How much torque can the Dewalt Trannies take?

[Tristan Lall]

Originally Posted by IKE

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.