pic: 20's IRI Carnage

[KrazyCarl92]

Andrew,

The bolt/pin sees none of the torsional loads because the hex transmits the torque. Therefore, the bolt failures are a result of shear stress.

You may be right that 7075 Aluminum is not up to the task. However, in this case, the task would have been 58.4 ft. lbs. of torque after the wheels broke. At a 40 Amp traction limit (normal conditions), the load is 17.6 ft. lbs. of torque. The fact that the dog broke indicates that the failure was a torsional failure. Normal conclusions about the comparative strength of the dogs are not applicable because of the wheel failure preceding the dog failure.

The video of the match shows these events happening in order:
1. Pushing match with 359, with 20 slamming on the ground on the end with the broken wheel
2. Wheel breaking off
3. Robot still performs multiple 0 turn radius pivots, indicating both sides of the drive train are moving
4. One side of the drive train stops moving

The transition between 3 and 4 indicates that the dog failure happened after the wheel failure. Therefore, the conditions which that dog saw were very different than most (if not all) conditions seen in previous failures. This, along with the different material and geometry of the part as compared to the standard AndyMark dog make it hard to determine if shock loads or stall torque could cause the torsional failure.

In what ways can the torque seen by the dog at the output shaft in shock loading exceed the traction limited torque of the gearbox by a factor of 3? The most I can physically make sense of is a factor of 2 when changing from full speed reverse to full speed forward or engaging in a pushing match at full speed to a stop.