<VV> Unique fan belt keeper-oner

[Dale Dewald]

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