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
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