Originally Posted by Jadium (Post 568774)

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.

Originally Posted by RaMoore (Post 568567)

Pictures of unhardened carrier as promised.

The damage is not as extensive as other teams have seen, but a clear difference from the hardened one.

As for the repeated cycles of heat, we tested with the knife scratch and it did seem to change it, for whatever the reason. I don't think any of us has had a class on the subject just some practical experience and some googling. I think we could try Kevin Sevcik suggested method for hardening if we can get it that hot.

Originally Posted by cbpetrovic (Post 568688)

Here are a couple of slides with a solution cooked up by one of our MEs...





C. B. Petrovic - Team 166

Originally Posted by Joe Johnson (Post 568784)

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

Originally Posted by dtengineering (Post 568572)

We have four 56mm BB trannies running our mecanum drive. The 8" AM mecanums are directly mounted to the BB drive shaft which is supported on the outboard end quite securely. (photos at http://www.chiefdelphi.com/forums/sh...highlight=1346 )We "broke in" the gearboxes for at least 30 minutes of continuous operation before installing them.

We have been running our robot for... well... less than 30 minutes and noticed one or two of the trannies were beginning to show significant backlash.

We took the transmission apart and noticed that while the final drive plate had not failed completly it was very close to failure as discussed in this thread.

This is after very low use with ONE CIM in a relatively low dynamic loading (no chain to snap) environment.

We have noticed that the two BB's that came in the KOP have less backlash. They also fit much more nicely onto the CIMs than the two we bought in early January. Perhaps there is variation from batch to batch to deal with as well.

In any case, here is one more report of BB 56mm failing (or at least coming close enough to failure to count) under ONE CIM load... and at extremely low cycles, too.

We are also pondering solutions, but it seems manufacturing a new plate, or having one manufactured is what we are going to have to do if we want to use these trannies.

Jason

Originally Posted by Joe Johnson (Post 568784)

#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

Originally Posted by eugenebrooks (Post 568887)

Joe,

A target harness approaching RC 40 should be fine. We have run
welded axles, 4130 flanges welded with 4130 rod onto 4130 axles
for several seasons. We harden these and then temper at 800F.
Tempered at 900F, a little softer, the RC is 36 and the tensile yield
strength is 161,000 psi. We temper at 800, slightly off the charts, so
to speak, but safely above the brittle zone, to get just a little more
strength. We have bent these axles in competition with direct robot
impact on a cantelevered wheel setup, but have never broken one.

Hardening the carrier plates to a value approaching RC 40 is probably
just right. Using 4140, or even better 4340, is best for this application.
We are using 4130 because this is the material we have on hand.

Here is a link to useful heat treatment info:
http://www.aerospacemetals.com/steelalloys.html


Eugene

Originally Posted by capnrmorgan (Post 568931)

Also my technical adviser suggests crucible packed with charcoal, bone, or other carbonized material and forge heated for 8 hrs or more.

Originally Posted by ZZII 527 (Post 568939)

I suspect the solution will be an entirely new plate made of something harder and with a tighter fit for the shaft.

Originally Posted by squirrel (Post 568950)

I was pondering making the shaft to plate joint a press fit. I think it would be relatively easy to get the size for round part of the hole figured out, but I wonder what the tolerance is on the flats on the shaft. If they vary by more than a thousandth of an inch, it would be challenging to get a reliable fit.

It would be really nice from the viewpoint of the wear we are seeing....I know in automotive transmissions, once splines start to get loose, they wear out quickly.

Originally Posted by squirrel (Post 568950)

I was pondering making the shaft to plate joint a press fit. I think it would be relatively easy to get the size for round part of the hole figured out, but I wonder what the tolerance is on the flats on the shaft. If they vary by more than a thousandth of an inch, it would be challenging to get a reliable fit.

It would be really nice from the viewpoint of the wear we are seeing....I know in automotive transmissions, once splines start to get loose, they wear out quickly.

Originally Posted by Joe Johnson (Post 568867)

Can anyone else see these slides? I can not.

Joe J.