pic: Another Belt-In-Tube Drivetrain

[1493kd]

We learned everything we know about belt in tube from 2791 but I will agree 100% that it is very robust and actually pretty simple once you get the hang of it... (And have machining capabilities)

2015- 1.5x3 side rails, 6" Colson wheels, 1/8" drop- 24 tooth pulley stock used to make custom pulleys that we counter bored to eliminate the need for a spacer on the bearing. Image below:
http://imgur.com/goFii7G


2016- 1.5x3 rails, 6" West Coast Pneumatic wheels, 1/8" drop, 24 tooth pulley with the counter bore.

The only negative I have with this set up is how much it weights, but the trade off of not having to touch it after we set it up is worth it to me.

[Chris is me]

Originally Posted by Cothron Theiss

What are your concerns with using smaller pulleys? I assume that the belts would slip before the pulleys or belt would fail, but is belt slippage your concern with a pulley smaller than 24t?

Belt skipping / ratcheting often damages the belts as the teeth are being loaded in an odd shocking manner, so from a robustness standpoint ratcheting is to be avoided as it is itself a precursor to belt failure.

Belt / pulley strength is a function of two things - the diameter (tooth count) of the pulley, and the width of the belt. A larger diameter pulley decreases the force put on the belt teeth a given output torque. A wider belt decreases the stress placed on the tooth by spreading the force out to a wider area. As belt sizes get thinner and as pulleys get smaller, eventually you start to skip teeth via ratcheting, and eventually you'll fail a belt.

In a drivetrain, with HTD belts, 24T pulleys and 9mm wide belts are riding on the edge of feasibility - these drives tend to fail belts by ratcheting and eventually tensile failure. I haven't experimentally determined how small of a pulley you can go to in order to safely use a 15mm wide belt - I'm sure I could estimate it with some math, I just haven't done it. I seem to recall 18T pulleys would cause 15mm belts to skip in a drivetrain, but I don't remember if that was speculation I heard someone else say or something actually based on real world experience.

[Cothron Theiss]

Originally Posted by Chris is me

In a drivetrain, with HTD belts, 24T pulleys and 9mm wide belts are riding on the edge of feasibility - these drives tend to fail belts by ratcheting and eventually tensile failure. I haven't experimentally determined how small of a pulley you can go to in order to safely use a 15mm wide belt - I'm sure I could estimate it with some math, I just haven't done it. I seem to recall 18T pulleys would cause 15mm belts to skip in a drivetrain, but I don't remember if that was speculation I heard someone else say or something actually based on real world experience.

Hm, it would be nice to know whether an 18t HTD pulley will work with 15mm wide belts. I'm currently working on a WCD that would be using that setup, so I have a bit of a vested interest. For your purposes, since you're already machining your pulleys out of stock, couldn't you switch to GT3 belts and GT2 pulleys? there's not a huge difference, but it would give you some margin of safety for the the setups that are on the cusp of failure.

[Mark Sheridan]

Originally Posted by Cothron Theiss

Hm, it would be nice to know whether an 18t HTD pulley will work with 15mm wide belts. I'm currently working on a WCD that would be using that setup, so I have a bit of a vested interest. For your purposes, since you're already machining your pulleys out of stock, couldn't you switch to GT3 belts and GT2 pulleys? there's not a huge difference, but it would give you some margin of safety for the the setups that are on the cusp of failure.

yeah we have broken HTD 15mm with 18 tooth. This year we made the pulleys almost the same diameter as our wheels to have a beefy safety factor with HTD.

Most likely, if we stick with belts, is to go with GT3/GT2 like 971.

[Ty Tremblay]

Originally Posted by Mark Sheridan

yeah we have broken HTD 15mm with 18 tooth. This year we made the pulleys almost the same diameter as our wheels to have a beefy safety factor with HTD.

Most likely, if we stick with belts, is to go with GT3/GT2 like 971.

How much stronger is GT3? Looking at the profiles, I can't tell the difference.

Edit: Google says 2x as strong.

[asid61]

Originally Posted by Ty Tremblay

How much stronger is GT3? Looking at the profiles, I can't tell the difference.

Edit: Google says 2x as strong.

Cripes, that's a ton! Who knew that such tiny changes could have such an effect...
The difference between trapezoidal and HTD is just as great somehow. It's a shame that GT2/GT3 is patented, otherwise Vex would be able to produce them (I think?). Does anybody know why Vex doesn't use GT2?
Also, can GT2/GT3 belts be used with HTD pulleys, and what effect does that have?

[Cothron Theiss]

Originally Posted by Mark Sheridan

yeah we have broken HTD 15mm with 18 tooth.

