<VV> Unique fan belt keeper-oner

[craig nicol]

Lon wrote:
The other is that the whole design is based on the assumption that the belt
can be retained by limiting its deflection. The factory thought that as
well, to some extent, by adding belt guards. But time has shown that the
belt guards do very little, while lightening the fan did everything.

  A good education can be had by watching our spring loaded idler on
multiple engines. Even when all things appear to be good (no bent pullies
etc) some idlers will vibrate substantially when at idle and others are
quite smooth. This says to me that there are more subtle issues that are yet
to be identified which cause distortions in the belt under operation.

Craig adds:
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.

Happy to chime in!
Craig Nicol


Sidebar: Modern engines (OBD II) detect misfiring by observing the
accelerations of the crankshaft using a high pulse-rate crankshaft position
sensor. The ECM expects a steady "sine wave" of accelerations. If
acceleration is missing, that means the cylinder has misfired. Enough
misfires and the ECM will set the MIL (check engine light).