Banebots 42mm gearbox: Recommendations for use

[ZZII 527]

Bad news: our tempered (HRC 49.5) 42mm carrier failed in the gearbox by fast fracture. More specifically, three of the pins fractured just below the plane of the plate. This occurred at almost no load.

To confirm that it was not just the pins (and to let out frustration) we subjected the carrier itself to the "hit it hard with a hammer" test and sure enough it fractured as well (picture attached).

Conclusion: It was too brittle, by far, at this tempering. I can try it again with a higher temperature temper and longer time in the oven to get it a little softer and less brittle, but I will need to get my hands on more and I don't think I want to buy whole gearboxes. I'm now less optimistic about the tempering solution since it seemed to fracture so easily, but there may still be hope.

[Joe Johnson]

Originally Posted by ZZII 527

Bad news: our tempered (HRC 49.5) 42mm carrier failed in the gearbox by fast fracture. More specifically, three of the pins fractured just below the plane of the plate. This occurred at almost no load.

To confirm that it was not just the pins (and to let out frustration) we subjected the carrier itself to the "hit it hard with a hammer" test and sure enough it fractured as well (picture attached).

Conclusion: It was too brittle, by far, at this tempering. I can try it again with a higher temperature temper and longer time in the oven to get it a little softer and less brittle, but I will need to get my hands on more and I don't think I want to buy whole gearboxes. I'm now less optimistic about the tempering solution since it seemed to fracture so easily, but there may still be hope.

You are beautiful. I am sorry about your failure but you give such great details that I love to read about it...

...I think you should dial down your targets a bit.

#1. Are you sure you need the extra strength? If you can make due with 350in-lbs I would highly recommend that you just leave them alone. Note that the 350in-lbs that I am predicting is not a single load failure it will take 100's of these cycles before the gearbox will fail to function.

#2 If you need that extra bit of safety margin, I recommend that the hardness of the shaft is a good point to start. Unless you were going to harden both, you will just push your failure to the shaft. So, I would say, push out the pins, harden and then temper back to RC 40-41. I think you will find that value a good compromise between brittle and ductile failure.

Good luck.

Joe J.

[ZZII 527]

Originally Posted by Joe Johnson

I think you will find that value a good compromise between brittle and ductile failure.

You mean brittle and ductile success, right.

I will try to get my broken sample to HRC 40-41 tomorrow with a different tempering. I won't be able to test it in the gearbox, obviously, but I'll break it (I'm good at that) and see what it looks like.

[Richard McClellan]

Our team is using two of these 256:1 gearboxes for our arm as well, and I've been thinking about heat treating the carrier plate and/or the output shaft. I talked to my materials professor today, and he said it could be done if I knew exactly what kind of tool steel the output shaft and the carrier plate were made of....does anyone know the material specifications? Without these specifications, I imagine we may experience the same sort of fast fracture that others are talking about if I can't figure out the exact type of tool steel.

[ZZII 527]

Originally Posted by richardmcc2

Our team is using two of these 256:1 gearboxes for our arm as well, and I've been thinking about heat treating the carrier plate and/or the output shaft. I talked to my materials professor today, and he said it could be done if I knew exactly what kind of tool steel the output shaft and the carrier plate were made of....does anyone know the material specifications? Without these specifications, I imagine we may experience the same sort of fast fracture that others are talking about if I can't figure out the exact type of tool steel.

I can't tell you the exact specs (I don't think anyone knows at this point), but I've been doing testing with the 42mm plate for the last two days and I have the following data:

Hardness Before Heat-Treatment: ~C24
900C, 15 min, water quench: ~C60
300C temper, 30 min: ~C50
This resulted in brittle fracture almost immediately in the gearbox (very low load).

800C, 15min, oil quench: ~C25
800C, 15min, air-cooled: ~C10

950C, 15min, air-cooled: ~C14
950C, 15min, water quench: ~C60
950C, 15min, oil quench: ~C50

The lab technician said his best guess is either O1 (oil cooled) or W1 (water cooled) tool steel. Definitely not A-anything (air cooled).

Many have said that oil quenching with a longer/hotter temper to get to ~C40 might work well. I will test this when I get more samples. I'm also getting some A2 tool steel to try.

