More flywheel shaft key problems
I only made it 2 weeks before
I hit another fatal stump with the mower which took out the new
flywheel shaft key that
was installed recently.
This time I used a different
guy who was a lot closer and 3 bucks cheaper. I was all ready with a
long screw driver/pry bar and mini-sledge to try to do the operation
myself, but once I started taking it all apart I realized the first guy
tightened down some of the nuts too tight. I remember him using an air
wrench, and not seeing a torque wrench being applied. Most experts
think they can gauge it by feel, but this nut was on so tight I broke
2 sockets trying to get it off.
The moral of this story is to
make sure your nuts don't get over tightened.
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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.
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Seems like a pretty poor design to break off so easily, especially since it's apparently quite hard to replace. While it is not uncommon to design a drivetrain with a part that fails when overloaded to protect the rest of the machinery from damage, it normally is a part which is easy to replace...
The thing with using torque wrenches for tightening bolts is that they are imprecise as a measure of bolt stress (or tension) which is what one wants to set. Friction between the bolt and the thread it is screwed in can vary due to rust or lubrication, and it comsumes between 85 and 95% of the torque. If possible it is preferable to measure bolt extension (e.g. with a caliper on an exposed bolt, or ultrasonically).
If it is not possible to measure bolt elongation directly one can use angle control, since after the pieces bolted together touch,one rotation of the bolt extends it by one pitch. Divide that one pitch by the length of the piece of the bolt that is not screwed in and you have the extension (ε). Multiply that by the Young's modulus of the bolt material (E) and you have the average bolt stem stress (σ = E·ε). Multiply that by the cross-section area (A) of the bolt, and you have the force pulling on the bolt. (F = σ·A = E·ε·A)
We've been thinking that it's a design flaw too (although, to be fair, we're not using the mower for its recommended use. We use it more like a bushhog than a lawn mower.) It seems like the flywheel key should be somewhere easy to get to!
I like your points about tension too. Your explanation made a lot of sense -- we never thought about that!
Mark says: "I like it when people talk over my head like that. I learn a lot. He should really get a free t-shirt for that."
Too bad our t-shirts haven't been printed yet.