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Re: Timing belt in drive success
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-Brando |
Re: Timing belt in drive success
Yeah I was looking at the strengths of the different belts and it seemed like 15mm was just kinda overkill, I really think the thinner belts look a lot cleaner too.
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Re: Timing belt in drive success
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Re: Timing belt in drive success
All of this discussion on timing belts is great. I have wanted to use them for a couple years, but availability and lead times has been an issue. Chain and sprockets are just so much easier to get.
That being said, we are close to finishing a 6wd drivetrain that uses a single 15mm wide 5mm pitch gt2 belt on each side. While the student was putting together the design, we kind of guessed at the belt width and pulley size. Actually, he wanted the pulleys to fit inside a 2" tube, so that dictated their size. Recently I have been looking through the gates design guide, and the proper way to select your belt and pulley size is to base it on the power input. A CIM motor's peak power is 340 watts, so the average gearbox is putting out 680 watts, or 0.91 hp. In the design guide there is a table that shows you the power rating of the belt for a given rpm and pulley size. In our case, we use a single belt, so it should be able to handle the full 0.91 hp. The chart says we should have used 30 tooth pulleys instead of 20 tooth. Oops! I'm sure it will run fine as a prototype, but we'll have to be more careful when designing an actual robot. I'm curious what design methodology others use? For those who are interested, you can download the Gates design guide here: http://www.gates.com/brochure.cfm?br...ation_id=11539 Also, for those making their own pulleys, you can buy flanges separately from SDP. |
Re: Timing belt in drive success
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I might be wrong but I think the Gates guide does not really account for the rapid starting and stopping that a FRC robot encounters either. Belt width is directly correlated to the ability to resist skipping/ratcheting. |
Re: Timing belt in drive success
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Re: Timing belt in drive success
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Without knowing your gear ratios I can't really tell much about the forces that are going to be on your belts. But if I were to assume that your wheels are between 3.5" and 4" and your top speed was going to be 15-17fps (fairly standard WCD numbers), then according to the gates catalog the smallest pulley you should run would be 30T, but that's just for high gear. On low gear (assuming you have a 256% spread, just for comparisons sake) the gates catalog says the minimum pulley size would be 56T. Also, gates has a minimum recommended sprocket OD chart which says that in my above hypothetical drivetrain the minimum sprocket OD in high gear would be 2.2", and low gear is off the chart, literally. I apologize in advance if I come off as bashing you here. All my above information comes from gates charts, not from experience, so chances are that you know something I don't when it comes to implementing a belt drive. |
Re: Timing belt in drive success
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Re: Timing belt in drive success
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I've used belts a lot on the team and in personal experience, and we know that for the reduced runtimes we experience in frc, you can get away with higher loading than gates recommends by a good margin. We know that design as posted doesn't pass the Gates' documentation, but that doesn't mean it won't work. It's a pretty marginal design, we don't expect it to work in an exact c-c implementation, but that's why we test. The sizes were determined as an easy size to integrate into our existing WCD. |
Re: Timing belt in drive success
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Also, why did you choose to use 9mm belt over 15mm? |
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Also, keep in mind that the amount of torque that the pullys will start slipping at is highly dependent on the tension on the belts. Perhaps all these stories of belts slipping is less because they exceeded their rated HP and torque, but because they were in a center to center design and couldn't be properly tensioned. |
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After the Suffield Shakedown scrimmage, New York City Regional, Boston Regional, Championships, Battlecry, Beantown Blitz, and IRI we've haven't had a single slip, ratchet, thrown belt or any other failure related to our belt selection. This was with an exact c-c setup. Not that I condone ignoring manufacturers data sheets and calculators, I just wanted to point out that its not always black and white. -Brando |
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Re: Timing belt in drive success
My limited experience with belt drives tells me that exact C-to-C is fantastic.
I designed and built a CNC plasma cutter drive (two identical units) when I was in HS, back in 2005, using exact C-to-C spacing belt drives. When the plasma cutter table was disassembled in 2011 in favor of a more modern setup the belts seemed as taut as the day I installed them. Granted they didn't see production-level usage, but they did suffer HS students leanring how to use CNC machinery for six years ;) |
Re: Timing belt in drive success
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For a given toothcount, beltwidth, etc... combination the strength is proportional to tension in a bell curve shape; too little or too much tension is a strength loss. With exact c-c you are assuming you're merely high enough on the peak, it would be foolish to believe you both picked the EXACT best c-c value and had zero error machining. The lack of a tensioner is certainly attractive with less work to fab, assemble and maintain. The cost of this however is reduced strength compared to a properly tensioned setup. Quote:
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