Banebot 56mm gearbox - double D related

[jskene]

We routinely add an acceleration limiter to our motor control software to lower the maximum torque that is transferred to the drivetrain during rapid forward/reverse speed changes. This should reduce the instantaneous load on the problematic gearbox plate, but I don't know whether it will be enough to solve the problem.

I'll have the team dissect gearbox to see what damage we have sustained so far (~ 2 hours of driving).

[Andy Baker]

I had a chance to read up on this issue last night and also talk to Joe this morning. I'm coming in late to this party, but am offering help or advice. Here are a couple of things I see...

1. Just how bad is this problem? What is the % of failure rate here?

2. What is the best fix? It's great to see that is what is focused on in this thread. Best means quickest and easiest for teams, not just the best exact design (for instance, the crossed-dowel fix is a nice design, but difficult to implement since the shaft is hard).

3. Specs for a fix are needed quickly. I'm very interested to see what would CB Petrovic comes up with the 4140 material solution. If I had to pick the "best" fix, that may be it... to re-create a bunch of plates out of 4140.

which brings us to this...

4. IF someone can get a fix (prints and a CAD file) done quickly, there could be ways to mass-produce new parts to get out to teams. I would suggest that a well-toleranced print be made, along with an acurrate CAD file (.stp version). Then, someone could go to http://www.mfg.com and post this as an RFQ. They could ask for quotes to be returned within 1-2 days, and finished parts to be required within one week. We at AndyMark have used www.mfg.com as a fabrication resource of multiple parts and have had good experiences 90% of the time. The bid could be awarded to multiple suppliers, each providing 1-2,000 parts. My guess would be that this part would cost anywhere between $8-$14 if someone was making 1,000 in this short lead time situation.

Once parts were made, boxes of them could be sent out to regions, and then teams could drive a couple of hours to get their part from a central location.

Andy B.

[MrForbes]

That sounds like a plan.....

Some things to consider: The 12:1 and 16:1 output planet carriers are different, because the planet gears are a different size, so the pins are in a different location. Would the replacements be available for both the KOP 12:1 and the upgrade 16:1 transmissions? or would teams have to settle for using 12:1 only?

If you're not familiar with the transmission design, it is a two stage planetary, with a 4:1 first stage, and a 3:1 second stage. An upgrade 4:1 second stage is available from Banebots, and apparently some teams have decided to use it, and apparently it has the same problem with planet carrier plate failure under some conditions. The stages are mostly interchangeable, except that the pinion for the CIM motor is only made for the 4:1 stage (as far as I know), and also I believe it is the only one that will fit in the motor mounting end of the transmission.

Should the replacement plate be made significantly softer than the shaft, to act as a sacrificial part?

Would it include the planet gear pins, or would the teams have to press them out of the old plate, and into the new plate?

Is anyone going to consider the end play issue now, or wait until it causes problems in regionals?

[Rosiebotboss]

Thanks Andy for taking the high road in your posting.

For the "newbies" that are keeping up with this thread: this is what FIRST is about, a perfect example of gracious professionalism. Someone recognized a problem and posted it here. Instead of finger pointing (yes, there was some in the beginning but that quickly subsided), the dialog quickly turned to "Ok, what now? Let's come up with a solution." That posting mobilized an army of engineers, students, teachers and teams working together for the common good.

Back to Andy.....Andy, thank you for taking the high road here. You very easily could have said, "Hey, teams!! AndyMark has an alternative design here. We can provide......." You can guess what the rest of the sales pitch would be. But, no. Andy has offered a solution instead of a sales pitch. THAT is GP in action!!

[Jadium]

Hi,

I know this will not help all of the teams, but since we were already machining our own shafts for a direct drive application we decided to hex the end of the shaft and the carrier plates. The parts will be made by Thursday and will hopefully be tested out Thursday night for a couple hours. We will be making the plates out of 4140.

I don't have access to modeling software until Thursday, so the only drawing I can post is a quick sketch, I will also post the IGES files we sent out for machining. The files will only be useful for the carrier pin hole dimensions as we have a hex in the plate instead of the double d. Also please note that the dimensions in this file were calculated using the SINE Law by measuring the pin diameter and the outer dimension between two pins of the pentagon, so if anyone can double check the dimensions...

By Thursday I will be able to post some results of the the strength of the modified shaft and carrier plate and ensure that the parts as dimensioned on this sketch and in the IGES files mate into the existing transmission.

[Joe Johnson]

Originally Posted by Andy Baker

snip
4. IF someone can get a fix (prints and a CAD file) done quickly, there could be ways to mass-produce new parts to get out to teams. I would suggest that a well-toleranced print be made, along with an acurrate CAD file (.stp version).
snip
Andy B.

