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

[RoboMan91324 at aol.com]

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