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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