Most bearing numbers, whether it is 6205 or 32006 or NU205, are short versions of a full technical description. They tell you what type of bearing you are holding, what size it is and often also in what kind of environment it can be used safely.
The problem is that when a bearing fails it often overheats or discolours and the number disappears from the ring. In many plants this is the point where guesswork starts, even though the logic behind bearing markings is simple and consistent.
Reading and understanding bearing numbers is not a theoretical exercise. For a maintenance technician it is part of everyday work. Once you know the system you can identify the type faster, avoid wrong orders and lose less time with searching and downtime.
Why it is useful to understand the system behind bearing numbers?
Bearing markings follow an international standard. The numbers and letters show what bearing you have in your hand and what dimensions belong to it. This is especially important when the marking is no longer readable. With the main dimensions most bearings can be identified quite easily, but for that you need to know how the code is built up.
A single bearing number does not only mean a size. It also gives information about the internal design and the load capacity. Anyone who understands the markings usually decides faster, orders the wrong bearing less often and has fewer unnecessary stoppages.
How a bearing number is structured?
In most cases bearing numbers are made up of three main elements.
- First element: the bearing type
- Second element: the size series
- Third element: the bore diameter
At the end of the code you often find letters. These indicate sealing, internal clearance, accuracy class or some special design feature.
The first number or letter that shows the bearing type
This part tells you what internal design you are dealing with. In industry these are the most common markings.
| Code | Type | Typical application |
|---|---|---|
| 1 | Self aligning ball bearing | Shafts where slight deflection or misalignment can occur |
| 2 | Spherical roller bearing | Heavy duty industrial units, vibrating equipment |
| 3 | Tapered roller bearing | Shafts, vehicle wheels, gearboxes |
| 4 | Double row ball bearing | Rigid, higher capacity, precision applications |
| 5 | Thrust ball bearing | Mainly axial load, axial support |
| 6 | Single row deep groove ball bearing | General machinery, drives, electric motors |
| 7 | Single row angular contact ball bearing | Combined load, ideal at higher speeds |
| N, NU, NJ, NUP | Cylindrical roller bearing | Industrial drives, high radial load |
| NK, NKI | Needle roller bearing | Small installation space, high load in gearboxes and machine tools |
Already from the first number or letter you can see if the bearing was designed for general purpose use, heavy load or a more specialised application.
The second bearing number shows the series and load range
This element defines the relation between bore, outside diameter and width. In other words, how slim or how robust the design is.
| Code | Series | Typical use |
|---|---|---|
| 0 | Extra light series | Lower loads, tight space |
| 2 | Light series | Common industrial applications |
| 3 | Medium series | More robust design, higher load capacity |
| 4 | Heavy series | Heavy duty, demanding environment |
| 8–9 | Thin section series | Compact constructions |
The series does not only define size. It also influences the load capacity and how sensitive the bearing is to mounting errors or shaft alignment.
The last two numbers show the bore size
The bore of standard bearings follows a simple rule. The last two digits give the bore diameter.
| Code | Bore |
|---|---|
| 00 | 10 millimetres |
| 01 | 12 millimetres |
| 02 | 15 millimetres |
| 03 | 17 millimetres |
| 04 and above | the number multiplied by five, in millimetres |
So code 05 means a 25 millimetre bore, 06 means 30 millimetres, 07 means 35 millimetres and so on.
What the letters at the end of bearing numbers mean
Manufacturers use these letters to give additional information about the design and the intended application.
| Code | Meaning | When it matters |
|---|---|---|
| 2RS, DDU | Rubber seals on both sides | Dusty or humid environment, better protection against contamination |
| ZZ, 2Z | Metal shields on both sides | High speed, cleaner and drier environment |
| C3 | Increased internal clearance | Machines with higher temperature or higher speed |
| K | Tapered bore | Where the bearing is clamped onto a shaft |
| NR | Groove and snap ring | Axial location in tight housings |
| P6, P5 | Precision class | Precision machines and machine tools |
These markings may look like small details but in practice they can decide the service life, running temperature or noise level of the bearing.
What to do when the bearing number is no longer readable?
Identifying worn or damaged bearings is a recurring task in many plants. In this situation three measurements give you a reliable reference.
- Inside diameter (d), the bore
- Outside diameter (D), the outer ring
- Width (B), the total bearing width
From these dimensions most types can be determined without difficulty.
For example, a bearing with 25 × 52 × 15 millimetres in practice almost always corresponds to type 6205.
The sealing design also helps. Rubber seals are usually black or brown, metal shields have a bright metallic surface. If the bearing is open and you can see the balls and the cage then the code normally does not contain letters for sealing.
