Banebots 42mm gearbox: Recommendations for use

[Joe Johnson]

As readers of these fora know, the 56mm Banebots transmission has a failure mode that involves the double D joint between the last carrier stage and the output shaft.

The 42mm gearbox shares this same failure mode.

I have done some testing and I have the following recommendation with regard to the use of these gearboxes:


My analysis is below. I think the numbers are somewhat conservative but I don’t think they are way out of line with what teams will see.

This will be my advice to teams in summary:
42mm w/FP: If you plan to stall the motor, use ratio less than 145:1
42mm w/BB: If you plan to stall the motor, use ratio less than 214:1

Note that even though you cannot BUY a 144:1 gearbox it is possible to build one if you buy a 256:1 and combine it with parts from a 36:1 BUT... BE WARNED doing so is probably marginal if used with a FP motor

Finally, I do not include dynamic effects in these calculations. If you are expecting the mechanism to have significant impact loads, I recommend even lower ratios.

As always, your mileage may vary.

Joe J.

************************************************** *********************************

Rockwell C to Tensile Yield:
RC 22 – 115 Ksi
RC 23 – 117 Ksi
RC 24 – 119 Ksi
RC 25 – 123 Ksi
RC 26 – 125 Ksi

42mm
Carrier hardness:
RC 23.6, 23.8, 24.3
Gear Brass:
RA 37.7, 35.4, 35.8 <<suspect due to small surface to test
Shaft:
RC 43.1, 40.6, 43.7

Given the analysis of the D on the 56mm gearbox, due to scaling, the 42mm D should take 54% of 350in-lbs failure for the 56mm gearbox. But the yield of the carrier on the 42mm gearbox is harder (119Ksi rather than 64Ksi) so it should 186% stronger due to better material. The net effect should be that the joint should fail at almost exactly the same value as the 56mm gearbox (350in-lbs).

If that is true, then I predict that the 42mm gearbox with a FP motor (with 12V stall = .42N-m = 3.7in-lbs) will fail if the effective ratio (the ratio including losses due to efficiency) is 95:1 or higher.

In order to get 95:1 it will take 4 stages. I usually use 85% per planetary stage as my efficiency when I design a gearbox output, but in this case I will use 90% per stage in order to be safe (more torque getting through means more stress on the output). Given that, I predict that the FP motor on the 42mm gearbox with a ratio of 145:1 or greater will fail.

Similarly for the BB motor in the kit (12V stall = .28N-m = 2.5in-lbs) will fail if the effective ratio (the ratio including losses due to efficiency) is higher than 140:1.

In order to get 140:1 it will take 4 stages. Again using 90% per, predict that the BB motor on the 42mm gearbox with a ratio of 214:1 or greater will fail.

Note that these are not one time failure predictions but a failure that will fail upon repeated cycling back and forth.

Now to the actual torque to failure test: I used a 256:1 gearbox. The input torque required to fail the gearbox with the output shaft locked was .6N-m (5.3in-lbs). The peak (after failure, the torque grows as the shaft plows through the carrier) was 1.0N-m

This is scarily close to the stall torque of the FP motor and not too far away from the BB motor. Also, this does not include any dynamic loading of the gearbox.

But, based on the 5.3in-lbs failure, I get that the D joint fails at 5.3in-lbs * (4 * .85)^4 = 710in-lbs. Note while this is higher than the 350in-lbs predicted above, that number was not 1 time failure load, but a load that if cycled caused failure, so I am not too worried at this difference.

[MrForbes]

I just opened up a 36mm gearbox, it has a 6mm diameter shaft at the DD joint. If you have done some measuring, could you please tell us:

What size is the shaft in the 42mm gearbox?

What size is the shaft in the 56mm gearbox?

Thanks!

[AdamHeard]

If we have our arm geared so that we have sufficiently more torque than we need, then we shouldn't be stalling the motors and shouldn't be tearing these out, right?

[ZZII 527]

Thanks so much for your efforts to analyze and solve this problem over the past few days, Dr. Joe. My team is in the "fortunate" situation of having a 12:1 single CIM drive system, but I am worried about our arm which is driven by the 256:1 42mm transmission with FP motor. While I don't anticipate the motor ever being driven to anywhere near stall, I tend to think that impulsive loading from the arm itself is the thing to watch out for.

I can easily imagine a large arm changing directions in a matter of milliseconds, hitting a dead stop, or gaining momentum before a chain engages and suddenly yanking on the drive shaft with much more torque the the FP could supply. And at 256:1, the gearbox would have very little give from backdrive at those time scales. A good test might be to short the motor terminals (worst-case scenario with Victor e-brakes on?) and drop a weight from an arm attached to the shaft. I bet it could do a lot of damage.

If this is the more likely way to get failure, then there are a bunch of ways for teams to alleviate the problem, many of which were mentioned in previous threads (gas springs, limit switches, etc.). Do you think this could be the more dangerous culprit, or am I missing something important?

[Rob]

Thanks so much for your efforts to analyze and solve this problem over the past few days, Dr. Joe. My team is in the "fortunate" situation of having a 12:1 single CIM drive system, but I am worried about our arm which is driven by the 256:1 42mm transmission with FP motor. While I don't anticipate the motor ever being driven to anywhere near stall, I tend to think that impulsive loading from the arm itself is the thing to watch out for.

I can easily imagine a large arm changing directions in a matter of milliseconds, hitting a dead stop, or gaining momentum before a chain engages and suddenly yanking on the drive shaft with much more torque the the FP could supply. And at 256:1, the gearbox would have very little give from backdrive at those time scales. A good test might be to short the motor terminals (worst-case scenario with Victor e-brakes on?) and drop a weight from an arm attached to the shaft. I bet it could do a lot of damage.

If this is the more likely way to get failure, then there are a bunch of ways for teams to alleviate the problem, many of which were mentioned in previous threads (gas springs, limit switches, etc.). Do you think this could be the more dangerous culprit, or am I missing something important?


