On Apr 12, 7:39*pm, Paul Drahn wrote:
On 4/11/2011 10:01 PM, 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

Hi, Roger.
Last week I bought an 11 year old Sears Craftsman 21 inch lawn mower.
Had,'t been used much and had a rear bag which I need.

The blade is mounted on an aluminum adapter on the engine shaft. Pretty
standard design. Th blade has a single center hole for a single bolt.
There are two slots on the blade adjacent to the hole. Again, pretty
standard.

The adapter is cast aluminum and is keyed to the motor shaft. The
distinctive part is the two "warts" on the adapter that fit tightly into
the blade slots. One wart in each slot. These appear to be designed to
shear off when the blade hits the immovable object.

I had not seen this design before. My other mowers all used two other
bolts in the blade slots and did not have any thing to save the engine
crank from sudden stops.

This mower is 6.5 hp rating, while the old ones are 3.5 hp. Perhaps that
is the difference.

Should the "warts" shear off, it looks to *be a simple matter to drill
the old remains out and press in new aluminum rods and the mower would
be back in business again.

Perhaps you would be able to use something from this design in your project.

Paul

My Craftsman 21" rear drive, rear bagger, with 6.75 advertised HP has
a steel or cast iron blade adapter the same as you describe but the
little warts are steel also. I suspect that the little warts as you
call them are there to encourage you to buy a genuine Craftsman blade
as opposed to brand X. A similar scam is with the drive belt. Rather
than a generic $4 v-belt, they call for a "special" belt for $20.
when you start looking around at the different manufacturers, they all
have a proprietary system for attaching the stupid blade.

I actually looked at the "warts" on my blade adapter and thought about
replacing them, but they are part of the problem where striking the
blade does more expensive damage than it would be if they would have
designed in a week point and allowed some slip/drag in the event of a
blade strike.

I ended up buying an after market blade adapter for less than Sears
wanted it took the guy 4 days to put the sucker in the mail, and when
I got it, I found that the sucker was not machined correctly as the ID
was tapered. Looks to me like the tool bit broke while they were
turning it. It took another 4 days for the guy to answer my e-mail
and then instead of sending a replacement he just gave me a refund.



It looks like things on the lawn mower DIY repair front are going to
get a lot worse. The Chinese have entered the small engine market and
a few of the low end mower manufacturers are using them. This is
probably going to mean a nearly zero parts availability.

Roger Shoaf