Banebot 56mm gearbox - double D related

[Joe Johnson]

There are threads here, here and here that are all discussing the same issue. Will the double D joint between the last carrier and the output shaft survive?

There are sub-questions:
Will the 12:1 1 CIM joint fail? If so what can be done to fix it?
Will the 16:1 1 CIM joint fail? If so what can be done to fix it?
Will the 12:1 2 CIM joint fail? If so what can be done to fix it?
Will the 16:1 2 CIM joint fail? If so what can be done to fix it?

We can debate who is to blame and how could this have happened all day long. It is not going to get us the answers to the question, nor is it going to get us to a solution.

Here is the state of things:


Data Point 1: Hardness test of shaft and carrier:

Output shaft: RC 23, RC 23.3, RC 23.2
Carrier Plate with D: RA 45.9, RA 46.9, RA 45.8

Rockwell C to Tensile Yield:
RC 22 – 115 Ksi
RC 23 – 117 Ksi
RC 24 – 119 Ksi
RC 25 – 123 Ksi
RC 26 – 125 Ksi

Rockwell A to Tensile Yeild:
RA 45.0 – 62 Ksi
RA 45.5 – 63 Ksi
RA 46.0 – 64 Ksi
RA 46.5 – 65 Ksi
RA 47.0 – 66 Ksi
RA 47.5 – 67 Ksi
RA 48.0 – 68 Ksi

Data Point 2: Calculations & Date = Prediction
Using calculations + above hardness tests of the joint, I predicted that with the 12:1 gearbox should fail at about 3.1N-m input into the gearbox. Since 3.1N-m = 28in-lbs = roughly 1.5X CIM_stall_torque, alarm bells began ringing.



Data Point 3: static failure test
Using a test set up and a torque wrench, I was able to determine that the join failed with a single loading of between 3X and 5X CIM_stall_torque. This is better than predicted but not that encouraging considering that the "failure" was basically a spungy type event, allowing for actual failure to begin at lower than 3X. See picture.

Data Point 4: Cycling Failure test
Using the same test set up, I was able to show that the joint will fail at 2X CIM_stall_torque with repeated loading. See Picture.

Data Point 5: Failure in the field
This still did not mean that teams would see failures in the field because we don't know the dyanmic loading that we expect to actually see during real world conditions. I typically use 2x as the dynamic load factor, but that is not a law of physics it is just a rule of thumb. So... ...we needed confirmation.

Team 166 uploaded a picture that confirmed this problem will be seen by teams. This is after 3 hours of driving. Team 166 did not say what ratio & how many CIMs they used, but I suspect it was 16:1 with 1 CIM. Will someone from 166 confirm this please.

Where do we go from here?
Now, what do we have to do going forward? I believe that we can get a quick 2X in the output by simply getting harder carrier plates (RC 23 or higher). I believe that will almost certainly get us to the 12:1 1-CIM case as well as the 16:1 1-CIM case.

What it does to the 2 CIM cases, I can't predict. My best educated guess is that for most teams the this will address the problem for the 2-CIM cases too, but I have to be honest, the picture from 166 has me worried. If they were in fact using the 16:1 1-CIM set up, then going to a 2X hardness will be exactly matched by a 2X in loading due to the 2-CIMs.

We need more testing and we need more thought on solutions.

More Information
For your information, Banebots is well aware of this. They are working hard with FIRST, their suppliers, and yours truly to get a good solution out there to teams. Everyone is sorry. Beating us all up will not help. Calling them will only take time away from fixing the problem. We are working on it.

Joe J.