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Re: Timing belt in drive success
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Re: Timing belt in drive success
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Re: Timing belt in drive success
We loved our octocanum drive last year. Loved. It. It was, however, too heavy, too expensive, and too hard to work on. This fall is dedicated to fixing all three. My answers are about last year's system (which was only possible to us because of Gates. They rule.)
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Sorry... Quoted first, saw the Google Docs part last, and am too lazy (and/or busy) to post there. :) |
Re: Timing belt in drive success
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The service factors gates provides are used by multiplying the factor by the horsepower and using the product to determine sizes. It's hard to fit an FRC robot into a service factor category, since the loads are so diverse and the runtimes so short. But the lesson to be learned from the service factor chart is how important it is to tailor the belt drive to the application and not just the HP. The gates service factor chart ranges from 1 all the way to 2.5 (2.5 is constant duty), with direction-changing applications having the highest service factor. All that being said, it's pretty clear that application is extremely important when designing a best drive and for that reason the gates charts are more of an indicator than something to be followed exactly. tl;dr belt drive failure comes from poor execution, wrong pulley size, and not having the experience and engineering intuition necessary to do it right. |
Re: Timing belt in drive success
[quote=JamesCH95;1184543]My limited experience with belt drives tells me that exact C-to-C is fantastic.
C to C will work for a lot of short run cases but when running belts in a drive train we would hesitate using permanent center to center distance without some adjustable tensioning device. You need to think of the assembly and maintenance of the system. Things like how a team would change out a belt needs to go into the design. Is the frame stiff enough to hold the center distance? Can you access the belts for inspection? How could a team tension the belt if the numbers didn't work out exact? Adrian hit the mark. Proper tension of a belt is needed to tune your drive train for maximum power |
Re: Timing belt in drive success
Only a few hours into drive practice so far, but we are happy with the performance.
Reposting specs for reference; 8wd, 7.5" - 15.5" - 7.5" wheel spacing with 3.5" colsons. Outer raised .3" 24T 9mm wide gt2 Pulleys/belt Andymark stock gear ratio (12:40 initial, 28:35 and 15:48) Exact c-c+.003 was applied. For the short runs this works really well, and was almost hard to install. For the longer runs it could be tighter, and we might add more c-c in future. No ratcheting issues so far. Robot competes in 9 days, and again 14 days later. After that we'll know for sure how we feel about this setup. |
Re: Timing belt in drive success
We re-did our drive system with gates HTD belts (15mm wide, 5 mm pitch) for Cal games and never had a problem. We are running AM Shifters (4:1) directly to our center 4" wheel and out to the front and back wheels with 34 tooth aluminum sprockets from sdp/si (faced the hubs off and broached the bore to hex). Used live .5" steel hex axles. Tensioning is done on the axles with bearing blocks within the frame. We were nervous about how they would do, especially when our bot is designed to carry another robot up and down the bridge. While it never happened at Cal Games, we performed the maneuver 20 or so times in practice. Belts never skipped... never. We even ran the bot (fully loaded to 250 lbs) into the wall a few times in low gear to see if we could force them to skip. Motors/breakers overheated before any skipping.
We're sold... we plan to use Gates belts on our 2013 drive system, it also saves about 3 lbs. The only thing we still need to perfect is the correct amount of tension. One side of our drive is over-tensioned we think because it doesn't drive straight any more and we were too worried about letting up on the tension when at Cal Games. Now is the time to experiment a bit. |
Re: Timing belt in drive success
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Can we see some pics/videos?? Glad to see you guys are happy with exact c-c. -Brando |
Re: Timing belt in drive success
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Re: Timing belt in drive success
Just an FYI. Equal tension is critical on the drive train belts. If any of the 4 belts are over tensioned stress is introduced to the belt. We have had a couple of belts break through practice wear and tear and over tensioning. When designing the drive train make sure you can easily replace the belts on the go and they all have the same tension.
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Re: Timing belt in drive success
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Re: Timing belt in drive success
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Playing around with an idea, the attached picture is a quick infographic of sorts detailing the pertinent dimensions, factors in the drivetrain. I'm trying to find a good compromise of minimal drawing, maximum information conveyed. It'd be cool to develop a psuedostandard for teams to compare/post drivetrains. Any comments?
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Re: Timing belt in drive success
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We had 0 issues with our belts all season. -Brando |
Re: Timing belt in drive success
These may be a bit too close to the edge of disaster but has anyone successfully used the 9mm width polycarbonate pulleys that SDP/SI?
In the 5mm pitch, they are only available in the HTD profile. Was considering doing some testing with an 8wd. They tend to be less expensive than their aluminum counterparts but still a considerable expense for test purposes. --Pat |
Re: Timing belt in drive success
Has anyone using belts for their drivetrain experienced belts that are too short to be assemble? If .003" is acceptable and assembleable, is there an upper limit for "add-on?" Is .003" it? In any case, how can adding a few mils get around the tolerance problem (+/- .012" center distance per a previous poster)? Does gates simply advertise a "tolerance" that is much larger than their usual deviations from nominal length?
I'm just a little surprised by the fact that anyone has had any success at all with exact c-c drivetrains. The setup just reeks of improper constraint to me, which typically causes many assembly headaches and binding in mechanical transmissions. |
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