This message was sent to me instead of triumphs@autox.team.net.
mjb.
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From: "Cindy and Terry@ESP" <esp@hypermax.net.au>
Subject: Dynamic Balance
Date: Mon, 9 Oct 2000 17:39:21 +1000
IMHO most of the replies have totally missed the mark with this critical
issue
The 4 cyl TR engine, like most, 4's, has an inherent imbalance that cannot
be totally solved by static balancing
In reality, all rotating items, and especially cranks, develop a twisting
motion, where the frequency is governed by the basic design, mostly the
firing order and number of cylinders. If the frequency of the impulse
matches the natural frequency of the shaft (determined mainly by the
material it is made of), then it will go into a cyclical resonance. If the
engine is held at these revs for extended periods, then the amount of
vibration is large and the crank will fail by overstressing. You can test
the natural frequency of the shaft by hanging from a chain or such and
"pinging" it with a light hammer, and measure the noise frequencies.
The solution to this is placing a torional damper at the free end of the
crank, which is where torional vibrations are greatest. There are 2 types of
damper, viscous and tuned. The viscous damper, which is becoming common in
race and very high performace road cars is designed to dampen the worst
credible vibration amplitude (amount of twitst). They work by forcing the
crank to try to shear a viscous fluid inside the damper. A tuned damper
however, which usually comprises two steel rings bonded by rubber, is tuned
for a single frequency only and works by absorbing the energy in trying to
shear the rubber. These are designed to handle the vibration the designers
see as most critical, eg , that that occurs at say 4000rpm (speed varies
with each engine). The engine revs are chosen based on the revs the designer
thinks an owner could hold for an extended period. If you look at a T2000
and T2500 engiens, you will see two different (weight) friction dampers.
It is for these reasons that TR's broke cranks when constantly revved to
4800rpm. The Standard designers never envisaged this when they designed the
vanguard (AKA TR) engine as max speed of 85mph was achieved at 3700 rpm (or
some such number) and they never considered an owner would hold the engine
at those revs long enough to destroy it, it would merely hit this value
during gear changes, momentary over-revs etc etc.
In reality what all this means is
1) you can't swap rubber harmonic balanceers and expect them to "just work"
2) you can raise the revs at which the harmonic occurs by lightening itmes
with high inertial mass like the flywheel and clutch , and to a very small
extent, the fan (thats one reason why designers changed to plastic fans)
3) the heavier the tuned friction damper, the greater the amplitude it was
designed to protect
3) if you want max protection, get a viscous damper
Terry O'Beirne, TR5, GT6 racer, 2500 racer etc etc
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