if anybody had done a true drop center in .1" wall tube. Also, on a related note, in a 6WD with the outer wheels shaved down, how does the different linear speed of the wheels affect the drive?

Re: pic: Manually machinable one-speed gearbox
 

#25 chain on a 17T sprocket offset 1/16" from the center of a 2x1 Versatube does clear the wall by ~1/32 of an inch.

Re: pic: Manually machinable one-speed gearbox
 

While I can't say for certain, from what I've seen I don't think this change in linear speed has much of an effect on the overall drive. 118's 2013 and 2014 drives performed exceptionally well even with the different linear speeds. That being said, they were both 8WD, and therefore the wheels running a different fps spent a significant amount of time not contacting the ground. I feel like the different speeds could be much more noticeable in a 6WD setup.

If you wanted to run a 6WD Texas Tube without moving your bearing holes off of the rail's center line, you could try running your center wheel on an 18t sprocket and your outer wheels on 17t sprockets inside 0.1" wall 2"x1". If I've run the numbers correctly, you should be able to put 4" wheels in the center and shaved down 3.8" wheels on the outer shafts to achieve equal linear speeds. However, that would only result in a 0.1" drop.

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That's an interesting idea for sure. Main problem I see with it really is that 2 18T VexPro COTS sprockets can't fit inside 2x1. It would require either the custom machining of a double 18t sprocket or shaving down the COTS 18t sprockets. (unless there's a thinner double 18t sprocket on the market that I'm unaware of)

A .1" drop should be plenty, at least by my observations. I've seen teams run 3/32 without a problem.

A nicer way to do it might be to run 17t sprockets on the center wheel and 16t on the corners. You also get almost exactly an 1/8" drop then. But at that point, you might as well run all 17t sprockets with drop center, since that gets more teeth meshing with chain.

Re: pic: Manually machinable one-speed gearbox
 

Would there be a problem if you ran the outer wheels slightly faster than the middle wheels?

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One solution, if a drop center is really desired, is to cut a pocket on the bottom of the center of the tube for the middle sprocket. IMHO, that's kind of defeating the point of chain in then.

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The outer wheels would be shaved down to 3.75" diameter to ensure equal linear speeds on all wheels.

What 118 currently does as far as I know is use all 17t sprockets and use smaller diameter wheels on the corner, causing their larger diameter wheels to have greater linear speeds than the outer wheels. Jack's idea is a possibility to fix that problem without having to implement a drop center.

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Is that a problem? Teams have been running uneven diameters for a long time.

Your wheels are slipping in one way or another in any motion other than going straight anyway.

The normal force is highest on the inner (larger diam) wheels as well.

Re: pic: Manually machinable one-speed gearbox
 

So what you are saying is that some one could run sprockets with 16t-17t-16t all on the same wheel size?

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I agree that it's not an issue. Game permitting I'd be surprised if we didn't run a setup very similar to 118's 2014 bot this upcoming season. The idea I presented would give an ideal set of linear speeds, but would likely lead to a few other issues down the line. For example, your center-to-center distances would no longer be nice easy numbers.

The width of the vexpro sprockets can easily be solved by shaving them down a little. Beware of the 16t sprockets, however, as we found that the chain would grind on the bearings, as Mason pointed out earlier.

All in all, 118 has it figured out. Their system is the easiest Texas Tube I've seen so far, no need to reinvent the wheel.

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To anyone who has run with "Texas Tube" in the past: How do you account for the stretching of the chain over the course of the season? There doesn't appear to be a lot of room for tensioners. Is this even a concern with the lengths of chain that are being used?

Re: pic: Manually machinable one-speed gearbox
 

This season we built our system with no tensioners and never had to touch it. It ran very smoothly through three regionals and champs. I belive others have had similar success.

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Alright, that's good to know! Now, since this is an awesome thread and I'm learning a lot from it, I'm going to ask another question.

Has anyone every thought about using timing belts instead of chain? Fitting it into a 2x1 box probably wouldn't be possible, but what about a 2x2 box. I'd imagine it would be even more of a "set it and forget it" scenario because a timing belt would be less likely to break than a chain.

