Re: nylon timing belts -- stretch or snap?

[bearbvd@domain.elided (Greg Hermann)]

At 6:36 PM 1/26/00, Cuadra wrote:
>Just curious -- do nylon timing belts stretch over time or do they just tear
>in half and break?
>
>A motorcycle chain, e.g.,  will stretch with age and mileage, and can
>eventually break off, too.
>
>Paul cuadra@domain.elided
>
>a working stiff with no TV/VCR/cable  (but I have a 6 year-old Aiwa boombox)
>
>164 LS 5-speed
>Moto Guzzi Sport (shaft drive)

Unless (and I doubt it) there have been serious changes in the technology
of how the belts are built since the time when I was involved in developing
and testing one, the cords in the belt are fiberglass, not nylon. The cloth
with which the toothed side of the belt is faced IS nylon.

Typically the first thing to look for is wear of the nylon cloth on the
loaded face of the teeth. If you have any, replace the belt. As the belt
ages, the rubber cracks. If it goes far enough, because the cords are wound
on the belt in a very fine pitch helix, eventually the rubber ceases to
hold the cords together, and they sort of come unwound until a loose end
get caught in something, then it's over very quickly.

Of course, any sign of age cracking or excessive hardening of the rubber is
cause for replacement.

The rubber compounds used are not particularly resistant to motor oil,
glycol (antifreeze), or fuel--all of which will soften the rubber over
time--so if you are in there to replace a leaky crank or cam seal, it's not
a bad idea to replace the belt.

The fiberglass cords are really surprisingly stable as to length. With good
manufacturing control over the tension of the cord as it is wound onto the
mold, the center distance length of several belts, as measured under a
specific tension over two toothed sprockets can be held within about a 0,1%
range. This belt length does NOT change with age or use.

Regards, Greg

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