Subject:
Re: Electric Fans / Crankshaft
Date:
Wed, 01 Jul 1998 21:34:42 -0400
From:
Pat B <sailnbail@worldnet.att.net>
To:
Egil Kvaleberg <egil@kvaleberg.no>
CC:
triumphs@autox.team.net
References:
1 , 2 , 3
Egil Kvaleberg wrote:
>
> On 1 Jul 1998, Gerhard Wiederholl wrote:
>
> > Where exactly is this said crankshaft damper ?
>
> It is at the front end of the engine -- it is the small flywheel with the
> markings for the ignition timing (BEFORE/AFTER) stamped along the edge.
>
> > 300 hz eaquals 300 RPM
>
> I think you will find that you are off by a factor of 60. Last time i
> checked, "Hz" meant revolutions/vibrations/whatever per SECOND, whereas
> "RPM" meant revolutions per MINUTE.
>
> Additionally, a general statement that crankshaft for inline sixes have a
> resonant frequency of 300 Hz must be entirely wrong. The resonant
> frequency will very much depend upon the weight of the crank shaft, as
> well as its radius. The 2.5 crankshaft is indeed a heavy one of
> significant throw, and has, according to Kastner, a very nasty resonance
> at 6200 RPM.
>
> To the subject at hand: It seems very unlikely that the weight or load of
> the fan would contribute in any significant way to the resonance of the
> crankshaft.
>
> Egil
> merrily running his Triumph without a crankshaft-driven fan for almost a
> decade.
> --
> Email: egil@kvaleberg.no Voice: +47 22523641, 92022780 Fax: +47 22525899
> Snail: Egil Kvaleberg, Husebybakken 14A, 0379 Oslo, Norway
> URL: http://www.kvaleberg.com/ PGP: finger:egil@kvaleberg.com
Egil,
A few more thoughts...
I am not an expert on Triumphs and therefore don't know what hardware is
installed on these vehicles. Perhaps I will become more educated during
the VTR Convention later this month.
I am knowledgeable in areas of noise and vibration in an automobile. I
may not be able to adequately explain the phenomonon in this note,
however there are a few conclusions I would make regarding this issue.
1) If it has a crankshaft damper, do not run the engine with out it.
2) A cooling fan is not a crankshaft damper. Most of the belt driven
fans I have seen are driven off the water pump(but this doesn't imply
that it couldn't be/or is not done)
There is some logic behind my previous statements. Let me try to
explain.
If the engine has a maximum rated engine speed of 6000 RPM
At a engine speed of 6000 RPM the rotational frequency of the crankshaft
is 100 rotations per second (or 100 Hz).
Engine vibration is excited by:
Engine Firing
Engine Unbalance
Crankshaft unbalance for the inline six cylinder is not a typical
vibration issue, however, engine firing excitation is major concern.
Engine firing for a six cylinder engine will occur 3 times every
crankshaft revolution (a four stroke engine only fires every other
crankshaft revolution). Therefore if a crankshaft is spinning at 6000
rpm, the firing frequency is 300 Hz.
These firing events, regardless of how often they happen will excite any
resonant frequency in the engine.
The problem occurs when the firing frequency and the resonant frequency
are the same. Since the firing frequency has so much energy the
amplitude of the resonating items will become extreme. Engineers will
add a damper (called a tuned mass damper), were needed to decrease the
amplitude and/or shift the resonant frequency to a "safer" frequency.
Regardless of the phenomena involved, DO NOT remove the crankshaft
damper!! It is there because the engineers who developed the engine
needed it to increase engine durability. If there is no reason for
having it, the damper would not have been installed. It may seem like
it is useless, but it is not.
--
Patrick Barber
74 Spitfire 1500 (FM14774U)
"Go Red Wings" ... 1998 Stanley Cup Champions!!!
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