Bearings explained


Whilst research, designing and building a P3Steel version 4 3D printer, I was a bit confused when it came to the various types of bearings required for the threaded rods, lead screws and GT2 belt pulleys: 624zz, 625zz, 605zz, F624zz, 608zz, etc. Fortunately I found this page, Notes on Ball Bearings, which explained the nomenclature. I have reproduced the page below.


Notes on Ball Bearings

Bearing Nomenclature

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:


which means:

  • Type Code
  • Series
  • Bore
  • Suffix

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 Self-Aligning 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 Double-Row Angular Contact Ball Bearing

Designed to take axial as well as radial loads.

4 Double-Row Ball Bearing

Designed for heavy radial loads.

5 Thrust Ball Bearing

Intended for exclusively axial loads.

6 Single-Row Deep Groove Ball Bearing

Typical ball bearing. Handles light axial loads as well as radial loads.

7 Single-Row 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 (Non-Metric) 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 Double-Row 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.


Type 6, “single-row 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 R8-2RS 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 non-contact seal
VV Double non-contact seal
DDU Double contact seals
NR Snap ring and groove
M Brass cage

And then there are the completely off-the-wall bearing numbers, like 499502H. I have no idea what that number is supposed to mean, but it applies to what is basically an R10-2RS bearing, only a bit thicker and with a groove and snap ring.


Common Skate Bearings

Number Bore
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 Single-row 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 Comparison

Consider the following three bearings:

Number Bore
6010-2RS 50 80 16
6210-2RS 50 90 20
6310-2RS 50 110 27

We can see from the part numbers that they’re all 50mm single-row 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
6904-2RS 20 37 9
6004-2RS 20 42 12
6204-2RS 20 47 14
6304-2RS 20 52 15

These are all 20mm single-row 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 Dimensions

Extra Light Bearings

Number Bore
6000-2RS 10 26 8
6001-2RS 12 28 8
6002-2RS 15 32 9
6003-2RS 17 35 10
6004-2RS 20 42 12
6005-2RS 25 47 12
6006-2RS 30 55 13
6007-2RS 35 62 14
6008-2RS 40 68 15
6009-2RS 45 75 16
6010-2RS 50 80 16
6011-2RS 55 90 18
6012-2RS 60 95 18
6013-2RS 65 100 18
6014-2RS 70 110 20
6015-2RS 75 115 20

Light Bearings

Number Bore
6200-2RS 10 30 9
6201-2RS 12 32 10
6202-2RS 15 35 11
6203-2RS 17 40 12
6204-2RS 20 47 14
6205-2RS 25 52 15
6206-2RS 30 62 16
6207-2RS 35 72 17
6208-2RS 40 80 18
6209-2RS 45 85 19
6210-2RS 50 90 20
6211-2RS 55 100 21
6212-2RS 60 110 22
6213-2RS 65 120 23
6214-2RS 70 125 24
6215-2RS 75 130 25
6216-2RS 80 140 26

Medium Bearings

Number Bore
6301-2RS 12 37 12
6302-2RS 15 42 13
6303-2RS 17 47 14
6304-2RS 20 52 15
6305-2RS 25 62 17
6306-2RS 30 72 19
6307-2RS 35 80 21
6308-2RS 40 90 23
6309-2RS 45 100 25
6310-2RS 50 110 27

Inch Bearings

Number Bore
SR3-2RS 0.1875 0.5000 0.1960
R4-2RS 0.2500 0.6250 0.1960
R4A-2RS 0.2500 0.7500 0.2813
R6-2RS 0.3750 0.8750 0.2813
R8-2RS 0.5000 1.1250 0.3125
R10-2RS 0.6250 1.3750 0.3438
R12-2RS 0.7500 1.6250 0.4375
R14-2RS 0.8750 1.8750 0.5000
R16-2RS 1.0000 2.0000 0.5000
R20-2RS 1.2500 2.2500 0.5000
1601-2RS 0.1875 0.6875 0.3125
1602-2RS 0.2500 0.6875 0.3125
1605-2RS 0.3125 0.9063 0.3125
1603-2RS 0.3125 0.8750 0.3438
1604-2RS 0.3750 0.8750 0.3438
1614-2RS 0.3750 1.1250 0.3750
1606-2RS 0.3750 0.9063 0.3125
1615-2RS 0.4375 1.1250 0.3750
1607-2RS 0.4375 0.9063 0.3125
1620-2RS 0.4375 1.3750 0.4375
1616-2RS 0.5000 1.1250 0.3750
1621-2RS 0.5000 1.3750 0.4375
1633-2RS 0.6250 1.7500 0.5000
1623-2RS 0.6250 1.3750 0.4375
1638-2RS 0.7500 2.0000 0.5625
1630-2RS 0.7500 1.6250 0.5000
1641-2RS 1.0000 2.0000 0.5625
1652-2RS 1.1250 2.5000 0.6250
1658-2RS 1.3125 2.5625 0.6875

Other Stuff

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

Design Life

The 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.

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.



01/01/2017 12:33:32

© 2003 W. E. Johns



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