Why is it important when dealing with flex plates?

 http://en.wikipedia.org/wiki/Moment_of_inertia

 "In classical mechanics, moment of inertia, also called mass moment
 of inertia, rotational inertia, polar moment of inertia of mass, or
 the angular mass, (SI units kg·m²) is a measure of an object's
 resistance to changes to its rotation. It is the inertia of a
 rotating body with respect to its rotation."

 Also read gyroscopic forces.
 http://www.gyroscopes.org/behaviour.asp

 "When the gyroscope is spinning it can contain large amounts of
 stored energy. Newton's first law of motion states that any body
 will continue in its state of motion (still or traveling) until
 outside forces change it. Because of the forces when the gyroscope
 is spinning, if it is moved the gyroscope will try to 'compensate'
 for this movement. If we take the example of a gyroscope's axle
 being clamped to the structure of the car (often the cars engine
 behaves as a gyroscope because of its shape, mass and rotations)."

 "If the gyroscope/engine is spinning at a fairly high speed in a clockwise
 direction, as seen from the back of the car and as shown in the diagram
 above. When the car is turned to the right (clockwise from above)
 forces A and B are applied to the structure of the car forcing the
 front end of the car down and the back end up. If the car is turned
 to the left then the front end of the car is forced up and the back
 end forced down. If the gyroscope is spinning in the opposite
 direction then the reverse will happen."

 "However if the car was moved directly upwards, downwards, forwards,
 back-wards or side-to-side the gyroscope would not apply any extra forces.
 The gyroscope only applies extra forces when the car is moved at an
angle

 (turning left/right or when the front/back of the car is moved
 up/down). This effect is known as gyroscopic precession."

 These forces go through the flex plate out to the ring gear
stressing the flex plate back  and fourth near it's hub thousands of
times per minute. In other words the ring gear attempts to continue
to rotate in it's original plain while the hub is being forced to
change direction.

 Paul Lamar

 I agree that there is gyroscopic forces that affect the rotation of
the flex plate. Also included with that is the toque converter which
has a much greater mass. Any movement that would take place is held
in check by the index hub on the converter  that goes into the back
of the e-shaft or crankshaft and the bearing in the transmission
pump. I am not sure the clearance of the bearing, but 0.002" seems
reasonable. That would imply 0.001 movement which would be minimal
flex in the flex plate. I am inclined to think the failures are due
to the torque pulses applied to the flex plate under high continuous
power loading. I believe you are right with the idea to make a flat
disc of water jet cut stainless and tack weld the ring gear on.
Lightening holes around the outer diameter will not fail in my
opinion. But then what do I know as a burnt out mechanic :-))).

Dale Davies
-

Dale,

I am lost with what you are trying to imply with the clearances. But
my conclusion on the RD-!B failure comes from this lecture on line.
As it is stated there, it is "VERY NOT-INTUITIVE". The part about
gyroscopic forces starts about 14:20 into the video.
http://www.youtube.com/watch?v=zLy0IQT8ssk

I must say I tend to agree with Paul that Torsional Vibration is not
the problem here, and Mazda Rotary does not seem to have a Torsional
Vibration Problem as is typical in piston based engine. So the RD-1B
failure should be mainly due to metal fatigue of the flex plate.

One solution is to use a very stiff flywheel from a manual
transmission instead of a flex plate/ring gear combination. If the
flex plate is eliminated, the precession force is transferred to the
shaft, onto the bearings, and eventually to the aircraft through the
engine mount, but the solution is heavier than a flex plate/ring gear
combination.

I think that for normal flying, a flex plate/ring gear combination is
sufficient. But for people with acrobatic in mind, a flywheel is a
better choice.

Henry Nee


My point is that the flex plate by itself without being coupled to the
transmission could flex from the gyroscopic forces. The torque converter and
the bearing in the transmission prevent any movement from taking place.
Without movement, there will be no fatigue from the gyroscopic forces. I was
asking about this earlier and Paul educated me that gyroscopic forces at Reno
will be minimal. Aerobatic will impart more, but that is controlled and
dissipated out through the bearing in the PSRU, mount and to the airframe.


I now believe the fatigue is starting from the twice per rev loading and
unloading with regard to the compression and subsequent combustion. This is
applied at the center of the flex plate and resisted close to the outside
diameter. The drive plate with isolator rubbers are bolted closer to the OD,
where the resistance to the rotation is applied. The holes near the e-shaft
mount bolts are a weak link due to the reduced amount of metal to transmit the
power pulses through. You do not need movement and flexing to generate fatigue
in metal. Look at conn rods that break. If they have not been subjectedto
stress from over revving the engine, the break started from a surface
imperfection that allowed loading and unloading of the rod to crack from
fatigue. When you look at a part that has been fatigued, the break looks like
it crystallized. When a part is torn apart from some other failure suchas
over rev, the separation point looks different.


Mark S has experienced a failed flex plate without the aerobatics, so that
bit showed me it is not a direct cause of the manner the plane is flown. It
must be a factor of design and the limits it was designed for. The factory
designs the flex plate with a low duty cycle in mind. 30Hp out of say 210Hp
available is light load. In a flying plane the engine is working at maxpower
for 5 to 10 minutes and the cruise at 60 to 75% possibly for hours. Theflex
plate was not designed for this.


With this in mind, I think Paul's re engineering with a flat SS plate of
0.090 to 0.125 thickness will cure the problems. It does not seem that Tracy's
isolators are a source of trouble, but a rethink on them may also be inorder
as Paul is proposing.


It all comes down to weight and designing for the application. Mazda is
concerned with weight in the cars as the builders of AC are. It is the duty
cycle that Mazda did not have in mind. They did not use these parts in the Le
Mans engine.

Dale Davies

Just more food for thoughtbecause there is a lot of talent on this letter
I was also thinking the same thing, and I pictured a much smaller
diameter 'flex plate' that is engaged by a worm gear from the starter
motor mounted at a 'right angle' to its usual orientation.  But the problem
would be DISengaging it quickly enough when engine starts.  Perhaps
could use a sprague clutch for this smaller flex plate to accomplish quick
disengagement.  Hey, if this is our only remaining problemmight be
worthwhile investigating.

Gene Kahn