Kevlar is actually a very weak fiber when separated from a woven mesh. Regardless, with that exception which typically is caused by uneven belt alignment and wear easily fixed, I’m surprised some claim filters get dirty.
"Easily fixed" is not really as easy as it may seem when a builder or owner installs an 80A to 100A + alternator on a 3/8" - 1/2" belt and then routinely deeply discharges a large house bank. Perfect alignment, even with perfectly clean pulleys, just can't solve an
overloaded belt from dusting.. The ideal fix would be dual v-pulleys (still can have dusting issues due to non-perfectly matched belts) or a multi-rib belt system (virtually no belt dust at all) but most owners don't want to invest at this level so a $25.00 K&N or replacement foam prevents the engine from sucking in belt fibers...
These are not uncommon levels of belt dust at all...
To minimize belt dust...
1- Practice good battery management. Charging in bulk takes a toll on not only alternators but also on belts. The larger a bank gets in relation to the alternator the longer the alternator is in full-field/bulk mode demanding max belt load and running hot. Don't regularly discharge below 50% SOC and try to tailor your alternator choice to what your belt can handle. With external regulation you can de-rate the alternator using a Balmar regulator and
Belt Load Manger until belt dusting is to a bare minimum.
2- Clean pulley grooves of rust at the beginning of each season. Rusty pulleys eat belts considerably faster.
3- When changing/upgrading alternators beware of the pivot bolt holes. We see lots and lots of *upgraded alternators running sloppy fitting pivot bolts. The pivot bolt should be properly sized for the holes. If the bolt is not sized properly it leads to misalignment issues and damage to the alternator foot/feet. *We also see many factory installations with sloppy pivot bolts.
4- On the adjustment arm tension bolt buy a longer bolt and back it up with a nut and lock washer. These tend to come loose over time in the soft aluminum alternator ears. Loose tension bolts, & "dished" washers clamping the adjuster arm, are perhaps one of the leading causes we see of belt dust/slipping..
5- If upgrading the alternator be aware that many larger aftermarket alternators ship with 1/2" pulleys and many small aux engines use 3/8" / 10mm belts. We see 1/2" pulleys in use with 3/8" belts quite frequently and the owner is often complaining of short belt life. Some 1/2" pulley's are machined deeper to accommodate a 3/8" belt but others are not..
6- Know what your belt can safely handle. We find 70-80A is about max for a 3/8" belt and 90-100A max for a 1/2" belt but this is with ideal pulley wrap and most engines do not have "ideal" pulley wrap. If you want to drive more than that you will need a multi-groove/serpentine type belt or a dual pulley set up. At 60-80A on a 3/8" belt there can still be some belt dust no matter how good the alignment. This is due to the lack of "belt wrap" on many small aux marine engines.
7- Pay attention to belt wrap around the crank and alternator pulley. The more wrap the more HP you can drive for a given belt size. If you have minimal wrap and a large bank consider a pulley upgrade.
8- Machined pulleys (billet steel) perform better with heat and driving large loads than do the stamped steel pulleys on many factory alternators.
9- With a new belt you'll need at least two adjustments. Let the belt "run in" then re-tension. Failing to re-tension the belt, after a run-in period, will also result in belt dust.
10- If you're not experienced at adjusting belts by feel, use a belt tension gauge..
11- Alignment, alignment, alignment... Perfect alignment can't solve it all but is critical to deriving the most HP from you belt system that you can with minimal belt dust.
There is more detail here:
Marine Alternator Installation Tips & Tricks