Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

We are using the 326:1 P60s, to articulate our arm, being driven by an RS395 and the other driven by a FisherPrice motor (For all you critics out there, this is not for power reasons but because we are limited to the number of BaneBot motors we can use - an RS395 will replace the FP and run the arm very happily). We have not experienced any failures yet but on both we have begun seeing signs of the gearboxes failing and although we have spares, we elected to try and "bulletproof" them rather than risk a breakdown. I am pretty confident we have mitigated the major modes of failure that we have been seeing from teams using these gearboxes.

The four "banes" of the banebot P60 include:
1) Rounding out of the output shaft and/or drive socket on the output carrier.
2) Bearing failure due to defective bearings (ungreased) and overloading the output shaft.
3) The roll pins acting against the flat sections of the ring gear that hold the rign gear torsionally captive galling, snapping and rounding out.
4) Binding of the gear train due to axial bending of the gearbox.

In order to rectify these we have:
1) Machined a new one-piece output shaft and final carrier from Stressproof steel to eliminate the interface between the output shaft and carrier.
2) Machined a new endplate with larger 1/4-20 mounting holes which houses larger (to accommodate the 1/2" output shaft) high quality NTN bearings spaced further apart to reduce loading caused by sideloads on the output shaft.
3) Bored sockets in the new endplate and stock ring gear to internally lock the ring gear to the end plate with 6 roll pins.
4) Replaced the non-graded 6-32 bolts which hold the assembly together with high strength socket head cap screws.

So far the end result is very promising. Obviously we have not been able to test it on our robot but we are hopeful this will eliminate the weak spots previously experienced with these gearboxes. All of these parts were made with a bench-top milling machine and 9x11 lathe and are well within the capabilities of even the most basic machine shops. The shafts for the planet gears on the final carrier are made from 1/8" hardened steel dowel pins. The output shaft is made of Stressproof Steel(http://www.niagaralasalle.com/product-stressproof.html) which is stronger than chromoly grades but machines like a dream (one of my favorite metals!). The bearings are 1/2" ID 1-1/8" OD, 1/4" W double sealed NTN bearings. The end plate/bearing block is just regular 6061-T6. The pins are 3/32" heavy wall roll pins.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Wow, I am so thankful for this post.

Our Arm Gearboxes failed during the semi-finals of NJ for what seems like the 10th time. This seems like it'll make them bullet proof.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Using such high reductions p60s is just all around a bad idea, even with these improvements. I imagine this now shifts the failure point to the gears in the final stage when the motor stalls.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Do you have drawings that you would be willing to post?

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Adam,

You're probably right that this will shift the failure point of the transmission elsewhere. If it's the ring gear, there's a steel version of that. I think you're wrong that the issue is stalling the motor, though. If you ever actually manage to stall one of these things, you're going the seriously damage the gearbox very quickly. We're experiencing a failure mode very similar to fox46's in that the gearbox gradually degrades over time. I think this is a combination of the weaknesses he's helpfully pointed out and some unique load conditions. We've never stalled our 256:1 RS-775 transmission, and yet we've deformed the carrier plate twice. I'm certain this is due simply to inertial loading from our arm as we lower the arm and stop it. The stop is invariably sharp and sudden thanks to the huge gear ratio, and this creates an instantaneous load on the gearbox that exceeds the capacity of the carrier plate. I expect fox46 is actually experiencing a similar problem.

His proposed solutions will definitely strengthen the gearbox and slow the degradation process, but I think they should be coupled with programming changes designed to limit the rate of change of voltage so as to lessen the sudden shocks when motion is stopped.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

All we had were some hand drawings when we built these but all the information is there to reverse engineer it. I'll see if I can find and scan them tomorrow- I think they are still floating around the shop.

No modification to any machine will eliminate the chance of failure. Shifting the failure mode to a stronger part of the mechanism was exactly what we wanted to accomplish. Planetary gear trains are incredibly robust. I suspect the method of failure is now the interferance fit of the sungear that drives the final reduction pressed into the 3rd reduction's carrier. In any case, it is much much stronger and can now be used as it was initially marketed to us.

