pic: 20's IRI Carnage

[apalrd]

I would guess that the gear would still fail before the stainless dog (or possibly at this point the 7075 hex shaft would become rounder), but the failure load would go up quite a bit relative to the 7075 dog, but that is just a guess. YMMV.

The dog is a tiny, high-load piece with many stress risers. It is subject to significantly more overall stress than either gear mating, as it sees loads when in both high and low (as opposed to only one of the two) in addition to shock loads during shifting (again, ~2x that of either high or low). It's small enough that, even in stainless, it only weighs 0.044 pounds (According to AM website).

I have never run aluminum shifting gears in a drive gearbox, so I can't comment on durability with good evidence. I have run aluminum gears elsewhere (in FIRST) and seen failures of many aluminum parts (not in FIRST).

[AdamHeard]

We ran 7075 Dogs w/ 6 motors (4 CIM, 2 775) and 2" wide roughtop wheels w/o issue in 2011.

Unsure how our dogs then compare in size to WCP's.

I'd have to check for exact ratios, but it was something like 7/18 fps.

[apalrd]

I was curious about this failure, so I pointed a more knowledgeable person to this thread. His suggestions:

-Without a small radii at the corners, there are huge stress risers at all of the corners.
-The cross section at the bolt hole is extremely small, and the stress concentration here will be extremely high.
-It would be interesting to try Ti for this part.

It is possible that the radi on the AM dog is larger than the WCP dog, which contributed to the failure. I can't tell from the pictures.

[Cory]

Originally Posted by apalrd

I still believe the cause of failure is improper material selection or component design more than anything else, not improper use of the gearbox (it should be able to handle this). 7075 is a significantly weaker material, and without changes to the dog design (e.g. increasing thickness, additional hardening, etc.) the overall strength is significantly lower vs stainless steel.

7075 is not the problem. We have used it or 7068 since 2007 with zero failures.

Most common grades of stainless are actually substantially weaker than 7075, unless AM is using a prehard grade (13-8, 15-5, 17-4) or 410/416

[KrazyCarl92]

Originally Posted by Cory

7075 is not the problem. We have used it since 2007 with zero failures.

Most common grades of stainless are actually substantially weaker than 7075, unless AM is using a prehard grade (13-8, 15-5, 17-4) or 410/416

Do you mean 7075 is stronger per unit weight, or stronger for a given cross-sectional area?

[AdamHeard]

Originally Posted by KrazyCarl92

Do you mean 7075 is stronger per unit weight, or stronger for a given cross-sectional area?

He means a stronger ultimate strength (psi).

[Cory]

Originally Posted by KrazyCarl92

Do you mean 7075 is stronger per unit weight, or stronger for a given cross-sectional area?

Stronger in an absolute sense. It has a yield strength of 70-75 ksi whereas the common stainless steels are in the 40 ksi range. The prehards are extremely strong (70-200 ksi). It all depends on the heat treat though, which could be highly variable for stainless, but 7075 is going to be T6, period. 7068 would be significantly stronger than 7075 at 90-95 ksi.

[KrazyCarl92]

Just did a quick matweb search and found that the following types of stainless steel all have ranges of yield strength beyond the 73000 psi listed for 7075 Aluminum:

302, 305, 308, 309, 310, 329, 347, 348, 403, 405, 410, 414, 416, 420, 422, 446, 450, 455, 630, 651, 660 & 675

Some of these appear to be dependent on the heat treatment while others are not. I have no feel for which of these types of stainless steel AM may be using or which kinds are even common, so it is tough to have a good basis of comparison for the different materials used in the dogs.

[apalrd]

After looking at a bunch of material sheets, I agree with Cory. Stainless steel is actually quite weak compared to many alloys of non-stainless steel. I surprised myself here.

Why stainless was chosen instead of hardened steel for the AM part is then a question as well (unless it is a prehard stainless alloy).

Which brings us to component design failure. Either the design load was exceeded (possible, but then design load should be increased) or design was not properly analyzed (also possible, likely shock loading or fatigue not correctly estimated in that case).

Is this picture the first dog to fail or the second? If it was the second that might explain the failure more, as the hole was drilled larger, at the point of highest stress.

[KrazyCarl92]

The dog depicted is the second one that failed (I can tell by the lack of threads in the through hole).

However, the first one that failed also fractured at three points: 1 at the threaded bolt hole and 2 other places exactly where the dog goes from thick to thin, as shown in this picture. I can post a picture of this failure as well later tonight.

[R.C.]

Originally Posted by AdamHeard

We ran 7075 Dogs w/ 6 motors (4 CIM, 2 775) and 2" wide roughtop wheels w/o issue in 2011.

Unsure how our dogs then compare in size to WCP's.

I'd have to check for exact ratios, but it was something like 7/18 fps.

Ditto,

This dog model is pretty much the exact same since the one we've used on 973/1323 bots since 2011. We didn't randomly pick 7075, its been tested on 254/968/973/1323/1477 etc... for a pretty long period of time.

