I am planning to
put more power here, although I don’t see that I will be
going much north of 5500-6000 RPM.
Just a guess, I haven’t run any numbers or anything, but I’d guess
that the lion’s share of the stresses on the flywheel come from
centrifugal effects, not engine torque.
I have experience with the design of jet engine disks, which are far
more highly stressed, the ones in the P&WA F-100 engine being
somewhat larger than our flywheel and turning a LOT faster, like
16,000 rpm for 4000 hours between overhauls. At such speeds, any
metal outboard of a particular radius is a negative – it adds more
stress than strength. A “hoop” larger than that would fly apart, the
metal simply isn’t strong enough to hold together as a hoop. The
strength comes from a much smaller, thicker hub at the ID of the
disk, and this transitions to a thin membrane that holds the OD in
place via radial tension.
Under such conditions, there are a few general guidelines to
remember:
-
The thinner and lighter the OD section of the disk, the better.
-
The ID section of the disk needs to be thick and strong. Of
course, jet engine designers want to make it as light as possible, so
very careful consideration goes into exactly how thick this “hub”
section needs to be. It’s still generally the most massive part of a
jet engine, though.
-
Holes are bad; they cause stress concentrations that will lead to
cracks after many cycles (engine start/stop events). For a durable
design, bolt holes near the OD would be fashioned as follows: They
would NOT be threaded but rather would be a clean hole with the edges
carefully rounded and polished. There might be a pair of smaller
holes, one on each side of the bolt hole itself, so that the three
holes form a o O o pattern aligned circumferentially around the
flywheel.
When I looked at my Jaguar flywheel, I was a bit surprised to find
NONE of this thinking in its design. Clearly, centrifugal stress and
fatigue life are not significant players in piston engine flywheel
design. Still, those guidelines would inform my attempts to modify
or lighten this flywheel, if only to ensure that I was improving its
durability rather than shortening it.
I never the less would not want any condition under which
that wheel can come apart.
That’d be bad. If you look at many military fighter aircraft, you’ll
see they sometimes actually paint a red stripe around the plane
indicating where the turbine disks are. The stripe is a warning not
to stand in line with them. If they came apart, that’d be bad.
Waaaaay back in the day – this was probably the 1970’s or 1980’s –
a friend of mine who was in the car biz attended something called
“The Great Engine Blow”. Some shop that prepared BMW engines (inline
sixes) for competition had decided they were going to test one of
their racing engines to failure – run it until it came apart. As
long as they were gonna do that, they decided to invite pretty much
anyone in the car biz that wanted to come watch.
The engine was mounted in a dyno and was in its own room, connected
to a zillion instruments and sensors. The operator and the crowd
were watching from behind a bulletproof glass window. The engine was
started, warmed up, and then proceeded to run at higher and higher
RPM’s for quite a while. After perhaps an hour of stepping ever
higher in RPM, they reached 12,000 RPM. At this point, the flywheel
came apart and buried itself in the walls, three or four pieces.
Fortunately, it didn’t hit the glass; nobody knows if it would have
actually stopped it. Anyhow, the engine kept right on running! The
guy in charge, perhaps tired of running far faster than anyone in his
right mind would race this thing anyway, said “@#$% it!” and jammed
the thing to full throttle/no load. The RPM’s immediately pegged the
tach which only went to 20,000 RPM. There it sat for six minutes,
after which it just quietly came to a stop, totally seized.
I coulda told ‘em why it seized. An automotive oil pump won’t pump
oil at those RPM’s, it just cavitates hopelessly. If, during that
run, they had let the speed drop down to 5000 about once a minute to
let the pump send some fresh oil through the bearings, there’s no
tellin’ how long it might have run. What’s more, I coulda told 'em
how to design an oil pump that wouldn’t cavitate. But, again, nobody
would run such an engine at those speeds anyway, so it was moot.
It’s also worth noting that Bywater once told us that the limiting
factor in the Jaguar V12’s redline was not the engine but rather the
torque converter bolted to it. The OEM welded torque converter is
relatively soft steel, and running that fast will eventually cause it
to distort. When that happens, vanes inside that are supposed to
miss each other start to hit each other, and the tranny quickly fills
up with metal shavings. Ungood. That’s why those interested in
running an A/T that fast are advised to upgrade to “furnace brazed”
torque converter. There’s nothing inherently better about brazing
vs. welding, but brazing allows you to make the housing of the
converter itself out of much harder steel and not lose the temper due
to welding it together. It’s the harder steel that gives the torque
converter its strength to resist distortion at high RPM.
– Kirbert
Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 22 Oct 2015 at 6:53, mike90 wrote: