One of the toughest things to find over the last few years has been bearings. Especially when I was looking for free or nearly free. I have a friend that would give me a couple every now and then but that was few and far between. Usually, when I was testing a windmill blade design or vertical axis wind turbine design, I would have to order some bearings. After getting a shaft and two bearings plus shipping, the cost was about $70 or so. That was just too expensive and just too limiting. For the kind of testing I do, I needed more than a couple of bearings. I need several sets. They need to be cheap and readily available without ordering through the mail or driving 30 minutes to go to a specialty store.
Well, a couple of months ago, I found just the place. A thrift store. Yep, a thrift store. I found a skateboard for $2 one day and thought I would try it. I found out that the skateboards and the roller blades use the "608" bearing. After some research I found that each bearing can support up to about 900 lbs radially and up to about 400 lbs axially. Each skateboard or rollerblade wheel has 2 bearings inside. The weight pushing down on the wheels would be radial load. In theory each wheel could handle 1800 lbs of downward thrust, except for the fact that the rubber in the wheels would squish around the edge of the bearing and stop it from turning due to friction. Not sure what "real world" weight limit would be with the rubber wheels, but it is significant for brief periods. When you slide sideways, while skating, that is the axial thrust. But don't be tricked here. The axial load limit isn't doubled due to two bearings in this case. Only one bearing at a time takes that load depending on which way the slide is. That is why they have two bearings.
The inside diameter of a 608 is 8 mm, but I found that I can use a 5/16 bolt to hold it. It is slightly smaller and that is what the skateboard chuck uses anyway. And you can find a skateboard every so often in a thrift store. But what you always find is roller blades. I saw 6 sets of rollerblades just 2 days ago in a thrift store for $5 a pair. That is 96 bearings for $30. You really can't beat that. I would recommend getting the adult sizes of roller blades because the smaller sizes may use cheaper bearings.
You can also find many videos on http://youtube.com about taking the bearings out and cleaning and relubing. A lot of people use a good bearing oil but they don't seem to understand that with oil you have to relube once a week or more often. Oil is light and does make the wheel spin faster with your hands, but real world testing with oil and grease show that grease is just as fast under load after the first 20 to 30 seconds of warmup. And a greased bearing can go for months or even years without having to be cleaned and relubed. So, if you are doing windmill applications, please grease the bearings. It waterproofs the bearings, it pulls dirt away from the roller surfaces, and requires much less maintenance.
Here is the bottom bearing for a 65 lb vertical windmill I was testing.
But remember, in this case the bottom bearing is supporting all the weight and that is axial load. So, 400 lbs would be the limit. Although, I would say 200 lbs would be the limit just to be safe. Especially since these bearings are meant to have radial thrust most of the time. The radial thrust in this case would be from the wind and up to 3600 lbs of that would cause the wood to snap very easily and the wind would be hurricane forces.
By the way, this test didn't work that great. Adding more blades helps, but a regular savonious works better.