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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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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 |
#2
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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
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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
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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
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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
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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
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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
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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
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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
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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 |
#11
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shear pin for lawn mower
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 |
#12
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shear pin for lawn mower
On Apr 13, 8:15*am, "Stormin Mormon"
wrote: 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? -- I agree, I an thinking about making my own from aluminum. I have no idea why this model uses the steel key, but I think it is a poor design. If the flywheel has to be aluminum for the OSHA nazis, then at least press the thing on to a steel hub so the key will shear rather than eating the flywheel. The more common flywheel has a wider slot and the key is some sort of die cast material. I went and tried one of these and my mower wouldn't start. I was unaware that mowers with light flywheels rely on the blade being attached, so when I tried my parts swap, I was without blade so I assumed that the timing was set different on the two models of flywheel. Now I am not so sure, but I think I am going to test it when I have both sitting in the driveway. I am also informed (but I am not sure of the reliability of the informant.) that mowers were detuned to meet emission requirements by retarding the spark. If I have the ambition I might play a bit with the timing to see if I can improve the situation. This of course is going to mean making my own timing marks and some how making an adjustable timing set up. Perhaps I could cut down the two bosses that the ignition module sits on and make a little plate so that the module could be adjusted like a distributor on a car. It might be that I will end up with a set up like the Model T where you pulled the spark lever back to start the engine, and then advanced it to run. Another way would be to do it like they do now on the newer cars, a halls effect sensor could pick up the crank position, calculate the rpm and the optimal time to fire the plug. (I probably sound like a guy with too much time on his hands. Mia culpa!) Yet another way would be to mount the flywheel without the key, relying only on the taper to keep it in position. Of all these schemes, I think the first may be the best even if the run timing is not changed if you spring loaded the plate a cable could then be run to the handle to retard the spark even further to make starting that much easier. Roger Shoaf |
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