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Timing belt in drive success
Splitting off of this thread here;
http://www.chiefdelphi.com/forums/sh...hreadid=107998 I'm curious to know the following (please, complete data sets so it's useful) from teams who have run timing belt in drive -Was it 1:1? -Toothcount on pulley -Profile and Pitch -Belt Width -Wheel Diameter and Type (roughtop, colson, etc...) -Gear ratio before the pulley (high and low gear) -tensioners? -If exact c-c, did you add/subtract from this number? Also, what type of machine was the part fabricated on. Once some information is posted, I'll tabulate it for comparison. 973 is prototyping belts for a wcd this fall (after exclusively using belts over chain in the 2012 season), and will update with our results. IF you could post directly in this google doc that'd be great! https://docs.google.com/spreadsheet/...2J4M 2c#gid=0 |
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Great idea:
Helpful resources I've been using up to this point in planning: GT2 Design Manual - http://www.gates.com/catalogs/file_d...cation_id=2999 Light Power and Precision Design Manual - http://www.gates.com/catalogs/file_d...cation_id=2999 The main catalog index - http://www.gates.com/catalogs/index....cation_id=2999 The torque/HP tables have been especially helpful, and can probably go a ways in explaining ratcheting (assuming proper tension/spacing). The Gates FIRST page also has some good basic references/calculators - http://robotics.gatesprograms.com/first |
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FRC836 (The RoboBees) used a Gates timing belt for drive in 2012. It was fairly reliable, considering it was a tensioner/idler system (we had one belt go off pulley three time during the season in competition due to ball interference). Ideally, you should design the system to not have this "feature".
The biggest issue with timing belt drive is supply. We had optimized the drive (pulleys and belt lengths) only to find that Gates and their supplier were out of stock. It was a frantic back-and-forth telephone ordering process to get a solution that worked. The above post has a lot of great resources. I would highly recommend using the off-season to develop a size chart for belt length with variables of drivetrain dimension and the torque/speed requirement (determines pulley diameter/teeth) to make the ordering process as painless as possible come build season. Specs (placeholder for later) Rear-biased, centrally mounted AndyMark SuperShifters 60 tooth pulley (6 wheels all had this pulley, rear axle drove center axle) 92 tooth pulley (main drive to rear axle) 25 tooth pulley at AM SuperShifter This is off memory, but here is a general layout of the drivetrain (I'll have to get a photo). - — - — - — - — - — - — Rear-----AM---------Front |90|-----25-------------- |60|---------60 -------------60--------60 |
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I've seen toothed belts both work well and completely fail.
The time I did see it work well was on 2783's 2009 Unicorn drive... The drive belts were between the CIMs and the crab modules (which had a geardown). The time I saw it fail was on 1747's 2010 drive... they went between supershifters and 8" plaction/plastic Omnis. The belts skipped teeth like crazy and eventually shredded. They were replaced with 35 Chain which solved the problem. What it came down to IMHO was that toothed belts do not torque well... They are better at higher speeds, making then better for interfacing motors to transmissions, not transmissions to wheels. |
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If you look at the gates spec sheet you'll see that belts can handle some really high loads. Can easily handle FRC loads but you have to size appropriately. What size belts were you using? What pitch? Width? Pulley count? -RC |
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I still think that chains are better for high torque situations... |
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We used Gates gt2 belts exclusively on our 2012 robot on nearly every system (ranging from tiny tiny 2mm to big 5mm) without issue. The gt2 profile is FAR superior to generical XL/MXL belts people are probably used to, and are better than it'd predecessor from gates (HTD) which comes in the kop Gates provides all the necessary equations to determine if the belt is strong enough. If the belt fails, you did something wrong; either it was under/over tensioned or it was simply too small to use (both are verifiable items). |
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11 has used belts as well the past two years and Adam is correct- if the belts fail, it is more than likely that it was user error, not a product issue. |
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This fall our team is looking into designing a few custom chassis utilizing newfound resources and skills our team as acquired. One of the areas I have been looking into is WCDs and more recently I am looking into using belts instead of chains.
