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AdamHeard 30-08-2012 17:50

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

scottandme 30-08-2012 18:40

Re: Timing belt in drive success
 
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

protoserge 04-09-2012 12:05

Re: Timing belt in drive success
 
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

ratdude747 04-09-2012 12:31

Re: Timing belt in drive success
 
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.

R.C. 04-09-2012 12:45

Re: Timing belt in drive success
 
Quote:

Originally Posted by ratdude747 (Post 1184075)
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.

Disagree,

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

Ether 04-09-2012 13:14

Re: Timing belt in drive success
 
Quote:

Originally Posted by ratdude747 (Post 1184075)
What it came down to IMHO was that toothed belts do not torque well...

I had an '83 Kawasaki 750 oversquare twin that had enough low-end torque to pull stumps out (hyperbole). It had a Kevlar toothed drive belt.



ratdude747 04-09-2012 18:00

Re: Timing belt in drive success
 
Quote:

Originally Posted by R.C. (Post 1184078)
Disagree,

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

IIRC it used all KOP sized belts and Pulleys. The ones with the really shallow teeth.

I still think that chains are better for high torque situations...

AdamHeard 04-09-2012 18:13

Re: Timing belt in drive success
 
Quote:

Originally Posted by ratdude747 (Post 1184121)
IIRC it used all KOP sized belts and Pulleys. The ones with the really shallow teeth.

I still think that chains are better for high torque situations...

This is unfounded opinion though. What is better? I can spec a chain and belt to handle the same power output no problem. This kind of opinion on chief could potentially misinform less experienced users.

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).

Akash Rastogi 04-09-2012 18:13

Re: Timing belt in drive success
 
Quote:

Originally Posted by ratdude747 (Post 1184121)
IIRC it used all KOP sized belts and Pulleys. The ones with the really shallow teeth.

Those are more likely to be used in manipulator design/conveyor design. I've never seen a team use the kit belts/pulleys in a drive.

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.

AdamHeard 04-09-2012 18:14

Re: Timing belt in drive success
 
Quote:

Originally Posted by Akash Rastogi (Post 1184125)
Those are more likely to be using in manipulator design/conveyor design. I've never seen a team use the kit belts/pulleys in a drive.

The kop pulleys are 5mm pitch HTD. Which 125 ran in drive this season (and possibly before?).

Akash Rastogi 04-09-2012 18:27

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184126)
The kop pulleys are 5mm pitch HTD. Which 125 ran in drive this season (and possibly before?).

Yup sorry I mean solely the pulleys that physically come in the kit, not the type. Meaning including the large pulley that comes in the kop. Silly assumption was that they didn't order more of the same kind as in the kit. Read/misread his post too literally when he said they used what was in the kit.

BrendanB 04-09-2012 21:28

Re: Timing belt in drive success
 
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

R.C. 04-09-2012 22:16

Re: Timing belt in drive success
 
Quote:

Originally Posted by BrendanB (Post 1184167)
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

Brendan,

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

BrendanB 04-09-2012 22:36

Re: Timing belt in drive success
 
Quote:

Originally Posted by R.C. (Post 1184174)
Brendan,

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

Using the SDP calculator for spacing will the belts loose tension overtime?

trilogy2826 04-09-2012 22:53

Re: Timing belt in drive success
 
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!

Chris is me 04-09-2012 23:10

Re: Timing belt in drive success
 
Quote:

Originally Posted by BrendanB (Post 1184187)
Using the SDP calculator for spacing will the belts loose tension overtime?

Not even a little bit. Use the GT2 tooth profile and do the math.

trilogy2826 04-09-2012 23:19

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184124)
We used Gates gt2 belts exclusively on our 2012 robot on nearly every system

Does anyone have experience manufacturing the GT2 sprockets? Gates specifically warns against using their CAD to make physical sprockets. The available sprockets seem inefficient from a weight perspective. We have the tools to do this, but without being able to confirm the tooth profile, it is a risk. Either way, I will make some up and let you know how they work out.

