@bob from maine: We regularly cold straighten hardened steel crankshafts as part of the production process, using a dial indicator to measure runout, as @Turbineman described. The crankshaft is held between centers and is spun around to measure the runout. The crankshafts are checked for cracks using Magnaflux, but someone doing it at home could use a can of Spotcheck.
In my experience, if you are straightening less than a certain amount of runout (depending on the shaft, but generally about .050"), you will never crack it, and if you have to straighten more than a certain amount (about .100"), you are 100% guaranteed to crack it. In between those extremes is a grey area, where you might or might not crack the shaft.
I would definitely not recommend trying to straighten a badly bent shaft -- or, really, trying to straighten a shaft at all, unless you know what you're doing. I also agree with @Davinci87 that you don't want to do this to a cast iron shaft. Trying to bend a brittle material is just a bad idea.
One process that might work on cast iron shafts with small amounts of runout is peen straightening. In this process, you use an air hammer to induce residual stresses on one side of the shaft, which causes the shaft to "grow" in that direction.
@OLD_CURMUDGEON's idea of heating up the crankshaft to make it easier to straighten sounds like a disaster to me. This will completely wreck the heat treatment of the shaft. You might be able to bend it more easily, but now your bearing journals will be dead soft. Good luck with that!
All of that being said, here is my own personal solution to bent crankshafts on lawnmowers.
I was thinking the same thing, Bob. Since this realtively high-end machine didn't have any protection, one wonders where else costs may have been taken out. I would be a bit nervous about the shaft giving up the ghost and some other time...and not only the next time the blade meets a rock.
Fixing a bent crankshaft on a lawn mower which runs at 3600 RPM is a bad practice. I've seen what happens when tractor mounted mower blades make an unanticipated dash for freedom and the damage has been significant. Once a cast shaft has been bent and cold-straightened you have no idea whether a crack has developed or not. Magnaflux and penetrating dye require the motor be disassembled and once you are that far into it, it's easier and cheaper to just replace the motor. A replacement motor still supports the repair-not-replace philosophy, plus you don't need to spend months recovering.
The "Fix-it" culture is totally dead in this country. I've maintained that position for decades. And, the younger generations have NO interest in repairing anything. They've been effectively indoctrinated by very cunning advertising that it IS more efficient to buy a newer, "more improved" product than to repair the existing one.
Try to explain the logic of repairing an item INSTEAD of landfilling that for a replacement to someone of this era. It is a physical impossibility, BUT that same person will sport a "Green" bumper sticker on their vehicle...... Hypocracy to the nth degree!
I forgot about TECUMSEH small engines....... They were the parent company to CLINTON & POWER PRODUCTS brands...... The CLINTON logo was an old stylized Indian arrow head, and the TECUMSEH logo was a likeness of the Indian Chief, Tecumseh. I remember exchanging several CLINTON engines on equipment back in the 1950s because the original engines just wore out in short time. POWER PRODUCTS used to make a series of 2 stroke cycle engines.... found their way into the go-karting craze, along with MacCULLOCH & WEST BEND.
We put a WEST BEND 2 stroke cycle engine on a machine. It was a replacement for another failed engine. The crankshaft was so hard that it had to be sent to a special machine shop which could cut a keyway slot with special tools. The local machine shop could not accommodate our needs.
Exactly Ann! Unless we keep that excellent "Fixit" mentality and transmit it to comming generations, it will be completely lost soon. Trashing any slightly damaged article is a despicable practice promoted by lazy and dumb people that justify themselves saying "I have enough money to pay for a new one". Another thing to have in mind, is that many items that were fabricated years ago were better made in respect to robustness and better designed too, and deserve to be fixed. The recent trend of intentionally designing for a short life and "throw away" mentality is consuming too much resources already, and recycling is not always completely "green" or efficient as prolonging useful life.
FINE! You deserve the title of a "TRUE ENGINEER" because you used the proper tool to achieve that "magical touch" on the pipe straightener that probably took the old repairman an entire lifetime to develop. I have an inexpensive (cheap) chinese dial indicator I bought at Harbor Freight tools. It has paid itself many times because by using it for many tasks (like properly finding the best disc brake rotor orientation). It has saved me a lot of money. I even carry it with me when I go to a tool store, where it helps me find the one article with the straight shaft in ten or so (the other nine were not truly straight!).
I don't know about Wisconsin ( a very good brand, along with Clinton, RIP.), but Kohler is alive and well. Check out the engines in the online catalogue of Northern Tool and Equipment.
In my younger years I went to the Clinton Engine factory technician school. We students were "drug over the coals". On final exam day, we spent the first 3 hours on a written exam. After lunch we were handed two pieces of Clinton powered equipment. I was issued a chainsaw and outboard motor. They both ran fine, but something was wrong with them only the examiner was aware of. We had to tear them down, find out what was wrong, write down the part numbers needed, go to the factory parts crib to get the parts, re-assemble the equipment and take them outside and test them.
I never went to another mfr's factory school, but other students said that, compared to Clinton's school, all they had to do was skate through Briggs & Stratton and Techumsa schools.
I've been there, Davinci87. I tried stratightening a crankshaft with my own crude methods (not with a pipe). I used that mower in fear for a short time afterwards, then gave up on it. I had this nagging fear that if I kept using it, I would soon end up being about six inches shorter.
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