You are probably right to be concerned about shock loading in your arm possibly causing damage. There are many ways shock loading can occur such as the arm hitting hard stops, the arm hitting another machine or field element, or a rapid change in direction caused by the operator or a control program.

You may be able to use software to alleviate shocks associated with rapid changes in direction by using an accelleration and decelleration curve. This will dampen the responsiveness of your arm, but will also reduce the shock loads that the system experiences.

Another option could be to install a clutch in the system somewhere between the arm and the BB gearbox. Check out page 993 of the Mcmaster catalog for a start at some off the shelf type options. The clutch would simply slip if torques that are too high are applied (like shock loads). Keep in mind however that this could wreak havoc on feedback systems that are attached to the arm.

Another possible solution similar to the clutch would be to link the drive system to the arm through a spring which would be strong enough to transfer "normal" loads but would absorb strong shock loads (I think team 571 did something like this on their 2004 machine to absorb shock loads through the arm).

Good luck everyone,

RAZ

[Joe Johnson]

350 in-lbs is nothing to sneeze at. And remember that it takes 100's of such cycles to fail the joint.

As others have mentioned, you potentially have some control via software that can help you keep from exceeding this output from the motor side.

Clutches and springs can help you from the arm side.

One way to drive through a spring. Think of a spring in general terms. One Year to save weight, we replaced a section of our arm drive chain with Spectra cable. This cable is very strong (nearly as strong as steel) but it was much much springier. Thing of that.

You can also think of perhaps using #25 chain to drive your arm vs. #35. If my memory serves me well, #25 chain breaks at 800 lbs or so rather than 3500 lbs or more for #35 chain, if you use a half inch sprocket radius for the output of the gearbox, the chain will break before the gearbox gets damaged -- not fun but at least you'd be alive again quickly.

Finally, you can back calculate what force on your arm will cause the joint to see 350 in-lbs or more. Is it reasonable that such a force would occur? If so, would it be enough to cause an instant failure or just a slight widenning of the bowtie. The second you can live with if it doesn't happen too often, the first one is something to worry about.

Joe J.

[ZZII 527]

Maybe a small bit of good news. We've been using the 42mm 256:1 w/ FP setup for our arm. After the output shaft, there is an aditional 72:10 chain reduction to the arm itself. The arm is just a light 1x1 Al box extrusion, no more than 2 lbs and 4 feet long. It hasn't been put through anything rigorous or a ton of cycles yet, but we have used it to right the whole robot a few times. We tore it down today to check out the internals and the carrier looks pretty good to me (picture attached). There is slight deformation at the corners, but it's only at the surface (maybe 10% of the total depth).

Obviously more demanding and cyclic loads need to be tested before this proves anything, but maybe with TLC the 42mm can be used effectively for light arms.

I tried a possible quick fix: pinning the shaft to the carrier, but after a couple broken bits, I can confirm that the shaft is, to use technical terms, really, really hard. The carrier hardness falls somewhere between a filing drawer hanger and a screwdriver. :cool: Note to self: leave the real engineering to Dr. Joe. But 119 ksi is pretty hard, am I right? Can we do a lot better with a different steel? Heat treatment?

What about shims to take out the small bit of play between the shaft and the carrier? This might help since the corners won't dig in so much. It would approach Dr. Joe's linearly-distributed loading condition more. I'm thinking of what happens when you use a metric hex wrench in an English hole. It might work, but will be a bit too small and as a result will strip the hole more easily.

[Joe Johnson]

Maybe a small bit of good news. We've been using the 42mm 256:1 w/ FP setup for our arm. After the output shaft, there is an aditional 72:10 chain reduction to the arm itself. The arm is just a light 1x1 Al box extrusion, no more than 2 lbs and 4 feet long. It hasn't been put through anything rigorous or a ton of cycles yet, but we have used it to right the whole robot a few times. We tore it down today to check out the internals and the carrier looks pretty good to me (picture attached). There is slight deformation at the corners, but it's only at the surface (maybe 10% of the total depth).

Obviously more demanding and cyclic loads need to be tested before this proves anything, but maybe with TLC the 42mm can be used effectively for light arms.

I tried a possible quick fix: pinning the shaft to the carrier, but after a couple broken bits, I can confirm that the shaft is, to use technical terms, really, really hard. The carrier hardness falls somewhere between a filing drawer hanger and a screwdriver. :cool: Note to self: leave the real engineering to Dr. Joe. But 119 ksi is pretty hard, am I right? Can we do a lot better with a different steel? Heat treatment?

What about shims to take out the small bit of play between the shaft and the carrier? This might help since the corners won't dig in so much. It would approach Dr. Joe's linearly-distributed loading condition more. I'm thinking of what happens when you use a metric hex wrench in an English hole. It might work, but will be a bit too small and as a result will strip the hole more easily.


Thanks for the data.

A reminder to teams (and I am going to shout, so plug your ears): I AM NOT SAYING THAT THESE GEARBOXES ARE FRAGILE LITTLE WIMPS THAT WILL BREAK IF YOU LOOK AT THEM CROSSWISE!!!

They have limitations. It is possible to use them over their limits and they are likely to have issues if you do.

Note that my recommendation of 350in-lbs max is the load that I predict they will fail at with 100's of cycles! It will not take a single or even a 100 such events to make the gearbox fail in the mode.

Now the good news/bad news. First the bad news: Banebots tells me they are out of the 256:1 42mm gearboxes. now the good news: with 64:1 being the highest ratio they sell, these gearboxes can not only take full stall loads of the FP or the Banebots motor, they have dynamic loading safety margins of 2.5 for the FP and 3.5:1 for the BB.

Here is the bottom line (and I am going to shout again so you can hear me): IF I WERE BUILDING A ROBOT THIS YEAR I WOULD ALMOST CERTAINLY HAVE FOUR (YES FOUR) OF THESE 42MM GEARBOXES ON MY ROBOT -- TWO FOR THE FP MOTOR AND TWO FOR THE BB MOTORS.

Joe J.

