I post a lot of stuff and as a result people PM me for advice on how to do things. That just happened and the question was how to replace wheel bearings. I thought it worthwhile to repost what I just sent to BCTECH. At the end is a video on the subject that is also useful.
Ball bearings don't tolerate lateral forces very well. They are intended to take radial forces almost exclusively. When you tighten the nuts that hold the axle to the forks, you are applying lateral force to the bearings. So that this doesn't happen, inside the wheel and between the left and right bearings and between the inner races is a spacer tube. It's exactly the length needed to not allow any of the lateral forces from the axle nuts to be applied to the balls in the bearings. All lateral forces are transmitted to the inner bearing races and the spacer tube. This allows the bearing to see only radial forces from standard wheel rotation.
So then, between your wheel bearings, is that tube. Right now it is centered between the 2 inner races on the existing bearings. You need to knock it out of place so that you can get at the inner races on your bearings. You are replacing the existing bearings because dislodging them is rather hard on the bearing since you will be hitting the bearing with something metal and hitting that with a hammer and applying lots of lateral forces to the bearings which will probably damage them. I use a long flat blade screw driver. I slide it down the center of that tube and just above the inner side of the opposite bearing. Hit the side of the screw driver that is sticking out of the wheel with a hammer and it will dislodge that steel tube so that you can see the inner race of the opposite bearing. Now place the screw driver blade on the bearing race and start tapping it out of the wheel. They are press fit into place. You want to be sure that you extract the bearing straight out. If it gets cocked, that is bad. Tap the opposite side of the inner bearing race to get it coming out straight. A few taps with the hammer and the bearing is loose. The spacer tube will fall out at the same time most likely. Once one bearing is out, getting at the opposite side is easy since it will be exposed inside the wheel and you can tap it out with a screw driver and hammer. Again make sure it comes out straight and even.
Once you do this with a bearing, it's best to NOT reuse that bearing. It's likely that you've caused it some damage with hitting the inner race and transferring that energy through the balls to the outer race. It might be OK, but probably not. I just assume the bearing is now damaged and replace them with new ones.
I remember a wheel a while back that had a spacer tube that had a shoulder on it for catching the inner bearing race and it also fit inside the bearings inner race. The bearings ID was 12mm, but because of the portion of the spacer tube that fit inside the bearing ID, the axle could only be 10mm. This spacer tube design meant that I couldn't dislodge is sideways a little to get to the inner bearing race of the opposite wheel bearing. Instead, I scrounged through my socket sets for a socket that had an OD that was just slightly smaller than 12mm, but larger than 10mm. I placed the socket over the end of the spacer tube and tapped the socket with my hammer. This pushed the spacer out the other side of the wheel and pushed the opposite bearing out at the same time. Once one bearing was extracted, I grabbed a screw driver and tapped out the remaining bearing as usual. I reused the same bearing spacer when I installed the new bearings. It didn't make extracting the bearings any harder, I just needed to go about doing it a little differently.
1. If they are exposed to the weather, always use sealed bearings.
2. If the bearings are enclosed and never get exposed to dirt and water, shielded bearings work fine.
3. Never apply lateral forces to a wheel bearing you care about.
4. You have 3 types of bearings. All steel (steel races and steel balls), hybrid (steel races and ceramic balls) and all ceramic (ceramic races and ceramic balls). All steel is the cheapest and have the highest amount of internal friction. All ceramic are the most expensive and the least amount of internal friction. I typically use hybrid bearings since they are much better than all steel bearings in every way and cost about half as much as all ceramic bearings.
Installing new bearings in your wheels...
1. You want the new bearing to go in straight. IF it gets cocked in the bearing seat, don't force it, straighten out the bearing and make sure it goes in straight.
2. Never hit the inner bearing race while installing a bearing in a wheel. This is bad and damages the bearing. You only want to apply force to the outer bearing race when installing a bearing. Some bearings are press fit on the inner race (not typical on wheels). In that case you don't want to apply lateral forces to the outer race. Only apply force on the race that is press fit.
3. A socket the same diameter as the outer bearing race placed on top of the bearing is a good thing to hit to insert a new bearing. The old bearing works too since it's exactly the right diameter.
4. Once one bearing is seated fully in the wheel, flip over the wheel, insert the spacer tube and then install the other bearing the same way...straight in and applying force only to the outer race where the bearing is press fit.
5. There is the possibility that the bearing seats are a little too deep into the wheel. When you insert the second bearing, if this is the case, then you will feel it when you try to turn the bearings. They will turn together since they are jammed against the spacer tube. They will turn roughly since they are jammed together and there is lateral forces on the balls. You won't have a choice here, you have to tap gently at the inner race of a bearing through the center of the spacer tube to slightly push it out a little to relieve this. The bearings should always turn smoothly. Better yet, pay attention to the second bearing that you insert it just the right amount so that it just sits on the spacer.
This is a good video on the subject. Start at about 18 minutes and end at about 35 minutes. That's the part that's relevant to most wheels. A lot of wheels don't use a secondary seal since the bearings are sealed. This is normal. In the video the motor cycle wheel has only one bearing seat and shoulder on one side. The bearing on the other side has nothing to stop it from going in too deep. I've run into this before. Pay attention to what he says about installing the second bearing and the spacer tube so that you can avoid installing the second bearing too tightly against the spacer tube. He puts grease on the outside of the bearings before he presses them back in place. I never do, but it's fine either way.
I needed to change out bearings in a new wheel so I recorded the process...
1 Removing bearings from a wheel: https://youtu.be/R83LZLUq7QI
2 Bearing placement tips: https://youtu.be/WXtD3ntKS0M
3 Installing bearings in a wheel: https://youtu.be/4Dh6-4_dge0