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shear pin for lawn mower



 
 
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  #1  
Old April 12th 11, 07:01 AM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 105
Default shear pin for lawn mower

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
Ads
  #2  
Old April 12th 11, 01:52 PM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 259
Default shear pin for lawn mower

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
  #3  
Old April 12th 11, 02:42 PM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 3,253
Default shear pin for lawn mower

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

  #4  
Old April 12th 11, 05:06 PM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 105
Default shear pin for lawn mower

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
  #5  
Old April 12th 11, 05:26 PM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 105
Default shear pin for lawn mower

On Apr 12, 5:42*am, Karl Townsend
wrote:
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


My worry on using "butter" bolts would be if some one replaced it with
a good grade bolt. The other day I tried to use a screw cutter on a #
10 Allen head screw and was unable to cut it, but the cheap screw cut
right off. What you might do is to rig something to keep some spares
right on the equipment. This would save a trip back to the shop and
discourage the use of a stronger bolt.

Roger Shoaf
  #6  
Old April 12th 11, 10:11 PM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 566
Default shear pin for lawn mower


"RS at work" wrote in message
...
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.


My ( wood's rm59 ) mower uses fiber washer and belleville springs under the
blades

IIRC, the stackup goes something like this :

hub--fiber washer--blade--steel washer--fiber washer--steel washer--2
bellevilles--lh bolt or threaded nut

Nice thing about this setup is that owing to the belleville springs the
blade can actually tilt off-axis some in the event of a collision .



  #7  
Old April 12th 11, 10:20 PM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 985
Default shear pin for lawn mower

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.

snip

Roger Shoaf


The traditional approach is to mount the blade via a spindle and then
drive it with a belt, pulley setup. The belt will buffer most of the
shock, grief of sudden stops...

You can buy replacement spindles for riding mowers and such. Would take
some thought though to offset the motor and install the spindle.

Personally I just watch for nasty stuff like cement blocks and make
sure I don't hit them I've had excellent success cutting off small
trees/saplings up to around two inch diameter by sneaking up on them
using the discharge chute. Toyed with the idea of cutting a small notch
or slot in the deck for same, but I'm lazy...

--
Leon Fisk
Grand Rapids MI/Zone 5b
Remove no.spam for email

  #8  
Old April 13th 11, 12:04 AM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 857
Default shear pin for lawn mower

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.
  #9  
Old April 13th 11, 04:39 AM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 485
Default shear pin for lawn mower

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
  #10  
Old April 13th 11, 05:15 PM posted to rec.crafts.metalworking
external usenet poster
 
Posts: 10,563
Default shear pin for lawn mower

Usually, the Tecumseh allignment key is soft metal, and is
sacrificial. You hit a rock, the flywheel is now out of time
with the cylinder stroke. On the points ignitions, you lose
spark. On the electronics, you still have spark, but it's
way out of time.

Maybe you need a softer flywheel key?

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"RS at work" wrote in message
...
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


 




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