Rhino track failier?

[callmecairo]

We are talking about replacing them with aluminum. Glad we found it before any competitions! It makes me wonder about all the other teams using them who don't know or haven't had issues yet.

[dradel]

Wish we had the resources to machine some out of aluminum

[MrBasse]

Why after two cases have come up with unknown cause of failure are people willing to tuck tail and run so fast? I would wait a day or two and see what comes of contact with Andymark. They understand build season and everything that goes into the process of building a robot, I'm sure they'll figure this out and help everyone know the cause and solution ASAP.

[bmammen]

Originally Posted by MrBasse

Why after two cases have come up with unknown cause of failure are people willing to tuck tail and run so fast? I would wait a day or two and see what comes of contact with Andymark. They understand build season and everything that goes into the process of building a robot, I'm sure they'll figure this out and help everyone know the cause and solution ASAP.

Make that at least 3 cases, after seeing this we check our chassis and sure enough many of our hubs have cracks and some chips. What's interesting is even our second chassis has them and it hasn't even had the tracks run by anything except hand turning. Here're a couple of pics: http://1drv.ms/1Q467jC One has a piece of paper in the crack to highlight it.

[callmecairo]

Originally Posted by dradel

Wish we had the resources to machine some out of aluminum

We've been really fortunate to have some wonderful parent helpers who can build stuff and know others who can build other stuff

[Andy A.]

Originally Posted by MrBasse

Do you have any fresh wheels to measure the bearing hole size prior to pressing a bearing in?

For those that do, the CAD model on AM website indicates a 1.124" bore with no draft.

Anyone want to place bets on the tool maker deciding to put a draft on that bore without telling anyone at Andymark?

[MrBasse]

Originally Posted by Andy A.

For those that do, the CAD model on AM website indicates a 1.124" bore with no draft.

Anyone want to place bets on the tool maker deciding to put a draft on that bore without telling anyone at Andymark?

That was my thought when I posted that... it should be noticeable if it's there. What's a standard draft? In college we did 2.5 degrees, but we were a bunch of noobs when it came to making a mold.

[Richard Wallace]

Originally Posted by Andy A.

For those that do, the CAD model on AM website indicates a 1.124" bore with no draft.

Anyone want to place bets on the tool maker deciding to put a draft on that bore without telling anyone at Andymark?

The layout print calls out that bore diameter as 1.1249 - 1.1254. Tight tolerance for a plastic molded part. Good fit range for a 1.125 bearing if the tolerance is held.

[BoilerMentor]

I wonder about the fillet at the bottom of the bearing pocket in conjunction with clamping loads from a traditional dead shaft configuration.

[Kevin Sevcik]

Thought I'd do some research, since I recall similar problems:
http://www.chiefdelphi.com/forums/showthread.php?t=84560&page=3&highlight=wheel+crac king
http://www.chiefdelphi.com/forums/sh...ht=wheel+crack
http://www.chiefdelphi.com/forums/sh...wheel+cracking

Most of the reports were from heavy shock loading from running into the old rock-wall like bump. Some were from applying loctite, which as noted in this thread, eats polycarb alive. Some reports were from over-tightening sprocket or assembly bolts. I think atleast one person blamed killing plaction wheels on hammering/forcing bearing into the wheel instead of hand pushing them in and stopping wherever they tightened up.

Not saying any of these are the are the actual solution, just putting it up to inform the speculation.

[Brandon Holley]

The glossiness and types of fractures I can tell from these pictures indicate the material may be glass-filled. Definitely would make the pulleys super stiff, but definitely suseptible to impact failures, as the glass in the resin makes the plastic surprisingly brittle.

Additionally, while the spec'd total tolerance may be 0.0005" (0.0127mm for those of us versed in those units), this is a basically an unmoldable range. It would be tough for an injection mold to hold that steady over many thousands of parts, even things like the HVAC of the factory or running at night vs running at day could have that effect. I envision few suppliers would sign up for that tight of a tolerance.

That being said, molded press fits have been common in AM and VP parts for years without issue- so this may not be part of the failure at all. Just wanted to add some realism to the discussion regarding tolerance and injection molding.

-Brando

[Andy A.]

