Mine's done exactly the same thing. It's caused by the gearbox bearings and PTO shaft getting worn, developing play, and vibrating back-and-forth. That shakes the vertical quill to the centre blade, which then fatigues, and breaks.
Just replacing the gearbox bearings and seals will correct SOME of the out-of-balance play and vibration; but not all of it. The gearbox input shaft is very poorly-designed: supported by two 6205 [25x52x15] ball bearings, that are mounted virtually back-to-back. There is only 9/16" separation between them; so their resistance to sideways forces is minimal.
And those sideways forces can be considerable, since the PTO shaft typically operated at an angle about 15 degrees, in the case of my mower. So the bearings wear out quickly, and the resultant play wrecks the seals. Then the oil runs out; and the gearbox seizes. LOVELY . . . .
UPGRADES
1: Make sure the bearings you fit are close-tolerance: at least ABEC-3; preferably ABEC-5. And close fit as well: C2 preferably, or C0, CM or CN, rather than the usual C3 [loose fit]. That will help - for a time.
2: Replace the ball bearings with cylindrical roller bearings: flanged NJ205 bearings are 25x52x15, so will fit right in. Limited resistance to axial thrust; but much higher sideways resistance.
3: Do away with the oil seal, and replace it with ANOTHER bearing, with its own oil seals. That's a 6205 2RS. That means leaving out the internal circlip, drilling and tapping 5/16" holes around the end face of the gearbox housing casting [I drill 6; you could go to 8]. Making up a clamp ring-plate to bolt round the shaft and onto the casting end. Using that to clamp and hold in the outer of the TWO bearings. If you want, you could then make the inner of the two bearings an NJ205 roller, for more strength - since the two ball bearings will take care of end thrust.
4: Carry out the same gearbox housing modification as [3] above. But, in place of an extra bearing replace BOTH ball bearings with 30205 Taper Roller bearings. They are 25x52x16.25 so, to get the gear wheel properly positioned, you have to machine 1.25mm off the inner and outer shells of the inside bearing. If you take a wider 25x52x22 taper roller for the outer bearing, and add a 10mm oilseal, that will protrude from the housing, and you use the ring clamp frame to apply the necessary preload - through the oilseal - for the taper rollers.
Four solutions, in rough order of effort needed, and resulting strength.
But the bearings may not be the whole story. There can also be play BETWEEN the bearing outer races and the gearbox casting. Mine certainly did; I had to use one thou brass shim, plus Bearing Mount, to get the bearings properly located. Not sure whether the bearing races had been fretting material way from the cast iron housing, or whether the housing was machined 'loose' from the factory. But loose they definitely were.
The original source of the vibration is the PTO shaft, which gets worn and starts flopping around. A new one from Kubota will run you $600-plus. But you can make one up using parts from Weasler or Neapco, and with standard Domestic Series 6 UJs [in place of the special - and weaker - Kubota ones] for about $200-250.
Finally, the deck belt also causes vibration, if it is unevenly worn. A new belt will be much more even, and run smoother.
But God knows why Kubota made such a crappy design in the first place . . . .
