How much torque can the Dewalt Trannies take?

[Tom Line]

Has anyone driven the stock dewalt transmission components to failure, and if so, what did it take?

We're planning an offseason project that will interface three cims into the input of a single Dewalt. I know 2 cims has been done through a single Dewalt, but I think I'm probably riding the hairy edge of failure adding the third.

Anyone with some info on what I should expect to fail first? Standard input per the NBD whitepaper.

[Brandon Holley]

You should check out the robonauts drivetrain from last year. I believe they routed 4 CIMs and 2 FPs through 1 dewalt.

[Lil' Lavery]

You should check out the robonauts drivetrain from last year. I believe they routed 4 CIMs and 2 FPs through 1 dewalt.

They did have back-ups, including a multi-DeWalt version. I do not believe they had to use them though.
Others teams have managed to "liquify" (their words) the insides of a DeWalt with many fewer motors running through them.

[sdcantrell56]

From talking to 118 they said they never had an actual problem with the gears in their dewalt transmissions, but they had problems shifting them with all the power.

We are planning on prototyping a transmission this summer with 2 cims into 1 dewalt which I think shouldn't be a problem at all.

One thing that 118 said is that all of the cutting on the dewalt housings and things like that needs to be very precise for them to work right.

[meaubry]

Perhaps Dennis from Team 27 could shed some light.
I think they found the limit this year, as Team 27 Team Rush was replacing Dewalt trannys after a few matches due to a catastrophic failure of some part in the system.
Maybe Den could figure out what type of load and torque they were seeing and let everyone know what the the limit is.

Mike

[R.C.]

1323 wants to use the dewalts. Can anyone shed some light on the pros and cons and how fast they went with one cim. Thanks

[Tom Line]

To the best of my knowledge Rush was twisting their driveshafts, not their trannies, but I could be wrong! Hope he can add some input as well. Rush was one of the faster heavy teams out there this year.

It was very educational watching battery voltages throughout the matches on the IFI screens - for instance during the 4 minute practice matches chicken's voltage was running down in the 6's and 7's, but they may have been using old batteries. Many other teams were dropping down that low as well at the end of 4 minutes. This year was a hard one on batteries - but that's for another topic.

RC - before someone beats you over the head with it, I suggest you do some searches on the forum. There's a white paper on dewalts to get the ratios and then it's a very easy excercise to use JVN's design calculator to figure out how fast you'll be moving.

[sdcantrell56]

I also believe that Rush was breaking drive shafts. I don't believe that the dewalt themselves were giving them trouble. Another great thing about the dewalt is that it is so cheap it is easy to have spares, but I think that the transmission itself is very strong. We are going to try 2 cims into 1 dewalt driving high traction treaded wheels this summer and report back on any problems.

[Josh Fox]

I'm not necessarily the best person on this subject but as far as I know we don't have any real numbers on the limit of the dewalts. Though the failures appeared to have similar simptoms, we managed to cause a different failure almost every time. This was running two cims through the dewalts on a 6 traction wheel drive (6 inch andymarks) with the center wheels dropped.

edit: originally we did break a few drive shafts but then we managed to chase the failures into the transmissions themselves

[Mr. Van]

Robodox also used 2 CIMs driving 1 Dewalt in a setup very similar to RUSH.

We used a 3:1 belted reduction from each CIM to the input of the Dewalt. The output of the Dewalt directly (via a coupler) drove the center wheel of a 6WD 1/8" center drop drive train.

(RUSH used a slightly lower geared reduction before the Dewalt.)

Some observations:

The Dewalts are planetary transmissions. As such, they have a considerably lower efficiency than a gearbox using only spur gears. (They also take up less space for the same reduction.)

You've got to make sure you hold the actual transmission really well. The original housing from the Dewalt drill is best for this. We found that our major point of failure was the actual transmission spinning inside the housing (we didn't use the entire drill casing - our mistake!) We found this out in testing and never had a problem in an actual match.

Shifting is not always smooth - especially using a single servo to hit all three gears. It is possible to fail to shift - leaving you in neutral unless you quickly shift back to your original gear. Shifting is easiest when under power with a light load. Shifting the Dewalts was our major problem this year. At one point, we thought we had broken a tooth on one of the gears in the Dewalt, but after taking it apart, we couldn't find our what was wrong. Our best guess was that it wasn't shifting completely for some reason. RUSH quickly figured out that they were better off just keeping them in a single gear for the 2008 game.

