>My first thought for an engineering solution would be a shear pin.

This is usually the way to do it.

>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.

The Flymo electric mower I have use the plastic spacers that adjust the
height of the blade as the shear pin item. Each spacer has a pair of "pins"
about a 1/4" diameter on each side, and a matching pair of holes for the
"pins" to sit in when two are mated up.

>Or maybe you could make the blade attachment squeeze between a
>couple of washers, sort of a torque limited clutch.

This is exactly what happens with the plastic spacers mentioned above when
the pins shear. The rough surface where they shear from has enough friction
against the other plastic face to allow you to keep mowing. Replacement
spacers are readily available from the local DIY shop.

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