Letters after a bearing number: K, E, C3, W33 and what they change

The digits of the number give the dimensions. The letters after them give the version. Some letters change the fit, and without them the bearing simply will not go on: K means a tapered bore, so the bearing sits on a sleeve, not directly on the shaft. Other letters change the working conditions: C3 is increased internal clearance for hot units and tight fits. A third group describes the cage, the seals and the lubrication groove. The main rule for a replacement: carry K and the clearance into the new number as they are, and check everything else against the manufacturer's catalog. If you do not understand a letter, do not drop it. Ask.

Why the letters differ between brands

The basic number (22208, 6205) is standardized and is the same at every manufacturer. Each plant sets the supplementary designation after it in its own way. NSK says so directly in its reference. So the same feature has different letters at different brands, and the same letter can mean different things. The meanings below come from the catalogs and references of the manufacturers themselves. For another brand or a Chinese plant, check the meaning of a letter in that plant's catalog.

Letters that change the fit

K: tapered bore. The inner ring is bored to a taper of 1:12. K30 is a 1:30 taper. Such a bearing is mounted on an adapter sleeve (a nut pulls it tight on a smooth shaft) or on a withdrawal sleeve on a stepped shaft. The benefits of a tapered fit: the bearing can be placed anywhere on a smooth shaft, it is easier to remove, and if something is damaged it is the sleeve that suffers, not the shaft.

An example from the catalog: NSK 22208 EAE4 and NSK 22208 EAKE4. The size is the same, 40×80×23 mm, and the load rating is the same. The difference is one letter, K. The first sits on a cylindrical shaft, the second on a sleeve. If the unit has a sleeve, a bearing without K will not fit it. The reverse is also true.

When ordering a bearing with K, check the sleeve. It wears out and is sold separately, with its own designation.

Clearance letters: C2, CN, C3, C4

Internal clearance is how far the rings can move relative to each other. It is chosen for the heat and the fit.

C3 is fitted, for example, on the bearings of the 1.5 kW motor from the catalog. If the old number says C3 and you order a bearing with normal clearance, the rings may clamp the rolling elements once hot. The unit will start to overheat and make noise. Clearance can be changed in a replacement only on the decision of the designer or a mechanic who knows the unit.

Version letters: E, EA, CC, CA, CD

These letters describe the internal design: roller shape, cage, guide ring.

• SKF: E is an improved internal design with a higher load rating. CC is a pressed steel cage, CA is a brass cage.
• NSK: spherical roller bearings come in versions EA, C, CD and CA. EA, C and CD have a pressed steel cage, CA has a brass one. Version EA belongs to NSK's HPS series with a higher load rating.

The version affects load rating and permitted speed. The dimensions are usually the same. Swapping a version of one brand for a version of another is a matter of comparing the data in two plants' catalogs, not a letter-for-letter table.

Lubrication letters: W33 and E4

In heavy units, lubricant is fed through the outer ring, which has an annular groove and holes.

• At SKF this is marked W33: a groove and three lubrication holes in the outer ring.
• At NSK it is E4: a groove and holes in the outer ring. NSK advises cutting a groove in the housing too, because the groove in the ring is shallow.

In meaning, W33 and E4 are close. If the housing is built to feed lubricant through the ring, the new bearing must have the groove. The number of holes can differ between brands and sizes.

Seal letters

A shield and a seal are different things. A contact seal rubs on the ring, so the permitted speed of such a version is usually lower. The exact value is in the manufacturer's catalog. The motor bearings in the catalog are marked 6205 2RZ C3. So a replacement needs the same type of protection and the same C3 clearance. What RZ means at a particular Chinese plant is checked in its catalog.

How to read a long number

Take NSK 22310 EAKE4 C3 (the catalog has no such version with C3 clearance; this is an example):

• 22310: spherical roller bearing, series 23, 50 mm shaft;
• EA: version with a steel cage, HPS series;
• K: tapered bore 1:12;
• E4: groove and holes in the outer ring;
• C3: increased clearance.

SKF letters come in their own order: internal design, seals, cage, material, clearance. The order differs between brands. So a number from one brand is "translated" into another by the meaning of each letter, not by position.

How we do it

• We copy the number from the old bearing in full, with all the letters. If something has worn off the ring, we ask for a photo of the face and of the packaging.
• We pick an item with the same bore (K or not), the same clearance and the same type of protection. We check the other letters against the manufacturer's catalog.
• If the exact version is not available, we send the closest one and write what differs. The decision stays with you.