I have a german built machine (Holzher eco-master) where there's some trouble with the spindle. 2 months ago the bearings suddenly failed. Had to buy a new spindle (machine 2 years old) since the warranty didn't covered it..($5000). Now the other day the same thing happened again.. the steel was burnt in a blueish color and everything was stuck..

How long is a motor like this suppose to last..? I must mention the engine is a water-cooled omlat..

What can cause this bearing-crashes..?
When the crash took place it was running a 3,2 mm router bit with very little stress..
Is it possible that this could happen if the machine works without a stop in several hours..(isn't thats why there's watercooling..?)

Are you sure your water cooling is working properly?

Spindle bearings need to be preloaded. Thermal changes can increase or decrease preload. Generally long runs heat the head and increase the preload. The more preload the more heat. When the runaway condition is reached bearing destruction is immediate.

That is why some Belleville spring makers, like KeyBellevilles, make spring washers especially designed to maintain constant preload despite temperature changes. I found them on the net by Googling "KeyBellevilles".