Understanding Your Bearing Code

In the world of rotating machinery, the humble bearing is a critical component. To the untrained eye, the string of numbers and letters stamped onto a new bearing or listed in a maintenance manual—like 6203-2RS or 33012 C3—can look like a random code. But for engineers and procurement specialists, this “bearing number” is a precise technical language .
Understanding what bearing numbers mean is essential for minimizing downtime, avoiding costly mismatches, and ensuring your equipment operates safely and efficiently. This guide will teach you how to read them, moving from the basic structure to the specific suffixes that define a bearing’s performance.

The Anatomy of a Bearing Number: Basic + Supplementary

Most bearing designations follow a logical system, largely standardized by the International Organization for Standardization (ISO) . This system consists of two main parts:

The Basic Designation: This is the core code that defines the bearing type, its critical dimensions (like width and outside diameter), and its bore size .

The Supplementary Designation: These are prefixes (before the basic number) and suffixes (after the basic number) that indicate specific design features, material variations, seals, cages, clearances, or precision classes. These often vary by manufacturer .

Part 1: Breaking Down the Basic Designation (The Core Identity)

Let’s dissect the basic designation using the common example of a deep groove ball bearing: 6 2 04.

1. The Prefix (Starting Point)

A prefix is less common but important. It usually modifies the basic design. For example, K often denotes a bearing with tapered bore (e.g., K 6203) . Prefixes like GS or WS are used for thrust bearing components (raceways) .

2. The Type Code (First Digit or Letter)

The first digit (or first letter/digits for some roller types) tells you what kind of bearing you are holding. This is the most fundamental part of the identification.

Table 1: Common Bearing Type Codes

3. The Dimension Series Code (The Second and Third Digits)

The second digit (and sometimes third) represents the dimension series. This code defines the relationship between the bearing’s width and its outside diameter for a given bore size . Think of it as the “heft” or robustness of the bearing.

• The first number of this series indicates the Width Series (e.g., 0, 1, 2, 3…).
• The second number indicates the Diameter Series (e.g., 7, 8, 9, 0, 1, 2, 3, 4…).

Generally, a higher number in this series means a larger cross-section, which translates to a higher load capacity. In our 6204 example, the ‘2’ signifies a specific dimension series (width series 0, diameter series 2) that is very common for light-to-medium applications .

4. The Bore Size Code (The Last Two Digits)

The last two digits of the basic designation are a code for the bearing’s inner diameter (the bore). This is where a simple calculation is needed.

The Golden Rule (for sizes 20mm to 480mm): For bore codes 04 and above, multiply the code number by 5 to get the bore diameter in millimeters.

• Code 04 → 04 x 5 = 20mm bore .
• Code 08 → 08 x 5 = 40mm bore.
• Code 20 → 20 x 5 = 100mm bore.

The Exception (for smaller sizes): For bore codes 00, 01, 02, and 03, the bore diameters are fixed standards.

Table 2: Small Bore Size Code Exceptions

So, for our example 6204, the ’04’ code means the bearing has a 20mm bore.

Part 2: Deciphering the Suffixes (The Specific Features)

This is where a bearing number becomes truly specific. Suffixes define how the bearing is sealed, its internal fit, and its precision. Since these can vary by brand, understanding them is a mark of true expertise . Here is a guide to the most common ones you will encounter.

1.Seals and Shields

These suffixes tell you how the bearing is protected from contaminants. A missing suffix usually means an open bearing.

*Table 3: Cross-Reference of Common Seal/Shield Suffixes*

2.Internal Clearance (C-Group)

Internal clearance is the amount of internal space between the rolling elements and the raceways. This is critical for managing thermal expansion.

• C2: Clearance smaller than Normal.
• CN (or no suffix): Normal clearance.
• C3: Clearance greater than Normal. Commonly used in applications like electric motors and automotive wheel hubs where the inner ring heats up and expands more than the outer ring .
• C4: Clearance greater than C3 .

3.Precision Tolerances (P-Class)

Most standard bearings are built to “Normal” (P0) tolerance. Higher precision classes are for high-speed or high-accuracy spindles.

• P6: Tolerance class better than P0.
• P5: Precision tolerance class, better than P6 .
• P4: Ultra-precision tolerance class.

4.Cage Materials

The cage (or retainer) separates the rolling elements. The material is often denoted by a suffix.

• M: Machined brass cage .
• TV / TB: Polyamide (nylon) cage.
• J: Stamped steel cage .
• F: Machined steel cage .

5.Other Important Suffixes

• K: Tapered bore (1:12 taper) .
• K30: Tapered bore (1:30 taper) .
• W33: Annular groove and lubrication holes in the outer ring .

Expert Tips for Accurate Bearing Interpretation

Read the Full String: Do not just look at the basic number. A 6203 is just a bearing; a 6203-2RS1 C3 is a specific component for a specific job.

Consult the Manufacturer’s Catalog: While the basic designation is standardized, suffixes are not. Always verify the meaning of a suffix in the official catalog of the brand (SKF, NTN, FAG, etc.) if you are unsure .

When in doubt, ask: If you are replacing a bearing and the number is worn or unclear, or if you are trying to cross-reference a number from one brand to another, ask an expert.

Frequently Asked Questions

What do bearing numbers mean?
Bearing numbers are a coded system that identifies a bearing’s type, dimension series, bore size, and specific design features like seals, cages, and internal clearances .

How do I read bearing numbers?
Start from the left. The first digit(s) indicate the type. The following digits define the dimension series (robustness). The last two digits of the basic number indicate the bore size (using a specific rule: multiply by 5 for codes 04 and up). Suffixes at the end detail specific features .

What is a bearing trade number?
It is essentially the same as the bearing part number or designation. It is the unique identifier used by manufacturers and suppliers to order and identify a specific bearing .

What is a bearing part number breakdown?
It is the process of dissecting the alphanumeric code into its constituent parts—prefix, basic designation (type, series, bore), and suffix—to understand the complete specifications of the bearing .

Conclusion
Mastering the language of bearing numbers transforms a seemingly random string of characters into a precise engineering specification. It empowers you to make informed decisions, avoid installation errors, and keep your machinery running smoothly.
With two decades of experience in manufacturing and a deep understanding of global bearing standards, DUHUI Bearing is your trusted partner. Whether you need help interpreting a complex code or sourcing a high-quality replacement, our team is here to assist.