BrendanB 04-09-2012 23:23

Re: Timing belt in drive success
 
Quote:

Originally Posted by Chris is me (Post 1184191)
Not even a little bit. Use the GT2 tooth profile and do the math.


Thank you Chris & RC!

Akash Rastogi 04-09-2012 23:27

Re: Timing belt in drive success
 
Quote:

Originally Posted by trilogy2826 (Post 1184190)
then the sprocket flanges popping off and the belts shifting over the sprocket edge


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.

AdamHeard 04-09-2012 23:30

Re: Timing belt in drive success
 
Quote:

Originally Posted by trilogy2826 (Post 1184194)
Does anyone have experience manufacturing the GT2 sprockets? Gates specifically warns against using their CAD to make physical sprockets. The available sprockets seem inefficient from a weight perspective. We have the tools to do this, but without being able to confirm the tooth profile, it is a risk. Either way, I will make some up and let you know how they work out.

We are machining some 24T 5mm gt2 9mm width pulleys currently that we will be using on a prototype. I'll update with results.

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.

s_forbes 04-09-2012 23:46

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184199)
We are machining some 24T 5mm gt2 9mm width pulleys currently that we will be using on a prototype. I'll update with results.

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.

I will be interested to see how that turns out.

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

Chris is me 05-09-2012 00:27

Re: Timing belt in drive success
 
Quote:

Originally Posted by trilogy2826 (Post 1184194)
Does anyone have experience manufacturing the GT2 sprockets? Gates specifically warns against using their CAD to make physical sprockets. The available sprockets seem inefficient from a weight perspective. We have the tools to do this, but without being able to confirm the tooth profile, it is a risk. Either way, I will make some up and let you know how they work out.

We bought pulley stock and machined it down at home. Not too hard to work with. You'll want a number of teeth divisible by three if you can in order to make it easier to hold in a three jaw chuck, but it machines easily and you can easily remove the hub or any other features you might not want.

This year, since we do belts-in-a-tube, we just had a single piece of pulley stock both belts ran on. Worked great.

steelerborn 05-09-2012 01:02

Re: Timing belt in drive success
 
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.

R.C. 05-09-2012 02:47

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184199)
We are machining some 24T 5mm gt2 9mm width pulleys currently that we will be using on a prototype. I'll update with results.

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.

We did the same thing with a few pulleys, looked pretty close. A Gates Rep said SDP-SI pays them to use their formula. So I figure the model should be super close? Maybe?

Quote:

Originally Posted by steelerborn (Post 1184212)
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.

For GT2 5mm, common widths are 9mm and 15mm. We plan to use a lot of 9mm for all our non drive items. Rollers/Conveyors/Grippers etc... For Drive, 15mm would be pretty safe, but I bet you can probably get away with 9mm.

Adam can comment on the other smaller pitches of GT2: 2mm and 3mm. I believe the widths are a lot smaller?

-RC

IndySam 05-09-2012 09:12

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184199)
We are machining some 24T 5mm gt2 9mm width pulleys currently that we will be using on a prototype. I'll update with results.

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.

Hmmm, we may have to try and waterjet some as an experiment. Being able to cut the profile and the hex might be a huge savings in time and cost.

Wonder if we could cut them out of 1/4 stock and stack them.

Chris is me 05-09-2012 10:53

Re: Timing belt in drive success
 
Quote:

Originally Posted by steelerborn (Post 1184212)
I would still use a tension system only because it makes mounting much easier.

We came up with a simple solution this year to make mounting stupid simple. I don't have any photos of it, but I can try to find one if someone wants it. Basically, we milled the bearing hole in the chassis with a small groove in it toward the center that would let us insert the axles and belts with slack. We then pulled the belts tight, riveted a bearing retainer on the outside, and placed the bearings in. This made installation super easy and after we did that, we never touched the belts again.

Quote:

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.
We used 15mm this year, which was strong and safe enough for our needs. Like RC said we would probably use 9mm for anything else. Always GT2 profile unless you need to save money or something.

