pic: 20's IRI Carnage

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The aftermath of our first match- and the reason we put up so few points all day.
Note to everyone: NEVER get into a pushing match with The Hawaiian Kids.
This is the broken dog gear that rendered our drive train useless all day.
We have now reinstalled our single speed DT from championships, so we should be good to go for Saturday.

Wow, that’s amassing!

What company’s shifter did that dog come from? What kind of ratios were you and Hawian Kids using? How did you end up fixing your DT?

Gives a whole new meaning to the term “Bad Dog”…:rolleyes:

That looks like it would make a great logo. Some team should change their name to “The Broken Dogs” and use that.

You could see the wheel rip off at around 1:08 in this video:

Wow, that’s amazing. We had a similar problem when we broke the bolt that holds the dog to the shifter pin. We weren’t going to cancel on the Girl’s Gen scrimmage happening the next day, so we drilled another hole and pushed a roll pin in there as a bandaid. After the scrimmage I rebuilt the trannie and put a new dog in. The old dog was about to break because the “lightening hole” that had been put in it to fit the roll pin had severely weakened it.

I guess you could say…it wasn’t very clutch?

From what I understand about the Hawaiian Kid’s drivetrain, they used 4 CIMS and 2 Banebots in their drivetrain. We had 4 CIMS. Both were shifting (I’m unsure of the speeds, but I’ll see if I can find it in our pit scouting data).

After the break, we asked around for a replacement dog- the only team that had the right dog was 868, who loaned it to us. Due to some issues, that one ended up breaking as well (incorrectly drilled hole, I think. I was in the stands scouting the whole time.)

We ended up replacing the shifting gearbox with our old single-speed from during the season (the 2-speed was an improvement to allow us to cycle better at IRI). Great improvement, right?
We also added a back-of-the-pyramid shot, a full-court blocker, and a centerline autonomous routine that we didn’t end up getting to use. Rather disappointing that we put in all that work for nothing.

Ouch.
Too Soon.
:wink:

On the bright side, we gained valuable experience! :rolleyes:
And the matches at IRI were incredible.
Those finals were unbelievable.

Some more back story on this failure:

During build season we decided to use a single speed gearbox in our drive train because of our lack of experience as a group with shifting gearboxes along with our desire to keep on schedule. Given that our strategy was floor loading to pick up discs scattered around the field, we did not deem high speeds necessary anyway. The 11 fps gearing with direct drive from the kit of parts gearboxes to 6 in. wheels was a nice sweet spot for a single speed that would allow us to still traverse the field when needed, but give us decent acceleration and not trash our CIMs. To make this happen, we “West Coasted” the tough boxes from the kit and ran 1/2" hex 7075 aluminum shafts in our 6WD. We used the 6 in. wheels from the kit because they performed well in 2012 and we wanted enough clearance for the frisbees on the carpet. This robot was the most reliable one we have ever built. For example, in Connecticut we had 4 total minutes of repairs all weekend.

Fast forward to after champs. We were applying for IRI, and in hopes that we would get in we started planning improvements for our robot as a good summer project and a way to really push the envelope with our design. These added features included a hockey stick/window motor blocker, an 1114 inspired pneumatic shot deflector for shooting from the back of the pyramid, and a 2-speed transmission from West Coast Products. We realized that the strategy of Ultimate Ascent had evolved, and especially at IRI it would require all teams to be proficient at cycling if they hoped to be effective in eliminations. Our limiting factors as a team were our speed to traverse the field when cycling and the sub-optimal drop on the floor loading. We couldn’t deal with the sub-optimal loading problem easily (4 sec. on average versus 1-2 sec. for most good cyclers at IRI), so we decided that speeding up our drive train while still protecting our motors and maintaining pushing ability would be important.

After getting all of these improvements finally installed with time before IRI, we started programming and drive practice. We got about 30 hours of solid drive practice in. By the end of drive practice, we were rather consistently hitting 5 cycles, sometimes 6, on an open field with our 3 disc auto from the back of the pyramid, along with cleaning up missed shots and scoring those with our end game hang. We also had a 4 disc center line auto that returned to our side of the field from the center line before anyone in the world got there in the first place…I really wanted to see that one in elims!

So then at IRI, we have our first match in Quals 6. We are with 2056 and 2826, and against 67, 359, and 3641. We knew we could not leave HOT entirely open for FCS, so we decided we would “puppy guard” the loading station. We would have our blocker up on them when they went to full court shoot, but in between pick up the deflected shots and alliance member misses and/or score our discs we had collected. As a result of this strategy we found ourselves in a pushing match with 359. Our robot was up on end at one point and then slammed down, presumably breaking one of our KoP wheels. The wheel ripped in half. Then our dog from our shifter shattered into 3 pieces, perhaps because the gearbox was subject to abnormal conditions in low gear due to the broken wheel.