[ZZII 527]

Is there any chance the BaneBots will make available for purchase the individual carriers? Two reasons: I would like some to test with that I'm not afraid to break. Also, I think teams would like to have spares if their carriers do fail due to some unforseen stall condition in a match.

I am going to take my 42mm carrier to a materials lab today and see if I can at least confirm the hardness values. Perhaps they'll also let me try some heat treatment. (Anneal -> Quench -> Temper would be best?)

[Jonathan Norris]

Now the good news/bad news. First the bad news: Banebots tells me they are out of the 256:1 42mm gearboxes. now the good news: with 64:1 being the highest ratio they sell, these gearboxes can not only take full stall loads of the FP or the Banebots motor, they have dynamic loading safety margins of 2.5 for the FP and 3.5:1 for the BB.

Joe I see where you are going with this, the lower gear ratio gearbox's are going to be much more robust because the fewer number of stages. But there is a good reason the 256:1 gearbox's have been selling like hotcakes, those are the only ones which are highly applicable to this year's game. Teams are looking to use the FP motors for high torque, low speed situations this year. Not like last year where some teams used them to power shooter wheels or run belting systems, in a higher speed less torque intensive situation. Making the 64:1 gearbox far less useful to teams than the 256:1 gearbox this year. Many teams looked at the 256:1 gearbox and thought "Wow that's a perfect ratio to use a FP motor to power an arm system". My team received our 256:1 gearbox's yesterday, and promptly dissected and inspected them, comparing the D design the design from last year. Lets say we are disappointed and concerned about the life time this system will work. We have designed our system to share the load across two of the gearbox's and sprocketed them down to a very reasonable level where we will will be using only 1/5 of the FP's stall torque. It now worries me that these gearbox's are no longer available, if we choose to use the ones we have will we be able to purchase replacements in the future?? Why are they no longer available?? simply out of stock, or quality problems?

[Joe Johnson]

snip

We have designed our system to share the load across two of the gearbox's and sprocketed them down to a very reasonable level where we will will be using only 1/5 of the FP's stall torque. It now worries me that these gearbox's are no longer available, if we choose to use the ones we have will we be able to purchase replacements in the future?? Why are they no longer available?? simply out of stock, or quality problems?

Using 1/5 of the stall torque of the FP would mean that you are pretty safe with respect to torque from the motor side. You have a pretty good shot at being able to limit the torque input by the motor.

If you can figure a way to limit the backdrive input from the arm to the gearbox, you'll be fine.

Now to your other points.

If you damage the gearbox, you can guy another ratio gearbox and rob parts to rebuild your 256:1 gearbox.

As to why they are no longer available, I didn't ask specifically, but I was under the impression that whatever stock they had in house has been sold. I don't know what or when or whether they will order more. My impression is that Banebots was not even planning on marketing these to FIRST folks. For our own reasons, we find them useful so we buy them but that was not their intent when they decided to add them to their catalog.

Joe J.

[ZZII 527]

Here are the results of some testing done at a materials lab today:

Before Heat Treatment:
HRC 21.4, 23.7, 23.8 - right on target with Dr. Joe's tests

Annealed at 900C for 15 minutes, then quenched in water:
HRC 59.9, 59.8

According to the lab technical instructor, who knows more about materials than I could ever imagine, this is tool steel. It comes annealed, which makes it easy to machine, but can be hardened quite a bit with heat treatment. (My chart says that as-quenched, the tensile stength has gone from ~119 ksi to almost 300 ksi.)

I have the carrier tempering now at 300C for a half hour. This will bring the hardness down a bit, but make it more resistant to fracture. Very good news for the 42mm gearboxes. I'm not sure how it applies to the 56mm, but hopefully those can be hardened too.

[MrForbes]

The planet carrier plate in the 56mm gearbox appears to be made of something other than tool steel, as it starts out about half as hard as the plate in the 42mm gearbox. It can probably be treated to result in a similar doubling of hardness.

I'm curious to know what you're doing with the planet gear pins, as I expect their strength characteristics, as well as the tightness of their press fit, will be affected by the hardening/annealing process.

[Justin M.]

We will be using two 64:1 42mm gearboxes to power our forlift, working in harmony...we'll see how that goes.

[ZZII 527]

Update: After tempering at 300C for 30 minutes, the new hardness numbers are:

HRC 49.5, 49.5 (~230 ksi tensile yield)

So it's still about twice the original hardness. I'm putting it back in the gearbox tonight and we'll test it a bunch. I'd still like to get some spares since this one has now been drilled/poked/prodded/baked/cooled/etc. If anyone from BaneBots is reading - selling these individually would be very helpful for the time being until a more permanent solution can be found.

Regarding the pins, all I can tell you is that they didn't fall out. :rolleyes: I'll check them again after testing.

In summary:
- annealed at 900C for 15 minutes
- quenched in water
- tempered at 300C for 30 minutes
- quenched in water
roughly doubles the hardness for the 42mm carrier plate. How this applies to the 56mm plate is still uncertain because it is apparently made of a different steel.

[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]

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]

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]

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]

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]

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]

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]

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]

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]

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.)

[gburlison]

Here are the dimensions I worked from:

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


I am not sure that some of your dimensions make sense, I think you meant:
Outside Diameter: 25mm

I measured the Double-D Diameter to be .493" or 12.49mm

[ZZII 527]

I am not sure that some of your dimensions make sense, I think you meant:
Outside Diameter: 25mm

I measured the Double-D Diameter to be .493" or 12.49mm

Woops. :eek: It's been a long week. Yes, outside diameter is 25mm and double-D diameter is 10mm. (I'm pretty sure it's 10mm or perhaps slightly larger. The 56mm plates have 12mm diameter double-Ds.)

[Mr.G]

:cool: Paper weight? :yikes:

From the title of the post "Banebots 42mm gearbox: Recommendations for use"

I could think of some others:
Weapon
Spacer
Barbell

Sorry couldn't resist. In all fairness I have designed many things that haven't worked as planned. This is a learning experience for us all.