[Richard McClellan]

Does this mean that you've purchased an entire gearbox for every one of those tests that you did? My materials professor said I could have a chemical analysis done for about $120 to find out the exact type of tool steel....I would imagine that would be a much easier (and cheaper) route than a trial and error approach of testing many gearboxes, would it not?

[MrForbes]

I believe one 256:1 gearbox contains 4 each of the 4:1 planet carriers....perhaps he made 4 tests with one gearbox?

[ZZII 527]

Originally Posted by richardmcc2

Does this mean that you've purchased an entire gearbox for every one of those tests that you did? My materials professor said I could have a chemical analysis done for about $120 to find out the exact type of tool steel....I would imagine that would be a much easier (and cheaper) route than a trial and error approach of testing many gearboxes, would it not?

I used one gearbox, but cut the plate into several pieces so I could test different samples. (Well, first I did an initial hardening to C50, then the plate fractured, then I took the pieces, cut them up further, and refired them to do the remaining tests.) A chemical/spectral analysis would definitely be faster and more definitive, though. I think the question I had was simply: will it harden? And the answer was yes for the 42mm, no for the 56mm. Getting the perfect combination of ductility and hardness is another challenge and knowing the exact composition of the steel would be helpful with that. Plus I'd be very interested to know the results just out of curiosity now.

[ZZII 527]

With all the work being done to solve the softer 56mm carrier plate issue, I haven't been really thinking about the 42mm plates. We still want to use them at 256:1 to drive our arm. Is anyone else still pursuing a 256:1-based arm?

I got a replacement carrier for the one I fractured, but it seems to be a slightly different version, so I am going to make new plates out of the left-over A2 tool steel I have from testing 56mm plates. At least I have a tempering process that I know works for getting this to HRC40. And this time I will take the pins out first. (To anyone just tuning in, I was able to harden the original 42mm carrier plate, which is, unlike the 56mm plate, tool steel, but the pins fractured.) I am going to make six (see attached), temper a few for testing, and save the rest as spares. If anyone wants the .dxf or an OMAX layout file, let me know.

[eugenebrooks]

Originally Posted by ZZII 527

With all the work being done to solve the softer 56mm carrier plate issue, I haven't been really thinking about the 42mm plates. We still want to use them at 256:1 to drive our arm. Is anyone else still pursuing a 256:1-based arm?

I got a replacement carrier for the one I fractured, but it seems to be a slightly different version, so I am going to make new plates out of the left-over A2 tool steel I have from testing 56mm plates. At least I have a tempering process that I know works for getting this to HRC40. And this time I will take the pins out first. (To anyone just tuning in, I was able to harden the original 42mm carrier plate, which is, unlike the 56mm plate, tool steel, but the pins fractured.) I am going to make six (see attached), temper a few for testing, and save the rest as spares. If anyone wants the .dxf or an OMAX layout file, let me know.

Tool steel is too brittle for this application.
You should be using 4130, 4340, or 4140.

Eugene

[ZZII 527]

Originally Posted by eugenebrooks

Tool steel is too brittle for this application.
You should be using 4130, 4340, or 4140.

Eugene

My earlier testing leads me to believe that the original carriers in the 42mm gearbox are tool steel. They are not hardened, however, so at worst I am making spares of similar hardness to the original. I've had some success, though, with hardening and tempering the A2 tool steel back to a Rockwell C40, the very low end of its useful hardness range, and I'm going to do some testing .

I'm not disagreeing with you, though; I also think steels you mentioned are more appropriate for this part. The steel I'm using is most noteably used for knife blades... I'm using what was quick and readily available, for now. (McMaster sells 5/32" stock of tool steel, but unfortunately not 4140.) I'm not recommending that anyone go out and buy a plate of tool steel for these, but if teams try hardening the original carrier and it is the same as ours, they will see tool-steel hardness. I'd like to see if the low end of that range can work.

[eugenebrooks]

Originally Posted by ZZII 527

I'm not disagreeing with you, though; I also think steels you mentioned are more appropriate for this part. The steel I'm using is most noteably used for knife blades... I'm using what was quick and readily available, for now. (McMaster sells 5/32" stock of tool steel, but unfortunately not 4140.) I'm not recommending that anyone go out and buy a plate of tool steel for these, but if teams try hardening the original carrier and it is the same as ours, they will see tool-steel hardness. I'd like to see if the low end of that range can work.

