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Unread 14-05-2015, 19:32
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Re: pic: Manually machinable one-speed gearbox

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Originally Posted by String View Post
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.
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.
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Unread 14-05-2015, 19:39
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Re: pic: Manually machinable one-speed gearbox

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Originally Posted by AdamHeard View Post
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.
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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Unread 14-05-2015, 19:55
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Re: pic: Manually machinable one-speed gearbox

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Originally Posted by AdamHeard View Post
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.
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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Unread 14-05-2015, 20:29
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Re: pic: Manually machinable one-speed gearbox

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?
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Unread 14-05-2015, 20:38
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Re: pic: Manually machinable one-speed gearbox

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Originally Posted by Dan Petrovic View Post
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?
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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Unread 14-05-2015, 21:49
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Re: pic: Manually machinable one-speed gearbox

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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Unread 14-05-2015, 21:55
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Re: pic: Manually machinable one-speed gearbox

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Originally Posted by Dan Petrovic View Post
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.
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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Unread 14-05-2015, 22:21
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Re: pic: Manually machinable one-speed gearbox

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.

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Originally Posted by Dan Petrovic View Post
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.
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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Unread 14-05-2015, 23:10
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Re: pic: Manually machinable one-speed gearbox

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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Unread 14-05-2015, 23:18
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Re: pic: Manually machinable one-speed gearbox

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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Unread 15-05-2015, 00:46
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Re: pic: Manually machinable one-speed gearbox

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With our set up and access points it's incredibly fast to assemble and tear down even though you'll never need to.
How are your access points for belt in tube set up? How is it assembled?
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Unread 15-05-2015, 00:55
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Re: pic: Manually machinable one-speed gearbox

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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Unread 15-05-2015, 01:15
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Re: pic: Manually machinable one-speed gearbox

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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Unread 15-05-2015, 07:28
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Re: pic: Manually machinable one-speed gearbox

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Originally Posted by Abhishek R View Post
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.
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.

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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.
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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Unread 15-05-2015, 01:43
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Re: pic: Manually machinable one-speed gearbox

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Originally Posted by z_beeblebrox View Post
How are your access points for belt in tube set up? How is it assembled?


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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