After the match, we quickly realized the dog was missing and started asking around for assistance. Luckily Team 868 had a spare dog for this shifter and gave it to us, along with helping us find the necessary tools for the job. When installing this dog, the aluminum threads for the 4-40 bolt hole stripped out using the non-torquey end of an allen wrench. We tried an 1/8th" roll pin, but the hole for the pin was drilled too large for it to grab. This spare dog subsequently shattered because of the oversize hole. At this point we began trying to lock our gearboxes into one of our gears in hopes of working Friday afternoon. More replacement dogs were nowhere to be found because the WCP dogs go on a 5/8" hex shaft, unlike the 1/2" AM ones. None of our jobs worked, so Friday evening the decision was made to go back to our original single speed gearboxes that worked all season. We got back working Saturday morning and were finally able to put up 70, 67, and 49 points in our 3 matches.

Looking at the wheels after the competition, we can clearly see bubbles in the plastic in the places where they broke. I spoke with Andy Baker at the competition and he did say that AndyMark will be looking to improve the wheels next season. This was not the first time this season the wheels have cracked, but it is the first time they ripped in half like this.

The WCP gearbox was actually a success. After 30 hours of drive practice, the gears held up great and our drivers love the fast, smooth ride. The shifting did bring an added layer of complexity to the failure which meant a very serious failure, but the dog only failed when it was subject to conditions it is not designed for. The prevailing theory is that rather than being traction limited, the gearbox was now limited by the stall torque of the motors in low gear. At least until the dog broke. This is roughly 3 times the torque that the dog is intended or expected to handle.

Thank you to Team 868, especially Charles, for your assistance in trying to get us back up and working. Even though we didn’t pull it off for the 2-speeds like we had hoped, it was really awesome to have competitors like yourselves helping us out even though we had a match against each other shortly after. THAT was gracious professionalism. Also thanks to 11, 71, 195, 1310, and 1625 as well as Chris Picone and Nick Lawrence for lending us tools and brainpower for the attempted repairs throughout the weekend. And thanks to 16 and 27 for being understanding pit neighbors when our stuff undoubtedly spilled into your space during the turmoil that was the Friday of IRI 2013 for Team 20. Additional thanks to all of the great teams and volunteers who provided the most inspirational product on that field. We were hoping to really help raise the level of competition, but we came away with a great experience nonetheless.

It was disappointing to finally earn our way to an event that my teammates and I have been dreaming of competing in for years, only to lose more than half of our competition within the first minute. It felt like we never got the chance to show the world what we had worked so hard on. Even more embarrassing was being the team to show up to IRI that didn’t move. I really didn’t get the chance to enjoy IRI this time around until the eliminations started (which were spectacular, by the way!) because of the stress surrounding the robot failure. It was encouraging to finally see our robot working well Saturday morning, and we got quite a few figh hives. You can bet we will be working hard in the next 9 months to earn our way back next year and hopefully actually compete in 2014.

I don’t know if amazing was quite the right word, but we did amass quite the amount of shattered parts by Friday evening. :frowning:

West Coast Products was kind enough to manufacture a modified version of their gearbox for us. We collaborated to find a solution that could replace the single speed gearboxes we had on for WPI, Hartford, and Archimedes, since our robot was not originally designed for a two-speed.

Even with the footage and the accounts from eyewitnesses, we still don’t quite know what the root cause of the failure is, though we have a few theories. We hope that once we fully understand what exactly happened that we can give them some feedback on this gearbox design that they graciously made for us, and we implemented as an offseason project/experiment. If anything, this was an unfortunate accident, but we hope it becomes a valuable learning experience for us and others.

Thank you to all the teams who helped us throughout the day – there were many teams who lent us drill bits, a spare dog gear, and gear pullers. Your gracious professionalism and coopertition was absolutely inspiring.

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Aloha everyone,
Sorry to hear about your transmission and wheel issues. We can only imagine the frustrations of trying to fix a robot during a competition, after spending so much time preparing for IRI.
Our robots right side transmission failed during semis 2-2 and we were quite disappointed to not be able to give it our best as well in that crucial match. We plan to fix it at TRR on Friday once we check in. Fortunately for us, we designed our robot for easy access to swap out the transmission with a spare. We always pack spares of everything given that we travel a few thousand miles away from home in order to compete.
Our driver did quite a bit of shifting during those situations, and its too bad it gave out as early as it did.
When our driver tried to push your robot out of the way from HOT’s full court shooting, we noticed your robot was going to flip so we backed off a little then continued to free HOT’s FC shots.
I guess people took notice since we had many teams visit our pit to ask us about our drivetrain and take pictures.
Our 2010 robot was arguably the best pushing robot in FRC that year. We won a lot of matches including 47 in a row (with ties) and IRI that year primarily due to that fact.
On a side note:
For our drivetrain, I posted a photo of our transmission that we were working on, on FB to FRC members to see which had the motor combination that was described. However, we elected not to use them and still utilize just a 4 CIM motor drive. We do however make custom/swap changes to our AM SS transmissions.
The drive we plan to use next year will effectively be much stronger (for pushing) and quicker. :slight_smile: We already have done observational tests on a prototype. As mentioned once by a 254 mentor, increasing the speed of a robot should only be done if your driver can handle it. We think he can as he has expressed to us when I asked him. Our driver is a sophomore who has already been to IRI 3 times as a drive team member. He has 2 years left.

