Banebot 56mm gearbox - double D related

[Joe Johnson]

Originally Posted by 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.

I have had good success with STEP files. IGES is fine but circles are often not circles, arcs are not arcs, etc. It is a pain sometimes -- especially if you have to modify the part or if you have to measure things.

For what it is worth.

Joe J.

[Joe Johnson]

Originally Posted by RaMoore

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.

This is the beginning of a failure but it is still a long way from a robot making a perfect circle. Will you please continue the test until a failure that stops the robot occurs? If you take pictures periodically and post them, all the better. If you send me your address, I will send you a replacement for the carrier.

This will be a very good set of data to capture the failure and to test possible solutions (with perhaps something to test coming in a few days).

Thanks.
Joe J.

[Joe Johnson]

Originally Posted by cbpetrovic

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





C. B. Petrovic - Team 166

Can anyone else see these slides? I can not.

Joe J.

[Springman]

Originally Posted by Joe Johnson

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

In reference to the hardening question, I have spoken to our local heat treating facility and they recommend a hardness of RC40. The implication of this is that you should probably start by using standard 4140 (vs. prehard @ approx. RC23) and have the parts heat treated professionally after machining. Speaking from my own experience, 4140 will become too brittle (break vs. deform) for this application at RC>45.

[Joe Johnson]

Originally Posted by dtengineering

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

This is a very strong case that the gearboxes will indeed fail in the field.

As to the low cycles, I have not observed this particular robot in action but based on other mechanum wheel robots I have seen (I can't get to YouTube from this PC), I am not sure that the loading was infact "low cycles"*

In these types of drives, even though the motion of the robot is smooth and continous, the voltages commanded to the motors are often rapid step changes from Full Forward to Full Reverse back to Full Forward -- Again, I am not denying that there is an issue, I am just saying that even though it is direct drive and only 30minutes, it may infact be many many high impact loadings on the gearbox.

For what it is worth.

Joe J.

*Don't jump on me here for burrying my head in the sand. I am have been working this issue since I first saw the joint the day after kickoff. It is only recently that I have been able to get others to join me.

[eugenebrooks]

Originally Posted by Joe Johnson

#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

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

[Joe Johnson]

Originally Posted by eugenebrooks

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

Great data. Thanks.

I have just spoken with a materials guy at Delphi. He has recommended 1040, hardened (he advises oil quench since the parts are relatively thin) then tempered to RC 40-42.

This is from a source I will call "usually reliable" but I am really out of my comfort zone on this one.

Everyone with expertise in this area please feel free to comment.

Joe J.

[capnrmorgan]

We are 2nd year team that is planning on using the 56 Trans in direct drive. I am new to posting as well. One possible solution we came up with is to reduce end play by adding a shim between the motor and the drive gear. This pushes the whole mechanism forward so ultimately the drive shaft is set "into" the Double D. We don't have a drive train to test yet. If other teams could make comment or possibly test this solution we be most grateful. We feel the end play is allowing the failure to occur. Also my technical adviser suggests crucible packed with charcoal, bone, or other carbonized material and forge heated for 8 hrs or more. We are concerned about warping the plate. We are opting to not heat treat and have andymark trannies just in case we have to redo the drive train.

[MrForbes]

Originally Posted by capnrmorgan

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

I always had a feeling metallurgy was one of the dark arts.....

As for the shim idea, it sounds reasonable to me at first glance, but I have not had a chance to take a close look at the fit of the motor and gear. I think it would be relatively easy to figure out the shim thickness required, although you'd have to pull the gear off the motor shaft to install it, or change it if it's wrong. Also putting the shim there would be best from the viewpoint of having full gear tooth engagement. Adding the shim in the second stage planetary would allow partial sun gear/planet gear contact.

[ZZII 527]

An update on the heat-treating front: Yesterday, I tempered the 42mm plate, which seems to be some type of tool steel, and managed to get a significant increase in hardness, although my first attempt was too brittle. I will try again when I get another sample, but in the mean time I tested the 56mm plates too. Unfortunately, they are definitely NOT tool steel and will not harden much without adding carbon. Some data:

56mm carrier before heat treatment: HRA 46.1, 46.5, 46.5
56mm carrier, 950C for 15 minutes, oil quenched: HRA 49.3, 49.9, 49.4
56mm carrier, 950C for 15 minutes, water quenched: HRA 57.1, 57.9, 58.5 (about a C16)

It's a slight improvement, but not the C40 we are looking for or even the C23 of the shaft. Case hardening is still an option, but not an easy one. I suspect the solution will be an entirely new plate made of something harder and with a tighter fit for the shaft. I will probably shift my focus to this avenue now, although hardening is still an easy solution for the tool-steel 42mm carrier.

Good luck all!

[MrForbes]

Originally Posted by ZZII 527

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

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.

[Joe Johnson]

Originally Posted by squirrel

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.

I think that we have pretty much put to bed the idea that the carriers can be easily hardened (by teams or even by a "swap service" of some kind).

I think the urgency of getting the mat'l analysis is lessened.

As to the tight fit, this is going to help some but my theoretical analysis/calculations do not show a significant improvent in this case (nothing like what I expect we need).

Here is my current thinking of most likely patches:
#1 harder carriers (= to the shaft hardness) address all the 1-CIM cases (16:1 & 12:1)
#2 harder (perhaps RC 40-45) carriers plus redesigned joint (e.g. square hole) plus harder (again perhaps RC 40-45) shafts (with mating joint) address all the 2-CIM cases (IF, and this is a big IF, tests show 2-CIMs stress the joint beyond what is done in #1 -- note that motor torque is not the only thing that determines the torque this joint sees, friction is another and it may limit the torque that the joint sees to something close to the 1-CIM number)

Stay tuned.

Joe J.

[ZZII 527]

Originally Posted by squirrel

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.

I'm ordering O1 and A2 tool steel to test with. I can cut them on the waterjet, but the tolerances will not be great. Maybe start small and work my way up to a nice press fit? I'm hoping with the same material as the 42mm plates, I can treat them to be just about any hardness I want. The A2 may be hard enough (B89-99) without any treatment.

I am not going to try 4140 because I think other people are on top of that and I don't know where to get 5/32" by tomorrow.

I should also mention that our original 56mm carriers showed only the very tiny beginnings of deformation after driving around yesterday evening (1 CIM, 12:1), although we haven't done high-cycle testing.

[cbpetrovic]

Originally Posted by Joe Johnson

Can anyone else see these slides? I can not.

Joe J.

Dunno what's u, I can see these from my house, I'm at the school now and cannot. Let's try this.

[Kevin Sevcik]

So, unbeknownst to me, one of our mentors has connections and is trying to get the 52mm carrier plate spectrally analyzed to determine alloying materials and carbon content, hopefully by tomorrow or friday if I'm really lucky. I'll keep everyone posted as soon as I have results.