Do you mind providing details on what parts you were using and how it failed? The 18t VexPro timing pulleys are plenty strong enough for use in an FRC drivetrain under normal conditions, but as Chris explained earlier, there must have been some slippage and racheting going on that lead to your failure.

Originally Posted by asid61

Cripes, that's a ton! Who knew that such tiny changes could have such an effect...
The difference between trapezoidal and HTD is just as great somehow. It's a shame that GT2/GT3 is patented, otherwise Vex would be able to produce them (I think?). Does anybody know why Vex doesn't use GT2?
Also, can GT2/GT3 belts be used with HTD pulleys, and what effect does that have?

Yes, the GT belt profile is trademarked. (GT stands for "Gates Tooth," so they made sure everyone would know who came up with it.) This means that Vex cannot produce ACTUAL GT belts or profiles. But there is nothing against measuring the belt teeth and coming up with a very very close approximation to the GT profile. So if you look at WCP's page on the GT2 belts and pulleys they sell, they refer to it as "GT2 compatible profile." I assume that this is just a legal workaround to use the GT profile.
And yes, I believe that GT (3mm) belts can be used with HTD (3mm) pulleys, and if I recall correctly, it performs about as well as HTD with HTD.

[Mark Sheridan]

Originally Posted by Cothron Theiss

Do you mind providing details on what parts you were using and how it failed? The 18t VexPro timing pulleys are plenty strong enough for use in an FRC drivetrain under normal conditions, but as Chris explained earlier, there must have been some slippage and racheting going on that lead to your failure.

No slippage or ratcheting. We have good control on build quality to get our Center to center right. its was overloading with too many cycles and the belts snapped on our drive. Our practice bot easily runs 100 simulated matches in a week. So we always have the practice bot on a lifecycle far ahead of the comp bot to catch these sort of failures.

971 helped us out and pointed out that these belts are being loaded beyond their recommended limits. We built a calculator just to double check belt loading. We also swapped a bunch of belts on the same drive rail and noticed despite the same CC, the tensions were different. We try to reduce belt load and build in tensioners which is what we did this year. The last conversation I had with the students is that we are in the mood of having the drive on the practice bot survive with minimal maintenance for over 1000 simulated matches. we are just sick of worry about this stuff, so keep in mind that perspective.

I think using 22 tooth pulley and 15mm wide GT profile belt will be a step in the right direction for Ty's design, which is pretty sweet.

[R.C.]

Originally Posted by Cothron Theiss

Yes, the GT belt profile is trademarked. (GT stands for "Gates Tooth," so they made sure everyone would know who came up with it.) This means that Vex cannot produce ACTUAL GT belts or profiles. But there is nothing against measuring the belt teeth and coming up with a very very close approximation to the GT profile. So if you look at WCP's page on the GT2 belts and pulleys they sell, they refer to it as "GT2 compatible profile." I assume that this is just a legal workaround to use the GT profile.
And yes, I believe that GT (3mm) belts can be used with HTD (3mm) pulleys, and if I recall correctly, it performs about as well as HTD with HTD.

Pretty much,the belts are not produced by Gates and the profile is not the same as the GT2 or GT3 profile by gates, it slightly different. Will be putting up test data over the next few months to compare the profile we sell vs Gates GT2/G3.

[Cothron Theiss]

Originally Posted by R.C.

Pretty much,the belts are not produced by Gates and the profile is not the same as the GT2 or GT3 profile by gates, it slightly different. Will be putting up test data over the next few months to compare the profile we sell vs Gates GT2/G3.

That's excellent, thank you! Any chance you can also throw in testing of HTD so that teams can compare your data against something most teams are more familiar with?

[R.C.]

Originally Posted by Cothron Theiss

That's excellent, thank you! Any chance you can also throw in testing of HTD so that teams can compare your data against something most teams are more familiar with?

Will do. Let me know if there is anything specific you'd like in terms of testing.

As a side note the pulleys we sell and future pulleys we will be releasing are the actual GT profile. The pulleys are free game for anyone to sell/make just not the belts sadly.

Thanks!

[Cothron Theiss]

Originally Posted by R.C.

Will do. Let me know if there is anything specific you'd like in terms of testing.

As a side note the pulleys we sell and future pulleys we will be releasing are the actual GT profile. The pulleys are free game for anyone to sell/make just not the belts sadly.

Thanks!

That's great news! It'll be interesting if teams can use the more compact GT pulleys to make belt-in-tube drivetrains using the 2x1 VersaFrame.

[Joey Milia]

Originally Posted by Ty Tremblay

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.

[Steven Smith]

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!

[asid61]

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.