A bearing designation packs a lot into a short code. 6205-2RS C3 names the type of bearing, its size, how it is sealed and how much internal play it has. The basic number is close to universal across makers; the suffixes are not. This glossary covers the parts of the code that maintenance engineers meet most often on deep groove ball bearings.
The basic number: type, series, bore
- Type: the first digit. 6 is a deep groove ball bearing, 7 an angular contact ball bearing, 1 a self-aligning ball bearing.
- Dimension series: the next digit. In 6205 the 2 marks a light series; a 6305 has the same bore with a larger outside diameter and width.
- Bore code: the last two digits. From 04 upwards, multiply by five for the bore in millimetres: 05 is 25 mm, 10 is 50 mm. Below that, 00, 01, 02 and 03 stand for 10, 12, 15 and 17 mm.
So a 6205 is a deep groove ball bearing with a 25 mm bore, a 52 mm outside diameter and a width of 15 mm, whoever made it. If the marking has worn off, measure the old bearing: those three dimensions and the type give you the basic number.
Shields and seals
- Z: a metal shield on one side, without contact. ZZ or 2Z: shields on both sides.
- RS: a rubber contact seal on one side. 2RS: seals on both sides. They keep out dirt and water better than shields and add a little friction.
- Each maker’s own letters: SKF writes 2RSH or 2RS1, FAG writes 2RSR, and NSK writes DDU for contact seals on both sides and VV for non-contact seals.
A bearing sealed or shielded on both sides is greased for life. One with a single shield is lubricated from the open side, so the way round it sits in the housing matters.
Internal clearance: C3 and its neighbours
Clearance is the play between the rings and the balls before the bearing is fitted. A tight fit on the shaft and heat in service both reduce it, so the class is chosen for the application:
- C2: less than normal.
- CN: normal, usually not marked at all.
- C3: more than normal, common in electric motors and other bearings that run warm.
- C4 and C5: more again, for higher temperatures or tighter fits.
Replace C3 with C3. A normal-clearance bearing in a motor designed for C3 can run hot and fail early, and on the shelf the two look identical.
Cage and other suffixes
- Cage: SKF uses J for pressed steel, M for machined brass and TN9 for glass-fibre reinforced polyamide; FAG writes TVH for its polyamide cage. A polyamide cage has a lower temperature limit than steel or brass.
- N and NR: a snap ring groove in the outer ring, and the same groove with the snap ring fitted.
- Tolerance class: P6 and P5 are tighter than normal; normal is not marked.
- Grease and heat treatment: these codes differ most between makers. Read them from the maker’s own designation key, not from a cross-reference table.
Ordering an equivalent from another maker
For a standard deep groove ball bearing, the basic number, the seal type and the clearance class make the request. Write all three: 6205-2RS C3 leaves less to guess than 6205. If the old bearing carried a cage or grease code you cannot read, say so and ask suppliers to quote the maker’s own designation. Search the full code, or send one request for quotes for every bearing of a motor overhaul. Bearings are among the most copied industrial parts; the guide to counterfeit checks covers what to look at when a branded bearing is suspiciously cheap.