Listers,
Not to through a damper on anyone's plans here, but ... there is a lot of
misunderstanding regarding these devices.
As Jim points out, they are dampers, not balancers -- they do not affect the
"balance" of the moving parts. It is unfortunate that Americans have come
to call these things "harmonic balancers" (but then again our suspensions
have "shock absorbers" that don't absorb shocks either).
The crank damper's purpose is to dampen TORSIONAL resonances in the
crankshaft/rod/piston/flywheel/clutch system. This has several
implications.
1) The conventional rubber/steel damper is tuned to a single resonant
frequency, the resonance of one particular crank/rod/clutch/etc system for
which it is designed. Hence, mounting this damper willy-nilly on another
engine will do no good. In fact, once you change one component in the
system (say, lighten the flywheel or install aftermarket rods) the damper
will be of little value -- you have changed the resonant frequency of the
system, and the damper is no longer damping the right frequency.
2) Fluid-type dampers are supposedly the solution to the above, as they are
claimed to dampen all frequencies, or a wider band of frequencies. I
haven't done the math, so can't swear to that claim, but it seems plausible
& accepted by many.
3) Both types (fluid & rubber donut) dampen TORSIONAL vibrations. There are
TWO common modes of failure in crankshafts: torsional, and bending. The
damper does nothing for bending.
Cranks that are poorly supported (e.g. 3-bearing 4 cyl), have long strokes
(e.g. most anything British), and/or heavy reciprocating masses (e.g. most
older stock pistons & rods) are susceptible to bending failure. It's quite
common in MGA's and Healey 100-4's. A crankshaft damper will do nothing for
this problem.
Cranks that are rather long (e.g. straight 6's, many V-8's, any layout
12-cyl) are more susceptible to torsional resonance, and can benefit from a
correctly designed damper.
4) By far, most of the vibration that one feels from a 4-cylinder engine is
unrelated to torsional. Regardless of layout (in-line, flat, or any
V-angle) all 4-cylinder engines have second-order shaking forces and/or
moments. A crankshaft damper cannot reduce those vibrations. Inline 6's,
or 60 deg V-6's, do not have second-order components and hence feel much
smoother. Ditto for 90 deg V-8's (or even straight 8's), and 60 deg V-12's.
Too many years since college to remember the outcome for boxer 12's,
H-16's, etc but then again I never expect to own any of those!
Regards,
Mark Palmer
>From: Jim Hayes <hayes@mediaone.net>
>Reply-To: Jim Hayes <hayes@mediaone.net>
>To: Brian Evans <brian@uunet.ca>
>CC: Joseph_Chimbolo@hyperion.com, Teamnet@hyperion.com,
>vintage-race@autox.team.net
>Subject: Re: Balancer
>Date: Mon, 14 Feb 2000 20:29:55 -0500
>
>Yesterday at Borders I picked up a copy of "The Design and Tuning of
>Competition Engines" by Philip H. Smith (Robert Bentley Books).
>I figure any book I spend 30 minutes in a bookstore reading a chapter
>(on balancing engines, no less) I should buy.
>See p 338. Balancers (or dampers as they should be called) are designed
>to reduce the harmonic vibration periods in a crank that can be harmful.
>The whole of chapter 5 covers crankshafts and balance - and it is worth
>a read - even understandable!
>Jim
>--
>Jim Hayes Winchester, MA, USA
>hayes@mediaone.net http://www.fotec.com/jim/jim.htm
>jeh@fotec.com http://www.fotec.com/
>All generalizations, with the possible exception of this one, are false!
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