After I replaced a relatively new inside rear wheel bearing the other day, I got to thinking about how the grease is supposed to stay in that bearing. It is an open ball bearing (17 mm id, 35 mm od, 17 mm thick ,NSK6207 C3 or KOYO 6207R) and when you are installing your supposed to pack the bearing itself with grease and also add grease in the housing (where the spacer is). I don't understand how adding grease to the housing helps, I can't see how grease there is supposed to get into the bearing.

So I wonder if using a sealed version of the bearing would be better? They are available, KOYO 6207 2RS for example. 

Am I missing something in my understanding of ball bearings in this application?

alistair