Originally Posted by Richard Wallace

The layout print calls out that bore diameter as 1.1249 - 1.1254. Tight tolerance for a plastic molded part. Good fit range for a 1.125 bearing if the tolerance is held.

Ah, I see what's up. The drawing (which I didn't bother looking at, of course) is up to rev 3, which creates the bearing pocket. The model does not have that feature, and shows a straight bore. I thought it was a little odd to have a inch long perfectly straight bore in a molded part.

[Kevin Sevcik]

Originally Posted by Brandon Holley

The glossiness and types of fractures I can tell from these pictures indicate the material may be glass-filled. Definitely would make the pulleys super stiff, but definitely suseptible to impact failures, as the glass in the resin makes the plastic surprisingly brittle.

AM specs the material as black polycarb. That's consistent with their other black plastic parts, none of which seem glass filled.

[Andy Baker]

All,

Thank you to the Ginger for starting this thread. During this past weekend, we have received emails from three teams concerned about the same issue. I won't list the team names, as they may consider it confidential information.

This post is a review what we are doing. I will post updates in this thread regarding how things are progressing.

Customer Service:
If anyone has cracked hubs for these Rhino Track Drive modules, please send a note to sales@andymark.com. We will send out* replacement pulley halves. It will help us if you take pictures of the pulley fracture and tell us which pulley it is. Please refer to the pulleys numbered as such in this picture:



In house testing:
We have a test mule here at AndyMark that has been tested many times. Our initial tests (before kickoff) did not show cracking. We drove this mule over the wooden defenses after kickoff and it seemed to do fine. As it drove more last week, seemed to have more resistance to driving the tread. Today, when we drove it, the 120a breaker tripped. We took it apart and found that there were pulley fractures on both sides, on pulleys 1 and 4. Pulleys 2 and 3 did not have fractures. They had a few hairline cracks, but nothing was wrong with pulleys 2 and 3 to affect performance. Here is a picture of our pulleys we took off today:



This picture shows that the front (#1) and back (#4) pulleys were fractured. Since the middle two pulleys fared much better, we think this fracturing is due to the higher forces on the front and back pulleys with regard to belt tension. We are building up the test mule with new pulleys in order to make a test today. We will be testing the chassis with a slightly looser belt tension to see if we can find a belt tension sweet spot between the belt not falling and the pulleys breaking.

Pulley improvements:
The bearing fit is good, in our opinion. It's a light press fit with no taper. The radius at the bottom of the bearing hole is smaller than the radius on the bearing. Since we are able to recreate this failure here, I don't suspect that a customer was inadvertently using thread locking material (which is always a no-no with polycarbonate).

Material: This material is black polycarbonate with no filler. We have tried fiberglass fill years ago with omni wheels, and we learned that while strength is increased with fiberglass, impact resistance is significantly reduced. We could try to make this part out of some sort of Nylon or a co-polymer using Nylon, but that would require an entirely new mold and would take much time to try.

Mold tool improvements: We can take away metal from the mold to add plastic to the pulley. We will look at improving the mold in 3 ways:

1. Adding a rib in the pocket between the mounting holes
2. Raising the counter-bored level for the screw head so that the top of the head is slightly under the face of the pulley (we believe that we can do this without changing the screw length)
3. Increase the radius on the ribs supporting the inner circle bigger

Corrective Action:
There is enough evidence here to justify that we need to improve this mold. Once we get a new part designed, I'll post a pic of it on this thread with the additions highlighted. Since our molder is here in Kokomo and works with us very closely, I am confident that we can make a mold improvement and have parts available within 8-10 days. Thank heavens this part wasn't made overseas.

* - While you may need cracked pulleys sent to you very soon, you may also wish to wait until we get this mold changed and have new, stronger pulleys. Please stay tuned for timing regarding when we can do this.

I am warning our ops and customer service folks that we may be sending out many replacement pulleys soon.

It will be an interesting week. Please stay patient with us as we solve this issue.

Sincerely,
Andy B.

[ChuckDickerson]

Andy, thank you for the prompt attention to this developing issue. I'm sure many teams are going to be watching this situation as it develops.

One additional data point request for Andy and others that have seen the failures. What was the total weight of your robot? Are these weighted 120lb machines or are they failing with lighter basic rolling chassis as well?