Andy,
I will have a STEP file and associated print with tolerances made by one of the best engineers I know (he's a Delphi guy that I could never quite get involved in FIRST... ...maybe some day).

I am also getting the shaft drawn up in CAD too.

Here is my thought process. I am looking for options here. We don't really have a bound on the problem yet. I believe it is likely that an RC 23 carrier plate will be the solution for teams with 1-CIM. But there are teams that have counted on using the 2-CIM with 16:1 ratio. Also, hoping for a good test outcome is not a plan -- it is just a hope. So, I am looking for options in case the tests show that an RC23 carrier is not enough. In this case, there is no more we can do with the carrier alone because we will just push the failure to the shaft. So... ...I am starting to think about the next step in case we have to take that step.


2 questions for folks with the right knowledge base:
#1 should we make the joint square to increase the surface area (and thereby lower the stress)? I have found an 11/32" square broach (8.73mm). Going from a 9mm D to a 8.73mm square will lower the stress to 46% of the current value (an increase in the failure torque of 2.15 (50% of the reduction is from 4 sides taking torque rather than 2 and the extra 4% reduction is that by going to the smaller distance between the flats, you get a larger load surface).

#2 I am thinking that we should have a target hardness of RC 40. This is a Tensile yield of 180Ksi (1250 Mpa). Is this too brittle? I am thinking it is probably ok. My reasoning is that I know that Forkbolts for car door latches are hardened to RC30-38. If RC38 is good enough to take the impact loading from a car crash, I think that RC38 is not too far. It is a small step from 38 to 40. Please share your educated gut feelings with us (provided you have an educated gut).

Joe J

[RaMoore]

Originally Posted by Joe Johnson

Some questions:
Why do it 3 times?

Did you "temper" the part after the final time you quenched it?

Will you do your test again only with the non-hardened carrier for a baseline?

Thanks for the data.

Joe J.

We did it three times to guarantee that we got hardening all the way to the core of the material. To harden the material further we would have to use higher temperatures and cool it faster and we felt this was unnecessary. Before hardening we were able to easily shave pieces of steel off the edges (using a pocket knife) and after the third quenching we could no longer shave large pieces of material off.

We didn’t temper the material following the third quenching because hardening the material to the point we would need to temper it is beyond our capabilities.

We will repeat the test with one of our unhardened carriers and post results soon.

[Joe Johnson]

Originally Posted by RaMoore

We did it three times to guarantee that we got hardening all the way to the core of the material. To harden the material further we would have to use higher temperatures and cool it faster and we felt this was unnecessary. Before hardening we were able to easily shave pieces of steel off the edges (using a pocket knife) and after the third quenching we could no longer shave large pieces of material off.

We didn’t temper the material following the third quenching because hardening the material to the point we would need to temper it is beyond our capabilities.

We will repeat the test with one of our unhardened carriers and post results soon.

Thanks for reminding me about the pocket knife test. I forgot that bit.

I am not a hardening expert by any stretch. From what I remembered from the one class I had that cover this, I thought that only the last one mattered. But I think I was mistaken. I will think some more about it and try to make a new hardened part tomorrow -- at least one will be using your 3rd time's the charm method.

Joe J.

[Dillon Compton]

Hi Joe,

We are using the 56mm gearboxes with the 16:1 reduction, no chains, one per wheel. I am currently very concerned about this issue, but as you keep re-iterating to do, we are avoiding panic (although I am currently investigating other transmission options, should we decide we need to change our design).

My team has very little machining capability, and no sponsors with the capability to do machining such as manufacturing harder plates for us.

A (sort of) solution that springs to mind is that perhaps Banebots could make the current plates with the "double D" available on their website for sale (preferably at "cost"). Given the ease with which the 56mm planetaries are dis-assembled and re-assembled, having spare plates would potentially function as a "fix" for those teams with a few extra dollars in their budget but did not want to/could not afford to buy a full set of replacement gearboxes. 1/2 way through the competition, one should be able to dismount the transmissions, check for the "bow-tie", and if needed swap the plates.

Just a suggestion - I look forward to an official response/fix/etc... from FIRST and BaneBots, as well as the results of your own, clearly extensive and dedicated, testing and trouble shooting.

Thank you for your continued support with this issue, Joe, and good luck to everyone else who is dealing with this issue.

//Dillon Compton
Team 1394

[RaMoore]

We just repeated our test with the unhardened carriers and we successfully broke our gearbox.

We could clearly notice the damage by the 50th cycle. I'll add pictures as soon as I get a chance. We feel that hardening definitely helped, but whether it is a "fix" more testing will tell.

