DANMAS@aol.com wrote:
>
> In a message dated 98-06-29 05:55:29 EDT, jmccarr@ibm.net writes:
>
> > Bill Piggot's, Original Triumph (New Edition, 1998) on page 71 talks
> > about the original fan as a crankshaft vibration damper. He makes the
> > following comment "Owners who have substituted an electric fan sometimes
> > experienced broken crankshafts owing to the loss of this damping
> > effect. Careful engine balancing appears to prevent this occurance."
>
> Jack,
>
> Piggot did a pretty good job with his book, but he does have a few errors. I'm
> afraid I will have to disagree with him on this one. In fact, if any thing, I
> beleive the opposite is true.
>
> Consider the construction of the crank damper, to which the fan is attached.
> It consists of three pieces - an inner hub, an outer ring, and a rubber fill
> holding the two pieces together. The fan attaches to the inner hub, which is
> bolted directly to the crank.
>
> Now, conside the operation of a piston type engine. On the power stroke, the
> piston drives the crank throw, and puts power to the crank. On the Compression
> stroke, just the opposite occurs - the crank throw is forcing the piston up in
> the cylinder. For each cylinder, the crank is alternating between these two
> forces, which produces vibration. The rubber between the two pieces of the
> damper tries to absorb these vibrations, and smooth out the crank forces.
>
> The more rotating mass there is to the crank, the harder it will be for the
> damper to perform its function. On the power stroke, any additional mass will
> resist the downward motion of the piston, and on the compression stroke, it
> will tend to "flywheel" the piston upwards. The difference between the
> opposing forces will be greater, the larger the rotating mass.
>
> Every book or magazine that I have read that touched on the subject advise
> putting as little weight on the crank snout as possible, just for the purpose
> of reducing the odds of the crank breaking.
>
> Additionally, if the fan blades are a little off balance, which I bet most
> are, as old as they are and as many knocks as they've had over the years, they
> will contribute even more to crank vibration.
>
> There is a whole lot more to the overall function of the damper, but this
> covers the basics.
>
> I may be all wet, but at least that's my opinion. Perhaps the TR3 uses a
> totally different damper construction than nearly any other car ever made?
>
> Dan Masters,
> Alcoa, TN
>
> '71 TR6---------3000mile/year driver, fully restored
> '71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
> http://www.sky.net/~boballen/mg/Masters/
> '74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
> '68 MGBGT---organ donor for the '74
Here's My 2 cents...
The reason dampers are used is to reduce amplitudes of items which are
resonating. If a crankshaft damper is being used, then the crank shaft
has a resonant frequency which is either excited by the firing frequency
or the rotational frequency of the engine. I assume that the TR6 has
inline six cylinder engine which is inheriently balanced for both shake
and couples. Since the Inline 6's have one of longest crankshafts made,
I would assume that there is a torsional resonance of the crankshaft,
somewhere below 300 Hz. If the engine were to operate at this resonant
frequency without the damper, the amount of time needed to fail the
crank would be rather short.
My suggestion... Do whatever you want with the fan. Be sure that the
damper is left intact and the rubber is in good shape.
BTW, in a modern inline 4 cyl or V6 engine, inertial forces can exceed
6000 lb. at redline.
--
Patrick Barber
74 Spitfire 1500 (FM14774U)
"Go Red Wings" ... 1998 Stanley Cup Champions!!!
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