WCP dog gear is 7075 Aluminum. Hardenening spec not listed on website. It’s 5/8" hex as stated earlier.

AM dog gear is stainless steel. No alloy spec on website. It’s 1/2" hex. Some use a 4/40 bolt, others use a roll pin to attach the dog to the shift linkage. We’ve always gone with the roll pin variant for the comp bot and never broken one.

Maybe 7075 isn’t up to the task? I’ve never seen an AM dog break like that. Every one I’ve seen break is because of the bolt/pin breaking off inside.

Shock loads can be brutal on drivetrain components. Just sayin. The dog can certainly handle the torque loads of the motors, I would guess indefinitely. But shock loading (especially high-speed impacts) can subject drivetrain components to many times the load the motors are capable of. Just think about the time (acceleration) required to get up to speed under motor power, vs the time (acceleration) required to get back to 0.

Andrew,

The bolt/pin sees none of the torsional loads because the hex transmits the torque. Therefore, the bolt failures are a result of shear stress.

You may be right that 7075 Aluminum is not up to the task. However, in this case, the task would have been 58.4 ft. lbs. of torque after the wheels broke. At a 40 Amp traction limit (normal conditions), the load is 17.6 ft. lbs. of torque. The fact that the dog broke indicates that the failure was a torsional failure. Normal conclusions about the comparative strength of the dogs are not applicable because of the wheel failure preceding the dog failure.

The video of the match shows these events happening in order:

  1. Pushing match with 359, with 20 slamming on the ground on the end with the broken wheel
  2. Wheel breaking off
  3. Robot still performs multiple 0 turn radius pivots, indicating both sides of the drive train are moving
  4. One side of the drive train stops moving

The transition between 3 and 4 indicates that the dog failure happened after the wheel failure. Therefore, the conditions which that dog saw were very different than most (if not all) conditions seen in previous failures. This, along with the different material and geometry of the part as compared to the standard AndyMark dog make it hard to determine if shock loads or stall torque could cause the torsional failure.

In what ways can the torque seen by the dog at the output shaft in shock loading exceed the traction limited torque of the gearbox by a factor of 3? The most I can physically make sense of is a factor of 2 when changing from full speed reverse to full speed forward or engaging in a pushing match at full speed to a stop.

I agree this is a torsion failure. I was saying previously that I had never seen such a failure of the AM stainless dog.

You are only considering torque originating from the motor. Hitting another robot at high speed can cause very fast acceleration on the robot, which results in a huge force on drive line components. While all of the steady-state math would see the wheels spinning and such, there is still a shock on the drive line components momentarily (and it is HUGE).

I would additionally guess the dog had been failing for some time, and there were already stress cracks at two or all three of the fracture locations, from repeated shock loading.

I still believe the cause of failure is improper material selection or component design more than anything else, not improper use of the gearbox (it should be able to handle this). 7075 is a significantly weaker material, and without changes to the dog design (e.g. increasing thickness, additional hardening, etc.) the overall strength is significantly lower vs stainless steel.

Thanks for the insight. Would there be any anticipated issues associated with making a part with similar geometry out of Stainless Steel and engaging it with the Aluminum gears or coupling to the 5/8" Aluminum hex shaft?

I would guess that the gear would still fail before the stainless dog (or possibly at this point the 7075 hex shaft would become rounder), but the failure load would go up quite a bit relative to the 7075 dog, but that is just a guess. YMMV.

The dog is a tiny, high-load piece with many stress risers. It is subject to significantly more overall stress than either gear mating, as it sees loads when in both high and low (as opposed to only one of the two) in addition to shock loads during shifting (again, ~2x that of either high or low). It’s small enough that, even in stainless, it only weighs 0.044 pounds (According to AM website).

I have never run aluminum shifting gears in a drive gearbox, so I can’t comment on durability with good evidence. I have run aluminum gears elsewhere (in FIRST) and seen failures of many aluminum parts (not in FIRST).

We ran 7075 Dogs w/ 6 motors (4 CIM, 2 775) and 2" wide roughtop wheels w/o issue in 2011.

Unsure how our dogs then compare in size to WCP’s.

I’d have to check for exact ratios, but it was something like 7/18 fps.

I was curious about this failure, so I pointed a more knowledgeable person to this thread. His suggestions:

-Without a small radii at the corners, there are huge stress risers at all of the corners.
-The cross section at the bolt hole is extremely small, and the stress concentration here will be extremely high.
-It would be interesting to try Ti for this part.

It is possible that the radi on the AM dog is larger than the WCP dog, which contributed to the failure. I can’t tell from the pictures.

7075 is not the problem. We have used it or 7068 since 2007 with zero failures.

Most common grades of stainless are actually substantially weaker than 7075, unless AM is using a prehard grade (13-8, 15-5, 17-4) or 410/416

Do you mean 7075 is stronger per unit weight, or stronger for a given cross-sectional area?