Where bearing identification usually goes wrong?
According to the experience of maintenance technicians most mistakes cluster around a few recurring points.
- Mixing up tapered and straight bores
This can cause fitting problems and the bearing will not sit properly on the shaft. - Ignoring internal clearance
The presence or absence of C3 clearance has a strong effect on temperature and service life. - Misjudging the sealing type
In a dusty environment metal shields often give too little protection. - Inaccurate measurement of width
On cylindrical roller bearings this often points to a completely different series.
Practical examples of interpreting bearing numbers
Code 6204
- Type: deep groove ball bearing
- Series: light
- Bore: 20 mm
Code 32006
- Type: tapered roller bearing
- Series: light
- Bore: 30 mm
Code NU205
- Type: cylindrical roller bearing
- Series: light
- Bore: 25 mm
These simple examples show that there is a clear and transparent system behind the markings. Once you understand this system a bearing can be decoded quite quickly using only the number and a few measurements.
Bearing numbers are part of a well defined standard. From the markings you can determine the type, the size series and the bore. If the number is no longer visible the three basic dimensions usually allow you to identify the bearing in a short time and avoid the wrong selection.
In practice accurate identification is rarely a complicated task. What it needs is attention and consistent measurement. The better you know the logic of bearing markings the less time you spend searching for the right bearing and the more time remains for reliable operation of the plant.
If you have any questions, please feel free to contact us or read the FAQ section below!
In our Csapagy.hu webshop, you will find bearings from more than 50 manufacturers, available immediately from stock.
Frequently asked questions – Bearing numbers, bearing identification
What if the dimensions match more than one bearing type?
It happens, especially when the bore and outside diameter appear in several bearing families. In this case look at the seal type, the cage design and the exact width. If all three match you can be confident about the type. If more than one option still remains check the machine environment, because that often shows which series was originally used.
How precise does the bore measurement need to be?
Even a tenth of a millimetre matters. Wear or tapering can mislead a measurement. If the bore is slightly tapered your caliper will show different values at different points. This is a good indication that the original bearing may have had a tapered inner ring.
How do I know whether the bearing needed C3 clearance or normal clearance?
C3 clearance is common in applications where the bearing heats up or runs at higher speed. Machines with long duty cycles or higher load typically require C3. If you are unsure it helps to check the other bearings on the same shaft or look at the machine documentation, because clearance choices are usually consistent along the shaft.
If I am not sure about the seal type what should I choose?
Rubber seals provide better protection against dust and moisture which makes them the safer option in most environments. Metal shields work well at higher speeds and in clean dry applications. If you see visible contamination on the old bearing or the machine runs in a dusty or humid area rubber seals are usually the better choice.
Are the three main dimensions enough or are there exceptions?
In most cases the three main dimensions are enough. There are rare exceptions such as special cages or unique series where the dimensions fit but the internal geometry is different. If more than one type fits the measurements, comparing the inner and outer ring shapes can help narrow it down.
How can I tell if the old bearing had a tapered bore?
If the inner ring is still available and you can measure the bore in several positions, a slight taper will be visible. A tapered bore does not have uniform wall thickness and the diameter changes slightly. If the inner ring is damaged or missing, check the hub or the shaft, because they often show clear marks that a tapered bearing was fitted.
The width measurement can be misleading. What is the correct way to measure it?
Always measure width on a clean undamaged ring. If the bearing has broken apart or the outer ring edge is damaged, the width may not be accurate. Cylindrical roller bearings are especially sensitive, because even a small deviation can indicate a different series. If you are unsure, measure a second component or confirm with a bearing table.
If I do not remember the clearance how do I choose between normal and C3?
The operating temperature of the machine is the key factor. Shafts that run hot or under constant load usually need C3 clearance. If the machine runs cool or at low speed normal clearance is often enough. If the old inner ring shows discolouration or heavy marks, it may indicate that increased clearance would have been required.
Can the wrong seal type cause problems?
Yes. Rubber seals provide better protection but create slightly more friction and heat. Metal shields protect less but allow easier rotation. If the machine is sensitive to temperature or runs at very high speed,
the wrong seal type can shorten bearing life. In wet or dusty environments rubber seals are generally the safer option.
Can I tell the manufacturer from the bearing number?
Not reliably. The number and letter combinations follow an international standard, so several manufacturers use the same markings. The brand is usually shown by the logo, but if that is no longer visible
only packaging or purchasing records can confirm it. The bearing number tells you the type, but not necessarily the manufacturer.