I have no doubt that this thread will quickly accumulate posts accusing teams of having poor designs or not using the boxes correctly but the undeniable fact of the matter is this: The Banebot P60 has a factor of safety of less than one, even for the pint-size RS395. The factor of safety decreases the larger the motor you put on it. No machine such as these gearboxes should ever be designed with a FOS of less than 1. As Kevin just mentioned, very few people are actually exceeding the ultimate strength of the unit, they are only comming very close to it so the failure mode presented seems to be more of a fatigue/degradation issue. Most people are just "marginally" exceeding the P60 capacities so even if these mods only added 10% more strength it is most likely enough to solve the problem

FYI: Banebots now have a P60 gearbox offering 627:1 :confused:
Edit: correction- it's 672:1! (thank you dislexia) I guess a suitable application for this would be powering a large clock?

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Kevin, based on earlier discussions we have had regarding the switching method used by Victors, is this necessary with a Victor with jumper set to coast mode?

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

(I know you asked Kevin but-) The coast mode would definitely help with reducing the load when stopping the motor but limiting the starting power while still keeping the speed up is our challenge. Also, we're using encoders to accurately index the arm and if the Jaguars werent in brake mode I suspect it would drive the software nuts. It would be even worse if it overshot the target position and the software then reversed the motors to bring it back into position. I've been cracking the whip trying to get our programming team to come up with a ramp function to ramp the power up and down relative to proximity to the target position of the arm but dealing with our programmers is like hearding cats and lately I can't convince them to do anything. They've been using the typical post-ship "theres no point to trying because we have no way of testing it" excuse.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Brake/Coast only ever enters the picture when you're sending a 0% command to a Jaguar/Victor. Then it will short or open the motor leads. If you're sending any command outside of the deadband, then one of the lower legs is always on and you're effectively braking during the low period of the PWM cycle.

Besides which, have you tried backdriving one of these things even in coast mode? The friction losses and cogging torque of the motor make a pretty effective brake all on their own at these ratios.

On the other hand, the huge ratio means you're mostly just pushing against the friction of the gearbox itself. So the motor operates a lot closer to no-load speeds than you might think. So "slowly" ramping down the voltage will translate pretty directly into slowly ramping down the speed of the load.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Those look like some great upgrades to a BB P60. You may well be able to get more torque out of the gearbox after the modifications than before, however it is important to remember that the purpose of the very high gear reductions is to reduce speed, not to increase torque. Note the following comment prominently displayed on BaneBot's product page for their high-reduction P60s...

"Maximum torque recommendation
We recommend maximum torque not exceed 35 ft-lb for all P60 Series Gearboxes. It is possible to mount motors that will exceed this in higher gear reductions. Higher reduction gearboxes should be utilized primarily for speed reduction. Designs utilizing a P60 gearbox / motor combination that will exceed 35 ft-lb should include a method of limiting torque to prevent damage to the gearbox."

As for the "safety factor" being at or near one, I always felt that as a robot designer that safety factor calculations were my job. If the manufacturer says "don't exceed 35 ft-lb", then it was my job to ensure that I didn't. If I needed a safety factor to make sure that I didn't, then that was my job to engineer it in. Sometimes, when pushing a machine to the limit, with no risk of injury to people or property, it made sense to design a system with a very low "safety factor".... but no matter what, I always knew that when you run equipment at the extremes of its design limits that it will sometimes break.

Looks like rather than dialling back the demands, you're cranking up the specs. It looks great and I hope it works well!

Jason

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Well done. Should have done this to ours.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Jason,

I don't think I specifically declared it above, but our arm design is fairly well inside BB's 35 ft-lb spec at about 8 ft-lbs, and we're still slowly failing our gearbox due to the shock loading I mentioned above. Yes, as an engineer I should've realized this was a possibility and demanded more than a 4:1 margin in the system. Still, BB's warning there doesn't really go nearly far enough. It's not that you have to limit the torque IF your motor-gearbox combo can exceed 35 ft-lbs. You have to limit torque if the load can exert more than 35 ft-lbs on the shaft. I think you could actually fail the gearboxes without a motor attached to them at all, in fact. So your only real option for using them safely as they come is to put a torque limiter on the output shaft and hope for the best.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

I was thinking back to this post:

... and wondering if the Victors would also behave as described above.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Ahh, I'm not surprised... either that you did a good design job, or that the little BB is getting beat around, depite that. I hadn't really appreciated the magnitude of the shock loading on the gear shafts until we built a mecanum drive machine and were trying to tune the PID loops for wheel speed with the robot sitting on blocks and the wheels spinning freely. As we slammed the motors from full forward to full reverse we hammered and hammered away at the gearbox. You're probably right that a warning about repeated impact loads should accompany the overall torque load.