We have made an alteration to the setup in the NBD paper. In NBD, the planet gears that mate with the gear on the drill motor are removed along with their supporting pins. Then the sun gear is reamed out to be fit onto a shaft. Instead of making this alteration, it is very possible to use the 4 pins that hold the planet gears to drive the sun gear by making a simple coupler. No machining of the Dewalt! This setup worked well for us the past two years and is especially useful when mating two motors to the Dewalt. (Any problems observed with our robots were from other sources.)

In the end, the difficulty in mounting the completed gearbox and the reduced efficiency as well as the costs required for additional hardware (belts, pulleys, bearings, etc.) have turned us away from the Dewalt for the near future.

Andy, the check will be sent soon.

-Mr. Van
Coach, Robodox

[R.C.]

Can you post some pics of the two cim dewalt gearbox?

[sdcantrell56]

I would like to see pictures of the transmission as well. Pictures of Rush's would be great too.

[roboticWanderor]

OOO *rubs hands together* my favorite subject!!!
Dewalts can hande A TON of abuse, they are designed to be used in 12 v cordless drills, used by professionals and craftsmen world wide, designed to take abuses and stress beyond the capabilities of one robot.
Team 418 has been using dewalt transmissions for 4 years now, and never had the transmission itself fail. we initially followed the NBD paper to the letter, but recently have taken to doing some things of our own including:
- change from reaming the carrier to machining a plate to fit on the planetary pins
- the carrier plate allows us to save time by not having to sand down the gearbox housing to a certain depth, just trim the CIM shaft down!

These things are bulletproof as long as they are thoughtfully put together (use the right screws, lock tight etc.) and we an recommend them to any other team looking for a cheap and easy multi speed transmission.

2 speed shifting is smooth, but 3 speeds gets a little tricky... but we have never actually seen a need for the 3rd speed (12:1, 4:1, 3:1 not much difference)

mounting them could not be much easier, a securely tightened shaft collar clamped around the gearbox holds the entire assembly very well.

118's V6 with the dewalt was a very powerful and innovative system that took full advantage of the compact and strong multi speed transmission that the dewalt offered. im not sure what part of their drive train broke during LSR finals that year though, rumor has it that it was the V6 though. Pavan, care to fill us in?

as a last testament to the robustness of the dewalt, our team has used the same model dewalt drills for about 5-6 years now and never had anything go bad besides the battery, and these things are still working great!

[Lowfategg]

Heres a question. If money did not matter as well as time, would you go with a Dewalt gearbox or a Andymark gearbox? Is there anything that would make the Dewalt better then say a Andymark supershifter?

[EricH]

Heres a question. If money did not matter as well as time, would you go with a Dewalt gearbox or a Andymark gearbox? Is there anything that would make the Dewalt better then say a Andymark supershifter?Depends on the application. 3-speed or 2? 3-speed, Dewalt. 2-speed, AndyMark.

I'd go with AndyMark if I was in a time crunch. Very little assembly, if any.

[Lowfategg]

Depends on the application. 3-speed or 2? 3-speed, Dewalt. 2-speed, AndyMark.

I'd go with AndyMark if I was in a time crunch. Very little assembly, if any.

Lets say we were only looking at the Dewalt as a two speed since the person before me said its hard to get 3rd gear to work.

[Lil' Lavery]

Lets say we were only looking at the Dewalt as a two speed since the person before me said its hard to get 3rd gear to work.

DeWalts are a planetary transmission, and as a result have all the advantages and disadvantages of it. The biggest advantage is the much smaller size of the transmission. This is especially important in less than "traditional" drive designs where transmissions might not have as much space. Non-coaxial swerve systems often are forced to have the motors and any reductions (transmissions) rotate with the swerve modules. If a team were to opt for a multi-speed, non-coaxial swerve system, DeWalts would definitely be the way to go. Of course there's plenty of other scenarios where DeWalts would be advantageous for their size as well, that's just one extreme case.

