Timing belt in drive success

Splitting off of this thread here;

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)
-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!

Great idea:

Helpful resources I’ve been using up to this point in planning:

GT2 Design Manual - http://www.gates.com/catalogs/file_display.cfm?file=PowerGrip_GT2_Design_Manual.pdf&thisPath=gates\catalogs&requesting=ptcatalog&location_id=2999

Light Power and Precision Design Manual - http://www.gates.com/catalogs/file_display.cfm?file=PT_GatesLPPdesignManual_2005.pdf&thisPath=gates\catalogs&requesting=ptcatalog&location_id=2999

The main catalog index - http://www.gates.com/catalogs/index.cfm?requesting=ptcatalog&location_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).

  • — - — - — - — - — - —


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.


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?


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.

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).

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.

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.

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!



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.


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

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!

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

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.

Thank you Chris & RC!

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.

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.