Brandon Holley 05-09-2012 11:01

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184126)
The kop pulleys are 5mm pitch HTD. Which 125 ran in drive this season (and possibly before?).

Sorry I'm late to the party on this one, I severed myself from the internet to enjoy the long weekend.

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

BrendanB 05-09-2012 11:10

Re: Timing belt in drive success
 
Quote:

Originally Posted by Chris is me (Post 1184262)
We came up with a simple solution this year to make mounting stupid simple. I don't have any photos of it, but I can try to find one if someone wants it. Basically, we milled the bearing hole in the chassis with a small groove in it toward the center that would let us insert the axles and belts with slack. We then pulled the belts tight, riveted a bearing retainer on the outside, and placed the bearings in. This made installation super easy and after we did that, we never touched the belts again.

Chris if you have a picture that would be most appreciated! Ingenious solution!

trilogy2826 05-09-2012 12:44

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184199)
We cut off of the sdp-si CAD.

Thanks for the point in the right direction. I was assuming (why do I keep doing that) that the Gates proprietary profile for GT-2 would not stray past their walls. I did an overlay of the SDP/SI 24T and the Gates 24T and they are indeed different in tooth geometry. Not much, but just enough to heed Gates warning about not using the CAD to machine.

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:

Originally Posted by IndySam (Post 1184255)
Hmmm, we may have to try and waterjet some as an experiment. Being able to cut the profile and the hex might be a huge savings in time and cost.

Wonder if we could cut them out of 1/4 stock and stack them.

We tried the following techniques in the 2011 and 2012 offseason to manufacture sprockets and gears:

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.

AdamHeard 05-09-2012 13:41

Re: Timing belt in drive success
 
1 Attachment(s)
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.

Brandon Holley 05-09-2012 13:53

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184284)
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.

I think you guys will be pleasantly surprised ;).

-Brando

steelerborn 05-09-2012 14:03

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.

sdcantrell56 05-09-2012 14:21

Re: Timing belt in drive success
 
Quote:

Originally Posted by steelerborn (Post 1184292)
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.

As someone who has designed and run belt drivetrains the past 3 years I will say 15mm width is not overkill. We tried 9mm width GT2 in 2010 with bigger pulleys than most people are talking about and had belt ratcheting issues every time we crossed the bumps. Swapping to 15mm width for 2011 and 2012 resulted in the belts never ratcheting even with larger wheels. Stick to 15mm width

Rob Stehlik 05-09-2012 14:37

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.

sdcantrell56 05-09-2012 16:11

Re: Timing belt in drive success
 
Quote:

Originally Posted by Rob Stehlik (Post 1184294)
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.


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.

Akash Rastogi 05-09-2012 17:30

Re: Timing belt in drive success
 
Quote:

Originally Posted by sdcantrell56 (Post 1184293)
As someone who has designed and run belt drivetrains the past 3 years I will say 15mm width is not overkill. We tried 9mm width GT2 in 2010 with bigger pulleys than most people are talking about and had belt ratcheting issues every time we crossed the bumps. Swapping to 15mm width for 2011 and 2012 resulted in the belts never ratcheting even with larger wheels. Stick to 15mm width

Did you happen to have any ratcheting balancing on the bridges this season Sean?

Adrian Clark 06-09-2012 22:10

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184284)
Our dream is to run 5mm pitch gt2 9mm wide in 24T

I might be a little late here, but could you elaborate on how you chose your width and tooth#?

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.

sdcantrell56 06-09-2012 23:22

Re: Timing belt in drive success
 
Quote:

Originally Posted by Akash Rastogi (Post 1184316)
Did you happen to have any ratcheting balancing on the bridges this season Sean?

No ratcheting that I ever heard and that even included driving onto the bridge and engaging mechanical brakes which locked the drive transmissions.

AdamHeard 07-09-2012 01:07

Re: Timing belt in drive success
 
Quote:

Originally Posted by Adrian Clark (Post 1184491)
I might be a little late here, but could you elaborate on how you chose your width and tooth#?

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.