Great job JOE and everyone else for pulling together.

[AdamHeard]

Last night we experienced a weird failure on our arm while we were tuning a PID loop.

We have two 42mm 256:1s on our shoulder joint with further reduction after that. One Fischer price and one banebots.

We were tuning, and therefore some oscillation of the arm was unavoidable.

After some test, we found the arm couldn't move. At all. We removed the chain and found the BB gearbox could spin, the FP one was jammed. We disassembled both and looked inside. The BB was fine, the FP however had sheared about 1/3- 1/2 of the teeth off each of the brass planetary gears in the final stage and had jammed shut. We replaced the damaged gears (The sun and carrier were fine), closed it up and it worked.

What was interesting about all this is that the Double-D joint was fine in both boxes, and had no more backlash than our untouched gearbox.

We found that the BB motor PWM had become loose, so we never gave current to the motor, and therefore were putting to much strain on the FP gearbox. This is just a guess, any insight would be appreciated as we hope this does not ruin this season if it has the potential to reoccur.

[ZZII 527]

Last night we experienced a weird failure on our arm while we were tuning a PID loop.

We have two 42mm 256:1s on our shoulder joint with further reduction after that. One Fischer price and one banebots.

We were tuning, and therefore some oscillation of the arm was unavoidable.

After some test, we found the arm couldn't move. At all. We removed the chain and found the BB gearbox could spin, the FP one was jammed. We disassembled both and looked inside. The BB was fine, the FP however had sheared about 1/3- 1/2 of the teeth off each of the brass planetary gears in the final stage and had jammed shut. We replaced the damaged gears (The sun and carrier were fine), closed it up and it worked.

What was interesting about all this is that the Double-D joint was fine in both boxes, and had no more backlash than our untouched gearbox.

We found that the BB motor PWM had become loose, so we never gave current to the motor, and therefore were putting to much strain on the FP gearbox. This is just a guess, any insight would be appreciated as we hope this does not ruin this season if it has the potential to reoccur.

The only thing I can think of (other than what you said about backdriving the BB with the FP) is something that happened to our 42mm gearbox: The four long screws that hold it together came loose little by little until eventually the entire housing would twist under the torque from the motor. This would not occur if you are using the holes on the sides of both bearing plates to mount the motor, but we are only using the front face of one bearing plate. Luckily, we spotted this before it became a problem, but I imagine it would really mess with the alignment of the gears and could cause them to bind. The simple solution is to Loctite them and tighten a lot.

Glad to hear that your carriers are holding up, though. Team that are using the 42mm with FP as speed sources with a lot of extra reduction should be okay.

[AdamHeard]

The only thing I can think of (other than what you said about backdriving the BB with the FP) is something that happened to our 42mm gearbox: The four long screws that hold it together came loose little by little until eventually the entire housing would twist under the torque from the motor. This would not occur if you are using the holes on the sides of both bearing plates to mount the motor, but we are only using the front face of one bearing plate. Luckily, we spotted this before it became a problem, but I imagine it would really mess with the alignment of the gears and could cause them to bind. The simple solution is to Loctite them and tighten a lot.

Glad to hear that your carriers are holding up, though. Team that are using the 42mm with FP as speed sources with a lot of extra reduction should be okay.

I can't believe I forgot that.

We noticed the gearbox (from front to back) twisted about 15 degrees when we tried to move the arm. That makes sense.

We are face mounting as well, but I think I have a simple solution; We will make a plate out of 1/4" (maybe a little thick, but we have the weight) that just bolts to the bottom holes and helps prevent that twist in case the bolts do loosen again.

Thanks

[ZZII 527]

We finally got a full, uninterrupted day of practice time to really put everything to the test. With four BaneBots gearboxes (2x56, 1x42, 1x36) on our robot, this was the true shakedown for the carrier plates.

My hardened carrier plates for the 56mm gearboxes held up fine, no noticeable backlash whatsoever. I believe they are of similar hardness to the BaneBots replacements, so I suspect that these will also be sufficient for the case of 1 CIM and hopefully also for the 2 CIM adaptor.

The soft 42mm replacement plate I made is just now starting to show backlash, to the tune of about 10-15 degrees more than usual. I took it apart and there is definitely a bit of a bowtie (sorry, no picture), although it is not in any danger of failing soon. Still, I will be more comfortable changing it out to one of similar hardness to my 56mm plates.

Which brings me to my next point. I have an entire strip (18"x4") of O1 tool steel 5/32" stock just waiting to be cut. I have been cutting small quantities on the waterjet, but I could potentially produce as many as will fit on that stock in one cut. A few issues:

As Dr. Brooks has noted, tool steel is not ideal for this application because it can become too brittle. Tempering back to RC40 can help this problem and the ones I've made have held up without fracturing, but this can't be gauranteed.

They would not be finish-machined (reaming pin holes and filing double-D). I would only have time for a rough cut on the waterjet.

They would not be hardened. (The steel comes annealed, actually slightly softer than the stock plates, and is supposed to be hardened after all machining is done.) That being said, they may be still useful as spares (I've had one in our gearbox for a week of driving practice and it is just now starting to show deformation) or perhaps others could do the hardening + tempering. (The materials lab I used originally cannot do large quantities and is now in use for classes anyway.)

If teams can live with this, I can start making the rough cut carriers this week and would just ask a PayPal payment or something to cover my costs. Since BaneBots won't have individual spares or hardened replacements for the 42mm, this might be a good option for the 42mm users. But I don't want to mislead people into thinking that these will be simple plug-and-play replacements. I can provide detailed instructions for all the finishing and heat treating I have done, which has held up well in the case of our gearboxes, but I would not have time to actually do it all.

Any suggestions/opinions/comments?

[gburlison]

If teams can live with this, I can start making the rough cut carriers this week and would just ask a PayPal payment or something to cover my costs. Since BaneBots won't have individual spares or hardened replacements for the 42mm, this might be a good option for the 42mm users. But I don't want to mislead people into thinking that these will be simple plug-and-play replacements. I can provide detailed instructions for all the finishing and heat treating I have done, which has held up well in the case of our gearboxes, but I would not have time to actually do it all.

