We did it again! This time not as bad. As it happened on the same gearbox, we’re concerned something might be wrong with it. We will tear down the old gearbox tomorrow, but for the meantime we swapped one in. Hoping to scrounge up from some steel gears to replace them when/if this one fails.
For other teams to identify potential failures before they happen, would you mind expanding on the circumstances that led to the failure?
- Wheel Size?
- Overall reduction in gearbox?
- Any additional reduction after the gearbox?
- Did the failure happen while driving on flat ground, or as a result of an impact?
We noticed a similar failure (but in a 14 tooth), that can probably be attributed to sudden impact combined with materials that don’t have impact resistance as a strong point.
There’s quite a lot of good learning (and fun!) to be had going down the failure root cause analysis road.
You have to see the ones we took out of our vexpro drive gearboxes. We had them break 3 times
Hey FRCGuy are you sure you don’t want my 2 lightly used 24 tooth Vex gears?
It’s probably too late for me to broach you steel gears.
I also don’t have 24 tooth steel gears to start with and I doubt I can get them fast enough.
Going to put these on my list of ‘coming soon’…
Just in case it hasn’t happened yet …
I’ll bet the VEX folks would like to hear from you. I’m guessing that they will want to know if
A) they need to either include a caution with their parts to help users avoid exceeding the loads the parts are designed to sustain, or
B) they need to investigate a batch of parts that might have been manufactured badly, or
C) they perhaps need to tweak their product’s design.
If you are at the championship, I’ll bet they are there too.
Ok, I’ll provide the details in order. Wheels are 8" AndyMark pneumatics. Overall reduction is the standard 2-CIM ball shifter from Vexpro with the 60:24 3rd stage. We’re pretty sure it happened while being pushed by other robots and hitting defenses. Thankfully, the guys from 254 came through with some extra gears for this. I’ll try to find Vexpro tomorrow to talk to them about it. Also, if you think that’s crazy, I’ll post a picture of one of that we stripped today that only has three teeth left :).
I was considering buying vex gearboxes for next season, but with stories like this I’m not so sure…
Steel final gears should be adequate to sort the issue.
Course they are heavier.
Every year I try to avoid broaching stuff but I finally got fed up and bought a #5 5 ton ratcheting arbor press and Dumont push broach. Built a nice stand out of 2x4 with caster’s for that 365lb monster.
I also bought a rotary broach and holder but I am sorting issues out on that.
I had 20 and 24 tooth Vex gears because I bought 2 dual CIM Vex ball shifters lightly used from an FRC team on eBay. So I actually have something to test with. Personally I just usually use AndyMark boxes.
If figure as CSA sooner or later I am going to run into these Vex products.
With 8" wheels, people aren’t quite taking into account sudden shocks that can be imposed on gears with the torque of 2-3 CIMs in a gearbox. Start doing some Lewis Gear equations and use a decent factor of safety. Vex gearboxes weren’t meant to do certain things, and as a designer, you need to determine whether the gearboxes can handle the loads you’re putting them through. If they can’t, you have two choices: find another gearbox or modify the COTS gearbox to make it work.
We had the same problem at waterloo, We sheared 3 Gears in 7 Matches, its the gears because we used two different gear boxes and they sheared on both
Congrats on making it to Worlds
I think the Vex box is actually pretty nice but:
If your team bought this COTS and managed to avoid buying the broach and press for it and you need to make a modification a few hundred bucks could really hurt. On the flip side of that you could buy extra gears early but first you would need to foresee the issue and then be able to source them…it looks like late in the season the supply chain got tight.
I rarely mind helping and I bet at CMP you might get the machine shop to broach steel gears you might buy just in case: but this could really hurt at competitions less equipped and might be pointless if you did not get suitable steel gears before you need them.
So really it is a design issue, but those 20 tooth steel gears I sent to CMP were AndyMark products broached. There are steel gears you can order on Amazon that could be broached (take longer to get).
BTW recognition to BeardyMentor and Philip for helping me out on this.
Or … It might be the way that you are using the gears (or it might not).
More stress than the part was designed to withstand = It breaks.
How do you know that you aren’t exceeding their advertised ability to withstand static or dynamic loads?
Instead of writing “it’s the gears”, you might need to write “it’s our robot”?
How would you test that without ruining gears?
I mean you’d have to test it to Vex’s specification and then risk testing it in a drivetrain.
Testing to Vex’s specification would rule out defective gears…testing your drive train would demonstrate a design issue.
You just have to calculate the theoretical max torque based on the Lewis gear equation, then include a factor of safety.
All I’m saying is, even the strongest gear teeth will break off when subjected to too much stress.
If that happens, you don’t write, “it’s the gears.” Instead you you would write, “it’s our robot.”, or “it’s our driver.”, or …
We had a VEX gearbox failure in the same type of gearbox. But it was the gears that failed, it was the casing. We took an especially rough underside impact and sheared the CIM right out of the black plastic. So… you know… we drove almost a whole match with a CIM hanging solely on its wires. (No way were we going to hit that E-Stop)
The same thing happened to us at the OKC regional. We believe that a bind was caused due to a broken PTO chain and resulted in a stripped gear. We are using these gears on our Vexpro ballshifter gearboxes.
As promised: http://www.chiefdelphi.com/media/photos/43781
When you say ‘broach’ them some steel gears, what exactly are you referring to?
You can Google “broach” to find some examples, but in this case, it is pushing a hex shaped cutting tool through a round hole to make a hex shaped hole.