A bearing number is a compressed specification, and the compression stops at the basic boundary dimensions. Everything that actually separates one product from another - cage material, closure type, internal clearance, precision class, paired arrangement and special heat treatment - hides in the letters before and after the number. Buyers who order a 6205 and receive a shielded C3 P6 part when they expected an open CN component are not victims of a typo. They read the size and not the designation. Decoding the whole string is a core purchasing skill.
Why a number is only a starting point
The basic number was designed for interchangeability, not for completeness. ISO 15 fixes the envelope - bore, outside diameter and width - so that a 25 mm bore bearing of a given series fits the same housing whoever makes it. That is a gift to the machine builder and a trap for the buyer, because it guarantees the part fits without guaranteeing it behaves. Two bearings can share every boundary dimension and still differ in speed limit, clearance, seal, noise grade and life.
Reading the full designation in the right order turns a string of digits and letters into a reliable order line, and it is the difference between buying a size and buying a part.
It also explains why quotations for the “same” bearing differ so widely. Two suppliers may quote the same basic number, then quietly fill the order with whatever suffix variant suits their stock - an open part where a sealed one was wanted, or a CN clearance where the running temperature demanded C3. The number matched, so nobody flagged the difference, and the failure appears months later as noise or heat rather than as a wrong part at the door.
A designation is therefore best read as a sentence. The basic code is the subject, and the prefixes and suffixes are everything else that changes the meaning without changing the noun.
The three blocks of a full designation
A complete designation splits into a prefix, a basic code and a suffix. The prefix names a structural feature or family; the basic code fixes type, dimension series and bore; the suffix modifies the finished product. Read them in that order and a part like 6205-2RS1/C3 P6 resolves into a clear specification.
| Block | What it sets | Example from 6205-2RS1/C3 |
|---|---|---|
| Prefix | Family, design or structural feature | None here; a leading N or NU appears in roller families |
| Basic code | Type, dimension series and bore | 6 = deep groove, 2 = light series, 05 = 25 mm bore |
| Suffix | Closure, clearance, precision, cage, pairing | 2RS1 = both sides sealed, C3 = greater-than-normal clearance |
Decoding the basic code
The basic code is the part a number is named for, and it carries three pieces of information in a fixed order: type, dimension series and bore.
The type digit
The first digit or digits encode the bearing type: 6 deep groove ball, 7 angular contact, 2 self-aligning ball, 3 tapered roller in the metric series, N cylindrical roller, and so on. The letter families use a prefix instead of a leading digit.
The dimension series
The next one or two digits are the ISO dimension series, which fix the outside diameter and width for a given bore. Change the middle digit and the envelope changes even though the bore does not: a 6305 shares the 25 mm bore of a 6205 but is heavier and wider, carrying more load at a lower speed.
The bore field and its exceptions
The bore field is the one part of the code a buyer always needs, and it has three rules that trip people up.
Bore field above 20 mm
For 20 mm and above, the last two digits give the bore multiplied by five. 6205 is 25 mm, 6308 is 40 mm, 6310 is 50 mm. The rule is mechanical and easy to apply once learned.
Bore field below 20 mm
Below 20 mm the convention changes: the number is often the bore directly, so 608 is an 8 mm bore, 609 a 9 mm bore and 625 a 5 mm bore. Reading these with the multiply-by-five rule gives nonsense, which is exactly the mistake to avoid.
Bores that are not multiples of five
A bore such as 22, 28 or 32 mm cannot be written by the two-digit encoding, so it is given directly after a slash - for example 6205/22 for a 22 mm bore variant. The slash is the signal that the usual multiplication does not apply.
Prefixes that change the geometry
Prefixes are shortened forms for structural features, and their meaning is family-specific. In the cylindrical roller family, NU, NJ, NUP and N describe which ring carries which rib and therefore whether the bearing can take thrust and in which direction. Elsewhere a leading letter may denote a material, a housing type or a design variant. Prefixes should always be read against the catalogue table for that family, never translated by memory across families.
Where the suffix carries the order
Most of the practical content of a designation lives in the suffix. Two bearings with the same basic number can be open or sealed, standard or C3, ordinary or P4, and the suffix is the only place that information lives.
| Suffix | What it changes | What to confirm before ordering |
|---|---|---|
| Z / 2Z | One or two non-contact shields | Shielded or sealed, and the resulting speed limit |
| RS / 2RS | One or two contact seals | Contact-seal friction and grease retention |
| C2 / CN / C3 / C4 | Internal clearance group | The running clearance your temperature rise needs |
| P6 / P5 / P4 | Tolerance (precision) class | Whether shaft and housing seats match the class |
| J / M / TN9 / TVP | Cage material and design | Temperature and speed compatibility of the cage |
| DB / DF / DT | Paired arrangement | A matched set with set preload, not two loose bearings |
Stacked suffixes and real variants
Real orders stack suffixes. A 6205-2RS1/C3 is a sealed deep groove bearing with greater-than-normal clearance; add a P6 code and it is also a precision part. Because each suffix is independent, the number of genuine variants of one nominal size is large - which is why a catalogue must be searched by the full designation. Closure, clearance and precision choices are each covered in their own guides: see the seal and shield suffixes, the clearance groups and the P-class tolerance ladder.
