On the subject of bearings...
The life of the bearings is dependent on two things - materials used and fit. Since our applications are not in a harsh environment, we can accept almost any materials based on hardened steels which will usually be a stainless type. Shields are not a problem since the amount of dirt needed to cause a problem will usually sneak by a rubber of steel shield. With the oil that seeps out, the dirt that sticks to it will be drawn past almost any seal. Furthermore, it is more likely that the wear on the cages will cause small particles to come off inside the bearing anyway.
Fit is the key here. When bearings are correctly assembled, there should be no clearance between the inner and outer races and the balls. If there is slack, then the bearings will bounce up and down a bit causing vibration and accelerate wear. As our bearings usually have a radial load on them at all times, this is less of an issue. If the inner and outer races are overly compressed (outer) or expanded (shaft) when the bearing is fitted, then the race is 'crowded' and the pressure on the races and balls will severely reduce the life.
Whatever ball bearings you choose, make sure that the fit in the housings and shafts is good, but not too good. By choice, the shaft fit should be close with no play and easy to push on by hand if there is any interference in the fit, and the housing fit should be a nice press fit with a thumb. If they are any harder to fit than that, the chances are you will crowd the races and shorten the life.
There's a lot of misconception about the use of ABEC grade bearings. This grading will tell you how much clearance there is in the bearing, and provide a guide as to the interference fits you should aim for. Putting an ABEC 9 bearing in a tight housing and crowding the races will shorten its life considerably. Putting an ABEC 3 bearing in a tight housing may give you the perfect clearance. Since we are talking about holding sizes of down to 0.0002" to get the right interference fit, you can see that a plastic moulding is never going to get you where you need to be as it is probably got a tolerance of 0.010"!!
If you want to get the most life from your bearings, focus on fit, not shields and ABEC numbers. And, of course, stop hitting the boards and denting (crowning) the races leading to a lot of wobble, friction and broken cages!! HTH
