paper: Belt Driven VersaChassis WCD - Step by Step guide

[Chris is me]

Originally Posted by asid61

From what I've read, you should put in tensioners regardless because of the tolerances required for belt engagement. Adding a cam tensioner is less than 0.1lbs anyway, so there's no reason not to have one.

If you put in tensioners, you could mess it up. If you don't, you can't. A very minor tradeoff, I know, but it's worth noting. It's surprisingly easy to overtension a belt.

You absolutely definitely do not need tensioners for a belt driven WCD, provided you can machine bearing holes in your drive tube with reasonably good accuracy. Our past three years of robots will attest to that.

Originally Posted by M. Mellott

I know that this system uses two 3-CIM transmissions but with only 2 motors each. Do you think that with the back-and-forth, high impact motion that a typical FIRST drive train will go through, the 9mm belts might not be strong enough? Would 15mm belts be a better choice for a drive train, especially for those who want to add on that third motor to each side?

We have had poor experiences with 9mm wide belts in the drivetrain using pulleys about this small. Last year several teams had belt failures doing this, though manufacturing issues at Vex may have contributed in addition to undersizing.

To me there's basically no benefit to not going 15mm, and the reliability is *much* higher. Even if it does work, why would you want to gamble on the edge of a safety factor with your drivetrain? All of our 15mm setups have been rock solid.

By the way, a good calculator for belt center distance is here. http://www.wcproducts.net/how-to-belts/ We have had good results with zero center distance added. Your mileage may vary based on your tolerances.

[Oblarg]

449 ran 9mm belts last year without any issue on their WCD.

Looking at the belt specs (which are conservative - you can get more than the rated torque out of them given how short the lifespan is in FRC compared to industrial applications), if I were running an 8WD I'd probably be happier with 15mm between the two lower wheels. For a 6WD, where the vast majority of the robot's weight is on the center wheel to begin with, I don't think it's a likely problem.

As for why bother with 9mm, it allows one to use the WCP gearboxes with pulleys-in-gearbox just as one would do with chain. Personally, I'm not a huge fan of that setup - if a belt does break (admittedly unlikely but it's better not to tempt Murphy if you can avoid it) it's a royal pain to fix - but even in other setups it's nice and much more compact to be able to fit 2 9mm belts on one pulley than have two pulleys.

[Chris is me]

Originally Posted by Oblarg

As for why bother with 9mm, it allows one to use the WCP gearboxes with pulleys-in-gearbox just as one would do with chain. Personally, I'm not a huge fan of that setup - if a belt does break (admittedly unlikely but it's better not to tempt Murphy if you can avoid it) it's a royal pain to fix - but even in other setups it's nice and much more compact to be able to fit 2 9mm belts on one pulley than have two pulleys.

Making your own pulleys from stock is pretty easy, and buying bigger spacers for a gearbox is even easier!

[Oblarg]

Originally Posted by Chris is me

Making your own pulleys from stock is pretty easy, and buying bigger spacers for a gearbox is even easier!

The former isn't so easy if you're working solely with a drill press and chopsaw.

[MichaelBick]

Originally Posted by Chris is me

Making your own pulleys from stock is pretty easy, and buying bigger spacers for a gearbox is even easier!

Shafts would have to be remade(non-trivial) or some how extended.

[thatprogrammer]

Originally Posted by Chris is me

If you put in tensioners, you could mess it up. If you don't, you can't. A very minor tradeoff, I know, but it's worth noting. It's surprisingly easy to overtension a belt.



To me there's basically no benefit to not going 15mm, and the reliability is *much* higher. Even if it does work, why would you want to gamble on the edge of a safety factor with your drivetrain? All of our 15mm setups have been rock solid.

By the way, a good calculator for belt center distance is here. http://www.wcproducts.net/how-to-belts/ We have had good results with zero center distance added. Your mileage may vary based on your tolerances.

Are larger spacers on the wheels required if going 15mm?

[Chris is me]

Originally Posted by thatprogrammer

Are larger spacers on the wheels required if going 15mm?

Yeah. If you're using 15mm wide pulleys, you'll need roughly 1/4" more in spacers (6mm to be exact) for the wheel with an offset pulley. You may also need larger standoffs for the Vex 3-CIM Ballshifter. The other wheel should be fine.

You do need a lathe to make custom 30+mm wide pulleys if you don't want two 15mm ones, and you may need to modify the outer wheel shafts a bit (may not be enough room for the standard locking collar retension, haven't checked).

[thatprogrammer]

Originally Posted by Chris is me

Yeah. If you're using 15mm wide pulleys, you'll need roughly 1/4" more in spacers (6mm to be exact) for the wheel with an offset pulley. You may also need larger standoffs for the Vex 3-CIM Ballshifter. The other wheel should be fine.

You do need a lathe to make custom 30+mm wide pulleys if you don't want two 15mm ones, and you may need to modify the outer wheel shafts a bit (may not be enough room for the standard locking collar retension, haven't checked).

Would I be correct in assuming the same things apply to the WCP dog shifter? I do know that both pulleys should fit in with no problem, but I'm not sure if the spacer size would change.