Any suggestions/opinions/comments?

I'll take 2 !!

Along with any advice that you may have.

[AdamHeard]

Since the failure mentioned above, and since we installed plates that eliminate twist, one of our gearboxes has seized up twice (well, two different ones once each but in the same arm location). The brass gears just shed teeth for some reason....

What confuses me is the Double -D joint is PERFECT?!?!?!

At a sunday scrimmage we noticed that the center gear on the early stages is the same as the brass gears and is pressed into that plate. We popped them out from 4 plates (we bought lots of spares) and machined them down (took about an hour) with the help of team 207 (their shop) and a local professor. We have ran these in the final stage of the problem gearbox through rigorous PID loop testing (I cringe every time our programmers test that thing.... shock load after shock load...).

We also noticed a FP motor in a stock 256:1 draws ~1.3 amps of current. The one with the new steel gears draws 1.0!

EDIT: looking at the motor stats, 1.0 seems unrealistic for a FP. but the fact that both gearboxes were tested on the same setup shows the steal gears run better, even though the actual draw may be inaccurate.

So far we have shown no signs of wear on any part on the inside, nor the double-d joint.

[ZZII 527]

EDIT: looking at the motor stats, 1.0 seems unrealistic for a FP. but the fact that both gearboxes were tested on the same setup shows the steal gears run better, even though the actual draw may be inaccurate.

So far we have shown no signs of wear on any part on the inside, nor the double-d joint.

Our 256:1 drew 1.0A under no load at the most new and lubricated time of its life. 1.0-1.3A is definitely a good number. I'm not sure why the 2005 motor spec sheet has such high no-load current draw numbers.

Whoever on your team decided to buy so many spares should get MVP this year. We have just one. Luckily, it has survived okay so far. I thought all I had to worry about was the carrier plates. Now I have to think about gear teeth being stripped too? :(

[Kit Gerhart]

This thread has provided a lot of good information, with all kinds of technical tidbits. I'll try to make my comments brief regarding our experience.

We bought two 256:1 to use with F/P motors for arm rotation. We used one gearbox with 1:4 reduction from the Banebots to the tower. The second was a spare. The first one failed almost immediately. We installed the second one, and it also failed almost immediately.

We, obviously, were asking way too much of the gearbox and ended up making our own gearing to rotate our tower with a big CIM. The bottom line is that anyone using a 256:1 Banebots with an F/P motor needs to isolate the output from all shocks, and probably avoid ever loading the motor to much below 80% of no-load rpm.

We are reliably using two 64:1 Banebots gearboxes for less demanding applications, but the 256:1 Banebots, F/P combination is a failure waiting to happen, and a very short wait.

[AdamHeard]

Well, we thought we had the problem fixed (read above) by the steel gears, anti twist plates and reducing the input sprocket size to reduce the stress on the gearboxes.


They totally, totally failed today. We had some other minor issues, but none like that.


Luckily, during the fix-it window we machined new motor mounts that will use the A-M planetarys with small CIMs.

The only three matches we were working in were against 330, then 254 and then 968. After that we were working awesomely on the practice field. Then... total failure.

Our drivetrain is still working great luckily and we play great defense, since we are currently ranked 52nd, we're hoping some of the top alliances realise our defensive abilities.

slightly off topic, but it's been a long day.

Do not use the high reduction BB gearboxes, especially if there is any significant shock load.

[ZZII 527]

Well, we thought we had the problem fixed (read above) by the steel gears, anti twist plates and reducing the input sprocket size to reduce the stress on the gearboxes.


They totally, totally failed today. We had some other minor issues, but none like that.


Luckily, during the fix-it window we machined new motor mounts that will use the A-M planetarys with small CIMs.

The only three matches we were working in were against 330, then 254 and then 968. After that we were working awesomely on the practice field. Then... total failure.

Our drivetrain is still working great luckily and we play great defense, since we are currently ranked 52nd, we're hoping some of the top alliances realise our defensive abilities.

slightly off topic, but it's been a long day.

Do not use the high reduction BB gearboxes, especially if there is any significant shock load.

That really, really stinks. I have been pulling for these little gearboxes, but I haven't heard any good stories recently. Could you elaborate on the nature of the failure? (Was it the double-D, the twising, the shearing of gear teeth, or all of the above?) My current thinking is that we will continue to use the softer-than-usual plates and just monitor the backlash carefully. At least it is a slow failure mode and I have an idea of how much the plates can handle. If I put in harder ones, it may just doom the next weakest link, whatever that is. Sigh...

Sorry to hear about it and I hope your AM solution works out.

[MrForbes]

That really, really stinks. I have been pulling for these little gearboxes, but I haven't heard any good stories recently.

1726 made it thru the whole weekend (AZ regional) with no problems with the 36mm 125:1 gearbox running our arm...hung 6 of the 7 tubes on the highest scored qualifying match...and made it to the finals....

Is that good enough?

[Kit Gerhart]

That really, really stinks. I have been pulling for these little gearboxes, but I haven't heard any good stories recently. Could you elaborate on the nature of the failure? (Was it the double-D, the twising, the shearing of gear teeth, or all of the above?) My current thinking is that we will continue to use the softer-than-usual plates and just monitor the backlash carefully. At least it is a slow failure mode and I have an idea of how much the plates can handle. If I put in harder ones, it may just doom the next weakest link, whatever that is. Sigh...

Our initial problem with the 256:1 BB was with the double D. We then made an output shaft out of one piece of steel and pressed in the original pins. The pins broke. I think we also lost a gear tooth or two. The bottom line is that the F/P is a powerful little motor, and when you multiply its torque by 256, it is way too much for that little planetary.

We are using the 64:1, 36mm BB's for our roller gripper with no problem, and I suspect the single- or two-stage 42mm would be ok with the Fisher Price, at least if you limit shock load.

