On Mon, 11 Apr 2011 22:01:32 -0700 (PDT), 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's good equations for shear force in a static situation. 35 years
ago, I could open my mec. of materials reference and quicly do this.
That would get you a bottom end number if you have a force you want to
hold. But you may not even have that.

I've done this in practice and there is a bazillion variables that you
don't have the answer. So, just try what looks a little small. Then
move up one step at a time. The most important part of your design is
easy change of shear pins. For example my post hole digger used to
break the shear pin on the auger and leave it come apart with the
auger stuck in the ground. So, I keyed that end and then made a keeper
on the PTO input so the shaft wouldn't drop when the input shear pin
breaks. Experimentaion found a normal hole could be drilled using a
5/16 butter bolt (grade 2) shear pin. And a rock broke it. The pin can
be quickly replaced so its no big deal and you aren't tempted to
oversize the pin.

Karl