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#1
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Re: How much torque can the Dewalt Trannies take?
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I'd go with AndyMark if I was in a time crunch. Very little assembly, if any. |
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#2
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Re: How much torque can the Dewalt Trannies take?
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.
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#3
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Re: How much torque can the Dewalt Trannies take?
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.
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#4
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Re: How much torque can the Dewalt Trannies take?
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#5
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Re: How much torque can the Dewalt Trannies take?
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For instance compare the banebots transmission to the AndyMark toughboxes. http://banebots.com/pc/P80A-nnnn-000...80A-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 to a DeWalt. http://en.wikipedia.org/wiki/Spur_gear#Spur_gears http://en.wikipedia.org/wiki/Planetary_transmission |
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#6
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Re: How much torque can the Dewalt Trannies take?
Ok thank you. Thats what I thought when I looked at planetary gearboxes.
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#7
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Re: How much torque can the Dewalt Trannies take?
To the teams that usually use these transmissions, do you typically use servos or pneumatics to shift? I can see advantages/disadvantages to both...
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#8
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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!
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#9
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Re: How much torque can the Dewalt Trannies take?
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. ![]() |
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#10
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Re: How much torque can the Dewalt Trannies take?
I would love to see those calculations, thanks
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#11
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Re: How much torque can the Dewalt Trannies take?
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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. |
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#12
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Re: How much torque can the Dewalt Trannies take?
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).
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#13
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Re: How much torque can the Dewalt Trannies take?
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. ![]() Last edited by ajlapp : 02-05-2008 at 20:17. |
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#14
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Re: How much torque can the Dewalt Trannies take?
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. |
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#15
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Re: How much torque can the Dewalt Trannies take?
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. |
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