I received some new ball bearings last week since my old ball bearings bought last year wore already.
In order to simplify the the ball bearing install difficulty, I used 1 flanged 2×5 flanged ball bearing and 2 non-flanged ball bearings for each wheel. In this case, a perfect size spacer is no longer needed to support inner race if I want to use 2 flanged ball bearings to support one wheel. Of course, the only disadvantages are you need spend a little bit more money on both flanged and non-flanged ball bearing, but you don’t need to worry if you can find proper spacer between 2 bearings anymore.
here is the a sketch how my motor mount was assembled:
there are 3 ball bearings inside of the bore of DS-0.5 40 gear, the left most one is a flanged ball bearing, the rest 2 are non-flanged. Since they are 2x5mm ball bearing, the outer diameter for spacing mini washer shouldn’t be larger than 4mm, I only found this one meet my requirement online http://www.helipal.com/h50187-m2-5-socket-collar-screw.html. The washer came with 4 socket collar screws, and there are 4 washers per package, so 8 washers will be needed for a 4 wheel micromouse at least.
Here are some pictures after I assembled my motor mount with pinion gear and ball bearing, secured by locker nuts.
3D printed wheel that fits the gear perfectly