[MrForbes]

There was a BB failure story in the pits next to us at AZ, the arm on the 1212 robot is attatched directly to the output shaft of a BB 56mm gearbox. And powering the 56mm gearbox is the KOP 36mm gearbox! I thought about that a while, and then looked up the numbers, and that little gearmotor puts out about 6 times as much torque as a CIM (according to my flaky unit conversion calculations). So it's no wonder they were ripping up carrier plates and output shafts in the 56mm when the arm was accidently pushed past the end of it's travel.

You need to be careful with the BB transmissions, they are not for high torque applications.

[ZZII 527]

I still think the higher gear ratios can be used, provided their function is speed reduction, not torque multiplication.

The torque that the final stage sees is entirely determined by the load. The FP motor can and will supply any amount of torque necessary to move your arm, whether it has to run near stall in a 64:1 gearbox or near full speed in a 256:1 gearbox. (Another way of saying this: If a 256:1 gearbox fails, switching it out to a 125:1 and/or changing to a weaker motor without changing anything else will not reduce the torque on the final stage and will probably fail even faster because of the additional speed of the arm.)

The teams for which the gearboxes have survived seem to be using them to drive light, well-counterweighted mechanisms through significant external reductions. Use your motors as speed sources and everything will be much happier.

I apologize if I am just restating the obvious.

[MrForbes]

You are not stating the obvious, you are stating a point that is rather hard to see. Thanks!

when we were picking the motor/gearbox for our arm we were looking at speed reduction first. 256:1 and 125:1 gearboxes were the only ones that would do what we needed....and we still required another stage of chain reduction. Then, looking at the torque, we though we were ok as far as driving the arm, but the backdriving problem led us to use a gas spring to counterbalance it. It turns out that this also greatly reduced the torque needed to move the arm, which probably is why the gearbox survives.

[AdamHeard]

That really, really stinks. I have been pulling for these little gearboxes, but I haven't heard any good stories recently. Could you elaborate on the nature of the failure? (Was it the double-D, the twising, the shearing of gear teeth, or all of the above?) My current thinking is that we will continue to use the softer-than-usual plates and just monitor the backlash carefully. At least it is a slow failure mode and I have an idea of how much the plates can handle. If I put in harder ones, it may just doom the next weakest link, whatever that is. Sigh...

Sorry to hear about it and I hope your AM solution works out.

Well, I haven't taken them apart to look. But, it appears that shaft misalignment may have been the issue. If you look at this picture (http://www.chiefdelphi.com/media/photos/26943) you can see the second plate which supports the shaft moves up and down to tension. They both do, but they are not connected; We think they appear to be aligned but may be slightly off.

We are in a predicament now though. We already bought the AM gearboxes so we will probably go with those anyway. We have added standoffs to the plates to keep them in unison though.

[Lil' Lavery]

Well, I haven't taken them apart to look. But, it appears that shaft misalignment may have been the issue. If you look at this picture (http://www.chiefdelphi.com/media/photos/26943) you can see the second plate which supports the shaft moves up and down to tension. They both do, but they are not connected; We think they appear to be aligned but may be slightly off.

We are in a predicament now though. We already bought the AM gearboxes so we will probably go with those anyway. We have added standoffs to the plates to keep them in unison though.

Are you stealing the CIM motor for the AM solution from your drive? And are you worried that the reduction of drive power will lower your ability to drive (keep in mind, lower torque not only means less pushing power, but less ability to turn and accelerate as well)? If you are replacing the CIM in your drive with a Large CIM and/or FP, it might be alright, but otherwise I'd suggest looking for an alternative arm solution that does not sacrifice your existing abilities.
Some alternatives for the 42mm Banebots include:
KoP FP gearbox (it may be a bit big and clunky, but it works better than anyone gives it credit for, I believe it is a 128:1 reduction)
AM Planetary (http://andymark.biz/am-0002.html) then reduced further by an AM single speed (http://andymark.biz/am-0011.html) (46.793:1 reduction, can accept two motors)
DeWalt XRP transmission (3-speed; 47:1, 15:1, or 12:1 reductions)

[AdamHeard]

Are you stealing the CIM motor for the AM solution from your drive? And are you worried that the reduction of drive power will lower your ability to drive (keep in mind, lower torque not only means less pushing power, but less ability to turn and accelerate as well)? If you are replacing the CIM in your drive with a Large CIM and/or FP, it might be alright, but otherwise I'd suggest looking for an alternative arm solution that does not sacrifice your existing abilities.
Some alternatives for the 42mm Banebots include:
KoP FP gearbox (it may be a bit big and clunky, but it works better than anyone gives it credit for, I believe it is a 128:1 reduction)
AM Planetary (http://andymark.biz/am-0002.html) then reduced further by an AM single speed (http://andymark.biz/am-0011.html) (46.793:1 reduction, can accept two motors)
DeWalt XRP transmission (3-speed; 47:1, 15:1, or 12:1 reductions)

Our Drive will be just fine.

We are swapping out one small CIM and adding in a large CIM and a FP in the AM planetary. We just slightly above traction limited now.

[Jonathan Norris]

We ran into a similar situation with our arm, we designed it to work with two FP in the 256:1 gearbox's (assisted by a 30lb gas spring). However, during ship night while testing and tuning PID loops one of the gearbox's broke down on us. We had spares of the last plate made for our first regional, but after our first experience didn't feel comfortable with the gearbox's.

So we decided to totally change motors and install a gas spring strong enough to hold all the weight of our arm (90 pounder). This meant we didn't need the torque of the FP anymore and could get away with using on window motor to power the arm. The unexpected advantages of using the window motor is that it made the movement of our arm much smoother and more controllable, plus the advantage of it using a worm gear so no back-driving.

These gearbox's can be useful for some purposes.. but driving the main joint of a FIRST arm is not one of them.

[ZZII 527]

I think I'm finally ready to put this one to bed.