[Lowfategg]

DeWalts are a planetary transmission, and as a result have all the advantages and disadvantages of it. The biggest advantage is the much smaller size of the transmission. This is especially important in less than "traditional" drive designs where transmissions might not have as much space. Non-coaxial swerve systems often are forced to have the motors and any reductions (transmissions) rotate with the swerve modules. If a team were to opt for a multi-speed, non-coaxial swerve system, DeWalts would definitely be the way to go. Of course there's plenty of other scenarios where DeWalts would be advantageous for their size as well, that's just one extreme case.

Being that I am new to FRC being from FTC I don't really know the advantages and disadvantages of a planetary transmission (since my FRC team did not play with them). From what I can see I love the idea of them and they seem like they would be very good. But from what your saying the Dewalts are good because of there size.

[Lil' Lavery]

Being that I am new to FRC being from FTC I don't really know the advantages and disadvantages of a planetary transmission (since my FRC team did not play with them). From what I can see I love the idea of them and they seem like they would be very good. But from what your saying the Dewalts are good because of there size.

In short, planetary gearboxes are a much more compact method of getting reductions than standard spur gears, but are far less efficient.
For instance compare the banebots transmission to the AndyMark toughboxes.
http://banebots.com/pc/P80A-nnnn-0005-Rn/P80A-43-0005-R2
http://andymark.biz/am-0145.html
While the banebot's transmissions had other issues in reliability that caused them to be unpopular, the differences in dimensions for gearboxes with essentially the same reduction are apparent. This is magnified even further when comparing a shifting transmission such as the AndyMarks (http://andymark.biz/am-0085.html) to a DeWalt (http://www.chiefdelphi.com/media/photos/29730).
http://en.wikipedia.org/wiki/Spur_gear#Spur_gears
http://en.wikipedia.org/wiki/Planetary_transmission

[Lowfategg]

In short, planetary gearboxes are a much more compact method of getting reductions than standard spur gears, but are far less efficient.

Ok thank you. Thats what I thought when I looked at planetary gearboxes.

[Nikhil Bajaj]

To the teams that usually use these transmissions, do you typically use servos or pneumatics to shift? I can see advantages/disadvantages to both...

[MrHance]

Haven read the thread up until now, and being one of the mentors from team # 647 that wrote the white pape for NBD. It is in my opion that the Dewalts are the way to go for many reasons. Just a few: 1) compact, 2) durable, 3) versital, 4) No need for pnumatics and if I took the time to think up a few more from Afghanistan, I'd put them in. The drawback is that they consume battery power because you use some many motors. The shifting issue can be addressed with proper program for the shifting of the tansmission. For the most part we use the shifting only on the drive system. When we use them for other applications, we selected the gear ration that we need and left them there. No Problems. You have to be very precise on you application and setup when using the Dewalts with any motor combination. Do not be afraid to experiment with them. Only your team can decide the best way to apply them in every setup that you do. Not everyone will come up with the same solution. Have a Nice Day!:)

[IKE]

This is a good problem to do the math on. When you go with a lower overall ratio and therefore more speed, the back drive torque into the transmission actually goes up. Where this applies to this year's game is that the ratios that people were using to get the really high speeds allow for more backdrive torque into the transmissions. We had a dual CIM Single Dewalt design that we were intending to use. This design had a reduction before the Dewalt as part of the 2to1 interface. We decided against it when we ran the stress calcs on the output shaft of the Dewalt. With the speeds we wanted the stress on that component would require a pretty exotic material (approximately 200ksi). While I am sure they are using an extremely strong alloy for that shaft, the factor of safety was just too low (1.2 top depending on exact alloy). One thing for teams to remember is that sometimes the forces coming back into the driveline (especially when a defender hits you at 10 fps) can be way higher than the forces going out. Again this was for our specific design. We ended up going with the old school AM shifter and with some small modifications. They were bullet proof in this design.

If there is a big request for it I will dig those calculations up and put together a paper otherwise I recommend that people run the exercise for themselves.

All that being said, I think the Dewalts are an excellent choice for typical FIRST games. We used them in 05, 06, 07, and 08 in various applications. We have had problems, but that was due to us thinking that we knew better than the NBD paper. Should you use DeWalt's follow that paper to a T and respect every detail. The biggest problem we had is that when we didn't follow the paper, we didn't get immediate failures, we got the kind that only show up after say 8 or 9 hard matches. This leads to a very stressful Saturday.:mad:

[R.C.]

