As I was trying to remember all the things I have had to do to my R50 on another post, I remembered a tip on breakage of fan shaft. We have determined this problem occurs when an object like a hard low moisture corn cob gets between the fan sprocket and the chain. We noticed we were seeing too many shafts break during late corn shelling season. Mine broke and took out the clean grain auger as well. The Fix to this is to make a spring loaded idler for the LH shoe drive chain. The later series have a belt drive and no reported failures. The RH idler off a L or M lower raddle drive works fine. Weld a Stud to the frame and attach idler and a spring and MAKE SURE CHAIN IS LENGHTENED BY AT LEAST 8 LINKS. Since we have started this we have had O failures. Again we only broke them in late season corn so correlation was our observation. Hope this might help.