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
Reply to
RS at work
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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.
Reply to
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.
Reply to
Karl Townsend
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
Reply to
RS at work
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
Reply to
RS at work
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 .
Reply to
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...
Reply to
Leon Fisk
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.
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.
Reply to
David Billington
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.
Reply to
Paul Drahn
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
Reply to
RS at work

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