We ran a BaneBots 42mm 256:1 gearbox at the Boston Regional on our main arm joint. It is followed by a 72:10 chain and sprocket reduction to the arm, which is fairly light, but not counterweighted and has no gas spring. We ran with Victor braking on and ran a position controller. We also used the arm to deploy our not-so-light ramp through a separate linkage, a movement during which the motor sees a significant load. The mounting looks like this:

http://www.chiefdelphi.com/forums/attachment.php?attachmentid=5082&d=1171338765

I had made hardened tool-steel carriers (see my many posts above), but we ran on annealed tool steel, which is actually slightly SOFTER than the stock plates, to see how long it would last. It lasted the entire regional. There is some backlash, maybe 15-20 degrees, but I don't think it is near failure. I will probably change it out for another soft carrier in Atlanta to be safe.

The bottom line is you absolutely can use a 256:1 gearbox to drive an arm, provided you send it through significant additional reduction. If you are using a FP motor, 256:1 in series with anything less than 4:1 is almost too fast for an arm to rotate anyway. (Ours rotates at about 30-40 degrees per second with the 72:10 additional reduction.) I am now convinced that the ratio of the gearbox itself does not matter nearly as much as what comes after it. The FP torque isn't what kills the final stage, it is the load from the arm. The only way to reduce this is to use additional reduction between the gearbox and the arm.

I am happy that we stayed with the BaneBots gearbox instead of jumping ship, because it was incredibly easy to mount compared to the plastic gearbox and the large reduction was exactly what we needed to slow down the FP enough for our arm. I would use it again, stock, in the future, and if they harden their carriers from now on, I would definitely use it.

[MrForbes]

The FP torque isn't what kills the final stage, it is the load from the arm.

There are two ways to look at this: either reduce the torque into the transmission (use a less powerfull motor) or reduce the torque load at the output of the transmission (further reduction and/or counterbalancing)

The FP torque IS what kills the final stage, when full load is applied.

Anyways, it's great to hear that the thing worked!

[ZZII 527]

There are two ways to look at this: either reduce the torque into the transmission (use a less powerfull motor) or reduce the torque load at the output of the transmission (further reduction and/or counterbalancing)

The FP torque IS what kills the final stage, when full load is applied.

Anyways, it's great to hear that the thing worked!

Yes, I should have worded it more carefully: the motor itself does not define how much torque is seen on the final stage, the load does too. (Especially in the range where the motor acts more like a speed source than a torque source, less than 1/2 the stall torque, which is where most arms reside on the torque-speed curve.) In the end, though, it is the motor which supplies the torque and breaks the gearbox.

Thanks for everyone's help with troubleshooting and posting your own experiences - good and bad. I said at the beginning that FIRST and Chief Delphi is the biggest, most knowledgeable consulting firm I know of and it definitely showed this year. Hopefully, BaneBots will take all of the data and stories and come up with an improved product for next year.

[John Wanninger]

We're also using the 42mm, 256:1 FisherPrice/Banebot combo to drive our arm through a a 54:14 (sprocket/chain/sprocket) reduction. Our arm load is approximately 100 in-lb max static load, with tube. We're using dynamic braking, no counterbalancing and closed loop proportional control. We limit maximum PWM signal to stay within the 100 to 160 command range. Our arm is fairly flexible and has some 'whip' to it - I think that may help to limit peak loads.

After a practice tournament, and two regional events (about 48 matches total), I opened the BaneBot for inspection. The internals look like new, with no observable cam out on the carrier plate. We're crossing our fingers that it holds up through the Championship Event. We do have a 16:1 and a 64:1 that I believe we can use for parts if need be. Another option is to go with a Globe motor-to-'lovejoy' coupling-to-shaft w/ dual outboard bearing set up.

[Kit Gerhart]

We're also using the 42mm, 256:1 FisherPrice/Banebot combo to drive our arm through a a 54:14 (sprocket/chain/sprocket) reduction. Our arm load is approximately 100 in-lb max static load, with tube. We're using dynamic braking, no counterbalancing and closed loop proportional control. We limit maximum PWM signal to stay within the 100 to 160 command range. Our arm is fairly flexible and has some 'whip' to it - I think that may help to limit peak loads.


It's interesting to hear that some of you have had much better success with the 256:1 F/P combo than we did. We were using about a 4:1 reduction from the BaneBots to our arm, but with very little flexibility anywhere in the system. Also, our closed-loop position control, at times, probably put the motor from full speed forward to near full speed reverse when the arm reaches position. Basically, we were putting extreme shock loads on the B/B. We switched to a home made gear set using a big CIM for our arm. The gear stuff is fine, but we ran into structural problems with the arm itself (now fixed) due to the shock loads. All of this gives me a real appreciation for the tricks they use to soften the shock load in car and motorcycle drive trains to keep from breaking things.

[Kevin Sevcik]

Just for reference, I'll relate our (on-going) experience with the BB trannies. We're using the FP 1:256 combo to lift our ramps. The peak load at the shaft runs about 250 in-lbs. For those at GLR, our ramps weren't working at all on Friday. They weren't put to use in the first two rounds, in the third round, they failed to work. After disassembly, one carrier was rounded, the other bowtied. Both binding because of it, both clearly showing the shaft was only half engaged. Our quick fix was to have the shaft TIG welded to the carrier. This actually worked surprisingly well, but the weld bead and alignment issues are too much of a concern to run with it permanently. For LSR, we've hardened* 2 more carriers and will machine the ring gear to improve engagement. All that said, I remain convinced that the actual lifting of the robots had less to do with the failure of the plates. My hypothesis is that, instead, the plates were initially damaged during systems integration when the lifts were being tested and were stalled several times. We'll know better after a full regional with the hardened plates and no stalling of the motors.

* This was a 1450*F soak for 30 min, a quench in warm brine, then an immediate temper at 900*F for 2 hours. I was working on the assumption that this was W1 as suggested by ZZII. Yes that's a really hot temper. The chart I found suggested that would give me RC 35-40. I'd really rather have another bowtie than a fracture. I'll be running them by a Rockwell tester on Monday so I'll update with what I've actually managed to do to them.