We would be running 3.5-4" wheel with the stock AM shifter ratio (11T pinions instead of 12T for 4" wheel).

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.

Adrian Clark 07-09-2012 03:18

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184506)
We would be running 3.5-4" wheel with the stock AM shifter ratio (11T pinions instead of 12T for 4" wheel).

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.

I'm sure the gates charts have a sizable safety margin, but without knowing the exact size of that margin its hard to tell if a belt drive will have ratcheting problems or not. But I guess that's what testings for. My biggest worry about using pulley below suggested size is that weather you're using exact c-c or using a tensioning system if the pulley is too small there's nothing that can be done to prevent it ratcheting, even with massive amounts of tension.

Also, why did you choose to use 9mm belt over 15mm?

DampRobot 07-09-2012 09:56

Re: Timing belt in drive success
 
Quote:

Originally Posted by Adrian Clark (Post 1184518)
I'm sure the gates charts have a sizable safety margin, but without knowing the exact size of that margin its hard to tell if a belt drive will have ratcheting problems or not.

Actually, I would guess not. I was doing some calculations for bevel gear max HP and torque, and my results were within rounding error of the published values. Perhaps Gates divides by a safety factor, but I would suspect not.

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.

Brandon Holley 07-09-2012 10:07

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184506)
We know that design as posted doesn't pass the Gates' documentation, but that doesn't mean it won't work.

For our drivetrain this past year, the Gates documentation vehemently showed our design would fail. We rolled the dice and decided to go for it anyway, based purely on intuition and previous experience.

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

Chris is me 07-09-2012 10:27

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184506)
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.

I'm a little confused. Are you saying the design won't work because of the exact center spacing? I don't see how that would make the belt any weaker than a tensioned system - if anything a tensioned system would be less foolproof and more finicky.

Quote:

Originally Posted by DampRobot (Post 1184538)
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.

Of the two belt designs we've run, our exact center design didn't ratchet / slip and our tensioned design did. It has a lot more to do with drive sizing than method of holding tension.

JamesCH95 07-09-2012 10:44

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 ;)

AdamHeard 07-09-2012 14:28

Re: Timing belt in drive success
 
Quote:

Originally Posted by Chris is me (Post 1184542)
I'm a little confused. Are you saying the design won't work because of the exact center spacing? I don't see how that would make the belt any weaker than a tensioned system - if anything a tensioned system would be less foolproof and more finicky.

You have to assume that an exact c-c will run at a reduced strength.

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:

Originally Posted by Adrian Clark (Post 1184518)
I'm sure the gates charts have a sizable safety margin, but without knowing the exact size of that margin its hard to tell if a belt drive will have ratcheting problems or not. But I guess that's what testings for. My biggest worry about using pulley below suggested size is that weather you're using exact c-c or using a tensioning system if the pulley is too small there's nothing that can be done to prevent it ratcheting, even with massive amounts of tension.

Also, why did you choose to use 9mm belt over 15mm?

Like i mentioned before, the toothcount, width, etc... were all picked so that they could integrate into our existing WCD (with zero modifications to the WCD). If substantial modifications are required, it diminishes the appeal as we have years of standardized drive parts useful for prototyping, practice bot, etc...

JamesCH95 07-09-2012 16:13

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184575)
You have to assume that an exact c-c will run at a reduced strength.

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.

The question to answer is which weighs and costs more (in a holistic sense): wider belts and pulleys, or tensioning mechanism?

Brian Selle 07-09-2012 16:39

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1184575)
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.

This. Earlier in the summer I emailed Gates about doing C-C drives and while their engineer said it could be made to work they recommended tensioners to dial things in and maximize performance. One of the biggest things I learned was that while stretch is almost negligible (0.00529" for a 700mm belt), the manufacturing tolerance is not (+/-0.024" for a 700mm belt). For this size belt it means +/-0.012" adjustment in center distance.

pfreivald 07-09-2012 17:58

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.)

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Was it 1:1?

