Quote:
Originally Posted by nuclearnerd
7075 could maybe find use in highly stressed structural plates, where you want to save weight on a big piece. But for small shafts & gears, don't be silly - just use steel!
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+1. The counterarguments might have sold me 36 hours ago, but not now.
As part of our pneumatic-actuated H-drive project, the students (for good reason having to do with floor clearance and bolt head thickness) wanted to go to a 6" dualie strafe wheel on the TB-mini. The tank wheels are 4", driven 8.45:1 in an AM 14U2 chassis, with two 4" wheels stripped of tread used as spacers. To compensate for the greater diameter, we decided to do a lower gear ratio. I knew that we had a 10.71 TB2 in stock from our 2014 (Aerial Assist) robot, so we located it and swapped out the gears (we wanted to use the long hex shaft). I told the students to move the shaft plate labeled 8.45:1 to whichever gearbox got those gears; they accepted this suggestion instantly, and took it as their own. (minor victory!)
Through 2014, we used keyed round shafts rather than hex, so the AA gearbox had a keyed round shaft. However, when they put the 8.45 gears together with the keyed round shaft, they couldn't put the pieces together. In particular, the shaft would not go cleanly through the bearing; I suspect they used a bit of force getting it out in the first place. They recognized that the shaft was slightly bent. We located a new matching shaft, and they put the 8.45:1 gearbox with a round shaft together. I pocketed the shaft for possible use in our "first four years" shadow box.
Yesterday evening, I verified that the mangled shaft is steel; a magnet sticks to it. I can't see using aluminum of any grade in this capacity. The shaft in question was
not used in the drive system; it was used to swing a "hammer" that "kicked" the yoga ball. Despite always supporting the other side of the hammer, somewhere along the way, the shaft deformed, most particularly near the key slot. Curiously, checking the hammer, there is less deformation of the aluminum hub than of the steel shaft.