pic: Stripped VexPro gear

[frcguy]

loading photo ...

[Maxwell777]

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

[cbale2000]

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?

[frcguy]

Originally Posted by cbale2000

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.

[Mechvet]

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.

[R.C.]

Originally Posted by Mechvet

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.

[nighterfighter]

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.

[pilleya]

Originally Posted by Mechvet

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

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.

Originally Posted by Mechvet

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

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

[EricH]

Originally Posted by pilleya

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.

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.

[billhcheese]

Originally Posted by R.C.

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.

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.

[pilleya]

Originally Posted by EricH

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.

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?

[EricH]

Originally Posted by pilleya

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.

[Mechvet]

Originally Posted by R.C.

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.

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.

Originally Posted by pilleya

<snip>

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.

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.

[Landonh12]

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.

[cbale2000]

Originally Posted by frcguy

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.

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

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.