Quote:
Originally Posted by ZZII 527
Maybe a small bit of good news. We've been using the 42mm 256:1 w/ FP setup for our arm. After the output shaft, there is an aditional 72:10 chain reduction to the arm itself. The arm is just a light 1x1 Al box extrusion, no more than 2 lbs and 4 feet long. It hasn't been put through anything rigorous or a ton of cycles yet, but we have used it to right the whole robot a few times. We tore it down today to check out the internals and the carrier looks pretty good to me (picture attached). There is slight deformation at the corners, but it's only at the surface (maybe 10% of the total depth).
Obviously more demanding and cyclic loads need to be tested before this proves anything, but maybe with TLC the 42mm can be used effectively for light arms.
I tried a possible quick fix: pinning the shaft to the carrier, but after a couple broken bits, I can confirm that the shaft is, to use technical terms, really, really hard. The carrier hardness falls somewhere between a filing drawer hanger and a screwdriver.  Note to self: leave the real engineering to Dr. Joe. But 119 ksi is pretty hard, am I right? Can we do a lot better with a different steel? Heat treatment?
What about shims to take out the small bit of play between the shaft and the carrier? This might help since the corners won't dig in so much. It would approach Dr. Joe's linearly-distributed loading condition more. I'm thinking of what happens when you use a metric hex wrench in an English hole. It might work, but will be a bit too small and as a result will strip the hole more easily. |
Thanks for the data.
A reminder to teams (and I am going to shout, so plug your ears): I AM NOT SAYING THAT THESE GEARBOXES ARE FRAGILE LITTLE WIMPS THAT WILL BREAK IF YOU LOOK AT THEM CROSSWISE!!!
They have limitations. It is possible to use them over their limits and they are likely to have issues if you do.
Note that my recommendation of 350in-lbs max is the load that I predict they will fail at with 100's of cycles! It will not take a single or even a 100 such events to make the gearbox fail in the mode.
Now the good news/bad news. First the bad news: Banebots tells me they are out of the 256:1 42mm gearboxes. now the good news: with 64:1 being the highest ratio they sell, these gearboxes can not only take full stall loads of the FP or the Banebots motor, they have dynamic loading safety margins of 2.5 for the FP and 3.5:1 for the BB.
Here is the bottom line (and I am going to shout again so you can hear me): IF I WERE BUILDING A ROBOT THIS YEAR I WOULD ALMOST CERTAINLY HAVE FOUR (YES FOUR) OF THESE 42MM GEARBOXES ON MY ROBOT -- TWO FOR THE FP MOTOR AND TWO FOR THE BB MOTORS.
Joe J.