TR6 flywheels & clutches
>Since the question of clutch &
fly wheels has arisen, I'm
> wondering if anyone has tried putting together a small button
clutch
> (Tilton or Quarter Master) with a flex plate,on a TR-6 ?
I can only
> imagine the rotating mass coming down to near nothing.
>David C. Wingett
Having had just a big bunch of history with the TR6 engine and
flywheels and clutches flying off, making an installation with
a light flywheel and the lightest smallest clutch makes a significant
difference in the durability of the crankshaft and the available
rpm. When making the flywheel (aluminum) have the center hub as
deep as possible with about a .002" interference fit. This hub
should then go onto the crank almost touching the seal. The problem
is that the flywheel actually wobbles as the vibration of the
crankshaft is transferred to it and this movement is then trying
to PRY the flywheel off the crank. When you lighten or reduce
the size of the clutch you reduce this movement by about one half.
I measured all this on the dnyo with a oscilloscope and potentiometers.
For the fitting you then heat the flywheel and it will slide
onto the crank end. I also drilled and taped the end of the crank
for FOUR more bolts. Use bolts that are HARD not just strong.
By hard I mean bolts that have a very limited stretch before breaking.
Bolts that are strong and stretch allow the flywheel to slide
around and wobble but the bolts don't break. Too bad, cause you
are still a DNF.. Machine bolts such as used for hold downs in
shop machinery such as a mill are hard and do not stretch.
Kas Kastner
> Kas
> Thanks for your response. Yes, I have also had a fly wheel
> come lose but, not disconnect ( last lap of the last race,
fastest lap
> of the season). Now, with the ATI balancer, and all the other
> balancing work done, shouldn't that take care of any odd
> vibration...? The Tilton and Quarter Master button clutches
are very
> state of the art & balanced to a Bone. The only problem
I would see
> is finding a source for the flex plate, using a stock ring
gear (of
> course it would have to be CUSTOM made, every thing else
is) and
> calibrating a throw out bearing to match the clutch.
>David C. Wingett
The problem is twofold. It is weight and it is size which of
course is surface speed. The larger the diameter the higher the
surface speed. To make the best of the bad rules and possibility
of keeping everything together I used an aluminum clutch from
a Porsche and an aluminum flywheel. I made a steel hub to fit
over the crank with an interference fit then bolted the flywheel
to the hub. This combination gave a TOTAL weight of 13 pounds.
Were I to do this again I would go to a multiple disc clutch.
Then you would have the light weight AND the small diameter.
To reduce the largest diameter weight, I even removed the ring
gear teeth in specific spots as I knew the engine always stopped
in certain areas and I could probably get away with it. Lots of
work to not much benefit. We then turned the engine to 7500 to
with pretty good reliability. The rpm range of the torsional vibration
in the crank can be moved around but is always there. The crank
in the TR6 has no overlap and thus is a whippy devil. I made the
weights as light as possible and this gave a problem at about
4400 RPM again at 6100 RPM and then the last vibration was over
8000 and was not of importance. This third vibration by the way
was so severe it was off the page on the pen recorder. The biggest
problem we had was holding the engine revs at the proper amount
on a yellow flag or the pace lap to not be in the range of the
vibration. I figured out that light was good by the simple theory
that if I tooled off the flywheel to nothing, then what could
fall off.
NOTHING. Therefore light is good. I would like to add that when
we whipped the clutch flywheel problem then the torsionals went
to the other end of the crank and broke the timing chain, we fixed
that with a double row, then it actually sheared the camshaft
bolts to where we had to add another bigger bolt to keep it all
intact. Worth doing though, we WON. Made 252 bhp at 7900 rpm in
1972. Boy this stuff is old isn't it.
Kas Kastner
> I pulled the transmission on my TR6 IT racecar Tuesday. The
flywheel was
held
on by two bolts(barely),
> and the other two bolts were broken.
The
crankshaft
looks like there is metal from the flywheel molecularly
>
bonded
to it!
Has
anyone ever experienced this before or have an explanation
for this
phenomenon?
>
The PO must have run it a while with the flywheel
loose.
This is the same trouble that I have sent severeal messaages
on. The TR-6
engine is prone to this because the crankshaft is a whip ( no
overlap).
Measured on a scope with potentometers on each side of the flywheel
at the
ring edge, the flywheel moves back and forth over .500" at
6200 revs. What
this does is actually stretch the bolts, then the flyweel is
loose and the
bolts break in shear and all is lost. The fix I found was the
lightest
combination flywheel and clutch,( mine was total of 13 pounds)
then put
7/16" FLYWHEEL bolts in the crank and add two more. The
bolts must be hard
not just strong....not grade 8 or stainless or most of the aircraft
stuff.
