> My first thought for an engineering solution would be a shear pin. Find a
> way to put something soft, cheap, and easily repairable between the motor
> and the blades. (think of lead for the key on the blade shaft) That way it
> limits the damage to the rest of the parts. Or maybe you could make the
> blade attachment squeeze between a couple of washers, sort of a torque
> limited clutch.

Not on rock sensor, but on the blade.  I am not sure is this practice 
widely in the state.

Here, there used to broken blades flying out and cut passer by.  Now the 
grass cutter uses a cable-tie type of "string", that replaces the blade. 
  One end with knot is inserted through a hole in the rotor.  The knot 
keep the "string" in place when cutting, countering the centrifugal 
force.  If it does break off, due to the small mass it cannot fly far. 
Needless to say, it is cheap.

Ling SM
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