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#1
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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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#2
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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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#3
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Re: How much torque can the Dewalt Trannies take?
Quote:
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#4
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Re: How much torque can the Dewalt Trannies take?
Quote:
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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#5
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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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#6
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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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#7
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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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#8
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Re: How much torque can the Dewalt Trannies take?
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??
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