Ray, that's a great solution. I suspect the lawnmower makers might be interested. On the downside, it makes their product a little more expensive, and also stops them from selling new mowers to people who own rock gardens.
On one occasion a wayward horseshoe found my lawnmower. I replaced the blade and the blade mount because the shear pins had done their job. I had trouble finding the right blade, though, because when I replaced the blade with the 22" proudly displayed on the mower deck, I could hear the slight "chirp" of the blade kissing the shield. No matter, I put in a 21" blade and I was on my way. As the years wore on, so did the mower, every so often a blade mount would crack, and at one point I had to replace the handle because it had sheared off from vibration. About 7 years later I was replacing another cracked mount, and as I rotated the blade around I found that the crankshaft was horribly bent.
Now it all made sense, the smaller blade, cracked mounts, and the handle breaking from vibration. I wished I had known about straightening the shaft with a pipe, rather than just taking the mower to the curb.
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