Ian Beadle wrote:
Hi Gang,
The picture is of my water pump pulley after it failed at 10,000ft.
Kinda
spoilt my day! I had
been in the air for 3 hours when the gyro tilted and the alternator
light
came on. When the
coolant temperature started skywards I knew I had a problem.
Fortunately I
was close to a country
town with an airstrip complete with windsock.
I have no idea why it failed. The only thing I can think apart from
it
being damaged in transit
on the engine from Japan is that there are 8 holes in the flange and
pulley
and I only have 4
bolts. Any ideas?
Ian RV6A Australia
Ok, went out to airport and checked my water pump. It has four bolts
retaining the pulley, however, there are holes for 8 bolts. I am pretty
certain it came with 4 bolts and the pump is an 86 model. I do have two
observations which may (or may not) have bearing on the failure mode. My
belts run on both sides of the pully. One wraps around the main pulley,
around the water pump pulley and then down to the airpump pulley. The other
one wraps around the main pulley, around the opposite side of the water pump
pulley and then around the alternator. Don't know if this presents more of
a balanced load on the pulley or not, but would think so.
Also, my main pulley is an underdrive pulley by 20% and I rarely run the
engine higher than 6000 rpm (cruise around 5300-5700), so those factors MAY
reduce the stress on the stock water pump pulley. Did not do a dry crack
check, will do that later.
Ed Anderson
Ed you may have higher side load. The side load is the vector sum
of the two side loads. The side load is also not just the
belt tension but is also a function of the torque being
absorbed by whatever device is being driven. Water, alternator
or air pump.
Here is how to calculate it. This gif is of a belt drive PSRU
of course. Estimate the HP needed of the device.
In the case of the water pump it could be up to 3 HP
at 6000 RPM or so. HP = torque X RPM / 5252 as I recall.
so torque = HP X 5252 /RPM = 2.6 ft pounds at 6000 RPM.
So side load in the case of the water pump pulley is
on the order of 24 X 2.6 Ft/lbs / 3 inches or about 20 pounds
if my calculations are correct. A bunch. Not counting preload
tension.
All you engineers out there put your thinking caps on and check me on this.
Things start to fatigue at around one million cycles as I recall so
5000 RPM into 1,000,000 is 200 minutes or 3.33 hours. So what
you are looking at is a pulley designed so the stress level is
below 15,000 psi for mild steel and below 65,000 psi for
HT treated 4130. Some say there is no such level for aluminum
and it may crack sooner or later.
Dan came up with a FEA peak stress level of about
100,000 psi with only four bolts. However Dan used 100 pounds side load.
So the peak stress may not be this high unless the belt
had a lot of pretension on it.
None of this is very a very precise
science but it does indicate we might have a problem.
Paul Lamar
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