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Unread 24-06-2016, 10:11
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Re: pic: Another Belt-In-Tube Drivetrain

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Originally Posted by Chris is me View Post
Shaker's drives haven't been welded for the past several years, so we have used 3" tubes to allow ample rivet clearance. In flat field games like 2014, we can use 4" wheels with 3" tubes no problem. In other years we had to switch to 6" wheels for extra clearance. With the 3" tubing, 1/8" drop was just fine (if anything a little more than we needed in 2014). If we went to 2.5" tubing or smaller we would have to stagger the drop or even switch to a zero drop drive, sanding outer wheels down to a smaller diameter if we needed to add drop after the fact.

The designers on the team are looking into ways to use smaller tubes in future drivetrains, to save weight and to allow adequate ground clearance with 4" wheels. 2.5" tall tall tubing, with rivets staggered to avoid the belts and pulleys, are a possibility. Experimenting with smaller tooth count pulleys is another option, albeit a risky one. 21T pulleys have a number of teeth divisible by 3 for easy machining, and they allow JUST enough clearance for a 2" tall tube if you stagger the rivets, but without testing I can't be certain they would be robust enough.
Here's a 22t 5mm HTD pulley with the counterbore in it. This only leaves .0195" of wall at its thinnest point, and only .005" of clearance from the radius of the bearing.

SDP/SI does sell 23t 5mm HTD pulley stock, however.
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Unread 24-06-2016, 10:18
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Re: pic: Another Belt-In-Tube Drivetrain

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Originally Posted by Ty Tremblay View Post
Here's a 22t 5mm HTD pulley with the counterbore in it. This only leaves .0195" of wall at its thinnest point, and only .005" of clearance from the radius of the bearing.

SDP/SI does sell 23t 5mm HTD pulley stock, however.
Ah, I didn't think about the counterboring issue. That makes things more difficult. On 2791 we just used 1/8 washers between the bearing flange and the tube to get everything to fit. This is a bit more compact than going to 2" wide tubing but lets you use smaller pulleys.
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Unread 24-06-2016, 23:24
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Re: pic: Another Belt-In-Tube Drivetrain

Quote:
Originally Posted by Ty Tremblay View Post
Here's a 22t 5mm HTD pulley with the counterbore in it. This only leaves .0195" of wall at its thinnest point, and only .005" of clearance from the radius of the bearing.

SDP/SI does sell 23t 5mm HTD pulley stock, however.
I don't think having that thin wall is an issue at all. I'd go with that.

For the correct belt thickness I made my belt drawings using the tooth heights and overall thicknesses of the belts given in the gates belt design manuals.
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Unread 27-06-2016, 14:20
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Re: pic: Another Belt-In-Tube Drivetrain

Hopefully not de-railing the conversation, but if anyone could humor me and take this discussion up a level, or link the relevant threads if it has been adequately discussed before, I'd certainly appreciate it.

#1: What are the pro/con of going to belt in tube if you are currently running chain in tube?
#2: Do you believe the differences are significant? And why?

As a data point, since I've been on 3005:

In 2014 we ran a WCD style 3.25" 6WD tank with internal 9mmxHTD5x20T pulleys (Vex) on the inside of the robot. Never had any ratcheting issues (or would expect). Used VexPro/WCP bearing blocks (without retention cams) and had enough slippage issues (block vs. tube, our fault) to cause pulley misalignment and the press-on walls of the pulleys to come off.

In 2015, we ran a chain in tube for an H drive configuration in 1x3x.125" extrusion, #25 chain, ~22-25T sprockets as I recall, geared at maybe ~10ft/sec. No issues to note.

In 2016, we ran a chain in tube, 8WD with #35 chain, 8" pneumatic tires, in a 2x3x.125" extrusion. No real issues to note.

Looking forward, though we may play around with "other" drive train options, it is highly likely we will prefer to similar drivetrain styles. We have the manufacturing capability to handle most designs (lathe, cnc mill, Al welding, etc), it just requires the investment of resources to create in the offseason and prove out. We value robustness over maneuverability, and think we can make up for any loss of mobility with extra driver practice in most games.

What advantages might we be missing out on by continuing to run chain in tube?

