Those who still believe that the water pump bearings are lubricated by the coolant, should have a look to then picture below. Of course, there are two ball-bearings which are provided with a life-long grease filling, and carefully sealed against outside influences. For the coolant there is an additional drain-hole, so that, in the event of the pump becoming leaky, under no circumstances, will the coolant reach the bearings.
In the above picture, you can see two different types of impeller construction. The closed system has of course, the disadvantage of clogging up easlier. A filter is now installed in heating systems where this type of impeller is also used. Why does one run the risk of having defects by using sensitive components like this in the first place? Once again, the reason lies in the more favourable efficiency and economy which, in the meantime, all constructions have to abide by. 11/11