The gearing seems about right, 7/17 or 7/18 should be it.

We haven't had a dog fail except:

https://fbcdn-sphotos-c-a.akamaihd.n...22823429_n.jpg - 2012 MTTD Offseason event - Practice Bot.

This guy was before WCP started making them, our sponsor made these and this dog was drilled off center. There was more material on one side than the other. We treated this as a random occurrence and really aggressive driving.

But for us, since 2011. 6-8+ robots (pbots included) have used this same dog design with probably over 1,000-2,000+ hours of driving with no issues. So we feel pretty safe with the material choice and run time, more or less we try to test all our products out for an extended amount of time before putting them into the market.

For this occurrence we'll be looking into it and trying to:

-Replicate Breakage
-Find the exact problem
-And a fix for it. (Going to 4140 or some other steel alloy)

I'll post back when we find something out.

-RC

[Adrian Clark]

I've often wondered about the safety of al dogs, so I find this thread very interesting!

R.C./OP, if either of you are interested I would love to run a simulation of this situation and post the simulation report as a whitepaper. To do this I would need to know a little more about what happened, mainly how much approximate torque the dog was subjected to.

-Adrian

[R.C.]

Originally Posted by Adrian Clark

I've often wondered about the safety of al dogs, so I find this thread very interesting!

R.C./OP, if either of you are interested I would love to run a simulation of this situation and post the simulation report as a whitepaper. To do this I would need to know a little more about what happened, mainly how much approximate torque the dog was subjected to.

-Adrian

Sure,

Email me when you can and I'll CC Carl in the email.

Thanks!

-RC

[JChang]

I would like to know how 254/973/1323 connect the dog to the moving shaft inside the output axle (roll pin, bolt or something else).

Our team used the WCP dual speed gearbox for this season, but a few days before ship date, the 4-40 bolt (threaded all the way) snapped. In the same week, the same thing happened to our two years old AM SuperShifter as well. We didn't want to have this problem on the field, so we drilled through the thread with an 1/8'' drill and punched a roll pin in. Our very old AM Shifter have the the roll pin setup and never failed on us. AndyMark now improved the dog design by custom making a bolt with threads only at the end (am-1272).

After 4 events (BMR, CRR, Champs, Indiana State) and weeks of testing for IRI, the modified dog gear finally failed, just 6 days before IRI. By that time the hole was enlarged by the shifting motion, causing the dog to break. We rush ordered the dog from WCP and thankfully, it came in time for us to complete the repair on IRI load in day. On the new dog, we drilled only halfway through the threaded part, leaving more materials on that side. I handed the partly drilled dog to Team 20, and they tried to repair it with the 4-40 bolt. There wasn't enough thread left for the bolt, so they went with the roll pin method. Unfortunately they used a bit that is to large, and when we went up against The Rocketeers, the second dog shattered. I hope this never happen to any team ever again.

Some suggestions for improvement:

• Make the OD of the dog larger, there is plenty of room before interfering with the 45t gear.
• Use roll pin or solid body 4-40, the current bolt will snap when inexperienced programmers attempt to write an auto-shift code.
• Thicken the thin part of the dog if there is room

-Josh

[Cory]

Originally Posted by JChang

I would like to know how 254/973/1323 connect the dog to the moving shaft inside the output axle (roll pin, bolt or something else).

Our team used the WCP dual speed gearbox for this season, but a few days before ship date, the 4-40 bolt (threaded all the way) snapped. In the same week, the same thing happened to our two years old AM SuperShifter as well. We didn't want to have this problem on the field, so we drilled through the thread with an 1/8'' drill and punched a roll pin in. Our very old AM Shifter have the the roll pin setup and never failed on us. AndyMark now improved the dog design by custom making a bolt with threads only at the end (am-1272).

After 4 events (BMR, CRR, Champs, Indiana State) and weeks of testing for IRI, the modified dog gear finally failed, just 6 days before IRI. By that time the hole was enlarged by the shifting motion, causing the dog to break. We rush ordered the dog from WCP and thankfully, it came in time for us to complete the repair on IRI load in day. On the new dog, we drilled only halfway through the threaded part, leaving more materials on that side. I handed the partly drilled dog to Team 20, and they tried to repair it with the 4-40 bolt. There wasn't enough thread left for the bolt, so they went with the roll pin method. Unfortunately they used a bit that is to large, and when we went up against The Rocketeers, the second dog shattered. I hope this never happen to any team ever again.

Some suggestions for improvement:

• Make the OD of the dog larger, there is plenty of room before interfering with the 45t gear.
• Use roll pin or solid body 4-40, the current bolt will snap when inexperienced programmers attempt to write an auto-shift code.
• Thicken the thin part of the dog if there is room

-Josh

There are good reasons to keep the dog as small as possible. It opens up the ability to use a lot of other custom reductions that would otherwise be interfered with. We had to actually turn ours down a touch to get enough clearance for the ratio we ran.

We used the new Andymark shoulder bolts for our dogs.