Why do plastic/nylon timing belt pulleys need to be changed so often?

Plastic and nylon timing belt pulleys in VAG 1.4 and 1.6 16v engines have been an area of concern for many. We all know that the plastic pulley wheels/bearings are more likely to fail than the belt itself. What neither you nor the manufacturer explains is why pulley failure is time related and not mileage related, as one would logically expect, and was the case in the original recommended service intervals books (four years or 40,000 miles, whichever comes first)
If this refers to the plastic/nylon timing belt pulleys that were an example of German engine-erring, firstly by GM Opel/Vauxhall in the 1990s, then, incredibly, by VAG in the early 2000s, then I will explain. The pulleys contain bearings. When pulleys are steel or cast iron alloy and the bearing begins to fail it first heats up, then starts to shriek, warning the driver that replacement is necessary. When pulleys are plastic or nylon and the bearing begins to fail it first heats up, then melts the pulley itself, throwing off the timing belt with no warning at all. That’s why the cautious four years or 40,000 miles is stipulated.
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