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That's a lot of weight you're gaining though (~2-4lbs depending on drivetrain), and you would have to settle for 1/8" wall probably. If teams say the chain doesn't break, then it doesn't break. That might be okay if you have the space and weight and mounting issues solved, because then you get a decent efficiency bonus.

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This season, 1477 used a Texas Tube style chassis. On a 4WD with all omni wheels, we used VexPro 16t #25 sprockets. We had tensioners on both our practice bot and our competition bot, and never used them on either robot. We had no issues with the 16t sprockets, and our chain runs were more than 200 links long. Game permitting, we will likely be running a Texas Tube style drive chassis again. Our experience with #25 chain in 2x1 tube has been outstanding thus far.

This is something I've been thinking about a lot lately. If using 1/16th wall 2x1 tube, it is theoretically possible to run 24t pulleys and 9mm belts from VexPro inside the 2x1. This could result in an extremely lightweight and efficient drive, at the cost of some structural integrity in the drive chassis. In my experience, #25 chain is less likely to break than 9mm timing belts, but is heavier and less efficient, assuming both the belt/pulley and chain/sprocket are assembled correctly.

http://www.chiefdelphi.com/forums/sh...ad.php?t=94701

Here is a picture of 2791 running timing belts inside tube extrusion.

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2791 has continued iterating on their belt-in-tube drive since then. Chris Picone would be the guy to talk to about that.

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192 has also used belt in tube for the past three years. 15mm wide GT2 in 2x2 .125 wall. With our set up and access points it's incredibly fast to assemble and tear down even though you'll never need to.

It is possible to run 15mm wide belts in 2x1.5 but you'd have to bore out the pulleys around the bearings or move the bearings out. I wouldn't recommend 9mm belt in the DT unless you had a proper tensioning system like 971 and even then they have had issues breaking belts.

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How are your access points for belt in tube set up? How is it assembled?

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Been running belt in a tube structure formed from sheet metal for the past three years with no issues. The sheet metal works great because we can start with a readily available belt/sprocket combo and size the tube accordingly, rather than the other way around. Access is achieved by opening up the entire side of the tube, which makes installation a lot easier than the through-the-end approach or small access pockets required by extrusion.

It is a fair bit larger than the texas tube approach, due to the increased sprocket diameter/width, and the need to add additional clearance for the rivets which hold the tube together, but it works well with our resources, and we like the insurance it provides.

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One thing I want to make clear is that with the Texas Tube approach, it is almost physically impossible for the chain to jump off the sprocket if assembled correctly, because of the constraints of the tube. For failure to occur, the chain would have to essentially break, the risk of which can be minimized by not using masterlinks. Even if the chain loosens and rubs against the tube after a lot of usage, it will still probably be safe (as the other teams have noted, all these robots have been through multiple regionals and championships, and in the case of 118 some very defense heavy games like 2013 and 2014).

Other benefits include:
-Low maintenance (basically assemble and never touch again for the rest of the season)
-More space and weight efficient
-Relatively simple, no special equipment besides a mill required
-Keeps things clean and out of harm's way (the number of times I saw noodles getting stuck in the chains of other teams was too much to count, because of Texas Tube, we had virtually no problems driving over and around the noodles).
-This has more applications than just the drive train!

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Above is an image of 192's 2014 base about to be welded up. There are two rectangular holes on the tops of each side as well as 4 holes in the side in addition to the bearing holes. First the belts go in the rectangular holes in the top, then the pulleys are slid into the side holes and rolled down the tube to their bearings holes. The gearbox is put on, axles are put through the pulleys, bearings popped in, and wheels are put on. Assembly time is about 5 minutes a side.

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Abhishek, I'm not doubting the effectiveness of Texas Tube using chain. The results speak for themselves. I'm merely exploring other options. My students expressed interest in trying out timing belts for the drive train, so I just wanted to see what experiences other teams had with it.

I don't understand. This thread talks about 2x1x.125 tube being used for Texas Tube. Why would we have to "settle" for 1/8" wall?

