On Sat, 29 Nov 2014 11:35:16 -0500, Bob Engelhardt
wrote:

On 11/18/2014 6:20 PM, wrote:
On Tue, 18 Nov 2014 15:24:07 -0600, Ignoramus29079
buy a new honda

A total waste of money if the design of the unit is bad and that
caused the damage to the original engine. The honda would likely fail
a few hours later than the original in that case.

What caused the break? If the impeller isn't ballanced it will break
the crank again.


Good point about the balancing. I did a static balance (fan on arbor on
2 level rails) & it was off by about 10 grams (on 20" diameter).

Then I got to thinking: an unbalanced fan would not cause a vibrating
load on the shaft. On the shaft bearing, yes, but a constant radial
load on the shaft itself; in the direction of the heavy spot on the fan.
A pulley & belt presents an oscillating radial load on a shaft and
that's a common use. So I don't think the design is a problem. Just a
POS crankshaft.

I hadn't thought about that before & it's interestingly the opposite of
casual thought.

Bob
10 gramms unballance on a 10 inch radius can cause a significant
oscillating force that WILL break a crankshaft. PARTICULATLY if it is
dynamically unballanced as well as statically. (not only acting
radially to the shaft, but having a component working back and forth
at the "end of the lever" as well. Static inballance is only a very
small part of the problem on something like this. You get a resonance
that, at the right speed, will re-enforce itself, amplifying the force
and acting tangentially to the crank centerline. Torsional vibration
is a different animal, and being belt driven it is self limitting.
Belt slip and belt stretch work to limit the amplitude of the torque
excursions. You will burn off the belt before you will torsionally
fail the crank.