Nope. Direct drive 6" mecanum (from BB p.80 9:1's) to 4" Andymark performance.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Toothcount on pulley

24 on the mecanum, 48 on the performance.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Profile and Pitch

Gates GT2, 5mm.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Belt Width

Uh... 1 cm?

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Wheel Diameter and Type (roughtop, colson, etc...)

"Glidey" AM HD Mecanum (6") on the primary drive, "Pushy" AM Performance (4") on the secondary drive.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-Gear ratio before the pulley (high and low gear)

9:1 (Banebots, direct drive). After the smaller wheel and larger sprocket we were at 15:1 on our roughtop AM performance wheels. We were not 100% happy with the pushy wheels on the bridge, though they were awesome on carpet.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-tensioners?

Nope.

Quote:

Originally Posted by AdamHeard (Post 1183704)
-If exact c-c, did you add/subtract from this number? Also, what type of machine was the part fabricated on.

This past year we used only COTS parts with no modification aside from drilling bore holes and making keyways in the sprockets.

Sorry... Quoted first, saw the Google Docs part last, and am too lazy (and/or busy) to post there. :)

Adrian Clark 07-09-2012 18:09

Re: Timing belt in drive success
 
Quote:

Originally Posted by DampRobot (Post 1184538)
Actually, I would guess not. I was doing some calculations for bevel gear max HP and torque, and my results were within rounding error of the published values. Perhaps Gates divides by a safety factor, but I would suspect not.

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.

Gates does use safety factors, they call it service factors, but I don't know if their service factors are incorporated into their charts. I would suspect that the numbers in the charts are not generated with any application in mind, they're just a very black-and-white representation of torque and speed capabilities of gates products possibly rounded up or divided by a safety factor.

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.

roystur44 09-09-2012 15:44

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

AdamHeard 19-10-2012 02:01

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.

Doug G 19-10-2012 02:55

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.

Brandon Holley 19-10-2012 08:58

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1190972)
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.


Can we see some pics/videos?? Glad to see you guys are happy with exact c-c.

-Brando

Akash Rastogi 19-10-2012 10:15

Re: Timing belt in drive success
 
Quote:

Originally Posted by AdamHeard (Post 1190972)
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.

Adam, why was the +.003 necessary in this case? Just to ensure tight tension/account for machining tolerances? How much can you add before the belt is over-tensioned? Just wondering.

roystur44 19-10-2012 11:22

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.

sdcantrell56 19-10-2012 11:27

Re: Timing belt in drive success
 
Quote:

Originally Posted by roystur44 (Post 1191001)
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.

This is one great reason why teams should look at running properly spaced belts without tensioners. This is of course only feasible if acceptable tolerances can be held.

AdamHeard 19-10-2012 13:30

Re: Timing belt in drive success
 
1 Attachment(s)
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?

Quote:

Originally Posted by roystur44 (Post 1191001)
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.

I don't think that will affect us the way we have it as an 8wd. Attached Picture shows it decently. For season we'd likely add more than .003 to the longer run to get more equal tension.

Quote:

Originally Posted by Brandon Holley (Post 1190982)
Can we see some pics/videos?? Glad to see you guys are happy with exact c-c.

-Brando

Sure! Probably not quite what you're looking for, but it's a video.

Brandon Holley 19-10-2012 13:39

Re: Timing belt in drive success
 
Quote:

Originally Posted by Akash Rastogi (Post 1190995)
Adam, why was the +.003 necessary in this case? Just to ensure tight tension/account for machining tolerances? How much can you add before the belt is over-tensioned? Just wondering.

We added +.002 to all our c-c spacings for our drive this past year. We did this to kind of artificially bias ourselves to the + side of our tolerance. On our plates, 3 of the 4 bearing holes for axles were machined in 1 step, with the last one being machined in a separate op. The +.002 helped ensure we didn't fall below the necessary c-c to keep proper tension on the belts.

We had 0 issues with our belts all season.

-Brando

waddell 22-10-2012 11:47

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

DampRobot 24-10-2012 01:48

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.

Gdeaver 24-10-2012 09:21

Re: Timing belt in drive success
 
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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