I personally inspected that PTO and there was no twist. The splines were straight.
The shaft bound up at the same point in all the tractors and we did not force the issue that day. However, when the tractor was assembled it only took 3 firm whacks with a 3 lb hammer to put it in place. That I can easly perceive the line guys doing.
Just examining the shaft I have a hard time seeing why a PTO would fail there. It is one of the stouted areas of the whole shaft. If a twist were to occur I can see it happening in the necked down section or at the aft end, just not at the front.
I can certainly not say it will necver happen. Based on my one real, and personally examined data point, I will, however, challenge the common wisdom that is quoted, and offer another explination.
Anyhow, that is what makes this all so fun, the learning, friendly debate and problem solving.
If I ever get a free momment at work perhaps I can do some load calculations or even FE model the shaft. That could be interesting.
I hope all is well with you and your family John,