McMaster has the steel you need in rod form, your choice of 4340, 4130 or 4140. If you really want to use the water jet, buy oversize, slice and face to make your plate, then cut it on the water jet. I don't want to dissuade you from experimentation because it is a good learning experience, but in the end you want a part that does not break. If you want your carrier plate to be bullet proof use the 4340 (or the 4130, or the 4140) that you can get from McMaster.

Eugene

[ZZII 527]

Okay, so I installed new 42mm carriers cut from A2 tool steel on Thursday. But in light of Dr. Brooks' comments and my own original failure (see earlier posts), I decided not to harden them for now. As they are (annealed), they are a bit softer than the stock 42mm plates, and so any deformation to them should resemble what would happen to the stock plates. The only other notable difference is that the flats are a closer fit to the shaft (I cut them undersized and filed them out).

A picture of our arm joint, showing the 256:1 gearbox and the additional 72:10 sprocket reduction, is attached.

We tested the arm Thursday and Saturday under normal usage conditions, with and without motor braking. The arm itself is relatively light and the FP motor draws about 4-6A to lift it at the worst angle. The most load it sees is during our ramp deployment, which is initiated by the arm motor. During this maneuver, the FP motor draws approximately 10-12A, corresponding to about 60 in-lbf on the carrier plate (anyone care to check my math on this? I took the motor specs from the sticky'd 2005 post and used 0.85 as the efficiency per stage), well under stall and the theoretical limit.

The result so far has been that there is LITTLE TO NO deformation. I thought I saw a bit more backlash today, so I opened the gearbox again and took the plate out (picture below from today). It is maybe the slightest bit looser on the shaft, but most of the backlash I saw was just from the fact that there are four gear stages.

So I am confident that the 42mm plates can hold up. Ours is not the most well-designed arm for reducing torque or shock loading (as of now, there is no counterweight) and we haven't implemented any software limits yet, and yet the plate has held fairly well. The 42mm plates are harder than the 56mm ones and I think they can take the torques involved in controlling a well-built arm with some additional chain and sprocket reduction. If you treat it right, I think this problem will be less drastic than the 56mm issue was / could have been.

But I would be interested to know: How many teams are still using the 42mm, 256:1 setup and if you are, is it holding up?

[gburlison]

Originally Posted by ZZII 527

But I would be interested to know: How many teams are still using the 42mm, 256:1 setup and if you are, is it holding up?

We are using the 42mm, 256:1 setup with a F-P motor. We have destroyed one carrier plate, but that was from a stupid mistake. We replaced that carrier, but would be very interested if you have a CAD model that we could use to have new carrier plates made. Thanks for all the research that you have done.

[ZZII 527]

Originally Posted by gburlison

We are using the 42mm, 256:1 setup with a F-P motor. We have destroyed one carrier plate, but that was from a stupid mistake. We replaced that carrier, but would be very interested if you have a CAD model that we could use to have new carrier plates made. Thanks for all the research that you have done.

Here are the dimensions I worked from:

Thickness: 4mm (5/32" stock works perfectly)
Outside Diameter: 25mm
Double-D Diameter: 10mm
Double-D Flats: 8mm
Pin Pattern Radius: 9mm
Pin Hole Diameter: press fit for 3mm (#32 drill, 0.116" works well)

Material: According to Dr. Brooks, 4130, 4140, 4340 seem to be the best choices, as they are easily hardened to something like HRC40. I used A2 tool steel (mostly because it was the first thing I could get my hands on in 5/32" ground stock) and I have both annealed (~HRC20) and tempered (~HRC40) carriers. So far, the softer annealed carrier has been holding up fine, but I suspect that to ensure longevity of the plate, hardness nearer HRC40 would be better.

I cut them on a waterjet, so I went way undersized on the pin holes and reamed them out later to avoid issues with taper. (My CAD files are dimensionally quirky for that reason, so I don't want to post them as they might be misleading.) I also had to file out the double-D a bit to get it to slip over the shaft. (I wouldn't suggest pressing it on. I tried this and the alignment issues are not worth the hassle.)