They are stong but will stretch. The NASCAR shops have the ticket.
Hard
bolts is the ticket, such as they use for hold down clamps on
a mill.
Kas Kastner
> Yes, I thought I'd switch to ARP bolts, anyway. I still don't
know how to
> evaluate whether a bolt is "hard" or "strong",
but at least I'm more
> comfortable that my flywheel bolts won't break... :-)
Hard versus strong is easy to explain, but I'm not sure what
physical
parameters denote it, certainly not grade 5, grade 8. A strong
bolt can
stretch but won't break, a hard bolt won't stretch but when it
reaches
yield it cracks rather than stretching. The ARP bolts seem to
be both.
From my limited metallurgy reading (mostly associated with knife
making) I
think the "psi" number is strength, that is, they stick
a sample in a
measuring tool that tries to pull it in two. Hardness is a different
number, which as I recall is measured by pressing a graduated
diamond
point into the metal and seeing how deep it penetrates with a
specific
force applied. Sometimes you see knives with little dimples where
they did
that. There are a bunch of other tests, like Charpy V-notch that
tests the resistance to shear forces. And Brinell that measures
another aspect of
hardness. But it all distills down to "buy some really good
bolts", and
that basically means "pay ARP what they ask".
Bill Babcock
I, too, got into some metallurgy when making knives. I think
the
essential quality required of the bolts mentioned by Kas is "toughness," which is a combination of the qualities of tensile strength,
to a great
degree, and to a smaller degree, resistance to deformation and
cracking.
The relationship between tensile strength and elasticity is
determined
by Young's modulus, which is, effectively, the multiplication
of stress
and strain (stress being the applied force, and strain the amount
of
stretch in the material for that applied force). Virtually all
alloys of
steel are very close to one figure -- 2.9 x 10^7 psi. So, the
lower the
tensile strength at the yield point, the more the bolt will stretch
before yield without permanent deformation (known as elongation).
The
higher the tensile strength, because of Young's modulus, the
less
elongation before the yield point. A grade 2 bolt will, under
load,
stretch by 15-20% before yielding, and still come back to its
original
dimensions when the load is removed. A grade 8 bolt will take
a much
higher load, but will elongate maybe 4% before the yield point.
Bolt hardness comes into play particularly in situations like
flywheel
bolts. Surface hardening a bolt does make that small amount of
the
surface material more brittle, but also helps prevent very small
nicks,
such as can happen when the edge of a bolt hole bangs into the
bolt
under big loads. Under large pulsating loads, nicks grow into
cracks,
causing the bolt to fail.
The relationship of pulsating or cycling loads (good examples
are
flywheel mountings and cylinder heads) to the fasteners used
is that the
tensile strength before yield of the fastener must not only be
greater
than the peak load, the fastener must also be able to be torqued
to a
high enough value to pre-stretch the bolt so that the load applied
on
the bolt when torqued exceeds the peak load by a comfortable
margin.
That way, in the case of a flywheel, the bolt absolutely restrains
the
mounting area of the flywheel from moving at all. And, because
the shear
loads at the flywheel are very high, the bolt also has to have
sufficient strength to resist shear.
The problem is complicated in the areas mentioned above by the
fact that
the block and the crankshaft are cast items, and the tensile
strength of
those castings is not equal to that of the bolts used. That means
there
has to be sufficient receiving thread area to spread the torqued
load so
that the threads don't yield, but are still put in tension enough
to
resist movement under peak load.
Getting back to metallurgy, bolt cracking is always of concern
with any
steel alloy, because of the cubic geometry of iron's crystal
lattice
structure. That makes for lots of flat planes to slide against
each
other if there's sufficient force to create a displacement (the
starting
point of a crack). By contrast, titanium alloyed with very small
amounts
of iron creates a hexagonal crystal structure, and those hexagons
lock
into each other (something like cells in a honeycomb), making
displacement in all planes much more difficult.
Unfortunately for us, commonly available titanium alloys do
not always
have the ultimate tensile strength of, say, a grade 8 bolt, so
a larger
bolt must often be used, and they are quite expensive and require
some
careful handling and protection. Certain alloys of titanium are
very
susceptible to environmental damage. When the first versions
of the
SR-71 spy plane (the airframe of which is virtually all titanium)
were
being assembled with titanium bolts, workers would assemble a
portion of
the frame, and come back the next day to find all the bolt heads
laying
on the floor. After some investigation, it was discovered that
they were
using cadmium-plated sockets to install the bolts, and the cadmium
transferred to the bolt heads during installation was enough
to cause a
chemical change in the alloy which reduced its strength to next
to
nothing. (!) Thereafter, every tool kit had only sockets with
a black
oxide finish.
Michael D. Porter
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