RS at work wrote:
I am thinking about making a blade adaptor for my lawn mower that has
a sacrificial shear pin so when I am out mowing down the tall weeds
and find that chunk of concrete or stump that someone tossed into the
field it will reduce the stress on the mower engine.

On my mower with a Tecumseh 195cc motor, I have had to replace the
flywheel as the shaft is steel and the spline key is steel but the
flywheel itself is cast aluminum. Although I found a new one on e-bay
for $30 if I had to get one from a dealer it would have run $60 or so,
and added to the cost of a new blade ($10-20) and a new blade adaptor
this gets really pricey.

My design is about the same as the factory set up except the torque
will be transmitted through the shear pin(s).

My question is how to size the pin or pins? I want them to be the
weakest link, but to hold up when mowing down the big nasty weeds.

My gut feeling says that two 3/16 brass pins ought to do the trick,
but I figured that some one here might have some experience with this
kind of calculation.

Roger Shoaf

There is a mower in the UK called a Hayterette which has a large disc
underneath and 4 short blades, about 4" cutting edge", attached to the
periphery and they can swing out of the way in the event of inpact with
a solid object like a rock. I've had one they work well.
http://www.google.co.uk/url?q=http://www.gardening-tools-direct.co.uk/garden_tools/hayter005010%281%29.pdf&sa=U&ei=lsqkTZ_qL8ix8QPMwM 25Dw&ved=0CB0QFjAF&usg=AFQjCNEjh4XA4dUPL7sSOfCBLvC LmWQqUg

One thing about your idea that springs to mind is you need to make sure
the blade assembly stays attached or constrained when the shear pin
fails so that the cutter blade doesn't wander away from the mower near
your feet.