Belt Driven Drive Trains

[Sh1ine]

Thank you for your feed back. If clearance were the only thing I was worried about I would just go with the 18 tooth pulley and not bother to ask how they would work here.

In my opinion, if you have the machining capabilities, belts are superior to chain. Proper tension is not an issue if you use Center to Center calculators and have the manufacturing capabilities to meet those tolerances. We have a HAAS mill at school and each rail is machined from one piece of aluminum tube. Using our construction technique we are able to keep our belt centers within a few thousands if not better. I think many teams assume chain needs a tensioner so a belts must also. For the purpose of our application adding tension adjustment just adds to the chance that the belts will fail, try Center to Center and you will never go back.

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) According to your calculations it seems like I could go to a 18 tooth pulley if I went with a GT3 system, which is good food for thought. A 21 tooth pulley would also be an option for us, we like to stick with teeth in multiples of three so that we can hold them in a 3 jaw chuck.

On different years we run different size wheels. Typically we use a 6" Colson wheel, but when we have a flat field we use a 4" wheel. With a 3" tube we get about 1/2" of clearance under the robot. I would love to be able to use a 4" wheel more often, so I want the team to develop a prototype this fall. We have always used a single speed gearbox, in 2014 we had a 6 CIM drive train with 4" wheels that traveled at about 13 f/s. Last year we used a Mecanum drive, so that does not count for this conversation. I will take a look through our old CAD to find a good representation of our drive train tube. It will shed some light to the design.

Again, thank you for your feed back. It is very valuable.

Originally Posted by AustinSchuh

To be honest, if clearance is the only thing you are worrying about, then you have all your priorities wrong.

971 always starts with the following document: http://www.gates.com/~/media/files/g...nual.pdf?la=en Bookmark it and read it. It is awesome and we've found it pretty accurate over the years.

The biggest challenges in a belt drive system are belt life, strength, and proper tension. We've struggled most with proper tension over the years. We also try to design for 0 belt failures over the year, since the drivetrain is such a key part of the robot. Our year is 3 competitions, and 3 offseasons on one robot, and many many weekends of practice on the other robot.

Lets work through some of the numbers. I'm going to start with numbers from 971's drivetrain, which I know well, and then lets extrapolate.

For the last couple years, we have been running 24 tooth pulleys with 3.5" wheels. In 2012 and 2013, we ran GT2 belts, 9mm wide, and they wore out by the end of the year and needed to be replaced. In 2014, we ran GT3, 9mm wide, and when they weren't perfectly tensioned, they broke. Too little tension and the teeth ripped off. To much and we broke the tensile element. It was a fine line. To me, that meant that we were on the edge of what they are capable of. We've since moved to 15mm wide GT3 so we have some safety margin.

Lets pick GT2, 9mm wide as the 971 suggested torque without a safety factor.
GT2, 24 tooth, 9mm wide -> 75 in-lb of torque.

Stall torques for the other configurations we've used in the past:
GT3, 24 tooth, 9mm wide -> 86 in-lb of torque.
GT3, 24 tooth, 15mm wide -> 159 in-lb of torque.

For grins, lets look at some other numbers. VexPro uses the HTD tooth profile.
HTD, 24 tooth, 9mm wide -> 36.9 in-lb of torque
HTD, 24 tooth, 15mm wide -> 70 in-lb of torque

You can run GT2/GT3 belts on HTD pulleys, and you will get performance somewhere between the two (Gates won't give you numbers on it, but will tell you that it is supported and is better than pure HTD). Food for thought.

So, lets analyze the proposed configuration

HTD, 18 tooth, 15mm wide -> 49 in-lb of torque

My opinion is that if you were to put that on one of 971's robots, you'd destroy the belt pretty fast. I don't have enough info from your original post to run those numbers. Wheel-size is another variable that I've been ignoring. Bigger wheels will require stronger belts.

Some more numbers to get you thinking:
HTD, 22 tooth, 15mm wide -> 62 in-lb of torque
GT3, 18 tooth, 15mm wide -> 101 in-lb of torque

(If you've already run these numbers, then this post will help others understand the tradeoffs we go through in belt selection).