9mm belt vs 15mm belt

My team is going to create a drive system over the summer and are to decide whether or not to us 9mm belts or 15mm belt.

AndyMark AM14U kits (the KoP drivetrain) uses 15 mm belts.

The shifting transmission from WCD products (sold by VexPro) appear to be able to handle wither 2 9mm belts (6-wheel drive, center direct driven) or one 15 mm belt (4 WD or end wheel driven).

I guess your question is, is a 9 mm Gates HTD belt strong enough to replace, say, #25 chain in an FRC drivetrain? My speculation is yes, but I have not done the math. You can also call Gates and ask them, they have a room full of engineers just waiting to talk about cool stuff like robots…

Dunno, never tried it. Betcha CD has a pile of teams that did this and can share their experience with you. Let us know what you find!

IIRC, 987 typically uses 9mm belts for their drives. We have used 15mm belts, but are really considering switching to 9mm in the near future.

In all reality, it really depends on the design (geometry, torque, pulley dia, etc). The loads stated by Gates (or other belt companies) are for continuous run for an extended number of hours with a known load. The FRC application is much different than the “designed” use.

This is a topic that has been beaten to death in recent times. There is a TON of misinformation out there, so I’d suggest treading lightly and do a lot of your own research to validate other people’s claims.

Here are a couple threads with plenty of info and links in them:


The best way to make an informed decision is to try it out yourself. Design a system that can utilize both and put it under heavy testing conditions.

As a general rule, 9mm belts are sufficient (source: I’ve designed and built two FRC robots that used 9mm belts-- GT2 and HTD), however if you’re a cautious type, or if you’re planning to push the limits of the belts (What are those, you ask? Check out the Gates website!), 15mm is “safer.” Certainly it feels a lot more comfortable.

In my opinion, if this is an offseason drive, it’s not a particularly huge deal which one you pick-- if your team has always used chain, there’s plenty of new things to adapt to without worrying about your belt choices. Once you have a grasp on [strike] thinking with portals[/strike] designing with belts, then you can start worrying more about what kind of belt best fits your application.

Don’t forget to change the pulleys. :wink:

Or don’t. Y’know, learning experience and all that…

(this year one of our mentors tried to convince the drive train group that we didn’t need new pulleys because the ones we had were the same size as the ones we needed… minor difference being the ones we had were GT2 and we just ordered $100+ of HTD belts. We almost ran it how he suggested to show him why that was a bad idea)

When talking about belts, it usually isn’t enough just to talk about “9mm vs 15mm”. To have a good idea if belts will or will not fail, we will have to know stuff like pulley toothcount, wheel diameter, wheel tread, drivetrain power, and tensioning method. Unlike chain, which can really handle almost anying FRC can throw at it, belts require care to select the correct power transmission components.

It depends on who made the belt. Gates GT2 and GT3 belts are able to replace 25 chain in most circumstances at 9mm widths without any strength issues. Unfortunately, the 2014 run of “9mm” Vex Pro belts had some quality issues, and had a tendency to snap even when strength calcs showed them well within their limits. I’d either use 15mm VP belts or hold off on using VP 9mm belts until the issue is addressed. I suspect the problem will be fixed for next year, but can’t be sure.

IMO, having done both belt and chain WCDs, belts aren’t really worth it. The lead time and serviceability drawbacks outweigh the noise and efficiency gains in my mind. If you’re going to try belts, the offseason is the right time, but all things being equal, I’d reccomend using 25 chain instead.