On Apr 12, 4:52*am, Dave__67 wrote:
On Apr 12, 1:01*am, 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

Some thoughts from someone who has never had to engineer a pin, but is
pretty good at breaking them:

A bit more work, but slightly oversize with grooves where it should
shear I think leads to better 'quick release' action, and helps keep
the pin from smearing into the gap between the parts (making
disassembly difficult, and perhaps even making the parts 're-grab').

Brass might be too malleable (and prone to smearing upon breakage),
depends I'd think on if the part is loose except for the pins or if
there is no movement between the parts until the pin breaks.

Dave



My design is a steel disc welded to a tube that has a keyway matching
the output shaft on the mower's engine. A hole is in the bottom of
the disc. A second disc has a slot milled across it and a hole in
the center. The depth of the slot is slightly less than the thickness
of the blade. When the bolt is tightened, the blade will be squeezed
in place. The pins are located between the two disks. (If you were
looking at the disk with the slot oriented at 3 and 9 O'Clock, the pin
holes would be at 12 and 6 O'Clock.

I suspect that in normal use, friction would be transmitting the
torque from the upper disc to the lower disc and then to the blade.
In the event of a blade strike, first the friction between the two
discs and the blade would be overcome, but there would be some drag,
then the pins would shear. Since there is a small gap between the two
discs, (Kind of like using a cheap pair of terminal crimpers to cut a
screw.) the pins would smear and the combined effect of the slip/drag
of the two discs and the smear of the deforming brass would soften the
blow enough to stall the motor but still cushioning the sudden stop.

To remove the pins, they would be driven out with a punch. (The holes
would have a shoulder in them.)

Roger Shoaf