Re: pic: Manually machinable one-speed gearbox
 

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?

Re: pic: Manually machinable one-speed gearbox
 

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

Re: pic: Manually machinable one-speed gearbox
 

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)

Re: pic: Manually machinable one-speed gearbox
 

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.

Re: pic: Manually machinable one-speed gearbox
 

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

Re: pic: Manually machinable one-speed gearbox
 

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

Re: pic: Manually machinable one-speed gearbox
 

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.