I would love to see those calculations, thanks

[Tristan Lall]

This design had a reduction before the Dewalt as part of the 2to1 interface. We decided against it when we ran the stress calcs on the output shaft of the Dewalt. With the speeds we wanted the stress on that component would require a pretty exotic material (approximately 200ksi). While I am sure they are using an extremely strong alloy for that shaft, the factor of safety was just too low (1.2 top depending on exact alloy). One thing for teams to remember is that sometimes the forces coming back into the driveline (especially when a defender hits you at 10 fps) can be way higher than the forces going out. Again this was for our specific design. In 2006, 188 had some similar issues with custom DeWalt outputs. Basically, we were breaking the output shaft after several matches worth of hard running. Although solidly mounted, the steel shafts we substituted for the stock ones were not heat-treated, and failed in torsion.

We actually used a different strategy to mount the whole thing. We eliminated all of the DeWalt plastic housing, removed the clutch selector and its stop, and removed the cosmetic cover on the shifter. Then we made a new adapter for the planet carrier out of acetal, with holes to match the pins on the planet carrier (instead of removing the pins). A turned plywood plug capped off the back of the transmission, and set the clearances for the planetary stage. We then tapped the existing holes on the cast aluminum housing for #6-32 UNC threaded rod, and used four tie rods to pull the entire assembly tight against an aluminum plate (part of a 2:1 gearbox with a pair of CIM motors attached, and options for other kit motors as well). We also removed much of the grease from the DeWalts, in a bid to boost efficiency somewhat (when you're looking at something like 70%, you do what you can...).

Apart from the output shaft issue (which would likely have been solved if we'd substituted hardened steel), and some manufacturing errors in the CIM gearbox, the design seemed solid. The DeWalts themselves showed no signs of wear. On the other hand, there was never any need for the top gear (in our configuration); it was set for around 16/12/4 ft/s (if I remember correctly), and truth be told, with that particular robot and without very careful deceleration, use of top gear would land the robot on its back. Servo-actuated shifting was sluggish compared to the pneumatic dog shifting on Woburn's 2004 two-speed, but because that robot spent most of its time in low gear, it didn't become an issue. We shifted it under full power in testing, without any problems.

[sdcantrell56]

Could you post up pictures of that transmission (188). That post is very informative and it is starting to confirm my belief that if at all possible we should try to use a custom dog shifting transmission, especially if cost is relatively close (within $100 per transmission).

[ajlapp]

As stated in another post Team RUSH is working on a short whitepaper about the engineering development we went through this season with our Dewalt trannies.

First off let me say that prior to this season we had zero Dewalt failures. They are truly robust and easy to implement.

That said, I'll give you a short run down of what we saw in general.

The format we used was two cims into a dewalt via about a 2.2:1 reduction....this then directly drove a 6" wheel with a custom shaft. Previous version had a final drive via another chain and sprocket...this increased the torque directly inside the transmission about 3 times..

We made this custom shaft from 1045 key stock available from mcmaster. Our original failure was to twist the double d off the shaft directly inside the dewalt.

We switched to a 4140 shaft...same result. We then made another shaft that threaded onto the dewalt shaft and was cross pinned. We managed to break a few pins, but the shaft strength issue was solved.

The dewalt then started dropping pins and gears from the inside of the planet set.

After much analysis and some dissection of various dewalt trannies we found the toughest variant sold in the 18v line. I don't recall all the suttle variations, but there are a few.

We then modified the second stage to hold larger pins. We also bored the planets to match.

The final failure was to actually spin the transmission carrier inside the yellow dewalt housing. Another fastener and pin solved this problem.

We also chopped the max pwm output of the system to staop the driver from being able to deliver max torque into the system while stalled or turning. This helped a lot, but cost us some speed on the back stretch.

