OH SNAP! Belt failures 2014
 

As the Championships grow nearer, a lot of people (myself included) have been going through design reviews and starting on new designs and revisions for future competitions. A topic that has been brought to my attention in regards to designs have been belt failures. With the new VEXPro/WCP COTS belts and pulleys that recently became available this season, more and more teams have started using belts in the place of chain. Of course with any new product, regardless of how much prior testing it goes through, there will be some unseen extraneous failure possibilities that are found when heavily used that will be fixed in the next iteration of the product.

We (256) snapped two belts this season in our drivetrain from what I believe was a fluke based on examination of the belts and some research on how belts break (the Gates website has so much useful information). That being said, I want to know what failures (if any) that others are experiencing, as well as a decent amount of detail as to the implementation of the belt system.

If you have had a belt or pulley system fail during the 2014 season (doesn't need to be from VEX/WCP - it can be from anywhere), could you please post the following:

1. List where you used belts and pulleys on your robot.
2. List the parts of the robot where belts failed, including how many belts failed in those positions.
3. Describe the different breaks on the belts (ie. clean, perpendicular to the belt, or jagged and messy diagonally, etc.).
4. What do YOU think caused the belt failure? Was it possibly user-error, or a tensioning issue?
5. Did the pulleys ever fail? If so, what size/tooth count?
6. How much load were the belts under? If no numbers are known, give a description and some estimates.
7. Did any of the belts share a pulley with another belt?
8. After using belts, what are your thoughts on them in general?

Also, if you have a picture of the setup where the belt failed, adding it would be greatly appreciated.


This is all for my own information, since I love the idea of belts instead of chain and know they have the potential to work extremely well (see: 971), and want to learn more about correct implementation of them to ensure future success.

Thank you very much for your input,
-Andrew Lawrence

Re: OH SNAP! Belt failures 2014
 

Andrew,
Although 3476 hasn't broken a belt this season it has been something that we were very careful to design against at the beginning of the season and something that we are very eager to prevent from happening.

That being said, could you tell us of 256's setup and what was this expected "fluke" that could have caused your failures? We sourced our belts from VEX this year and we have been quite happy with them so far. Seeing this is our first year using belts we are still learning. :)

Thanks!

Re: OH SNAP! Belt failures 2014
 

We snapped 6 belts as of now. We snapped one on the practice bot, one on the competition bot at our second competition, and 4 on our competition bot at our 3rd competition.

We are using belts on our drive train. It is a 6 CIM 6WD WCD setup with 4" wheels. We are using 9MM 130T belts, using 24T pulleys at each end. The robot only weights 100 lbs, so it is fairly light.
At our 3rd competition, the belt going to each wheel snapped once each. The practice bot snap and the 1st competition bot snap were both on the same wheel.
All of the breaks were very similar. The were all very rough and messy, with alot of jagged edges. We emailed Vex, and they said that we were over torquing the belt, because we have too small of pulleys, with too much power on the drive. Didn't seem like a tension error, because the teeth were all fine, and the belts looked like stress snaps.
The pulleys never failed, and still look brand new.
The belts were under the load a 6 CIM drive would give. I don't have exact numbers.
We had 15MM pulleys in the center, and then 9MM pulleys on each end. The 2 belts shared the 15MM in the middle.

For worlds, we plan on putting chain back on so we don't have to worry about snapping. This summer, we will put more work into doing calculations, and redesign the chassis so it will fit larger pulleys.
But otherwise, we loved how quiet and smooth the belts were. We are glad we tried them this year, and will use them again next year if we can fix the snapping issues.

Re: OH SNAP! Belt failures 2014
 

Everyone with belt questions/issues should look at this thread and this thread. Also, I wrote a post in the first thread (long I know) about what 971 has learned about belts.

I summary: use Gates GT2 and not the Vex HTD. It's my experience that they are better in terms of manufacturing quality and usefulness for FRC applications. Also, look up Gates/other companies documentation. There is quite a bit of information out there if you know where to look (I linked some in my original post). Knowing what a properly tensioned belt is is important, and knowing the signs of an undertensioned or overtensioned belt can save you many headaches in the future.

Andrew,
One question I have for your belt failures is how high is your center of gravity? 971 speculates that having a high cg could affect the failure of drivetrain belts. (see my post for more details)

If you post pictures of your belt failure, I or someone else might be able to help you diagnose what happened.

