Timing Belt Pitch

In preparation for next year, team 1836 and 1515 are collaborating, and thinking of using belt. We were thinking of using gates belt, specifically the GT2 belt, as we have heard of many teams using it, and because we can get it free from gates through the FRC kit of parts coupons. We were wondering though, what pitches have other teams used. We are specifically thinking of using 3mm pitch belt. Would this be to small of a pitch. It it is, would 5mm or 9mm pitch be ok?

Important question. Are you thinking about using this in your drivetrain, or somewhere else on the bot? Drivetrain loadings are usually significantly higher than any other system on your bot.

While the order page is currently closed for the season, the free Gates belts are all 5mm pitch, HTD profile belts.

While GT2 is a better torque-carrying profile than HTD, I’ve used 5mm pitch, 9mm wide, HTD profile belts* on multiple FRC robots since 2009 without any problems. The most important thing for smooth operation of timing belts is to ensure they are properly aligned and tensioned. I try to avoid the need for belt tensioners, and always design for the exact center-to-center distance.

The nice thing about using 5mm pitch, 9mm wide, HTD profile belts is that SDP/SI has a great assortment of polycarbonate timing belt sprockets with aluminum hubs that make for some really lightweight (non-drivetrain) mechanisms. Just pay careful attention to the dimensions in the product spec sheet, as the size of the inner aluminum hub varies depending on the number of teeth. It is possible to crack some of the smaller sprockets (usually less than 20t) if you get to aggressive on drilling out/broaching the stock bore ID. For those sizes, I prefer to use either COTS aluminum sprockets or custom machined from timing belt pulley stock (also available from SDP/SI).

*To me, having one standard belt pitch/profile in the shop was more important than using some GT2 components and possibly having it get mixed up with free Gates HTD parts. To me, if the application needs something stronger than the “standard” size timing belt, then I use roller chain or 20DP gears.

This logic is sound - but why not just use GT2 for everything? You can also get GT2 parts with the Gates donation, and we have taken advantage of that for two years now.

I’ll write up a big post about 2791’s experiences with timing belt tomorrow, but the short version is “go for it”.

We used 9mm wide, 5mm pitch, HDT belts on our 2011 robot, which was geared for ~12 fps. Whenever we went from full forward to full reverse the belts would ratchet from slipping. Despite this, they lasted the season with minimal wear.

However for caution reasons we used 15mm GT2 on this year’s robot and we feel much better about the extra security for minimal weight. We will continue to use 15mm belts in the future for drive train applications and 9mm belts elsewhere.

Also pulley size is a factor, and there is potential that the pulleys we used had too few teeth, as this year we went for pulleys with about 7 more teeth (I don’t remember the actual numbers off the top of my head.)

We are using it for a drive application, and gearing to about 17 fps with a low gear of around 7 fps. We were thinking of not building to the exact center to center distance, but going with a tensioner, because we felt it would be beneficial to get the extra chain wrap, and also because it would allow us to change ratios. Is the general consensus that building for exact center to center distance is better?

This was our first year attempting belts, and even though we encountered some hurdles, it’s very likely we’ll be using them again next year. We went with a 15mm wide, 5mm pitch GT2 profile belt from GATES, with 32 tooth pulleys on all the wheels. We took a serpentine approach for the belt path with power driven through the middle wheel; the belt is on the outside of the frame for ease of maintenance. After some testing, we put in an additional tensioner (on the left) to take up some of the slack from our imprecise tolerances and the deflection we saw in the fiberglass frame material.

We did see some belt skipping on the middle pulley on the practice bot, most likely a combination of the low belt engagement and the deflection of the cantilevered idlers when under load. Fiberglass is flexy stuff! We’ll likely be switching to aluminum for the drivetrain next year if we use belts again. I’d be wary of designing to exact dimensions if a serpentine approach is taken… we’ll probably be using a tensioner next year as well.

The belts served us well in our first regional. No issues with the drivetrain, and the minimal backlash let us pull off some neat autonomous maneuvers.

We used GT2 belt on our elevator and roller claw in 2011. It really worked out well. This year we went with the belting for our 6 wheel drive train and all our rollers. The belting works way better than poly cord. We used a tensioner, dead axle and a shaft to drop our wheels. Really cool design.

Here is a link to our Picasa web albums to get a feel of how we used the belting in our designs. Chain is dead

2012 Build
https://picasaweb.google.com/117769834305511597729/2012BuildSeason?authkey=Gv1sRgCO3O7rTmmaX3Wg

2011 Build
https://picasaweb.google.com/117769834305511597729/2011RobotMechanismCloseups

Thanks for all the help everyone. So for driving applications, we should be using gt2? Some more info on the run: it is actually very short, under 3 in center to center distance, for a swerve drive actually. It isn’t a serpentine run, very direct. Roy, was 971 using tensioners for the drive belts, I couldn’t see in the pictures, though it didn’t look like it? Any problems with belt slippage?

Yes we use a u shaped tensioner that is pushing the wheels outward. There is a tension screw mounted on the frame. No problems with skipping if the tension on the belt is adjusted correctly.

Ok, thank you so much. Sounds lie 1836 and 1515 will be sporting 5mm GT2 belts next year.

IF you ever get a chance to see our intake take a look at it. Inside the tubes is belting that drives our over the bumber intake. Really cool design the kids came up with.

https://picasaweb.google.com/104549320706687649408/InstantUpload#5726648333113049666