When you remove a worn bearing from equipment and wipe off the grease, the marking stamped on the outer ring is not a random string of characters. It follows an ISO-standard coding logic. Mastering this system allows you to quickly grasp the bearing’s specifications and structural features, supporting proper spare part selection and replacement. A full designation consists of a prefix, a basic designation and suffixes. The basic designation defines core dimensions, while suffixes specify operational characteristics such as sealing, internal clearance and cage design, which are often overlooked by maintenance personnel.
The basic designation identifies the bearing type, size series and bore size in sequence. The widely used deep groove ball bearing starts with the digit 6 and is commonly found in motors, pumps, fans and other general machinery. Angular contact ball bearings and roller bearings follow separate coding rules. The second digit stands for the size series. For bearings with the same bore, a higher series number means a larger outer diameter and width, together with a higher load capacity. The last two digits form the bore code. For codes 04 and above, multiply the number by five to get the bore diameter. Codes 00, 01, 02 and 03 correspond to 10 mm, 12 mm, 15 mm and 17 mm respectively. For miniature bearings below 10 mm bore, the final digit directly indicates the diameter. For large bearings of 500 mm bore or more, the dimension is marked after a slash.
While the basic designation sets the bearing’s boundary dimensions, suffixes determine interchangeability. Sealing codes appear most frequently. Z denotes a single metal shield, and ZZ means shields on both sides. These non-contact shields create minimal friction and suit clean, high-speed environments yet offer limited defence against dust and moisture. RS marks a single rubber contact seal, and 2RS indicates seals on both sides. Rubber seals lightly contact the inner ring for better protection in dusty and humid conditions, at the cost of slightly higher friction and a reduced speed limit. Equivalent sealing marks may vary across coding systems, so working conditions should always be considered during selection.
Internal clearance codes are equally critical. Clearance refers to the small free space between rolling elements and raceways. An incorrect clearance grade causes overheating, noise and premature failure. The normal CN clearance is usually unmarked. C2 means reduced clearance for quiet, precision applications. C3 has greater clearance than normal and is widely used in motors and pumps. Heat buildup and interference fits shrink operating clearance, and the extra space in C3 bearings prevents binding. C4 and C5 provide progressively larger clearance for high-temperature or heavy-interference applications. Unless equipment specifications state otherwise, replacements should retain the original clearance suffix.
Suffixes also indicate cage material, precision class and tapered bore. Steel cages serve most general purposes. Polyamide cages run quietly but have temperature limits, while brass cages withstand heavy loads, vibration and high heat. The suffix K identifies a tapered bore bearing, typically mounted with an adapter sleeve. Higher precision grades are reserved for precision spindles. Prefixes are uncommon on standard bearings and mostly indicate special materials or split constructions.
Take 6205‑2RS C3 as an example: 6 stands for a single-row deep groove ball bearing, 2 for its size series, and 05 converts to a 25 mm bore. 2RS represents double-sided rubber seals and C3 is the increased clearance grade, with an outer diameter of 52 mm and width of 15 mm. It is a typical choice for motors and fans. Many maintenance workers only check overall dimensions before replacement, ignoring suffix differences. Bearings with identical basic numbers are not always interchangeable if sealing or clearance differs. When markings wear away, measure dimensions with calipers and select suffixes based on service conditions. Learning to decode bearing designations helps match spare parts accurately, avoid selection errors and reduce unplanned downtime.