|
Message
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
--
Homepage: http://www.flyrotary.com/
Archive and
UnSub:
http://mail.lancaironline.net/lists/flyrotary/
|