The things I can think of:
- We have run 3" high tubing two years in a row, to accommodate extra center drop as well as a slightly larger sprocket to reduce chain loads/sprocket wear/etc. I have justified this in my head by saying the extra profile yields a stronger frame (torsional), but is it needlessly stronger? Would running belts make it easier to go to 2" profile and maintain the high safety factor I would like, even at large diameter wheels?
- Are belts (when properly tensioned) more accurate with less slop when it comes to measuring distance traveled (for auton)? Or is the difference to chain not worth mentioning?
- Is the system more efficient? More robust? Overall lighter (I know the belts are, but the hubs look heavier than an equivalent plate sprocket).
- Other?

Any rate, we will probably do another iteration of our drivetrain before build season next year, and the discussions on the belt in tube have me intrigued. Thanks for any input!
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Unread 27-06-2016, 14:35
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Re: pic: Another Belt-In-Tube Drivetrain

Chain is a bit less efficient than belts. Look up Team 234's paper on Chain vs. Belt, it has some good info in it.
Because both chain and belt are positive interference/have engaging teeth, neither is more accurate in auton. Timing belts might give you better performance by a couple hundreths of an inch, but that's about it.

Belts are lighter than chain, but you're right that the pulleys are not. In my experience the weight difference is negligible.

If you're considering switching drivetrains, running 9mm belts on 36 tooth pulleys or something could be a good swap for you. If you're already using 3" tall tubing, using the largest pulley (around 36 tooth) would net you a good factor of safety. I'm only going off the "24 tooth 9mm" being the absolute dangerous bare minimum for belts, so doing your own testing in the offseason would be a good idea.

Personally, I prefer chain in tube for the compact factor and the strength that it offers; I've never broken a #25 chain (well I did once, but not in a drivetrain, and certainly not in a normal application). But if you're already used to designing with 3" tall tubing, maybe large pulley belts are the way to go for you. 2x2" tubing, or a setup like the one in this thread, are also options you can pursue. The low noise of belts is also a big appeal factor for me personally.
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Unread 27-06-2016, 14:42
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Re: pic: Another Belt-In-Tube Drivetrain

Quote:
Originally Posted by Steven Smith View Post
Hopefully not de-railing the conversation, but if anyone could humor me and take this discussion up a level, or link the relevant threads if it has been adequately discussed before, I'd certainly appreciate it.

#1: What are the pro/con of going to belt in tube if you are currently running chain in tube?
#2: Do you believe the differences are significant? And why?

snip

What advantages might we be missing out on by continuing to run chain in tube?

The things I can think of:
- We have run 3" high tubing two years in a row, to accommodate extra center drop as well as a slightly larger sprocket to reduce chain loads/sprocket wear/etc. I have justified this in my head by saying the extra profile yields a stronger frame (torsional), but is it needlessly stronger? Would running belts make it easier to go to 2" profile and maintain the high safety factor I would like, even at large diameter wheels?
- Are belts (when properly tensioned) more accurate with less slop when it comes to measuring distance traveled (for auton)? Or is the difference to chain not worth mentioning?
- Is the system more efficient? More robust? Overall lighter (I know the belts are, but the hubs look heavier than an equivalent plate sprocket).
- Other?

Any rate, we will probably do another iteration of our drivetrain before build season next year, and the discussions on the belt in tube have me intrigued. Thanks for any input!
In no particular order:
  • There really isn't a weight savings with this specific design. The pulleys are about .5lbs heavier than the sprockets would be, and the belts are about .5lbs lighter than the chain would be.
  • While you should always strive to do it right, chain can be a little more forgiving in terms of tension, proper spacing, and alignment, in my opinion.
  • You can run chain-in-tube with 1.5"x2" rails like this design. There are some great posts and videos by 2363 in the forums.
  • I would say yes, 3" tall tube is needlessly strong. If you've got a welded chassis, 2"x1" is plenty strong even if the front and rear tubes are .0625" wall. A lot of the strength of your chassis comes from a properly designed and attached belly pan.
  • When properly tensioned, belts are no more or less accurate than chain in terms of odometry.
  • Chain wears in (which some people perceive as stretching) over time. This can often lead to less tension in the chain later in the season, and can sometimes be a cause of issues depending on chain load. Belts wear too, but I don't think they wear as fast and the effects of belt wear aren't as apparent in an FRC robot.
  • My main reason for wanting to go with belt-in-tube over chain-in-tube is for pulley wear. In FRC, a chain drive will wear both the chain and the sprocket because most of our sprockets are made from AL. In a belt drive, the AL pulley is much harder than the rubber belt and won't wear nearly as fast (if at all).
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