The Walden Effect: Farming, simple living, permaculture, and invention.

Flywheel shaft key mistakes

replacing the fly wheel shaft key on a lawn mower

I had to replace the flywheel shaft key on the lawn mower today.

It's my second flywheel shaft key, and it went faster this time. Less than 30 minutes.

The things I'd like to remember for next time would be try not to forget the bell shaped spacer that goes on top of the flywheel and the oil spout goes first before putting the gas tank back on.

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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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Mark, I'm surprised that you have flywheel keys shear. The frictional force of the two tapers should be sufficient, with the key only in place to control spark timing. Typically, the only reason a key shears is if the nut gets loose and the blade comes to a sudden stop, resulting in a high inertia impulse. Are you able to get sufficient torque on the nut during reassembly? Does it have a lock washer? Sometimes an impact driver (manual or electric) can help tighten it. Good luck. David
Comment by David From Alabama Mon Jul 28 18:31:43 2014

For the tapered surface to give enough friction, the nut has to be very tight so the bolt is stretched out to give enough compression on the taper. The most accurate way to do this is to measure bolt elongation, but that is usually difficult. Another way is to specify how much the nut should be rotated from hand-tight.

The most common method is to use the torque applied to the nut as a proxy for the extension of the bolt. So I would suspect there to me a tightening torque mentioned in the service manual.

BTW: Do not lubricate either the taper or the threads! It will screw up the necessary friction that prevents stuff from coming loose. And it could lead to overtightening the nut.

To prevent the nut from coming loose, apply some thread locking fluid.

Comment by Roland_Smith Tue Jul 29 06:18:40 2014
David and Roland --- Great tips! I suspect the problem isn't lack of tightening, though, since the flysheel shaft keys only break when I accidentally hit a couple of stumps in a row.... (Oops.) Our "lawn" is getting flatter, but it's not quite there yet.
Comment by anna Tue Jul 29 07:10:59 2014

Anna, the fact that the key shears of is exactly what indicates that there is a problem.

Normally when the flywheel is pressed down on the tapered axle it should generate enough friction to keep the flywheel in place, even if you jam the mower. The flywheel should be designed for that. It is also made to withstand the repeated torque pulses it gets from the exploding fuel/air mixture.

However, the taper on the axle and in the flywheel should have solid contact without play. If you put the flywheel on the taper without the key and you can tilt the flywheel with your hands, it doesn't sit correctly and there is probably not enough surface to generate enough friction. Maybe the axle is bent, or the taper is worn.

A slightly worn or damaged taper might be corrected by putting some abrasive paste between the shaft and the flywheel. Rotate the flywheel while keeping the shaft stationary until the play is gone. That wil grind down the surfaces to fit again. Clean both surfaces thoroughly before assembling them again. This might affect the interchangability of the parts; you might have to re-grind the taper if you ever want to replace the flywheel.

Comment by Roland_Smith Tue Jul 29 10:46:15 2014

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