I must just be missing something here, but wouldn't the best thing be to not
drive over the rock in the first place? 

How about a guard on the front of the unit that stops the blade if it runs
into something solid? It could be one unit or multiple smaller units so that
if any one were pushed up your existing blade clutch gets tripped.

It can be pretty heavy and spring loaded so that nothing short of a rock is
going to move it. If it starts to push things along in front like a dozer
blade, then mount it at an angle so the crap gets pushed off to the side, or
make it a v like a train "cow catcher" to deflect junk to either side. Even
if the rock gets moved out of the ground by the guard, it will then be
setting on the mowed grass and be quite obvious to see and remove.

After it trips, the blade stays cut out for some number of seconds then
automatically re-engages. The result is a number of little unmowed spots
which indicate where the rocks are. You go along after and remove the rocks
and then mow over those spots.

---
James.
 


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