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

[Teamcodeorange]

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!

Originally Posted by Teamcodeorange

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!

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.

[Thad House]

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.

[Alan Anderson]

Originally Posted by Thad House

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.

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.

[DampRobot]

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.

[Wayne Doenges]

Here's a picture of two of our belts that failed:
http://farm4.static.flickr.com/3781/...6ec44e388b.jpg
We figured that the carpet thread wound itself around the drive pulley and increased it diameter, of said pulley, which caused the belts to tighten up and break.
I asked the student if they didn't notice that the field was disappearing from behind them
During the finals at St. Louis, we broke several drive belts. The last very last match we played with only 3 drive belts instead of 4. We won St. Louis.

[Oblarg]

I've (thankfully) only seen one belt failure (a snapped HTD belt) during my time in FRC, which occurred on 449's 2008 robot towards the end of Battle O' Baltimore and was likely the product of our extremely shoddy belt tensioning system that year (everything on the drive was on 80/20 sliders, so between every match we'd loosen the bolts going to the gearbox and pull until the belts were tight, then re-tighten - ah, the things you do when you don't know any better...). The belts this year on all the teams I've worked with have held up perfectly.

On the flip side, I've given up trying to keep track of all the drive chain failures I've witnessed...

[Chris Fultz]

Originally Posted by DampRobot

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.

What gear reductions on the ball shifter?

[DampRobot]

Originally Posted by Chris Fultz

What gear reductions on the ball shifter?

I don't remember the actual reductions off the top of my head, but I know we were 18fps high, and 8fps low, both adjusted speeds.

[electroken]

We've had one belt failure so far this year. It was a VEX 130 tooth, 15mm belt in our drivetrain which broke straight across, a textbook crimp failure.

I didn't see that belt get installed, but I'm willing to bet it involved prying it over the sprocket with a screwdriver.

[Kevin Kolodziej]

Team 1675 used 8 belts on our 2014 robot. This was our second experience with belts. Our first use was in 2011 on our roller claw (I don't remember the ratios but they were 15mm HTD belts from Gates on fairly small pulleys). This year we used 8 belts on our robot:
Drive: (4) VexPro 150t 9mm belts, 30t pulley on each end. 6 Wheel WCD Gearboxes with 2CIM shifter, 15:1 / 5.13:1 ratios, 4" wheels. Center pulley was a double pulley, versablocks with cam tension on the outside wheels.
Arm Shoulder: (2) Gates HTD 239t 15mm belts, 18t pulley to 60t pulley. 18t pulleys were driven by 100:1 versaplanetary with 775 motor and additional 18:24 gear reduction. Versablock with cam tension on the shoulder joint. 18t pulleys were on a shaft supported by drive gearboxes.
Roller Claw: (2) VexPro 150t 9mm belts, 30t pulley to 42t pulley, 4" Versawheels on intake. 30t pulley driven by 15:1 Versaplanetary with BAG motor and bevel gears. Versablocks with cam tension on intake wheels.

We experienced 2 belt failures during the 2014 season, both on the drive. On our practice bot, one of the drive belts was shredded (messy, frayed break). The second failure was on our competition bot on Friday afternoon at our second regional. One of our rear drive belts snapped (perpendicular clean break as far as I can remember).

We believe the first failure was due to a pulley failure. The inside flange of the pulley in the gearbox (the one with bolt belts wrapping it) popped off. When the belt walked to the edge of the teeth, it shredded on the sharp edge. We believe the second failure was due to over-tensioning of that particular belt.

After talking with Aren Hill at Vex about our pulley failure, we took a careful look at our alignment and determined that our belts were not straight and that was putting stress on the flanges of the pulleys. I believe it is common practice for smaller timing pulleys to have the flanges pressed on so any misalignment is likely to cause this failure over time. After replacing spacers to achieve better alignment, our walking issues seemed to disappear, however, we still had 4 more pulley failures (total of 5) over the course of the season (3 total on practice bot, 2 on competition bot, both at the gearbox and at the wheel). The failures always seemed to occur on the same half of the drive, which leads me to believe we had an inherent flaw in our fabrication causing a misalignment that we could not detect. For what its worth, however, we did notice that at least one of our drive axles was always towed in towards the gearbox when the belt was tensioned (not overly tensioned). We believe this was a flaw in either the versablock at that wheel or the bearings or some combination of the two. We disassembled the block and bearings and reassembled and the problem persisted. We did not have additional bearings to try swapping out. This was not the corner that we saw our pulley failures in, however, and it did not give us any noticeable problems.

We had no issues with our printed pulleys on our roller claw.

We had some limited belt slipping on our shoulder joint, but this was due to the bottom shaft flexing as the drive gearboxes flexed. A brace between the two gearboxes would likely solve this.

As for loads on the belts, our estimated pushing force was 170 lbs, and the 30t belts on our drive I think had about a 1.5" diameter, so potentially one belt could have seen approximately 250 lb of tension if we were in a pushing match, plus the pre-tension. The shoulder belts saw some shock loads as our arm bounced around a bit, but I couldn't begin to estimate that load. Perhaps the most surprising, however, was on our roller claw. Though generally under very low loads (lightly tensioned, not much load from the ball), there was a match when the screw from the versablock backed out and interfered with the spokes of the wheel (VexPro DT 4"). The belts held up just fine as the motor pulled the wheel through the screw, creating a notch in the wheel from the head of the bolt and bending the 10-32 bolt.

As mentioned above, our drive belts shared a common pulley in the gearbox, and this is where our initial failures occured. When we popped a flange off at this pulley on our competition bot, it was actually the outside flange, not the inside flange, that came off, but the belts did not walk off the pulley.

Final thoughts: We love belts. There's a few things we learned this year about proper tension that we will implement in the future. One disadvantage is when we popped a flange or snapped the belt on the drive, the only way to replace the internal pulley or replace the belt was to remove the entire drive gearbox from the robot, which also involved removing the belts for our shoulder because of the shared mounting. After having to do this a few times, we got pretty good at it and had it down to a 30 minute repair, but this doesn't work for elims. Simple solution to this is to move the output pulley to outside of the gearbox, but that does create other design challenges. But I certainly do not miss trying to put a masterlink on a chain in a cramped part of the robot, upside down! We are going to explore using the printed pulleys in more applications as well. Tooth wear was practically zero and having the flanges not pressed on is a bit more forgiving for alignment issues. I was initially concerned with our choice to use 9mm belts on the drive, instead of 15mm, but the larger pulleys (30t instead of 24t or 18t) seemed to be the right combination for use with the 9mm belts.

[Oblarg]

Originally Posted by Kevin Kolodziej

One disadvantage is when we popped a flange or snapped the belt on the drive, the only way to replace the internal pulley or replace the belt was to remove the entire drive gearbox from the robot, which also involved removing the belts for our shoulder because of the shared mounting. After having to do this a few times, we got pretty good at it and had it down to a 30 minute repair, but this doesn't work for elims. Simple solution to this is to move the output pulley to outside of the gearbox, but that does create other design challenges.

I'm currently playing around with a few drive train ideas with this very issue in mind - send me a PM if you're interested in discussing a few potential designs.

[who716]

We initially planed on using gates belts to pivot our ball pickup but the force it took to pivot it made the belts slip way to much for us too feel confident so we Changed that to chains. That was one of our first ever attempts to use belts on our final design.

[Thad House]

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.