You can learn a lot about a bearing just from its part number.
A typical bearing is the 6203ZZ bearing. This part number can be divided into it's components:
6203ZZ
which means:
The type code indicates the type of bearing. While each manufacturer uses their own numbers, there are a few numbers that could be considered standard in the industry.
1 
SelfAligning Ball Bearing This kind of ball bearing has a spherical outer race, allowing the axis of the bearing to "wander around". This is important because misalignment is one of the big causes of bearing failure. 

2  Spherical Roller Bearing  
3 
DoubleRow Angular Contact Ball Bearing Designed to take axial as well as radial loads. 

4 
DoubleRow Ball Bearing Designed for heavy radial loads. 

5 
Thrust Ball Bearing Intended for exclusively axial loads. 

6 
SingleRow Deep Groove Ball Bearing Typical ball bearing. Handles light axial loads as well as radial loads. 

7 
SingleRow Angular Contact Bearing For axial (one direction only!) as well as radial loads. 

8 
Felt Seal To assure that the entire inside edge of the seal touches the inner ring, the inner ring is enlarged. If a bearing of more normal proportions is required, the outer ring is also enlarged, and the bearing is referred to as a "wide cup" bearing. 

32 
Tapered Roller Bearing This is the kind of wheel bearings used in cars. The rollers are not cylindrical, but conical. They handle large raidal and axial loads. 

R 
Inch (NonMetric) Bearing 
Varies 
N 
Cylindrical Roller Bearing Instead of balls, cylindrical rollers are used. These bearings can handle much more radial load, but can handle much less axial load, than ball bearings. 

NN 
DoubleRow Roller Bearing Handles greater radial loads than standard cylindrical roller bearings. 

NA 
Needle Roller Bearing Needle bearings are basically roller bearings, but the rollers are much smaller, making the bearing more compact. 
Varies 
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:
8  Extra thin section 
9  Very thin section 
0  Extra light 
1  Extra light thrust 
2  Light 
3  Medium 
4  Heavy 
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:
00  10mm 
01  12mm 
02  15mm 
03  17mm 
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:
Z  Single shielded 
ZZ  Double shielded 
RS  Single sealed 
2RS  Double sealed 
V  Single noncontact seal 
VV  Double noncontact seal 
DDU  Double contact seals 
NR  Snap ring and groove 
M  Brass cage 
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.
Number  Bore (mm) 
O.D. (mm) 
Width (mm) 

608  8  22  7 
627  7  22  7 
688  8  16  4 
698  8  19  6 
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".
Consider the following three bearings:
Number  Bore mm 
O.D. mm 
Thickness mm 

60102RS  50  80  16 
62102RS  50  90  20 
63102RS  50  110  27 
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.
Number  Bore mm 
O.D. mm 
Thickness mm 

69042RS  20  37  9 
60042RS  20  42  12 
62042RS  20  47  14 
63042RS  20  52  15 
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.
Number  Bore mm 
O.D. mm 
Thickness mm 

60002RS  10  26  8 
60012RS  12  28  8 
60022RS  15  32  9 
60032RS  17  35  10 
60042RS  20  42  12 
60052RS  25  47  12 
60062RS  30  55  13 
60072RS  35  62  14 
60082RS  40  68  15 
60092RS  45  75  16 
60102RS  50  80  16 
60112RS  55  90  18 
60122RS  60  95  18 
60132RS  65  100  18 
60142RS  70  110  20 
60152RS  75  115  20 
Number  Bore mm 
O.D. mm 
Thickness mm 

62002RS  10  30  9 
62012RS  12  32  10 
62022RS  15  35  11 
62032RS  17  40  12 
62042RS  20  47  14 
62052RS  25  52  15 
62062RS  30  62  16 
62072RS  35  72  17 
62082RS  40  80  18 
62092RS  45  85  19 
62102RS  50  90  20 
62112RS  55  100  21 
62122RS  60  110  22 
62132RS  65  120  23 
62142RS  70  125  24 
62152RS  75  130  25 
62162RS  80  140  26 
Number  Bore mm 
O.D. mm 
Thickness mm 

63012RS  12  37  12 
63022RS  15  42  13 
63032RS  17  47  14 
63042RS  20  52  15 
63052RS  25  62  17 
63062RS  30  72  19 
63072RS  35  80  21 
63082RS  40  90  23 
63092RS  45  100  25 
63102RS  50  110  27 
Number  Bore inch 
O.D. inch 
Thickness inch 

SR32RS  0.1875  0.5000  0.1960 
R42RS  0.2500  0.6250  0.1960 
R4A2RS  0.2500  0.7500  0.2813 
R62RS  0.3750  0.8750  0.2813 
R82RS  0.5000  1.1250  0.3125 
R102RS  0.6250  1.3750  0.3438 
R122RS  0.7500  1.6250  0.4375 
R142RS  0.8750  1.8750  0.5000 
R162RS  1.0000  2.0000  0.5000 
R202RS  1.2500  2.2500  0.5000 
16012RS  0.1875  0.6875  0.3125 
16022RS  0.2500  0.6875  0.3125 
16052RS  0.3125  0.9063  0.3125 
16032RS  0.3125  0.8750  0.3438 
16042RS  0.3750  0.8750  0.3438 
16142RS  0.3750  1.1250  0.3750 
16062RS  0.3750  0.9063  0.3125 
16152RS  0.4375  1.1250  0.3750 
16072RS  0.4375  0.9063  0.3125 
16202RS  0.4375  1.3750  0.4375 
16162RS  0.5000  1.1250  0.3750 
16212RS  0.5000  1.3750  0.4375 
16332RS  0.6250  1.7500  0.5000 
16232RS  0.6250  1.3750  0.4375 
16382RS  0.7500  2.0000  0.5625 
16302RS  0.7500  1.6250  0.5000 
16412RS  1.0000  2.0000  0.5625 
16522RS  1.1250  2.5000  0.6250 
16582RS  1.3125  2.5625  0.6875 
Ever 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.
Maximum Capacity Bearing
The design life of a bearing depends on rated load and the equivalent radial load.
Deep Groove: L_{10} = (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 = (XF_{r} + YF_{a}) × s
where F_{r} is actual radial load, F_{a} 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 F_{a} is zero (no axial load) you can ignore all this folderol, and P = F_{r}. Likewise, if F_{r} is zero (no radial load), then P = F_{a}.
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.
If you really want to try calculating X and Y, start here.
These are some places that sell bearings and give satisfactory service for a good price, at least in my experience.
© 2003 W. E. Johns