[eugenebrooks]

Once we got to a proper design for the gearbox on our arm, we have had no problem at all with our 64:1 banebots gearboxes driving it. We are running a FP motor into this gearbox because the FP motor has better fans to drive air cooling and internal thermal protection that prevents motor burnout.

We are driving our arm with the 64:1 banebots/FP setup through a 40:1 worm gear from Boston Gear, and are using latex tubing to cancel the torque due to gravity on the arm. The result is a very smoothly functioning arm that can be operated in both a manual mode with switches on the motor power and with an on-off feedback setup (not PID) using pots to control position. The BB gearbox drives the worm through a shaft with a hardened pin and the torque is sufficient to snap the pin if the arm gets tangled up in the rack and the driver team miss-handles the power to the arm. This is the only thing that we need to address for future competitons. The worm gear setup has the advantage that no power is applied to the motor in order to hold the arm in a static position, so the motor stays cool. I am not sure that I would want to be running a 256:1 bane bots gearbox and a 10:1 worm setup...

We had no end of trouble before switched to a setup that provided for cancellation of the torque produced by gravity on the arm. If you design your arm drive with a worm gear to get lockup with the power off, and you cancell gravity with latex tubing, you will have no trouble at all with your gearbox. Run current through the motor at stall, or hammer the gearbox with oscillations in the arm and the gear box will die quickly.

We are using the 56mm gearbox, with a 12:1 ratio, and two CIM motors and the two motor adapter for our main drive. We did make our own plate and shaft out of properly heat treated 4130 (with a square drive), and we did carefully check the gear box for any interference problems before breaking it in in both direcitons on the bench. We then tore it down, cleaned it, checked all parts for abnormal wear problmems, and then greased it before putting them back together. These 56mm gear boxes have been through two regionals now without problems, and we are headed to a third regional and the nationals, so I hope that I am not speaking too soon, but we have not had problems with them.

Eugene

[Kevin Sevcik]

Update on my hardening results. First, my process:

Soak at 1450*F for 30 min.
Quench in a warm/hot brine. (Hot tap water + rock salt)
Temper at 900*F for 2 hours.

The carriers came out at RC 35. If I were making one more go at it, I'd probably keep the soak and quench the same and drop the temp to 800* - 850*. For the curious, the brine was to even out the quench action out of paranoia over warping or cracking. It was hot due to further cracking paranoia, but mostly to soften the quench somewhat to make sure the carriers weren't too hard. These were the only carriers we had, and I didn't want to botch them.

[gburlison]

I would like to give a big THANK YOU to Shane Colton from team 97. We had the carrier plates you made for the 42mm banebot transmission hardened and they performed extremely well. As expected, the harder plate just moved the failure to a different part of the transmission. After 7 matches at the Colorado Regional, our 256:1, 42mm tranny seized. Upon disassembly, it was hard to determine what failed first, but the brass gears in the final stage were missing teeth, the pins in the carrier plate had sheared, and the ring gear had some damaged teeth from the damaged brass planet gears. We had an extra carrier plate with pins (thanks Shane) and extra planet gears. The ring gear was our biggest concern. We turned the barrel over so that the damaged portion was at the first stage of the transmission where it would see less torque and we were pleasantly surprised to find that due to the spacing of the stages, the damaged portion was almost entirely between the stages. We had to play one match without an arm, but had the whole thing back together and made it through 5 elimination matches before losing in the semi-finals. I would recommend that you pay close attention to the entire output stage, replace or rearrange the brass planet gears, and flip the barrel to keep the transmission in good shape.

Let me state that I would use this transmission in the future, I would just be more careful in my design and increase the safety factor to avoid this type of failure.

[ZZII 527]

I would like to give a big THANK YOU to Shane Colton from team 97. We had the carrier plates you made for the 42mm banebot transmission hardened and they performed extremely well. As expected, the harder plate just moved the failure to a different part of the transmission. After 7 matches at the Colorado Regional, our 256:1, 42mm tranny seized. Upon disassembly, it was hard to determine what failed first, but the brass gears in the final stage were missing teeth, the pins in the carrier plate had sheared, and the ring gear had some damaged teeth from the damaged brass planet gears. We had an extra carrier plate with pins (thanks Shane) and extra planet gears. The ring gear was our biggest concern. We turned the barrel over so that the damaged portion was at the first stage of the transmission where it would see less torque and we were pleasantly surprised to find that due to the spacing of the stages, the damaged portion was almost entirely between the stages. We had to play one match without an arm, but had the whole thing back together and made it through 5 elimination matches before losing in the semi-finals. I would recommend that you pay close attention to the entire output stage, replace or rearrange the brass planet gears, and flip the barrel to keep the transmission in good shape.

Let me state that I would use this transmission in the future, I would just be more careful in my design and increase the safety factor to avoid this type of failure.

Glad I could help. I think the advice you give is valuable: keep a close eye on the transmission. All it takes is one snag and the FP motor will gladly tear the last stage apart. Add it to your pre-flight checklist to make sure the gearbox isn't binding, the backlash isn't unreasonably large, and the housing isn't rotated out of place (see earlier posts). It may also be a good idea to tear the gearbox down and inspect it carefully whenever you have a long break.

I wonder...what ever happened to the motor current sensors from '04? I would love to have one of those on the FP so that if it sees abnormal currents it will shut down and minimize damage to the gears. Or better yet...a Victor with built-in current feedback!

Edit: I know I've been the one saying the whole time that the load isn't determined by the motor, but by the arm, and that the double-D failure is more preventable by limiting the output load via springs/counterweights/extra reduction, so I should probably clarify a bit at this point. I still think that is the way to prevent long-term, cyclic failure modes, like the double-D widening out. But for the catastrophic failures that occur when the gears bind or the housing goes out of alignment, limiting the motor current so that it cannot provide anywhere near stall torque would be extremely helpful.