My question for teams who use belts is: how do you attach the pulleys live axles? Do you hex, key, or use set screws (:ahh: )? Additionally, is there a healthy range of movement for your bearing blocks when using belts? I understand chain stretches overtime and I heard pulley aren't known to stretch but do you leave extra room to make taking belts on and off easier? Sorry for all of the questions, our team is small and young but we are trying to absorb as much as possible! Thank you! BrendanB |
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If your doing a WCD like 254/968 have done. Then broach the pulleys. If you really wanna save some weight and lower part count just do center to center on belts. Don't even put in the bearing blocks. Sdp has a real nice calculator on their site to determine the correct c-c. -RC |
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2826 used XL belts with 18 tooth sprockets in an offseason 8WD. We used an idler tension system. All parts were SPD/SI. The result was an unmitigated disaster. The failure mode began with the belts skipping no mayter the tension applied, then the sprocket flanges popping off and the belts shifting over the sprocket edge, then shredding the belt.
We based our original belt decisions on anecdotes and basic CD research. Liking our lessons learned, we followed the design process and actually performed the proper load calculations. What resulted was that the sprockets were 60% too small and the XL tooth simply could not handle the dynamic load at the width by almost a factor of 3. Lesson: Always do the math! |
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Thank you Chris & RC! |
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Since you mentioned this, and we had that problem too (3929's belt driven shooter), what was your fix? Our quick fix at a competition was just to loctite it in place and it held up after that. |
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We cut 2 test items on our router that felt really nice on the belt by hand, and the full set is being cut on a production cnc so I can only a high quality. We cut off of the sdp-si CAD. |
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Our belt system was sketchy last year, but sufficient enough to last through a regional and champs with no failures. Lots of lessons learned, primarily that the tension required in the belts necessitates a stiff frame (our fiberglass material is good stuff, but flexes way too much). Especially with a serpentine path and cantilevered idlers. Interesting to note, we used 5mm pitch GT2 belt profile coupled with HTD sprockets. Not optimal, but functional enough to work well if the rest of the system is designed properly. We did this for cost purposes, the HTD sprockets were given to us as a sponsorship from Gates. Had we used the proper profile for the pulleys the drivetrain may have performed better, but it worked great at competition. An entertaining video from when we were testing the first iteration of our idler system. We fixed this with a better idler design. http://www.youtube.com/watch?v=8YQk0fjhk-o |
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This year, since we do belts-in-a-tube, we just had a single piece of pulley stock both belts ran on. Worked great. |
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A lot of people have a bias toward chains, and like to give belts a lot of negatives they don't really have. Just do some research and you will be surprised all of the different applications belts were used in.
