pic: Broken Output Shaft

[Jon Jack]

The initial design of our robot included a hanger that was powered off a PTO from our drivetrain. Step hexing is really pushing the limits of our machining resources and was something we wanted to try and avoid. By having a single speed transmission we figured we could get away with using a 1/2" hex shaft instead of the 5/8" hex shaft commonly found in other 2 speed transmissions. However, we still needed a way to shift the drivetrain into neutral. The hole at the end of the shaft is there for a shifter to shift the output shaft into neutral.

The shaft failed right where the snap ring groove is. We actually continued to run the same (old) shaft design on our practice robot until just before Atlanta when both shafts failed in exactly the same spot.

In further analysis, we had a couple things working against us:

1) The hole diameter for the shifting pin was made too big and the hole was drilled too deep (past the snap ring groove).

2) The snap ring groove further compromised the strength of the shaft. I don't have the numbers in front of me at the moment, but I believe the wall thickness would have been just under 1/8".

A few days before ship day, we made a decision to ditch the hanger so that we could put more effort into making our ball grabber and kicker better. When we ditched the hanger we no-longer had a need for a transmission with a PTO. We also didn't want to remove the PTO entirely just in case we wanted to add a hanger later.

After San Diego when we sat down to begin redesigning the transmission we stripped out the entire PTO. Since now we were now running a pure one speed, we could get rid of the slot and hole for the shifting assembly.

Before we crated in our robot in San Diego we pulled a couple drive axles (which were also 7075 Aluminum) from the robot just check and make sure that they weren't breaking or bending. Those shafts were okay so we knew that 7075AL would be fine for our redesigned output shaft.