04-17-2016 01:35 PM

Maxwell777

Re: pic: Stripped VexPro gear

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

04-17-2016 03:44 PM

cbale2000

Re: pic: Stripped VexPro 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?

04-17-2016 03:55 PM

frcguy

Re: pic: Stripped VexPro gear

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.

04-17-2016 07:53 PM

Mechvet

Re: pic: Stripped VexPro gear

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.

04-17-2016 08:00 PM

R.C.

Re: pic: Stripped VexPro gear

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.

04-17-2016 08:00 PM

nighterfighter

Re: pic: Stripped VexPro gear

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.

04-17-2016 08:12 PM

pilleya

Re: pic: Stripped VexPro gear

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

04-17-2016 08:15 PM

EricH

Re: pic: Stripped VexPro gear

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.

04-17-2016 08:22 PM

billhcheese

Re: pic: Stripped VexPro gear

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.

04-17-2016 08:27 PM

pilleya

Re: pic: Stripped VexPro gear

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?

04-17-2016 08:29 PM

EricH

Re: pic: Stripped VexPro gear

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.

04-17-2016 11:19 PM

Mechvet

Re: pic: Stripped VexPro gear

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.

04-18-2016 12:10 AM

Landonh12

Re: pic: Stripped VexPro gear

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.

04-18-2016 12:18 AM

cbale2000

Re: pic: Stripped VexPro gear

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.

04-18-2016 12:40 AM

TheModMaster8

Re: pic: Stripped VexPro gear

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

04-18-2016 12:48 AM

Mechvet

Re: pic: Stripped VexPro gear

"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.

04-18-2016 12:48 AM

pilleya

Re: pic: Stripped VexPro gear

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/

04-18-2016 02:12 AM

s_forbes

Re: pic: Stripped VexPro gear

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)

04-18-2016 02:31 AM

waialua359

Re: pic: Stripped VexPro gear

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. :/

04-18-2016 07:46 AM

pfreivald

Re: pic: Stripped VexPro gear

This is exactly why we lost the quarterfinals at Tech Valley; we saw that the gear was starting to strip, but the repair/swap would take too long. We hoped to survive through the tournament and would have made the semifinals if we could have driven ten more feet--but alas, it gave up the ghost on the wrong side of the (open) Sally Port.

We switched to toughboxes with aggressively-lightened steel gears for FLR and had no drivetrain issues throughout.

04-18-2016 08:06 AM

CJ_Elliott

Re: pic: Stripped VexPro gear

It's ok. We too lost teeth in semis and couldn't run at 10k... after having already replaced the 14 tooth gears in both gearboxes during quals. I feel like this whole 14 tooth aluminum gears thing is an issue.

04-18-2016 08:58 AM

Chris is me

Re: pic: Stripped VexPro gear

Personally I wish that VexPro would make all of their pinion gears (say 14-24T) available in steel as well as aluminum. They are a higher quality gear than AndyMark in terms of tooth profile, manufacturing tolerances, and quality in general, but being stuck with aluminum pinions can be somewhat limiting when you don't have the ability to compensate for the loss in strength in other ways (ie doubling the face width, different DP of gear, etc).

04-18-2016 10:09 AM

JesseK

Re: pic: Stripped VexPro gear

Can anyone confirm, via math, that the steel teeth would have been strong enough to prevent total tooth failure? At the time this happened, we had to move fast to fix it for our first event:

For our tilt motor we ran a BAG -> 100:1 VP -> 20T/40T reduction and kept stripping the teeth on the 20T. This was because of the cantilever on the VP - the shaft would flex just enough to cause a LOT of stress on the teeth, and they'd strip. By the time this happened VEX was out of 20T gears, so we swapped to belts. We also added a plate on the other side of the cantilever with bearings to support it.

We're running WCP gearboxes with an extra 18->36T sprocket reduction (non direct-drive) and they have been flawless this season with no signs of wear on the sprockets. So I don't know that aluminum vs steel is the issue.

04-18-2016 01:27 PM

waialua359

Re: pic: Stripped VexPro gear

Is that really what happened? Holy Moly. Sorry to hear that.....I had wondered what happened to you folks when the backup robot was used.
For our situation, we had hoped to come back in and decided not to bring in the backup bot.

04-18-2016 01:35 PM

Chris is me

Re: pic: Stripped VexPro gear

Gears versus sprockets are apples and oranges comparisons - you can't draw conclusions about material choice for one by looking at the other. Sprocket load is shared by several teeth at once, while gear loads are borne by just a few teeth at a time. Chain also dampens some degree of shock load, to an extent.

04-18-2016 02:22 PM

JesseK

Re: pic: Stripped VexPro gear

Hmm, fair point.

04-18-2016 03:05 PM

CJ_Elliott

Re: pic: Stripped VexPro gear

Yea... 10k was a bit of a cluster. We broke every system on our bot, but we digressed. 3 grenaded drivetrain gears at 1 competition... I think that might be a record.

04-18-2016 05:58 PM

Jeffrafa

Re: pic: Stripped VexPro gear

We also learned this lesson the hard way on the WCP SS gearbox. Fortunately it first happened during our 6-hour unbag window prior to our first competition, so it didn't cost us any matches. It did cost us 2x 45 minute teardown sessions (1 during unbag, and 1 at the event) to upgrade both competition gearboxes - We were also fortunate to already have the 14T steel gears on-hand, spared for another mechanism.

We also helped get 4469 back up and running with steel gears at PNW DCMP after they stripped the same 14T aluminum gear in the Vex 2-stage SS gearbox.

R.C. - It would be great if these two gearboxes were configurable with steel 14T gears, or even came with them by default for the relevant gear ratios. Currently only aluminum is available when building a gearbox for purchase through Vex or WCP - steel replacements have to be purchased separately.

04-18-2016 06:13 PM

R.C.

Re: pic: Stripped VexPro gear

Jeff,

I totally agree with you, I thought we made this change over. I will check and get this implemented, also have a few other changes planned.

04-19-2016 07:51 AM

pfreivald

Re: pic: Stripped VexPro gear

That's awesome, thanks!

(I'll probably still drill a bunch of holes in the steel, but will never go with aluminum gears in a drivetrain again.)

04-19-2016 08:15 AM

BoilerMentor

Re: pic: Stripped VexPro gear

We broke one at our district championship event. The interesting and slightly concerning thing is that it wasn't at a particularly high load, just a sustained, consistent torque load due to a misaligned limit switch. The gear that failed is driven by a PG27 with a 16t steel gear on the output shaft and is only driven at partial power.

Unfortunately there isn't a steel option like these: http://www.vexrobotics.com/vexpro/mo...ore-gears.html

We only use a portion of the gear's circumference to actuate our intake, so rotating the gear is an option we'll likely pursue at champs if we have the down time.

04-19-2016 08:21 AM

pilleya

Re: pic: Stripped VexPro gear

Does the PG27 have a 9015 or a RS775-125, as there is a massive difference in torque and power output.

04-19-2016 08:31 AM

BoilerMentor

Re: pic: Stripped VexPro gear

RS775. It broke on the "down" actuation though, which is much lower applied power systematically. My guess is it's actually more related to shock load of the intake coming up against the hard stop that sets the height. It didn't affect our performance this time, so it isn't nearly as big of a deal.