It actually sounds like the fact that your system worked through a regional but is showing signs of wear is a sign that you designed it perfectly... after all, if it isn't overloaded, it's overbuilt!

Jason

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Our arm is biased with gas shocks to the point that you can release it from full height with the drive chains removed and it will "float" back down to rest position and we still were seeing signs of failure.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Here's another way to take the Bane out, if you have room to do it:

Instead of 326:1, use a 64:1 with 5:1 external gearing (or sprockets or pulleys or winch etc). This will reduce the torque load on the tiny P60 by a factor of 5.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

We actually already have an external 5.4:1 reduction. The P60 drives a 10 tooth sprocket which turns a 54 tooth sprocket connected to the fulcrum of the arm. If we experience any further problems though, plan "C" is to put a stud in the side of the arm to support an intermediate 10t and 30t sprocket to provide a second external reduction and then machine the ring gears down to a 3 stages and use them to power the intermediate reduction.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

pseudo-code for rate-limiting the command to the motors:

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

This isn't balanced. A balanced arm should remain stationary at maximum extension (i.e. horizontal)

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Correct-
I never said it was balanced. The shocks only relieve some of the load, they do not remove it.

Ether: We may be thinking the same thing but I am just having a little bit of trouble visualizing what this pseudo code will do. Are newCmd and oldCmd encoder values or motor commands? (bear in mind I have very limited programming knowlege). I was thinking some sort of function like this:

What I am trying to accomplish would be to start the motors off slowly, throttle them up to full speed as the arm gets moving, and then as it approaches its destination point, to throttle them back down. Basically a soft start and soft stop function. Is this the same sort of thing your pseudocode does?

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Sounds like you want a closed-loop controller with a limit on the max allowed rate-of-change of the output.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

I see what you are trying to accomplish here but at this point I would not recommend any team trying to drive a long arm off a banebots gear box. The gear boxes simply can't handle the shock loads of an arm, even when dampened by a gas spring. You've definitely solved some of the issues with the planetaries, but there are other issues I would be concerned with:

- with that high of a planetary reduction, the loads on last couple stages are pretty significant, to the point where I don't think those mild steel planetary gears will last very long.
- I would also be very concerned with stage plates in the last couple stages, the pins that the planetary gears rotate on are only press fit into the mild steel and have been known to come loose and destroy the gearboxes.

I would highly recommend teams looking into using the Fisher Price Gearboxes included in the kit, they make for a robust arm gearbox. They are a pain to mount, but the Fisher Price is still a beast of a motor and should handle the loads well as long as the arm is dampened.

If that's not an option I would recommend teams looking into cobbling together a AM gearboxes with a cimulator and sprocket reductions to get the desired reduction on a bane bots motor.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

You're not sitting there stalling it, but when you first get the motor going, or rapidly change direction, the motor is stalling momentarily and putting out much more torque. As you already know, this probably isn't near full stall torque of the motor at 12V, but multiplied by the 256:1 it's still something rough.

Such high reduction p60s really shouldn't be used for arms, even with proper counterbalancing and software limits they are just waiting to tear themselves apart.

We've used our 4:1 p60s in our arm rough and hard, with aggressive cyclic direction changes and even full stall on a few occasions for a few seconds without damage.

Banebots isn't at fault here, they designed a gearbox that is mechanically nice, and very, very affordable. For people who want higher reduction but are aware the gearbox could damage itself from higher loads or cyclic loads, they offer a higher reduction. User error is not the manufacturer's fault.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

A limitation of the amount of change that can occur between loops is really all that is needed, similar to what Ether suggested. It will make calibrating a PID a bit difficult, but not impossible.

If you are building a proportional only controller, then adjusting the gain for a slower approach to the target will be simple. Then, use the torque limiting function, like Ether posted, to smoothly start the motion. Combined, these two should get you close to what you want.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Not quite actually, it is the 326:1 gearbox that drives the 10 tooth sprocket- we are indeed using the 326:1 P60. From the motor to to the fulcrum of the arm, overall we have a 1630:1 reduction.

