Quote:
Originally Posted by squirrel
The quick reversing of direction can be avoided with careful programming and control setup, and I have a feeling that this is much more likely the cause of the problem than the 5 ft lbs of torque.
It sure is nice to have this early failure report, so we know to be careful with the BB gearmotors, and have contingency plans in mind using either the Globe or window motors.
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The last joint double D* on the Banebots 56mm Trans has given me heartburn since kickoff when the joint first caught my attention (actually I am worried about the same joint on the 42mm & 36mm too but only at the higher ratio --
read more here).
I have done some calculations, some FEA's and finally this past week some tests. I am still working on my "final report" but here is my initial report. The joint fails under static loading when the input to the 12:1 gearbox is about 3X the 12V stall torque of the CIM motor. The joint fails under back and forth cycling at about 2X CIMstall.
From my point of view, the joint as shipped is not strong enough to be used with the 2 CIM adaptor from BB. It seems like it should be okay for use with 1 CIM with the 12:1 ratio but I am not sure because I usually estimate the dynamic loading to be twice the static loading. That would make the 12:1 gearbox right on the edge of acceptable. BUT, in this case, with teams rapidly switching from forward to reverse, I am not sure that a factor of 2X is enough.
The 56mm carrier plates are pretty soft. They are Rockwell A hardness of about 46 -- corresponding to a yield stress of about 64,000psi. It is not too hard to get a material with a yield stress of about 2-3X that number.
I think that that is probably where I am going to go with my recommendation. More to come.
Joe J.
*In many ways, double D's are not very effective torque transmitters, they cause stress risers and concentrate stresses on small sections of the joint. Splines are better because the distribute the stress to more material but not as easy to make.