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I don't think the problem is power, by itself. The auto transmission was designed to accommodate the high-output engine*. Drag racers often use the stock flex plate, of course the duty cycle for drag racing is very different, but they have plenty of power.
Comparing the RX-8 to the earlier rotaries, the engine produces less torque, but it operates at substantially higher RPM. Since flex plates don't generally break in the RX-7 or Cosmo (I have never heard of it happening), the improvements are probably to handle the higher RPM - which could point to either gyro loading or to warping as the problem (but probably not cracking as a root cause, IMO). It seems like holes always have cracks with them and usually it is not a big deal, but when subjected to additional stress - either impact, flexing, thermal or whatever - the cracks can go from microscopic hairline to broken part in relatively short order.
I'd be inclined to say the light steel flywheel is the better bet, even independent of cracking (I am not convinced that it would not suffer the same problem). Aluminum flywheels have a reputation for being a little more problematic in the cars. At least in the US, auto transmissions are an also-ran, and with the overall declining parts availability for non-Renesis rotaries, the ability to get brand new flywheels in a variety of weights & materials seems like an advantage, compared to dealing with flex plates.
The overall fact that the flex plate is not a weak part in the car but does seem to be a problem here means that there is something going on that is not well understood. Understanding it resolves the whole discussion.
* Historical note. Mazda didn't design their own auto transmissions, they bought them from JATCO like most other Japanese manufacturers. Since most of the automatic transmissions were sold in Japan, and all RX-7s and Cosmos from 1986 on were turbo in Japan, the auto trans design was actually built for the high output engines, and adapted for non-turbo in the US! This explains why the flex plate is "turbo size," and some other quirks in the 2nd gen cars, like why the auto trans driveshaft in the US has a turbo style U-joint on one end, and a NA style U-joint on the other end.
On Fri, Jul 15, 2011 at 12:41 PM, Tracy <rwstracy@gmail.com> wrote:
I'm still in analysis mode on this myself.
So far the possibilities are:
1. The flex plate may have been warped when installed ( I bought several that were) I think the standard should be close to zero runout or wobble on the flexplate in view of this failure.
2. The flexplate could have had cracks when installed (I also bought several that had cracks and they were not visible unless you looked closely)
3. The dang things (RX-7 flexplates) are not strong enough for boosted 13Bs or 20Bs.
I do NOT think the problem is fore & aft travel of the input shaft. The RD-1x drives have had thrust bearings from day one. There is no thrust load at all on the damper, flexplate or crankshaft imposed by the drive. This was not the case with the Ross drive I used at first.
I have over 1000 hours on the same flexplate on my NA 13B. I have only 55 hours on the 20B. But I'll be checking both. I did inspect both flexplates VERY carefully before installing.
If I found that a change were necessary, I would consider two possibilities.
1. Use a modified aluminum racing flywheel used with an auto counterweight. (Or possibly a lightweight steel one if there is such a thing)
2. Use an RX-8 flexplate with an adapter plate made of aluminum or steel sheet to go between the flexplate and damper. The RX-8 version looks stronger than the RX-7. I briefly studied this when good RX-7 flexplates were getting hard to find. I did not develop a formal procedure for doing this but it looked like it would be pretty easy. Anybody with spare time out there want to take a closer look? Hint: The damper bolt pattern would be offset 45 degrees from the RX-8 torque converter bolt pattern.
Tracy Crook, RWSOn Fri, Jul 15, 2011 at 1:47 PM, Bobby J. Hughes <bhughes@qnsi.net> wrote:
Bill summed up what I’ve been
thinking the last few days. I’m not changing anything until Tracy makes a recommendation.
It could be fore and aft movement on the shaft, flex plate out of round, worn
rubber isolators or a combination of all these items. We may need a scheduled maintenance
plan to replace items like the rubber isolators or other items that wear and
loosen tolerances.
I will however be inspecting for
this problem before most flights.
Bobby Hughes
(playing with fiberglass again)
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Bill Bradburry
Sent: Friday, July 15, 2011 10:03
AM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: Ok Here
are the important flywheel measurements
David,
I can’t help but think that you may
be fixing something that is not broken. I realize that the flexplate came
apart on your plane, but that doesn’t mean that the flexplate is the
cause of the problem. That flexplate holds up quite well in the car with
a heavy torque converter attached to it. Think of the forces when a car
under heavy acceleration changes gear! As long as the forces are along the
plane of the plate it is very strong. Not so much in the forward/rearward
directions. In fact that would cause metal fatigue and cause it to break
just like yours did. I think that what ever is causing the endplay is
what you should be looking at. I suspect that if you check out the
transmission on the car you will find that the endplay of the input shaft is
controlled by thrust bearings or some such device. Lynn may know the answer to this??
That msg from Al Wick really resonates
with me. Tracy
had to install a thrust bearing to prevent the drive shaft from hitting the
bottom of the pilot bearing and causing an eccentric shaft failure on the
engine. Maybe you now need to add a cage similar to what Al was
discussing that will trap the shaft end play completely. That, to me,
makes more sense than adding a bigger flywheel. If you strengthen the
weak spot, you will then discover where the next weakest spot is because that
will be the next failure!
This is a real opportunity for you and the
rest of us with the same drive. Please study this a little more before
you begin the fix!
Bill B
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of David Leonard
Sent: Friday, July 15, 2011 12:42
AM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Ok Here are the
important flywheel measurements
I finally got to the hangar and measured out my S5 turbo flywheel and
the remains of my S5 flexplate. The results are very encouraging, even as
good as the possibly could be to make the swap, but I only had ten min at the
hangar before I had to leave to pick up my daughter from daycare, so they are
one try and slightly sloppy... I make no guarantees. The height
measurements of the flex plate were particularly suspect because, well, mine is
all warped and broken.
Ring Gear Diameter - Perfect
fit. As we suspected, same size, same tooth spacing.
Ring Gear Fore-Aft Placement - As
measured from the underside of the big nut. (for all further reference, aft
means aft in the CAR). The flex plate puts the aft side of the ring gear
1.290 inches from the front of the big nut. The flywheel puts it 0.913.
So the flex plate puts the ring gear (appx) 0.377 in. further aft. This
is actually fortuitous because it just so happens that the ring gear thickness
is .365 ... So all we have to do to fix the discrepancy is
weld and extra ring gears onto to the flywheel ring gear and there will be
teeth that the starter can reach:(bottom of the page.. the cost $80)
http://mazdatrix.com/flywheel.htm
It may also be possible to fix the discrepancy by changing the starter... will
look into that.
Damper Plate Placement - On the
flywheel, the mounting surface is proud (or aft) of the ring gear. On the
flex plate, the mounting surface is recessed (or forward) from the ring gear.
And it all pretty much works out. That position on the flywheel will vary
somewhat depending on which flywheel you have and how much wear you have on the
clutch slip surface. But on my (I assume well-worn) flywheel the placement was
0.965 while the flex plate was 0.950. Pretty damn close and given
measurement error it might me meant to be right on. Or, any difference
could be made up by trimming down the spline on the damper plate or removing or
adding spacers between the two.
So in my mind, a flywheel is the way to go. Stout, easy to get, pick your
weight, only mods seem to be drilling the mounting holes and welding on a
second gear ring.
--
David Leonard
Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net
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