I would still use a tension system only because it makes mounting much easier. I used sdp-si for all my designs my only negative is sometimes they are out of stock for belts sprockets and can take quite some time for you to get your parts, and that would be horrible if it was in the middle of the build season . Just got to be sure that you have a good selection of sprockets in stock or modify your design for what is in stock. Just out of curiosity what width belts does every one use, I see alot of 3/8 inch belt out there. But I think you could probably go smaller without the fear of breaking a belt. |
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Adam can comment on the other smaller pitches of GT2: 2mm and 3mm. I believe the widths are a lot smaller? -RC |
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Wonder if we could cut them out of 1/4 stock and stack them. |
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We ran 5mm pitch HTDs, 15mm width in our drive this year with remarkable success. We assembled our tensioner-free drivetrain and didn't touch the belts or wheels all season (that includes the tread too, thanks Colsons). The main reason we used HTD vs GT2 was to take advantage of the KOP options and save a little bit of money. HTD profiles also appear to be a lot easier to find, especially in plastic if thats something you are interested in doing. We loved the belts this year and I see no reason we will not continue using them in our drive and in other areas for a long time. We may switch over to 9mm width GT2s just to save some real estate, but either way, belts are here to stay for us. -Brando |
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We will be running a batch using low-voltage EDM, which will (I believe) give the most accurate resolution and accuracy. I am hoping the smoothness spec can still be met with this technique. We will probably be teflon coating these. so it should not matter. If you want to trade an EDM for a milled sprocket sometime, let me know and we can compare/contrast. Also, if you don't go down the Shaker sprocket route and want to add flanges, what is your technique? Our current method is to laser a flange, press in the flange angle guide, and bolt it to the sprocket. If there is a way to do this without post machining the sprocket, that would be handy. Quote:
1. Laminate plates riveted, glued or bolted together to a consistent thickness: Laser and waterjet 0.04, 0.08, 0.125 and 0.25 6061 AL and Steel. Result: Any thickness over 0.08 resulted in a kerf that ended up damaging the belt. 0.04 and 0.08 cut accurately, especially in steel, but the multiple laminations required for a given thickness were difficult to align, even with a complex process and a custom jig. Moreover, the laminations presented a "roughness" that drastically decreased the life of the belt. Overall, not recommended. For what it's worth, this technique has worked very well for us in gear manufacture from small to large (thanks 148 for the inspiration) 2. EDM the sprocket profile to the desired thickness. Works great with gears and I will be trying them on GT-2 belts in the next couple weeks I would like to do a cost/benefit analysis of make VS buy and machine Vs EDM... eventually. |
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Jake, it must be awful nice to be able to get the finish from the EDM.
I'm hoping the milled profile works for us, I can't imagine it won't with how low our runtimes are (even on practice bot) compared to industry. I wouldn't be surprised if it leads to a bit of belt wear. I'd be willing to swap some down the road. Our dream is to run 5mm pitch gt2 9mm wide in 24T with exact c-c; per the math this is marginal, so our expectations is that our prototype will fail. We would then run with tensioners. We're machining our pulleys in one shot into a large block (as attached). We then will hold these on a mandrel to lathe the rear side to length and diameter at size. At this point a flange is generated. For the other flange we will bolt on a .032" plate (easy for us to router) with #4-40s. 987 had a very cool process for their large pulley that we plan to copy for large and less critical pulleys in the future. They picked the drillsize closest to the proper radius for the root of the teeth, and drilled all the teeth. They then used a mill cutter capable of an undercut to remove the top of this material all the way around leaving a flange. We used the same process as you for cutting plate gears this season, in 24, 20 and 16DP. They initially look nasty, but quickly wore smooth. We LOVED it, and probably cut over 100 gears. I'm sure you guys did as well. RC, for 2mm and 3mm pitch gt2 we primarily used 6mm wide. I really need to clean up Encore's CAD to a quality I can post so teams can see how timing belt is really easy to effectively integrate. Utilizing this smaller belts (which are still KILLER strong if you use them right) we were able to package much larger reductions in smaller packages than possible with #25 chain. |
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-Brando |
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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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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. |
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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. |
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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. |
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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. |
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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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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 ;) |
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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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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. :) |
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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. |
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[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 |
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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. |
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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. |
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Can we see some pics/videos?? Glad to see you guys are happy with exact c-c. -Brando |
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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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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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We had 0 issues with our belts all season. -Brando |
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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 |
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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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We have used HTD5-15 short belts with exact C to C and have had no issues.
However this year we used a long HTD5-15 to transmit power across the robot. The short of it- things bend, flex, and stretch. This is a belt tensioner we had to add after several competition. This belt was perfectly tensioned by C to C in the beginning of the season. What changed? A little bit of every thing. With long belts and flexing frames, Tensioners from the beginning may be a good idea. http://wiki.team1640.com/index.php?t...0815_csm-2.jpg It maybe a little complex but, it was a good cad - machining exercise. I believe that the Gates GT-2 belts are superior to the HTDs we use but, the availability and lead times force us to go with HTD. We have not had a problem with plastic pulleys at all. |
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