<VV> Unique fan belt keeper-oner (Now, Those Flapping Belts)

Jay Pitchford jay.pitchford at gmail.com
Thu Jan 28 22:40:13 EST 2010


Yeah, what he said.

On Thu, Jan 28, 2010 at 7:14 PM, <RoboMan91324 at aol.com> wrote:

> Dale,
>
> You are not exactly correct.  Analogies such as you describe are used to
> understand similarities in different disciplines.  However, the analogy you
> draw between the mechanical components and an electrical circuit though
> correct in general is not the cause of the flapping belt.  I believe we
> were
> talking about the flapping and not so much the variable looseness and
> tightness
> of the belt.  Under the steady conditions of idle, the tightness and
> looseness variance caused by the crank is minimal but does excite a
> particular
> frequency.
>
> The belt flapping is caused by sympathetic vibration that is excited by the
> crank pulses Craig described.  The pulse frequency (or a multiple of the
> pulses) matches the frequency of the section of belt that is vibrating.  To
> draw a musical analogy, piano tuners often start the process by striking a
> tuning fork of known frequency and holding it close to a piano string.  If
> the
> string vibrates, its natural frequency is the same as the tuning fork.  (A
> multiple of the natural frequency is not likely to happen in this
> application.)
>
> If the vibration of the belt at idle is disturbing, you can change it by
> one or more of several methods.
>
> 1.  You can change the idle speed so that the belt and crank pulses no
> longer match.
>
> 2.  You can change the tightness of the belt.  To use another musical
> analogy, think about the strings on a guitar.  As you tighten or loosen the
> string, its natural frequency changes.  You can easily hear the difference.
>
> 3.  You can change the mass of the belt.  A thicker belt will have more
> mass per unit length and will want to vibrate at a lower frequency.  Again,
> think of guitar strings.  The thicker strings (more mass) vibrate at a
> lower
> frequency and the thinner strings (lower mass) vibrate at a higher pitch.
>
> Keep in mind that the sections of the belt are different lengths and can
> change tightness depending on whether you are accelerating or decelerating
> as
> well as other variables.  As an example, you might vibrate a section of
> belt
> under normal idle but it might not vibrate once you load the
> alternator/generator with headlights, radio, wipers, defrost fan and such
> turned on.  This
> alone could change the tightness of the belt enough to cause a visible
> change in vibration.  Also, if you make a change as suggested above, one
> section
> might stop vibrating and another start vibrating.
>
> Best regards,
>
> Doc
> 1960 Corvette; 1961 Rampside; 1962 Rampside; 1964 Spyder coupe; 1965
> Greenbrier; 1966 Corsa turbo coupe; 1967 Nova SS; 1968 Camaro ragtop
> ~~~~~~~~~~~~~~~~~~~~~~~~
> In a message dated 1/28/2010 3:15:52 PM Pacific Standard Time,
> virtualvairs-request at corvair.org writes:
>
> > Message: 4
> > Date: Thu, 28 Jan 2010 15:35:59 -0500
> > From: Dale Dewald <dkdewald at pasty.net>
> > Subject: Re: <VV> Unique fan belt keeper-oner
> > To: virtualvairs at corvair.org
> > Message-ID: <4.3.1.2.20100128145444.00f91130 at mail.pasty.net>
> > Content-Type: text/plain; charset="us-ascii"; format=flowed
> >
> > At 07:20 1/28/2010 -0500, craig nicol wrote:
> >
> > >Here's what causes the belt to deflect and causes the idler to react:
> The
> > >crankshaft does not rotate at a constant speed. Three times during every
> > >rotation of the crankshaft, one of the pistons radically accelerates the
> > >crankshaft and pulley as it passes through its powerstroke.
> > >
> > >The belt is somewhat elastic and the combined mass and load from the fan
> > and
> > >generator don't react immediately. Instead, the belt stretches tight on
> > the
> > >driver's side as the crank tries to accelerate the fan and generator and
> > it
> > >correspondingly goes slack on the passenger side of the engine. That's
> > why
> > >GM installed deep groove pulleys and belt guides to keep the belt from
> > >jumping out of the pulley on the slack side.
> > >
> > >Engines with strong idle power pulses, lighter flywheels, heavier fans,
> > >higher generator loads, and more elastic belts will all contribute to
> > >increased "flapping" of the belt at idle speeds.
> >
> > Hello, I am a metallurgist, not an ME or EE, so those who are better at
> > this please correct the following.
> >
> > As is often the case, an electrical analogy can be used to describe belt
> > action as a first approximation.  Think of a LRC circuit.  The crankshaft
> > is a source of electric power; a DC source (RPM) with an added AC
> > component
> > (power pulses).  The fan is a capacitor (rotational inertia) with a large
> > variable resistor across (airflow as a function of speed).  The
> alternator
> >
> > is a resistor (electrical load), and the belt is an inductor
> > (stretch).  The bearings and pulley friction are additional resistances.
> >
> > LRC circuits can be either partially or fully damped (preferable in this
> > case) and/or have a resonant frequency.  What Craig adequately stated is
> > that a certain combination of components might mechanically resonate due
> > to
> > the power pulses.  Using the electrical analogy we might predict how the
> > change of one component (e.g. steel vs magnesium fan) might affect the
> > belt
> > action.
> >
> > Dale Dewald
> > Hancock, MI
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