All Papst fans with the letters "NG" behind the numbers indicating the size and voltage of the fan (612, 8412, ...) use a special Sintec Sleeve bearing no other fan manufacturer I know uses (I think "N" stands for "New" or something like that and I'm pretty sure "G" stands for "Gleitlager", the German word for "Sleeve Bearing"...so you get "New Sleeve Bearing" or something similar). Some people call this fans Papst-Sintec fans to separate them from the other more standard ones.
These Sintec Sleeve Bearing fans are of very high quality compared to usual Sleeve Bearing fans : They have the low noise most Sleeve Bearing fans have at the beginning, but they are as durable as good 2-Ball Bearing fans, so they will stay quiet for a very long time while most standard sleeve bearing fans become noisy after a very short time. I can't explain how these Sintec bearings work in detail though...would have to do some research for that...