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If you designed to package your drive in a 2x1 tube you could use the .1" wall tubing from Vexpro to shave a little weight down. 2x2 doesn't come in .1 wall and it wouldn't be wise to press bearings into .0625" wall 2x2 in a drivebase.

Drawing a quick 36" tube in Inventor I had the following weights:

.1" wall 2x1 Versa Tube: 1.97lbs. (0.65lbs. per foot)
.125" wall 2x1 tube: 2.4lbs. (.8lbs. per foot)
.125" wall 2x2 tube: 3.3lbs. (1.1lbs. per foot)

It all depends on what you have planned and are building on top. We had 2x1 Versa tube uprights coming off of our setup so 2x2 tubing worked very well for us. With the extra space it also can make mounting off of the tube a little easier and you can lighten the tube to bring the weight down a little. My personal preference would be a lightened 2x2 tube but that's just me.

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I know, I wasn't referring to the belt system. I just wanted to say that in case people who haven't seen the method are reading this thread, because when talking to people in the pits at competition, they ask if we are ever afraid of the chain failing inside the tube where we don't have easy access to it and if we should cut access holes.

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Thanks for doing this. Honestly, I'm not overly concerned with the extra weight. You're likely to save a little weight by using belts instead of chain, so it may cancel itself out.

Okay, it just seemed like you were on the defensive a little bit. Thank you for spelling out the benefits. I love the idea of Texas Tube just because I really like the efficient use of space, but there are always those on every team that question its ease of use.

I do have more questions though. How would you get a continuous loop of chain without a master link? My team has only ever used master links with our chain.

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Buy one of these for 25 chain (also available at bike shops if you google enough):
http://www.team221.com/viewproduct.php?id=70

Follow these steps:
http://www.billetboard.com/showthrea...oul-Chain-Tool

First year using one and it was one of the best things we purchased this year. :) We had 50+ feet of 25 chain on our robot and only had four master links where our elevator platform connected to the chains. Using lighter chain (25 over 35), no master links, and C-C distances has really make working with chains an easy process and eliminated a lot of my worries in the past two seasons in our mechanisms.

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We did #25 chain in 2x2x.125 tube on our drop-center 6WD this season and had a good experience. No maintenance all season. There was some chain slap against the tube initially (16T sprockets and long chain runs; not a real problem but noisy), so we slid in a strip of UHMW to dampen that and now you'd never know.

We used a 2x2 because the frame rails in our design had minimal cross bracing due to the large opening for acquiring game pieces.

Re: pic: Manually machinable one-speed gearbox
 

So it sounds like the consensus here is that to do a Texas Tube design, you must keep all of the axles horizontally inline, and just put smaller diameter wheels on the outside to create a "drop center." I've been throwing together designs in CAD for different drivetrain styles, and I wonder if someone can explain to me if this would or wouldn't work:


(sorry for the poor render quality)

Rather than dropping the center or raising the outsides, I did both. I lowered the middle 1/16" and raised the outsides by 1/16", to end up with a
"drop" of 1/8". From everything I can see and read, this would fit very snugly into a 1x2x.125 tube. It's completely COTS and would let you use 4", 6", or really any other size wheel anywhere on the drivetrain -- which is big for teams with few resources.

General specs:
2"x1"x.125" aluminum tube
AndyMark Toughbox-Mini gearbox (alum plate cut down slightly)
17t #25 Team 221 dual sprocket

Re: pic: Manually machinable one-speed gearbox
 

You can also use omni wheels on the outside wheels, especially in a game with little or no pushing and defense like this year.

Re: pic: Manually machinable one-speed gearbox
 

Yes, we've done this. The 1/8" total drop is "shared" equally between the outside and inside axles.

In fact, our entire setup is very similar to this, with only a couple differences:
-- we used 2x1.5x.125 tube
-- we used VexPro 16t #25 sprockets

Re: pic: Manually machinable one-speed gearbox
 

Did you have the issue that 118 mentioned earlier of the chain grinding on the outer bearing race?

Re: pic: Manually machinable one-speed gearbox
 

No, in fact we deliberately chose the 2x1.5" tube to avoid this issue with the 16t sprockets.

Here is a technical drawing of (an early version of) our drive tube.