Belt Driven Drive Trains

[AustinSchuh]

Originally Posted by Sh1ine

Proper tension is not an issue if you use Center to Center calculators and have the manufacturing capabilities to meet those tolerances.

See below.

Originally Posted by Sh1ine

Thank you for running the calculations and directing me to the Gates manual. I need to go through that. I am not an engineer, calculations are not my strong point, but it is something that I need to work on. I know that we are able to run a 15mm HTD belt with a 24 tooth gear for an entire season, plus with out an issue. (In fact we have used one set of belts two seasons in a row without a break)

You don't need to be an engineer to understand this stuff (though it does help with some of the language and what they care about). We have students every year who become belt experts and work through the calcs.

Knowing that a 24 tooth pulley, 15mm wide, HTD works for your specific application is good data. Make sure that is from a year with heavy D. In the end, the design of your drivetrain, the aggressiveness of your drivers, and the duration of your season all play into whether or not a specific belt design will be good enough for you. What works for us may not work for others.

Make sure to scale the torques accordingly with wheel size changes. Bigger wheels will require higher torques to drive.

Originally Posted by Thad House

Even still, that is 4x greater then what the numbers Austin posted say belts can take. If a 15mm 24T pulley can only take 70 in-lb, teams would be snapping them all the time even with 2 CIM drives.

Those numbers are in in-lb. If they look scary, you are right. We were hesitant to try belts for that reason, and only after talking with a Gates rep and 987 were we willing to even try it. We are really on the edge. Wheel traction and robot weight transfer are all key parts of the equation. We make sure to drive our center wheel with a gear, and then spread the torque to the outer wheels from there with belts. This reduces the torque requirements some. See below for more analysis.

Originally Posted by GeeTwo

First of all, exceeding the torques specified in the Gates guidelines will not cause the belt to break, but to jump teeth.

That's not fully true. The Gates guidelines are for 'infinite life', or close to it. You can exceed that torque for short periods of time, but you will dramatically shorten the lifetime of your belt. The exact failure mode of course all depends on the tension of the belts. Too little tension (for the desired load), and you'll see belts jump. To much tension and you risk breaking the tensile elements.

Not to digress too much, but you can tell a lot about how a belt is performing by what happens when it fails. There are 2 classic failure modes. 1) The teeth fall off the belt. This means that you were under tensioned. Each time the belt jumps teeth, it fatigues the teeth. This eventually causes the teeth to de-laminate from the tensile elements and fall off. 2) The 'valleys' in the belt, the space between the teeth, gets crushed and the fibers break. This is a cleaner break. This means that the belt is over-tensioned. 3) I've actually seen failures where both happened at once. This tends to mean that you are just putting too much power through the belt, and can't tension it enough.

Originally Posted by GeeTwo

Finally, I believe that the numbers in Gates' document are applicable for exact C-C calculated pulley spacing. I suspect that many teams with adjustable tension in their belt setups over-tension them. This will shorten the lifespan of the belt (from thousands of hours to hundreds or even tens), but increase the torque that a given size sprocket can deliver to (or receive from) the belt by providing a controlling tension on the "return side" of the belt. With enough tension in the belt, the limiting factor really does become the tensile strength of the belt and sheaves.

Gates does _not_ recommend exact C-C. They have some pretty choice words about it. Please read them. It will make you think twice.

Exact C-C only works if your application is forgiving. There are a bunch of manufacturing tolerances that come into play. Actual pulley diameter, pulley runout, and belt pitch tolerance are all key parts of the equation. The tolerance of the belt pitch can be enough for longer runs that some belts will run over-tensioned on exact C-C drives, and some belts will run under-tensioned. We haven't viewed it as worth the risk to do a study and see if we could get away with exact C-C on a drivetrain. For our manipulator gearboxes, we over-design things enough that we feel comfortable doing an exact C-C for some systems.

Read Page 67 for what Gates has to say about exact C-C. Their manual is very good.

Page 65 has the equations for determining the recommended static tension for a given operating point. It basically boils down to the ratio of the tension in the loaded portion of the belt to the unloaded portion of the belt at your desired operating point. You want something like a 6:1 ratio (check the equations to be sure). Nowhere in there does it say that one tension is best for everything.

There are 3 main ways to test belt tension that I know about. 1) pluck the belt and listen to the note it makes. You can compute the natural frequency of the belt and determine the desired tension. 2) apply a known force to the center of the span and measure the deflection. 3) buy a fancy tool. We actually use 1. It was fun to see the students all running around with tone generators on their phones plucking all the belts and trying to match the pitch. Everyone learned a lot, and the tools are cheap.

At one point, I got Gates to give me rough static break numbers for the tensile elements. The numbers are comparable with #25 chain for a 9mm wide belt, GT2. Food for thought. That is the reason why we can actually get away with the belts in a high peak load system like a DT.

If you actually respect the ratings, #25 chain is only good for a working load of 140 lbs. That works out to 122 in-lb for a 22 tooth sprocket.

Belts are pretty cool. We love their performance, damping, and sound characteristics. Our experiences and safety margins may be a bit higher than other teams due to the long and hard life of our robots. In 2014, we probably approached 100+ hours of runtime on each of our robots. Most of that was hard focused practice under heavy D. My goal is to be able to design systems for FRC that survive those lifetimes from the beginning without over-designing too much.