[Kevin Sevcik]

About hardening, I don't think the 3 times repeating will get you a lot of extra depth, but I'm going off my one class a couple years ago, same as Dr. Joe. If you're not heating it high enough to transform the metal and break your previous hardening, then your hardening isn't going to work. If you are heating it hot enough, then you're breaking down the previous hardening. I suspect what you might be doing is a poor man's version of carburizing, building up a layer of carbon from the volatilized motor oil, then letting it soak in by heating up the part again.

At any rate, some brief googling gives me a little better info for those trying the homebrew hardening. The austenizing point is apparently slightly past where the metal stops being magnetic. So heat a bit past where a magnet won't stick to it anymore. I'm also curious if the carrier can withstand a water quench if anyone has a carrier they wouldn't mind possibly warping or cracking.

[eugenebrooks]

Kevin is right about this. We have a heat treating furnace in our
shop that we have used to heat treat axles and shafts for several
seasons now. You soak the metal at a temperature above the transition
temperature to non-magnetic long enough for complete transformation
and then you quench, once, in oil or water as required by the material.
If you are not getting the metal above the temperature that ceases
to stick to a magnet, you are not doing much useful. We in fact use
a magnet on a soaked sample in the furnace to check that our
temperature meter is not off, we crank it up 25 degrees at a time
until the sample goes non-magnetic, and then give it another 50 degrees
for margin.

The shaft in the 56 mm transmission is already hardened. A high
speed steel cutter makes absolutely no progress on it. We learned
this when we attemped to turn a shaft into a square hole to mate to
a 4130 carrier plate that we made for a test of hardended material.
We ended up having to make a new shaft from 4130, and we will
heat treat the material tomorrow. If you heat the shaft from the
transmission you are likely to only temper it to a softer state.

Contrary to prior posts, water quench cools more quickly than oil
quench. Oil quench is dicated for some materials because water
cools too quickly and causes distortion. I would use oil for machined
parts that you do not want to risk distortion during quench. This part
is very thin. There is absolutely no need to water quench it. Oil will
cool it off quickly enough.

After a proper heat soak, and getting the part into the quench
before exposure to air cools the metal below the transition temperature
is really important, you have to temper. A temper at 800 degrees
is what we use on our axles. We do this in the furnace to get a
precise temperature, but they come out a dark blue and this is one
way you can tell what temper you have.

I don't know what the carrier plate in the bane bots tranny is made
of, so I don't know if it can be hardened. We made a carrier plate
and axle of 4130, with a square hole, and we will see what a proper
heat treatment of these parts gets us.

Eugene

Originally Posted by Kevin Sevcik

About hardening, I don't think the 3 times repeating will get you a lot of extra depth, but I'm going off my one class a couple years ago, same as Dr. Joe. If you're not heating it high enough to transform the metal and break your previous hardening, then your hardening isn't going to work. If you are heating it hot enough, then you're breaking down the previous hardening. I suspect what you might be doing is a poor man's version of carburizing, building up a layer of carbon from the volatilized motor oil, then letting it soak in by heating up the part again.

At any rate, some brief googling gives me a little better info for those trying the homebrew hardening. The austenizing point is apparently slightly past where the metal stops being magnetic. So heat a bit past where a magnet won't stick to it anymore. I'm also curious if the carrier can withstand a water quench if anyone has a carrier they wouldn't mind possibly warping or cracking.

[Joe Johnson]

Originally Posted by RaMoore

We just repeated our test with the unhardened carriers and we successfully broke our gearbox.

We could clearly notice the damage by the 50th cycle. I'll add pictures as soon as I get a chance. We feel that hardening definitely helped, but whether it is a "fix" more testing will tell.

This is pretty great news. Now we have a clear this fails / this does not case.

Well Done. I had forgotten to tell RaMoore when I asked them to do this that we will get them new parts to replace the damaged ones.

As to pictures. Yes, we need them.

What we need now is a mat'l lab that can reverse engineer the mat'l of the shaft and of the carrier plate ASAP. Does anyone know of a lab that will do this quick and free?

If we know the material it may be possible to design a heat treating process that can give us the harness we need. If that is possible, then one possible path toward a solution is to locate a heat treat source that will set up a carrier exchange program.

But, first things first, anyone Got Lab?

Joe J.

[Lil' Lavery]

I read in one of the close threads (please forgive me for being lazy and not finding the specific post) that when a team tried to pin the two pieces together, they actually broke their drill bits on the output shaft. I'm not sure what bits they were using specifically (as they didn't mention), so this may or may not be problem in terms of teams doing this themselves.

[cbpetrovic]

This plate looks like it could be manufactured out of 4140 pre hard (Rc 28 - 32) fairly easily. 4140 Rc 28 - 32 should give Sy of around 120 - 130 ksi. A wire EDM could do all features with high precision. Comments?

Could you provide drawing for us to use?

thanks

Ron Reich 166