Belts in a Drivetrain

[DampRobot]

Originally Posted by Gregor

Could you put your information in the spreadsheet? With the new versachassis + belts + pulleys from VEXpro your specific setup will become more common, and your data point would be useful.

Done. Thanks for reminding me, I've found looking at that spreadsheet very useful in the past.

[BBray_T1296]

This year we went with a 6 CIM 2 speed 6 wheel drive using colsons and geared with a high speed of ~15.5fps and low of ~5.5fps. We used 15mm belts and pulleys with nearly everything purchased from VexPro.

We were the single strongest drivetrain in terms of pushing power of any other robot in both Dallas and Oklahoma. We faced a self-proclaimed "unpushable robot" and bulldozed them from one side of the field to the other.

While all of the above is true, and the belts did well, I personally would rather recommend using chain for the following reasons:

-Breaking:
At Oklahoma (2nd regional), we broke 3 different belts. I think (I am not 100%) that they were a set used either partway in Dallas, or on our practice bot, so they were not brand new. The cause of the breaking is known. In Dallas and on our practice bot, we used encoders as servos (Im not a coder dont get mad at me). The class in the code also contained a system that would quickly ramp up the power from 0 to 100%, even if full yoke was suddenly applied. In early Oklahoma, one of our encoders broke (the mechanical linkage broke), so the servo class was temporarily removed, and thus the power ramp function. With 3 CIMS geared to 5.5fps, there is an utterly massive amount of torque. Without the power ramp, going from 100 to -100 instantaneously is little problem for the CIMs, but a major problem for the momentum, so somewhere something had to give. The belts did. I would expect the larger gauge of chain would be capable of handling that torque, even if it meant we were spinning the tires.

-Replacing:
In many cases, replacing a chain is much easier than a belt. In our drive you had to remove both pulleys to slip a new belt on, which took a fair amount of time. With chain you can replace one with everything else still fixed. If you break a belt, you throw it away. You break a chain, it is usually repairable.

-Buying:
On our robot, each pair of belts (there were 3 on each side) was a different length. This was mostly due to the fact that with our compact robot, the gearboxes had to go where there was room for everything else. We bought 4 pair of belts (8 of each size). The problem was, when we broke belts, we kept breaking the same one (even with our redundant system: if any one breaks, at least 1 wheel would still turn). When we ran out of that one kind of belt, there were no more, even though we had 2 extra of a different belt. With chain, you just chop it to length. Bring 20' of chain and you could replace each loop once, or one single loop 10 times, depending on your needs.

-Space:
We needed 15mm belts for our drive, as already stated we still snapped them. 9mm would have been worse. ANSI 35 chain is 0.1" thinner than 15mm, and both require slight clearances (guides on pulleys!). While you don't save much, it does add up. Using ANSI 25 chain like we did last year (with no breakages on our 4 CIM 2 Speed 6wd robot) you save 1/8" vs 15mm

-Weight:
While belts are lighter than chain, I think it is not light enough to be worth it.



Overall I was satisfied with the belts this year, and they definitely do work, but as much as I hate to say it, I prefer chain much, much more.

[FrankJ]

Added our experience with Versa blocks. We like them. Easy to tension. Sprocket centers just where the calculator said they should be. Two speed versa gearbox with 36/30 belt drive to the center wheel which drives the outboard wheels. Outboard wheels are driven by 30 tooth pulleys Started with 9mm belts all around. We changed the primary drive belt to 15mm because the 9mm belts were breaking. We have left the outboard drive 9mm with slightly less tension that allows them to ratchet rather than break.



We are using Cheesy Poof style control which seems to help with ramping the violent direction changes.