I am very confident that these modifications will last. As it is, we have quite alot of practice with our robot- I would say it has had more run time than it will in the regional. We have NOT failed one of these transmissions yet. We only began noticing some backlash in the shaft and one of the ring gears turning back and forth against its pins. I have total confidence in the arm once these modifications have been added. Even now I would run matches as it sits in its crate. In my team's case, this mod is insurance against what we have been seeing from other teams. Trust me- our P60s have a very easy life compared to some of the other applications I have seen.

My analogy and opinion of the situation is this: Banebots has built a car. It bears resemblence to a Geo Metro- Cheap, compact, practical, simple etc etc.. But they are also marketing it to be robust and powerful. What they have in fact done is built a Geo Metro but swapped the engine to that of a V12 Lamborghini motor. This looks like an awesome little car to drive! BUT on their website they say "Do not depress the accelerator any more than 1/4" or you will ruin your car" What do you think would happen if such a car was to be sold to the public? Exactly what is happening with the P60s. Now who would you say is at fault here? The drivers for not keeping their right foot in check, or the manufacturer/designers? Its a ludicrous concept, and thats what I think of the high ratio P60s.

Banebots should stop producing P60s in any higher than ~60:1. SHOULD teams decide they want a P60 with a higher ratio, they can buy the planetary stages seperately and stack them together with longer bolts. If the thing fails, it is the team's fault for modifying the stock P60 with extra ratios but BB should have never marketed their high ratio gearboxes under the pretense that it is an appropriate reduction method for a speed 550 motor.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Blaming Banebots because a team ignored the manufacturer's product use guidelines and the gearbox subsequently imploded is inane. It's like blaming McDonald's for selling hot coffee because you burnt your crotch while trying to drink their coffee while driving on the highway.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

What if they sell that cup of coffee to you at a drive through? They are endorsing driving with said cup of coffee. If the coffee is indeed hot enough to cause second degree burns and they endorse driving with it - by all means they can get sued - even despite the "Caution Contents Hot" on the side of the cup. - That isnt the best analogy. We all know that many establishments have been quite successfully sued over this.

Yes they have the disclaimer not to use them with any more than 35lb-ft but obviously this spec is overrated. When using the RS395 motors on the 326:1 gearbox, at stall the combination produces ~28lb-ft of torque which is less than the rated loading of the gearbox. I don't know how you can claim a team is overloading a 326:1 with an RS395.

In our team's case, we have a 5.4:1 reduction after the P60 on our arm. At it's maximum extension in the horizontal plane, it reaches about 5 feet. Given this spec, the arm should have a capacity of 37.8 lbs. Since the whole arm itself weighs 15 lbs the P60 should reliably tolerate upwards of 20lbs of force at the end of the arm. This doesn't even consider the gas shock we have to assist it. I don't know but I doubt any of the drivers have been practicing scoring steel belted radials.

Yes the impulse loading on the arm will indeed approach this limit of 35 lb-ft but if BB was prudent they would have allowed at least a factor of safety of 2. Alas, declaring that these P60s are only good to ~17 lb-ft would definitely deter many teams from buying them. I know we would have gone with something else if this was the case. Of course this would severly impact their sales to the FIRST community so it seems they either chose to favor their bottom line rather than the reliability of their product or they have over estimated the strength of their product.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Ether, here are the hand drawings that the guys have been working from. Not all the dimensions are on them but anything that is missing is pretty easy to figure out. Hope this helps.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Thanks for posting these up. I showed your pictures to on of our Sponsors yesterday and it looks like we'll be able to get these done in time for our next event. I'll update with pictures and results when we're done.