Final verdict...the system works well now, but the edm process used to bulk up the trannies gets away from the ease of use. Another section of chain, a final drive stage or some other means of soaking up the torque spikes could make this system perfect.

p.s. 118 does put 6 motors through one dewalt, but they have tons of chains all zapping some of the torque way before it ever gets back to the dewalt; and they never go in reverse. :)

[fimmel]

team 885 had a few issues with them this year, it was simply error, TAKE THE BACKDRIVE PINS OUT if your going to use it in a drive train. they managed to grind the insides on thursday. (this is what i have heard from some of their mentors)


...forest

[roboticWanderor]

Final verdict...the system works well now, but the edm process used to bulk up the trannies gets away from the ease of use. Another section of chain, a final drive stage or some other means of soaking up the torque spikes could make this system perfect.

WOW that is a lot of work there guys! great job, cant say i haven't fantasized about the dual cim dewalt transmissions a few times myself. i have probably mentioned this before, but the carrier adapter plate provides a much much better solution compared to EDMing that stage. Like you said, i would also recommend putting the further gear reduction After the dewalt to make sure you don't run into those torque problems anymore.

[Aren_Hill]

i can attest after last years bot with 1 cim dewalts are practically bulletproof if you follow every part of the NBD whitepaper.

And this year we a fp motor run through one to pull back our shooter, and the antibackdrive pins really came in handy, that setup was installed once and left, our shooter never broke.

[R.C.]

Are there any others mods that aren't on the white paper besides:

Taking the back pin out?

[IKE]

Simple torsional stress calculation:
I couldn't find my old notes, but here is an example if you are direct drive with a 6" wheel that only the torque is causing stress (i.e. no bending load). The shear stress is equal 16*Torque/(pi*d^3) for a round shaft. So back to our 6" wheel example a scenario might go like this: because you are pushing against an opponent all of your robots weight is on one side and your wheels are slipping. Axle torque is the Normal force (weight) * traction coefficient (1.3 peak from team 494 testing)* radius of wheel 3" or about 500 in*lbs (132lbs*1.3*3in). You throw this into the equation above with a 5/16" diamter shaft (I think that is the dewalt output shaft minor diameter) and you get ......... a stress of 83 KSI. So that people get some perspective mild steel (1010) is around 26 KSi (hot rolled) 44 KSI (cold rolled). If you made your direct drive output shaft out of that material you would probably be able to drive your robot around just fine since the loads would be split with the other side. You wouldn't break a shaft until someone hit you. If you went with something like 1050 cold rolled you would begin yeilding a shaft when pushed. Do this a few times and eventually you will break a shaft. For fully reversed torque, it is common to use a factor of safety around 2 when you want to make sure you won't break that part. 170+KSI steel tends to be special grades of heat treated 4130.
Moral of the story for robotics: If your design starts requiring above 50KSI material you either want to rethink your design or be very careful with your design AND your material selection. I have often seen failed designs where the designer was designing for 1010 Cold Rolled (44 KSI), and part was made with 1010 Hot Roller (26 KSI). What was once a factor of safety of 2 goes to 1 and eventually the shaft fails.

This is a very simple version of a much more complicated shaft. In reality the D-shaped shaft has a stress concentration factor that makes the actual stress a lot higher than the what you would see in a round shaft. There are a lot of good books out there about machine design The one I have is "Machine Design: an integrated approach". If has a ton of examples that are easy to follow once you have a good understanding of what Stress and Strain are and how to apply loads. If you don't know what Stress and Straing are, read the book from cover to cover. This was the book for Purdue's Machine Design class while I was there. While figuring out the exact load scenario is best, simple calcs like the one above will tell you if you are in the ball park. I tend to use calculations like that to sort out should never fail, might fail, or garuanteed to break.

[Justin Ridley]

A quick note on the Robonaut V6. We did put six motors through a dewalt in '07 and I was completely shocked that it handled the torque. It's important to mention that we did see a couple sun gear failures that year that seemed to be caused by hard impacts (sudden stopping of drive). We made the design easily replaceable since we knew we were pushing the limits of the gear box. Overall I think it was as reliable as most (but not all) drivetrains that year.

Another important point... we were only running the drive in one direction. If the gearbox had to withstand the back and forth seen with tank steer I have little doubt it would have failed.

[R.C.]

What about using omni's in the front and putting a dewalt to each of the 4 wheels left. Direct drive or shaft, sprocket and chain to the wheels??