Many years ago, I purchased a lawnmower from a reputable national chain. I expected it to have a long life, yet shortly after the one-year warrantee ran out, I hit a large rock that was sticking up in the lawn. The mower began to vibrate severely. Obviously, something had gone terribly wrong.
I stopped the lawnmower and turned it upside down. I rotated the shaft by hand, and I could see that the shaft was bent. When I removed the blade I could also see there were no shear pins nor any other way for the blade to take the shock without transmitting it to the whole mower. Once it hit the rock, the shaft took all of the impact. I was now stuck with an almost-new mower that had a bent shaft. I was facing the option of either rebuilding the engine or buying another mower.
I live in a rural farming community where it is common to fix the oldest and most decrepit machinery -- it’s part of our lifestyle. I remembered having seen a house nearby that had a bunch of old lawnmowers sitting out for sale. The house also had a sign posted advertising lawnmower repair. So I took the mower to the gentleman who ran the mower shop and asked if he could repair my mower’s shaft. He took the mower and turned it over. He grabbed a pipe and put it on the bent shaft. He pushed the pipe in just the right direction, and he pushed with just the right amount of pressure. He was able to straighten the shaft. Just like that it was fixed.
The original problem was still there, however. There was no give if the blade hit a fixed and hard object -- which was bound to happen again. So I modified the shaft-to-blade adapter so that the blade would spin on the adapter if I hit another object. Voila, a solution. I used the lawnmower with great success for another 24 years.
This entry was submitted by Ray Mainer and edited by Jennifer Campbell.
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I straightened the cranshaft on 2 mowers that way. I put a dial indicator against the end of the shaft and keep bending it little by little until the indicator reads <.002". It doesn't take that long, and you quickly learn how much force and movement is required with that particular pipe. Just keep the same pipe around for next time.
Small engines have always had a flywheel key, and an aluminum one to boot to prevent damage to expensive parts. Key replacement has always been a common repair for blade impacts. In this case, however, the poster is talking about the blade adapter, not the flywheel. There are many different designs, some with shear protection and some without.
Lawnmowers use a lightweight flywheel because the blade also acts as a flywheel. heavier flywheels are used fo engines with low rotating masses on the output end, such as belt pulleys.
As a kid we had a small engine on a yard vac with not much spark plug hole thread left in the AL cylinder head. We'd screw in the spark plug and start the engine to launch the spark plug like a bullet. Take cover!
The mower I have today is 30 yrs. old with the too-thin tubular steel push-handle attachment to the deck being the weak point. After the first couple of years I had no choice but to weld smashed 3/4" iron pipe in parallel to the tubular steel handle and down to the deck's anchor points. No problems since.
Small mower crankshhafts can or cannot be formed using cast iron. Some may be nodular iron, and some, steel. I have been down this road with small engines for decades. One of the problems in a vertical shaft engine with a bent crank is removing the (aluminum) base plate from the shaft, even one with only a slight bend. Forcing the baseplate over the radius puts enormous pressure onto the bearing. In many small engines, these are not pressed-in roller bearings, but more commonly are simply pressed-in or cast-in aluminum bushing-types. They almost solely depend on the total immersion of lubricating oil for any expectation of life.
The BEST way to straighten a bent crank is to jig it in a lathe, determine the amount of runout, and then heat it sufficiently to anneal it and then "work" the bend out. Using pipes or other brute force methods is chancey at best.
Changing a flywheel for an iron replacement is also a dangerous endeavor, since it drastically changes the rotational inertia of the system. Replacing flywheel keys is far safer. I DO agree that it would have been a good design to include a breakaway key @ the blade end of a typical garden lawnmower. Years ago we had a large tractor-style mower. The mowing deck was approx 48" wide with three staggered, belt-driven blades off the main PTO. However, each blade also included a hub mechanism which included an easy-to-replace key in the event that a blade hit an immovable object. Even small tree stumps CAN bend a shaft, so it's NOT only large stones or rocks or bricks, etc.
Decades ago, small engine manufacturers DID carry replacement parts. Today, however, I doubt whether you can purchase any replacement items for a BRIGGS & STRATTON or HONDA small engine. IF you move up the ladder to engines included on industrial equipment, then the picture changes.
Does anyone remember CLINTON, POWER PRODUCTS, REO, KOHLER, WISCONSIN? They all made small engines. and are mostly all part of history today.
Straightening a bent crankshaft with a pipe may sound like a good idea, but keep in mind that the crankshaft is cast iron, not steel. CI is much less forgiving about being bent and bent back. Having a compromised crankshaft spinning a 21" steel blade at 3,600 rpm only 6" from my ankles is not particularly appealing to me. Especially on a mower with an aluminum or plastic mower deck.
I love it when the simplest fix can overcome a design flaw in a product. Still, you think there would be some kind of safeguard against this sort of thing! I don't have a lawn that needs mowing, so I don't use a lawnmower. Have modern mowers been adapted so they don't succumb to something as simple and common as a rock?
As a kid, I mowed a rock which bent the shaft and the blade on our mower. Attempting to replace the shaft was my first experience with 4 stroke engines. While I never did get the mower working correctly, I did learn about the inner workings of the engine which kickstarted my engineering desire to see how things work.
I agree: The pipe method is a great solution. I have to admit, my son hit a rock a few years ago and I could never get the crankshaft quite right (close, but not satisfactory). I eventually ended up tossing the mower and getting a new one.
when the blade hits a solid ground object. But most owners either don't know about the the breakaway key or do not have the mechanical ability/ tool to fix the mower, so the mower gets quickly thrown out when the flyway key is broken and the mower will not run or runs badly after hitting an object.
Nothing in the average owner's manual about this either... (Sarcasically) It must be much too dangerous to even ~inform~ the mower user about the flywheel shaft key even to write "This is what could have happened but take mower to service shop because the shop will have special tools to fix it."
"Buy a new mower!" is what the mower manufacturers want.
I, however, have benefited from this by getting the supposed dead mower for free, removing the flywheel, installing a new key, and, bingo!, an almost new mower with low hours on it!
[And customizing the mower by replacing the standard aluminum mower flywheel with a heavier cast steel one from a brush cutter mower (from the same type engine) provided a lot more root cutting fun prior to the flywheel key breaking away...]
I, too, live in a rural (although non-farming) community where fixing tools and machines is common--or used to be. Glad to say I don't have a lawn to bother with. But we do like to repair our stuff, or take it to the local fixit guy. There's something really satisfying about not throwing things away, and especially so about fixing something yourself.
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