For anyone else who was looking in to doing this, Stressproof Steel is sold as Alloy 1144 and should be pretty easy to find at your local steel supplier.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

When is your next event? Be sure to post how they work out for you and if you experience any other failures.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Point of order. The gears are definitely mild steel, but the ring gear is aluminum and likely to fail first.
Also, the carrier plates are definitely NOT mild steel. The guys that can run our pmi gun are out of office this week, but I had a scrap carrier plate to play with. 20 minutes, a butane soldering iron, and a cup of water took the carrier plate from 22 HRC to 40+ HRC. Definitely not a mild steel.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Now THIS is some good information. Did it warp or deform at all to where it would become unusable? I bet a map gas or oxy-map torch would be able to get it up nice and glowing in a bit less time. I totally wish I had a spare one to attempt making a bulletproof P60 with some mods.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Didn't bother measuring it, but completely subjective testing in a P60 suggests no significant warping. It felt slightly rougher than a virgin one, but my hardened plate was already slightly bowtied and deformed from the above mentioned shocks.
Also, 20 minutes encompassed the whole heat treatment, including a 5ish minute soak at something above 1550F/bright red. Don't be knockin' my butane iron/torch.

Before anyone tries this for realsies, I'd recommend a little more research on someone's part. Possibly mine. I'd like to know the actual alloy so I can figure out a real heat treat procedure for it, including a temper. I said 40+ HRC for a reason. I tested multiple locations on the thing and my haphazard heat treat gave me hardnesses ranging from 40 to 60. This does not inspire confidence, and I'd really recommend a good tempering to relieve some of those stresses and reduce the chances of fracturing as your exciting new failure mode. Brittle fracture is obviously an even worse failure mode, since you've moved from a gradual semi-predictable failure to an instant an catastrophic one.

Finally, you're going to want to harden to shaft as well if you're expecting this to increase the life of your transmission. A stock transmission fails by deforming the shaft as well as the plate.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

All excellent feedback regarding the banebot mods. That is what this "project" is all about. Some of us want a Corrvette for the price of a Fiat. Some things I noticed for those of you planning on modifying the banebots:

The drawing is missing the rear guide pin on the output shaft that centers in the 3rd stage carrier plate. Is it machined integral with the output shaft, or a pin pressed into the shaft.

Are you using extended inner race bearings where the spacer is located?

Is there any problem with the spacer moving off center when you grease and reassemble?

Banebots has been very responsive to suggestions for improvement. You should talk to them. Thank you for doing this. :) :)

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

I am not sure I follow you on this- our P60s didn't come with any kind of a guide pin. I think I can imagine what you are talking about though- is it a pin that locates the 3rd carrier to the output shaft? I noticed there is indeed a hole through the center of the carriers but there is no bore machined in the output shaft.

No, they are just regular bearings. I suppose you could put a spacer between the races but we are just leaving it up to the spacer on the output shaft to space them.

The spacer can sometimes slide out of line but all you have to do is use a screwdriver to poke it back into position from the other side before sliding the shaft on. The spacer is machined with chamfers so that it is somewhat self-centering and will slide onto the shaft as long as you get it close. The fit between the shaft and spacer does not have to be tight as it just keeps the bearings from wandering along the shaft.

I like the hardening- it will probably solve some of the issues but I would be very leery about the possibility of making the parts brittle- especially the output shaft interface. let us know how it works out!

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

So, today we had our first BaneBots failure. It's a 256:1 P60. It slightly spun the ring gear and really seized the whole thing, before we even made it to the field. It sees only about 10 or 12 lb-ft average load, not accounting for acceleration of the arm or shock loading. The shaft is well supported. It has probably only 50 cycles of our arm on it. We took it apart, and found the DD hole in the final carrier plate and the DD shape on the output shaft to be significantly yielding/deforming. We had that welded up at the regional. Also, in place of the roll pins on the end block, we had them put in a couple weld beads, and file it flat, to match the flats on the gear. It seems to be okay now, we'll see how long it lasts.

Also, one interesting thing we discovered is that the optional Steel ring gear has slightly different dimensions than the Aluminum one. It shouldn't be different, but it is. The distance across the flats is different.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

When you have time, take a look at this possible solution, it might just do the trick. There is a question as to it's legality according to this year's rules, so please think it through before you implement it.

Re: Take the "Bane" out of your Banebot P60s - Solution to Banebot P60 Weaknesses
 

Well we just got back from our competition in Boston and this mod DOES INDEED FIX THE P-60 ISSUES! We ran the whole competition without a single failure with these modifications. In fact, while testing our autonomous program we hyperextended our arm and one these little guys stalled with a fisher price motor on it and then proceeded to rip our counter-balancing gas shock right off its mountings without any damage to the gearbox. If you are having P-60 woes, here's your way out.

PS: We elected to run the steel ring geas.

Best of luck!