Vermeer 7030

I have a Vermeer 7030 disc mower that has started giving me problems and I was hoping for some help.  Last year a bearing went out and locked up the gearbox and somehow the 14mm hex drive shaft for the cutters heads broke.  Over the winter we rebuilt it, replacing all the bearings in the gearbox.  It feels very smooth now, but it has already broken 2 of the hex shafts this year.  Both are broken just outside the gearbox.  When turning the cutter heads after the shaft breaks, they turn freely and are smooth with no obvious tight spots that would suggest twine or anything wrapping around the pods. 

 

After taking the bar apart (removing all the cutter heads in the process) I can't find any reason for the shaft to break.  I asked the operator if what speed he was going and today he told me the hay was very thick so he was in low range, 2nd gear, which means a ground speed of about 2 mph, because I had told him after breaking the first shaft the only reason this shaft should break was either very high torque or hitting something with the cutter.  He also told me he was at the end of the row, turning to start cutting the next strip and not even cutting at the time.

 

When we rebuilt the gearbox, we went through each cutter unit to check the bearings and replaced all we found bad.   As I mentioned earlier, I turned the end cutter and all the cutters turned easily and smoothly, which suggests to me there is no problem in the bar.  

 

I can't find any information that is not several years old  on the mower.  Does anybody have any newer information regarding these mowers?  I am wondering if this could be a design flaw because of where it breaks, just outside the gearbox.  This is the location that would take all the stress for the entire cutter bar.

 

Any ideas would be appreciated as these shafts are a couple hundred each.

Vermeer 7030