My point is that just simply removing the blade key will prevent damage to the mower when a rock or something stops the blade. And the fix of retightening the bolt is not that much work. Replacing the flywheel key is a lot of bolts to remove and replace.
I have had that thermal fuse fail simply because I ran a second batch of coffee too soon after running a batch. The thermal fuse got a wire jumper and the thing worked another year before the heater finally failed. Yes, I understand the potential hazard of removing the fuse, but we never run the thing unattended anyway, so it was not a big deal.
The key which connects the shaft to the flywheel on the top side of the mower engine is made out of a soft metal. Not aluminum but some type of alloy.
The flywheel, at least on Briggs and Stratton and out-of-production Tecumheh engines, is used for the engine ignition, timing, and balance. When the blade hits something, the key shears off, the flywheel keeps going and spins on the shaft. This takes the engine out of timing which stops the engine. If the flywheel hasn't moved too much, the engine maybe able to re-started to run badly but most of the time the engine does not re-start or run at all.
This is when most people give up now and scrap the lawnmower. The flywheel keys are able to be purchased at stores which actually handle small engine repair parts, usually not at big box stores which only have replacement maintenance parts.
This is where I came in to get the free mower. In one mower I got, the flywheel had sheared the pin but not moved enough to totally shut the engine down. But this mower had blade mount which the threads and locator pins had been damaged because the bolt had loosened up- probably a forgotten lock washer. The mower felt and sounded like garbage when it ran due to the off timing, loose and knocking blade, and vibration from the loose blade/off timing. I installed a new shaft key, got a brand new blade, and replaced the blade mount and with one from a another dead mower (which had sat outside for years getting water into the engine). The mower then ran like a charm! I used the stink out of it for about three years later giving it to a relative who is still using on a small lawn...
I agree in principle with you. However, products are being designed to be non-servicable. Lawn mowers (for the most part) are still servicable items, but how many other devices are simply made to be thrown away. For example, I have a Bunn coffee maker that quit. I disassembled it to find out what was wrong to discover a set of thermal fuses. I tried to order these from Bunn, but no-way. I eventually (with great effort) found a suitable replacement.
The point is, I spent many hours on a $1 part and a $40 coffee maker. Ten minutes in Wal-mart would have replaced it. Young people are not as motivated or dedicated as I am to fix it (and economically I could not blame them as I am just stubbornly cheap).
How many Ipad/Android teardowns has DN shared? Were these ever put back together?
The problem is always that when the blade gets stopped instantly that the rest of the rotating mas twists until something is damaged. That problem is very obvious indeed. Many years ago, while I was still in high school and Kenedy was president, I came up with a simple, cheap, and very effective solution, which has saved me from lawnmower damage many times. Here is the simple solution: whenever I get a rotary mower with a gas engine drive, I remove the blade and the blade attachment adapter that is ke6yed to the crankshaft. Then I remove the key and put a dab of anti-sieze grease on the shaft, and reassemble it without the key in place. I tighten the blot and the job is done. When I eventually hit something that stops the blade the coupling slips a bit and often the engine stalls. ON quite a few of those occasios I have had to tighten the bolt. BUT the engine has never been damaged enough to notice. Using an aluminum key might also work, but using no key at all has proven itself to work quite well.
Of course if the blade ever comes loose during normal operation I would need to stop the engine immediately and tighten the blade, and that has indeed happened twice in 45 years. But that is a very small cost compared to having to replace an engine, since usually when the crank is damaged the lower bearing plate is damaged as well.
Well, in shops that I have had familiarity with, that was the procedure they used very successfully. But, I didn't claim that it was a carte blanche solution, and certainly NOT for cast iron shafts..... nodular iron, yes! And, steel, of course. Furthermore, most small engine cranks are NOT seriously heat treated, so I disagree with you on that point. But, that also depends on the ultimate quality of the engine...... small lawn mower & outdoor power equipment engines are mostly considered "throw-aways" nowadays..... I have taken a file to the counterweights of these genre of engines, and the file penetrates very easily, so that adds more credence to their non-hardened state. They VERY well could be "spot hardened" at the points of bearings however.
A bent shaft on a mower usually means one end of the shaft stopped while the other end didn't. The 'fly-wheels' masses are at opposite ends of the crankshaft, when the blade stops, the engine fly-wheel continues to try to rotate causing a twist in the crankshaft. Fifty thousandths of run-out may represent only a few degrees of twist, or the crankshaft may be 75% fractured. The risk is significant.
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