Omni Wheels Destroyed by Steel Bars

This thread has two purposes: to warn you about using omni wheels on your robot this year, and to solicit any suggestions to mitigate these dangers.

We used 4 omni wheels (https://www.vexrobotics.com/omni-wheels.html 6 in) on our drivetrain this year, because having a consistent center of turning was quite important to our software folks. After dropping our robot of Level 2 last year without issue, I guess we assumed the omnis would be fine. To be sure, we ran our 40lbs test chassis over some 1 in bars many times, without issue. We have blue nitrile tread on the center wheels.

Well, we just got back from our Week 1 competition, and the results are in:

On Friday afternoon, we noticed that some of the rollers on the wheels didn’t spin well. They got progressively more jammed over the course of the day, and then they began to break. Top picture is of one of these wheels. Seeing this, we decided to swap them after our last match of the day.

The next morning, we rolled out onto the field with brand new, never used omni wheels. 8 matches later, they looked like the bottom photo. The back omnis, which had more weight on them, each had several rollers broken off. The front ones had all their rollers, but some of the rollers were jammed and wouldn’t roll, which seemed like a precursor to breaking based on our experiences the previous day.

Steel 1 in Bars: 8, Omni Wheels: 0

The driving worked fine on the broken omnis, except for the fact that our autonomous seemed to be less accurate and our driver thought the driving felt weird (although those could be unrelated software issues). However, it seems like once one roller breaks off, the ones around it do as well, so letting this process go on for too long seems dangerous. If we hadn’t been able to swap them, I’m not sure what would have happened.

We’re currently brainstorming ways to get around this issue for our next competition. Have any other teams experienced similar issues, and if so, what did you do to solve it? Right now, we are leaning towards getting rid of the omnis, but if anyone has any suggestions for more durable omni wheels or anything else, we would love to hear it.

If you have omni wheels on your robot, you might experience this too, so beware.

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I think this is a testament to the durability of these omnis! Consider the impact load they have to survive! OUch!

Jason

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I suggest looking into wheel pans (look up how bomb squad protected their swerve modules in 2016). You could do something similar by making a deflector plate that impacts the bar before the omni.

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Adding on, this is a classic six wheel drive robot with middle direct driven 6” Andymark performance wheel with Blue Nitrile tread. The robot was driven as you would normally expect through the rendezvous under defense. It was indeed fine in practice.

Our robot does a lot of fancy SW controlled auto-aiming and, since we have no turret, needs smooth controlled turning around a known center.

Unfortunately, it seems my premonitions were right. I might suggest a low horizontal CoF tread (like pebbletop)

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Last year, we manage to break 2 rollers on 2 of our wheels just as our competitions were almost over (1 wheel at each event). It didn’t feel too weird when driving (i’m guessing since it’s only 1 roller) but when nearly half your wheel is out, I wouldn’t be surprised if that was the problem.

During an off season demo, our key manage to somehow get loose? It got all chewed up and fell out. Wheel was no longer powered, motor shaft just spun. That was really crazy since I had no idea why the robot wasn’t able to drive straight all of a sudden, and this was with little children around.

The Rendezvous zone did break 3 of our Omni s today but it was just the rollers falling of the wheels not them completely breaking

And we are using the 6 in vex pro ones

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We noticed our 6" DuraOmnis getting splits after the end of week 0.

We swapped to metal ones instead… held up over GSD, I think. We’ll need to inspect our rollers. We don’t pass through the center field much though.

I think it’s worth noting that you guys are a tall robot that did cycles across the rendezvous zone, so the impact on your wheels is greater that a low trench bot.

We re-used our Vex 6in Omnis from last year, and had no issues, however we only went through the rendezvous zone for about 10% of our cycles, and when climbing. We definitely don’t want them to disassemble without warning, so we’re probably going to swap ours out for brand new ones before our next event.

We certainly did do a lot of aggressive driving over the bumps, even for a rendezvous-cycler; our omnis lasted fairly well for the amount of abuse they were taking.

Another theory we came up with was that the omni damage might have been more from side impacts (e.g. being pushed into the bars). Side impacts would have loaded the omnis in a different way and probably would be more likely to break rollers as the omnis have to take the full side load instead of just running up and over.

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We broke 4 wheels today. We believe the side impacts of intaking balls from the dance floor caused most of the damage. April and the rest of the field staff at Dripping Springs were kind enough to hand us our broken rollers after many matches.

We got a couple AM high grips from Spring Konstant and we swapped those in for tomorrow.

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Just put steel/hdpe rims on the outside of the wheel to protect it from side impacts. It probably won’t be perfect, but that was my first thought for protecting them.

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In looking at the game video we shot again it doesn’t look like she impacts were our issue. It was just speedy driving over the 2x1 beams, like you would expect in close matches.

This is why we believe it is mostly the side impacts for our breaks. That is the same wheel, only the outside of the wheels are being broken. We did have another wheel that had one side with many breaks and a single roller broken on the inside.

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Worked so well in 2016 that we are using them again this year

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Thanks for posting. This is an issue we anticipated. To make it worse we have two omnis in an H drive under the robot. Our solutions - to be tested later this week.

  1. We have a pneumatic H drive. Don’t hit the bars with it down. Our driver station now has an indicator UP/DOWN and one of our drive team will have as their job keeping an eye on this.
  2. Go slow. On day one of build we went over a test bar and proved it could be done. But at speed it will not be done forever.
  3. Spares packed. Our design allows for rapid changes.
  4. Don’t go much at all. Our design is predicated on decent auto, above average climb and level climb ability. In between we expect to either rebound our teams shots or play defense. I suggest, not that they pay much heed to mentors!, they not go through the middle any more often than necessary. Probably we’ll have a role on one end and try to stay there until 45 seconds left in the match.

Good to know we can run on damaged wheels. That has happened a time or two last season.

T. Wolter
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We are using these pretty successfully as they are all metal and thus sturdier. Thus team with these could use these (though they are no longer sold): https://www.andymark.com/products/8-in-aluminum-omni-wheel

Early on in the build season we took last year’s robot and ran over the 1" x 2" aluminum bars that came in the KOP about 100 times at full speed with omni wheels in the front. More damage was done to the robot than the wheels - they came out ok. I didn’t notice any damage from our omnis from this past weekend either, but I don’t think we side loaded them at all.

2471 and 4488 both made custom aluminum Omnis in 2018. I know you can find them in 4488’s CAD, they worked great and will be far more durable than the plastic. But it doesnt seem that Andymark carries the spare omni rollers anymore, but they do have the sleeves. try contacting Andymark and asking if they have any spare… or use the wheels you already broke and take the rollers from them.