Just for reference take a look at this John Deere axle shaft. This was from a machine with 80 hours on it that I was asked to take a look at. The shaft had a dense fine grains structure on the outside and on the splines indicating a very hard surface. The grain structure in the center was coarse indicating a softer tougher result. Exactly what you want in a tractor axle. We concluded the operator side stepped the clutch with the wheels buried and the engine revved up. Was a good thing the axle broke instead of blowing the whole transmission.

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