The chatter marks are certainly apparent, Chuck.
Whoa! 150 hours is certainly not much out of a brand new
housing. It could be as you suggest that the two piece seal played a
role. However, I have another suggestion. Measure the apex seal
slot at the top (across the gap) and see what the distance is. Do it both on
the good rotor and the damaged one if you can find an
undamaged spot. If like my rotor, the slots are worn, I
believe they may contribute to the chatter and breaking of the apex
seal.
Here's the theory. As the slot "V"s there
is less support for the seal near its top, the bottom is still fairly well
constrained. As a consequence, the top portion of the seal is more
able to be swayed back and forth by combustion and friction
forces. In other words, instead of just rubbing on the rotor
housing wall with the smooth top of the seal, the edge of the seal starts to
drag along the wall. The seal (with less support at the top)
acts like a stiff spring (up to a limit) and "vibrates" as it slides along
the housing wall leaving the chatter marks. At some point the forces
become too much and the seal breaks.
It might be interesting to determine the vibration
frequency of a seal ( given its dimensions and composition) and then measure
the average distance between the peaks (or valleys) of the chatter.
Then given the resonant frequency of the seal determine at what rotor
rpm it would leave chatter marks of the measured distance. {:>).
No, I don't have a clue about how to do this, but I'll bet some mechanical
engineer could come up with something.
Just a theory, of course, but if your apex slot
tops are within the specifications called for by Mazda then that would
discredit the theory. If they are worn then it might tend to give it
support.
When my seal broke it also caught the next seal down
in is slot and smeared the top of the slot such that it could not come back
up.
You and I are apparently the only ones (that I know of
) that seem to have had this failure mode of an apex seal disintegrating for
no apparent reason.
Thanks for sharing the photos and
information.
Are you going to try for Sun & Fun?
Ed
Ed
----- Original Message -----
Sent: Wednesday, March 15, 2006 10:44
PM
Subject: [FlyRotary] Motor teardown
results
I finally got
time to get my failed 13b apart. The rear rotor was missing one apex seal,
the other 2 were intact but jammed due to the fragments of the broken one.
The rotor housing has the classic chatter marks all around. Doing a Google
of "13B apex seal chatter" results in plenty of examples of this failure
mode. Apparently the chatter is from the natural resonant frequency of the
seal, it eventually fatigues the seal causing failure.
The front
rotor was in good shape, but the housing had the same chatter marks, but
not as severe as the rear rotor. The front rotor housing was brand new 150
hours ago when I rebuilt this motor after the Grand Canyon forced landing.
At that time the good rotor housing had slight chatter marks but I reused
it. The front rotor was destroyed at that time due to foreign object
entering the rotor.
Both rotors
were missing the corner seal rubber plugs on the side with the short
apex seal segment. Perhaps the small segment vibrated more, disintegrating
the rubber seals?
The first 260
hours of flying my rotary bird was with stock 3-piece seals, and I never
saw any chatter marks. The last 250 hours has been flown with 2 piece
seals and now I have chatter marks .
I now have
Tracy's 2 piece apex seals and I wonder how they will perform, it appears
that the heavier weight of the 2 piece seals puts the resonant frequency
in the cruising range of 6000RPM?
Chuck
Dunlap
RV6
13B
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