pic: Stripped VexPro gear



Our stripped gear from our gearbox. Badly damaged after getting caught in the lowbar during auto before we refined our positioning.

On the plus side, now you have a brand new dual sector gear.

I’m curious about the circumstances that caused this; I’m having a hard time visualizing how getting stuck in the low bar could cause this kind of damage to a gearbox. Any more details you can share?

Sure. During auto the robot would go through the low bar, and then attempt to back over it. If our alignment was off, when backing up, the robot went in at a slight angle and was pushing against one of the side supports for the bar. The motors proceeded to run while the robot was pushing against the support for a good 5-7 seconds, causing the gears to grind. This happened once or twice until we dialed in our positioning. For context, this gear is part of the third stage on the 2 CIM ball shifter. It actually drives the output shaft.

We blew every tooth off of two of the VexPro 14tooth 3/8 hex gears.

Turns out aluminum gears just aren’t quite rated to the level of impact we were subjecting them to.

For the 14T I would personally only use the steel in anything that takes moderate to high load and the aluminum one for low load. Or just use the steel one all the time since the weight difference is minor.

1771 had this exact thing happen to them at the PCH DCMP.

Actually, the gear stripped, we replaced it, and it stripped again. So we replaced it, again.

The robot still managed to drive, the driver barely noticed any difference in performance.

It only really takes one tooth to break or chip to cause all the teeth to be stripped off the gear, as I believe that the shrapnel causes the next tooth to be clogged and break.

Aluminium gears are usually fine, especially 7075 ones. But if you can, it can be good practise to use 4140 for the smallest gears (isn’t a huge weight increase for small gears)

With gears that small there is only a small amount of tooth contact, meaning that the forces aren’t spread out over a huge area. Something we found with the impacts of stronghold is that it can cause cantilevered gearboxes( CIM’s are heavy and have a lot of inertia) to move up and down, which can mean that in gearbox 3rd stages the amount of tooth to tooth contact can be decreased. Also teams are putting much more torque on them than usual, for example if a team normally uses a shifting gearbox with a 11fps low gear on a year with an open field and 4inch or 3.25inch wheels, and then they change to 8inch wheels and a 6fps low gear, then the final stage is experiencing almost 4 times the torque.

Depending on how you are using it, a 4140 Steel gear http://www.andymark.com/product-p/am-2354.htm might be better for your purpose

Not quite…

What usually happens is that if one tooth breaks off, the next tooth takes more force than it would ordinarily (fewer points of the same force being applied = more stress as well), plus a chance of shock loading. Then that one takes off… repeat until you’re out of gear teeth.

We learned that the hard way lol, went kaput in elims during finals our first district event, we now use steel 14t on our drivertrain lol.:cool:

When only a small number of teeth are removed, there is normally a lot of damage to the other teeth. But this damage doesn’t seem that it could be all caused by the normal meshing of the other gear. In this case is it the shrapnel that is the cause of this?

In that case, yes, it would be any remaining bits. You’ll also want to check the mating gear(s) for damage, as damage to metal tends to transfer to any metal of similar softness that it hits… I wouldn’t be surprised if there was some “hard whacks” causing damage as well.

Absolutely. Our fix was to go grab some 3/8 AM hex gears and face them down to the required width. No issues after we switched out the aluminum gears.

This is the differing factor.

However, two years in a row of COTS gearboxes failing us has prompted the decision to return to custom boxes. They’re heavier (and a touch more expensive), but they never broke.

We had the same thing happen to us on our scaling mechanism. I believe we were using the 60 tooth 1/2 hex bore gear from VexPro. We have a CIM Motor that has 3 stages of AndyMark GEM Planetaries on it, along with a 1:3 gear ratio coming out of it… totaling up to around 900 ft/lbs of torque. At Bayou, we went out in the quarter finals because we had 4 teeth shear on that gear. We couldn’t scale because the robot sagged and got caught in that section of the gear that had stripped.

We ordered a steel gear from McMasterCarr and welded in a 1/2 inch socket. It worked throughout Rocket City flawlessly.

That would certainly do the trick. Makes me glad we traction-limited the low-gear on our drive system. :rolleyes:

That said though, of the 100+ VEX aluminum gears we’ve used on our competition robots over the past 3 years, the only time I’ve had issues with chipping teeth was on the choo-choo mechanism on our 2014 shooter (which, too be fair, was poorly designed, and over tensioned) so don’t let them scare anyone away for most normal uses.

those gears are made out of 7075 Aluminum which is as strong as many types of steel, thats one impressive feet lol

“Strong” is a rather non-precise word for material descriptions.

Steel and 7075 may have similar load capability and shear strength, but when it comes to a sudden impact (ie a 145lb robot at ~12fps slamming into a barrier), the more brittle aluminum gear doesn’t stand a chance.

In comparison with 4140( normal steel gear material) it has a higher yield strength, but a lower ultimate tensile strength. It also has a much lower %elongation. The percentage elongation means that it can’t bend and deform as much before it fails

http://www.wcproducts.net/how-to-gears/

The material of the gear is not the end-all deciding factor in whether or not it’s strong enough for the job. If you stick enough torque on it, you can break anything!

Each stage of gear reduction adds up fast. Whenever we need a gearbox that needs more than 3 stages of spur gear reduction, I start to get a little paranoid. We underdesigned the 3rd stage of our stacking mechanism last year and separated several gears from their teeth when operating it with the braking mechanism engaged.

Always make sure each stage can handle the maximum torque the motor can output, and add however much cushion you think you need. (Then test test test… break it at your shop so you don’t have to break it at competition)

We found out the hard way losing in the Hawaii regional this year.
Our 14T aluminum hex gear and another of our aluminum VEXPro gear lost teeth in one of our trans. Could not swap out the trans in time for the next semis match.
In the past we always used steel and could see noticeable wear after running our robot for a lot of practice and real matches.
I guess with the aluminum ones, they just break teeth. :confused: