Ty, I'm told that people manage to bend crankshafts by hitting things, whether they have a bar blade or a blade disk. It seems that some of them don't give up easily after they hit the rock, they try to machine it away to a flat surface, with the mower.
With 2-stroke Victa's, and a few others, there is the added protection that the blade disk is held on by pressure, rather than locking, however the downside to this is a slight increase in difficulty getting the setup right, and as a result a few home improvers can end up with a sore arm from kickback.
On the other end, rust, rope, grass, muck and age will do quite well to seize the blade disk bolt, and the blades to the extent where they do not provide adequate protection, when the mower is not maintained well over it's life.
But yes, even swingback systems working perfectly are not fool proof, there are always those out there who think there mower can grind rocks.
