Type 6, "singlerow deep groove", is perhaps the most common type of bearing. If the bearing is an inch bearing (the first digit in the number is an R), then the size is the digit or digits immediately following the R, in 16ths of an inch. An R82RS bearing, for example, has an 8/16th or 1/2 inch bore. If the first digit is a number, however, it is a metric bearing, and the second digit is the series, which reflects the robustness of the bearing. The series are, from lightest to heaviest:
Yes, they go in that order. Gotta keep things simple, you know. Each of these series also establishes a relationship between the bore size, outer diameter, and thickness of the bearing, in accordance with ISO standards. I have no idea what they are. The third and fourth digits indicate the bore size in millimeters. Except for 0 through 3, the bore size is simply five times the third and fourth digits together. 0 through 3, however, are different:
If there is no fourth digit  for example, a 608 bearing, a common roller skate bearing  then the size is the last digit in millimeters. The last letters indicate something special about the bearing. For example:
And then there are the completely offthewall bearing numbers, like 499502H. I have no idea what that number is supposed to mean, but it applies to what is basically an R102RS bearing, only a bit thicker and with a groove and snap ring. ExamplesCommon Skate Bearings
All these bearing numbers start with 6, which tells us they're Singlerow deep groove ball bearings. The second digits tell us the robustness of the bearings. The last two, in series 8 and 9, are very thin and lightweight bearings, while the first, in series 0, is an "extra light" bearing without being abnormally thin. The third bearing, in series 2, is the most robust of all, being merely "light". Light vs Heavy ComparisonConsider the following three bearings:
We can see from the part numbers that they're all 50mm singlerow deep groove ball bearings. However, we can also see that they're each a different series; specifically, Extra Light, Light, and Medium. Compare the O.D. and thickness of each bearing, and you can see how the Extra Light bearing (series 0) is the smallest, and the Medium Bearing (series 3) is the largest. The larger bearing can take much more load than the smaller bearing, though how much depends on the manufacturer and the RPM the bearing is run at.
These are all 20mm singlerow deep groove ball bearings of different series. The first, of series 9, is a "very thin section" bearing, meaning it is much thinner than usual  it is only 25% as thick as its O.D., while the others are approximately 30% as thick as their O.D. Common Bearing DimensionsExtra Light Bearings
Light Bearings
Medium Bearings
Inch Bearings
Other StuffEver wonder how they assemble ball bearings? There are two ways. The typical ball bearing, called a Conrad bearing. There is enough space between the balls that if they're all pushed over to one side, the inner ring can be pushed to the opposite side, into the space left by moving the balls. This increases the space on the side where the balls are, letting them be removed. The bearing cage usually keeps the balls evenly spaced so this doesn't happen by accident. Conrad Type Bearing Assembly The other kind of ball bearing is called a maximum capacity bearing, and has a special notch cut in the side of the rings, into which the balls are placed during assembly. As a result of this notch, the axial loads this kind of bearing can take are quite small, and must be in combination with a large radial load. However, the increased number of balls that can be fit into the bearing means the maximum capacity type bearing can handle a larger radial load.
Design LifeThe design life of a bearing depends on rated load and the equivalent radial load. Deep Groove: L10 = (C/P)n The rated load, C, is the load at which 10% of bearings fail after one million revolutions. The manufacturer will provide this number. One million revolutions may sound like a lot, but it's not. A car engine typically has one million revolutions on it after only eight hours. The equivalent load, P, is a combination of axial load and radial load, times some factor to account for shock loading, acceptable noise levels, lubrication quality, cleanliness, speed, temperature, etc. Calculating it can be a pain. The exponent, n, is 3 for radial bearings, and 3.33 for thrust bearings. This large an exponent means that doubling the load on a bearing will decrease its life by a factor of eight or ten, depending on the type of bearing. Don't overload your bearings! The formula for calculating equivalent load is P = (XFr + YFa) × s where Fr is actual radial load, Fa is actual axial load, X is the static radial factor, and Y is the static axial factor, and s is the service factor, which varies from 1 on up. If Fa is zero (no axial load) you can ignore all this folderol, and P = Fr. Likewise, if Fr is zero (no radial load), then P = Fa. Calculating X and Y is so complicated that I avoid it when I can  by using separate thrust and radial bearings, by assuming X is 1 and Y is 3 (values which far exceed anything realistic), or by using software. SKF has an online bearing calculator here. 


