29-04-2016 14:07

Mechvet

Re: pic: Another stripped VexPro gear

For other teams to identify potential failures before they happen, would you mind expanding on the circumstances that led to the failure?

Specifically:

• 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.

29-04-2016 15:27

chrisfl

Re: pic: Another stripped VexPro gear

You have to see the ones we took out of our vexpro drive gearboxes. We had them break 3 times

29-04-2016 15:55

techhelpbb

Re: pic: Another stripped VexPro gear

Hey FRCGuy are you sure you don't want my 2 lightly used 24 tooth Vex gears?
Cause...

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'...

29-04-2016 16:15

gblake

Re: pic: Another stripped VexPro gear

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.

Blake

29-04-2016 19:12

frcguy

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 .

29-04-2016 20:11

ollien

Re: pic: Another stripped VexPro gear

I was considering buying vex gearboxes for next season, but with stories like this I'm not so sure...

29-04-2016 20:16

techhelpbb

Re: pic: Another stripped VexPro gear

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.

29-04-2016 20:20

Michael Hill

Re: pic: Another stripped VexPro gear

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.

29-04-2016 20:21

31Josh61

Re: pic: Another stripped VexPro gear

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

29-04-2016 20:28

techhelpbb

Re: pic: Another stripped VexPro gear

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.

29-04-2016 20:53

gblake

Re: pic: Another stripped VexPro gear

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"?

Blake

29-04-2016 20:57

techhelpbb

Re: pic: Another stripped VexPro gear

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.

29-04-2016 21:00

Michael Hill

Re: pic: Another stripped VexPro gear

You just have to calculate the theoretical max torque based on the Lewis gear equation, then include a factor of safety.

29-04-2016 21:06

gblake

Re: pic: Another stripped VexPro gear

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 ...

29-04-2016 21:48

Thayer McCollum

Re: pic: Another stripped VexPro gear

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)

29-04-2016 23:03

lethc

Re: pic: Another stripped VexPro gear

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.

29-04-2016 23:30

frcguy

Re: pic: Another stripped VexPro gear

As promised: http://www.chiefdelphi.com/media/photos/43781

02-05-2016 10:27

LSanchez439

Re: pic: Another stripped VexPro gear

When you say 'broach' them some steel gears, what exactly are you referring to?

02-05-2016 10:39

FrankJ

Re: pic: Another stripped VexPro gear

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.

02-05-2016 11:23

techhelpbb

Re: pic: Another stripped VexPro gear

Buy a gear with a 1/2" round hole (also maybe a keyway) and push broach it 1/2" hexagonal.

The push broach looks like this (this one is from Dumont):
http://www.andymark.com/product-p/am-2204.htm

It fits in either a hydraulic or arbor press.

The 20 tooth steel gear I did for other teams look like this before I broach them:
http://www.andymark.com/Gear-p/am-0186.htm

You know that intelligence test where you put the round peg, in the round hole and the square peg, in the square hole?
In this case you push with about 1,100lbs of force until the round hole chisels out to a hexagon
Proving once and for all - brute force can solve problems.

02-05-2016 12:16

MVCI

Re: pic: Another stripped VexPro gear

Is the output direct driving a wheel (ie output shaft going through a rigid frame)? Look into the possibility for relative motion between the output gear and its driving gear. This could be caused by loose transmission mounting or flexing of the plastic case. With extra load/impacts on the gears being part of this game and improper gear mesh (caused by relative motion between gears) I imagine this could easily happen.

02-05-2016 12:18

Reptilefox

Re: pic: Another stripped VexPro gear

That exact thing happened to us (3230) at the Idaho Regional!