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

[Mel Francis]

This discussion of the belt frequency harmonics gave me the alternate idea
of assembling a fixed, two-piece sheet metal sleeve, which would form a
close-fitting guide tube around the belt, from the idler to the fan pulley.
Has this approach ever been tried?

The Wright brothers were able to get past these same harmonic problems in 
the
very long chain-drives to their two propellers using metal guide tubes and
they never had any failures that involved thrown chains.

Mel

----- Original Message ----- 
From: <RoboMan91324 at aol.com>
To: <virtualvairs at corvair.org>; <dkdewald at pasty.net>
Sent: Thursday, January 28, 2010 6:14 PM
Subject: <VV> Unique fan belt keeper-oner (Now, Those Flapping Belts)


> 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
> ~~~~~~~~~~~~~~~~~~~~~~~~
>