Suffixes that describe service conditions
Beyond geometry and fit, suffixes also encode service conditions. A bearing may be dimensionally standard yet carry an S-code for heat stabilisation above the usual 120 °C limit, a low-noise suffix for quiet running, a grease code for a factory fill, or a locating feature such as a snap ring. None of these changes the basic number, and all of them change what the bearing can do, which is why the suffix list has to be read against the maker’s catalogue and not against habit.
Reading a designation before you order
Designation literacy is a discipline, and it pays off every time a part is ordered or received.
- Split the string. Separate prefix, basic code and suffix before interpreting anything.
- Confirm type and series. Read the first two digits against the family table.
- Extract the bore. Apply multiply-by-five above 20 mm and switch rules below it.
- List every suffix. Check each against the catalogue for that family, not against memory.
- Quote the full designation. A short note asking the supplier to confirm the code prevents a functionally wrong part that happens to fit.
- Check the arrival, not just the order. Read the designation on the box and on the bearing against the order before the part enters stores.
- Keep a standard set. Standardising on a short list of full designations means a part pulled from the shelf is predictable without re-reading its code.
- Record substitutions. When a suffix variant is accepted in place of another, write it on the job so the next reader sees the change.
The same discipline applies to cross-referencing. Core coding is internationally compatible, but cage, seal and preload letters are only partly harmonised between makers, so the safe route is to cross-reference the full designation rather than the basic number - which is what a part comparison does. Suffix conventions differ enough that a Japanese and a European maker can describe the same bearing with different letters.
Designation literacy in the storeroom
When a plant standardises on a small set of full designations rather than loose sizes, storeroom staff can trust that any part pulled from the shelf behaves as expected, and reordering becomes a matter of matching a complete code. Mixing sizes that share a basic number but differ in suffix is a quiet source of failures: the wrong part fits the hole perfectly and only reveals itself later as noise, heat or short life.
Confirm the family against the deep groove and cylindrical roller category pages, verify boundary dimensions with the ISO 15 envelope, and check stock against the full code in the catalogue search. Three identical-looking numbers can describe three different products; a minute spent reading the code at the door is cheaper than a machine that runs hot because a C3 was substituted for a CN.
If a plant can read a designation, it can also write one. A purchase order that spells out the basic code and every required suffix - closure, clearance, precision and pairing - gives a supplier nothing to interpret and a buyer nothing to argue about later. The short habit of copying the full code from a drawing rather than typing a bare size is what keeps a stores catalogue honest, and it is the same habit that makes a cross-reference trustworthy when a second brand has to be accepted.
- Three blocks. Prefix, basic code and suffix together form the specification.
- The basic code is only size. Type, dimension series and bore - nothing else.
- Suffixes decide the order. Closure, clearance, precision, cage and pairing all live there.
- Bore rules change at 20 mm. Multiply by five above it, read directly below it, and use a slash for odd bores.
- Order the full code. Cross-reference the whole designation, never the bare number.
Frequently asked questions
What do the first digits of a bearing number tell me?
They give the bearing type before the dimension series - 6 for deep groove ball, 7 for angular contact, 2 for self-aligning ball, 3 for metric tapered roller and N for cylindrical roller. The next one or two digits then set the dimension series and the envelope.
Why do suffix letters differ between two brands for the same part?
Core codes are harmonised internationally, but cage, seal and preload letters are not fully standardised. Each maker publishes its own suffix list, and the same letter can mean a material in one family and something else in another, so the full designation must be cross-referenced rather than assumed.
Can I stack a clearance suffix and a precision suffix on one order line?
Yes, and real orders often do. A 6205-2RS1/C3 P6 combines a contact seal, greater-than-normal clearance and a precision class. Each suffix is independent, so the finished variant is defined by the whole string, not by the basic number.
How is a 22, 28 or 32 mm bore written in a designation?
A bore that is not a multiple of five cannot use the two-digit multiply-by-five encoding, so it is written directly after a slash - for example 6205/22 for a 22 mm bore variant. The slash is the signal that the usual rule does not apply.
Do I need the full designation if the basic number matches my old part?
Yes. The basic number guarantees the envelope, not the behaviour. Two parts with the same number can differ in closure, clearance, precision and cage, and the wrong one fits the housing perfectly while running hot or loud. Quote and cross-reference the complete code.
Which suffix tells me a bearing is heat-stabilised?
A heat-stabilisation suffix, commonly an S-code, marks a bearing dimensionally stable above the standard limit of about 120 °C. It does not appear in the basic number, so a high-temperature duty should always be specified by its suffix rather than by size alone.