Bearing Nomenclature: Decoding the Secret Language of SKF/FAG

Why replacing a 6205 with a 6205-C3 can destroy your motor

Bearings are the joints of the machine world. To the untrained eye, the code on the side is gibberish. To an engineer, it contains the entire specification of the component. Let’s break down the ISO standard used by giants like SKF, FAG, and NTN.

🔍 The Anatomy of the Code

A standard metric bearing code is built like this: Prefix + Basic Designation + Suffix

A. The Basic Designation (The Core Identity)

Usually 3 to 5 digits. Let’s look at a common example: 22218

1. First Digit (The Type): Tells you the construction.
   • 6: Deep Groove Ball Bearing (The most common).
   • 7: Angular Contact Ball Bearing (Thrust + Radial load).
   • 2: Spherical Roller Bearing (Self-aligning, heavy load).
   • N: Cylindrical Roller Bearing.
2. Second Digit (The Width/Series): Tells you how “beefy” it is.
   • 0 or 1: Very light/Extra light.
   • 2: Light.
   • 3: Medium.
   • 4: Heavy.
3. Last Two Digits (The Bore Size): The Rule of 5.
   • Multiply the last two digits by 5 to get the bore diameter in millimeters.
   • Example: 18 x 5 = 90mm Bore.
   • Exceptions: 00=10mm, 01=12mm, 02=15mm, 03=17mm.

B. The Suffix (Where Mistakes Happen)

This is where maintenance engineers often go wrong.

C4: Clearance greater than C3.

Sealing:

Z or 2Z: Metal Shield (High speed, keeps large particles out).

RS or 2RSH: Rubber Seal (Lower speed, waterproof).

Cage Material:

M: Brass cage (Heavy duty).

P: Polymer/Nylon cage (General purpose).

Internal Clearance (CRITICAL):

CN: Normal clearance (often omitted).

C3: Clearance greater than normal.

🛠 Real-Life Engineering Scenario

The Failure: An induction motor in a steel plant kept seizing. The mechanic replaced the original bearing 6310-C3 with a generic 6310 found in the store. The Physics: The motor runs hot (80°C). The shaft steel expands due to thermal expansion. The Root Cause: The C3 rating provides extra internal room for this expansion. The standard 6310 had “Normal” clearance. When the shaft expanded, it consumed all the clearance, crushing the balls against the race. The Fix: Always match the C-rating. If the application is hot or vibrating, you likely need C3 or C4.