How to Read Bearing Numbers and What to do When the Marking is no Longer Visible?

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.

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.

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.

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.

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.

Source: Bearing Express