Re: OH SNAP! Belt failures 2014
 

We used gates gt2 belt on our shooter when we had a motorpolt, however this happened:

after we killed the first belt, we switched to chain. user error probably killed the belt, as we were hard stopping the shooter while still running the belt. The belts were driven by 2 cim's and 2 775's geared 5.33:1 at the section that sheared. We did not break the pulleys, but the belt was dead. The belts we use on our drivetrain, however, are still in good shape after two regionals. We used VEXpro 9mm Timing Belts (two of them are attached to two cims each, and two to a cim and a minicim each), and they have held up great.

Re: OH SNAP! Belt failures 2014
 

Our setup was a standard WCD driven by a 6 CIM transmission. We believe it is a fluke because the belts tore in a jagged pattern, which Gates says is very uncommon. I believe part of the cause of the fluke is on our part, since our drive wasn't assembled with the care it should have been. We used belts in the offseason just fine in the same setup.

Re: OH SNAP! Belt failures 2014
 

I found a picture of our first break. This was the belt on the practice bot. The competition ones were not quite as diagonal of a cut, but they were still really jagged.

Re: OH SNAP! Belt failures 2014
 

Kevin, Our center of gravity this year was about 8-9 inches off of the floor. I don't have pictures, unfortunately, but I plan on stopping by your pit at champs and asking about your drive, if you have the time.

Re: OH SNAP! Belt failures 2014
 

Please do. We might be able to give you more detailed information in person.

Re: OH SNAP! Belt failures 2014
 

I might have to stop by and take a look as well. Your robot is incredible.

Re: OH SNAP! Belt failures 2014
 

It looks like the teeth are coming off this belt, which means that it is likely undertensioned. Teeth coming off belts is a pretty common failure for undertensioning.

It looks like these belts might have been overtensioned. Just because all the teeth line up fine doesn't mean you can't overtension belts. Also, excessive wear is a telltale sign that you've been overtensioning. Most teams running belts are actually overtensioning belts without realizing it. Taught is too tight when it comes to these belts.

Re: OH SNAP! Belt failures 2014
 

Although that could be a reason that our belt sheared teeth, it was overtensioned if anything (it was extremely difficult to get on), and made the nice tension'd sound. The cause of failure was the sheer force put on the belt.

Re: OH SNAP! Belt failures 2014
 

Interesting. But it was possible to get on without detensioning before? Do you have a tensioner on your drivetrain that you use to get your belts on? Are these gt2 or vex belts? What brand of pulleys did you use?

Re: OH SNAP! Belt failures 2014
 

This kind of tear looks like what you might get from overstressing a belt. Perhaps it was too tight, or perhaps it was kinked or forced over the edge of a pulley when it was installed.

Re: OH SNAP! Belt failures 2014
 

We had similar experiences at SVR.

We run a 6WD WCD VersaChassis setup with 4" wheels, omnis in front, and either VersaWheels or VersaWheels DT as the center and back wheels. Dropped center, VersaTrucks, and CAM tensioning. We used 4 CIM, 2 Minis in a 3 CIM ball shifter at Davis and took out the Minis at SVR. The center wheel was direct driven off the ball shifter output shaft, and the front and back belts were driven by 130T 9mm wide VP belts on 24T pulleys. We were traction limited in both low and high gear.

At Davis, we played really, really hard defense, and didn't have any problem with the belts (although we tripped the main breaker a lot more than we would have liked).

At SVR, we suffered 3 separate belt failures. The first belt broke after about 4 matches. It was a classic overtension failure, with the tensile elements of the belt pulled out, and no real wear on the teeth to speak of. We replaced the belt, and assumed that it broke from a combination of a regional and a half of really hard play and overtensioning. I went through all the belts and made sure they were on the loose side of correctly tensioned. We suffered 2 more belt failures throughout the rest of the regional, with nearly identical failure modes, and in different locations in the drivetrain. I inspected the drivetrain, and there were no obvious alignment problems, debris, etc. that might be causing the problem. The second two breaks occurred with belts that I'd consider "quite undertensioned", yet they looked like classic overtension failures. I brought the belts by 971's pits, and they agreed that these all looked like overtension failures. When I took a look at their drivetrain later, I noticed they had the same diameter wheels, similar pulley diameter, the same belt width, and tensioned their drive belts more than we did, yet suffered far fewer belt failures.

I've heard through the grapevine that VP did have quality issues with their drive belts this season, and my own experience and anecdotal evidence seems to support this. Multiple teams seem to have seen belts fail in this way, and that teams using Gates belts that I talked to didn't see these types of failures anywhere near as frequently. Of course, anecdotal evidence does not a strong conclusion make. If this is a quality issue, I hope VP addresses it (and lets us know that they have). If this is more a user issue, I hope the community learns how to use these products without having to worry about them breaking.