Bearings Industry: Online B2B Portal for Bearings

Every bearing number, tolerance class and clearance code traces back to a published standard. This hub explains how those standards fit together, how to read a bearing designation, and what the precision and clearance codes mean in practice — for buyers, engineers and newcomers alike.

On this page

The Standards Families
How a bearing is sized
Reading a bearing number
Precision & tolerance classes
Internal clearance (C2–C5)
Suffix decoder
Key standards at a glance
Glossary

The Standards Families

Bearings are made interchangeable across the world because their dimensions, tolerances and clearances are fixed by international standards rather than by each manufacturer. A 6205 from one brand fits the same shaft and housing as a 6205 from another. The main bodies are:

Body	Scope	Where it dominates
ISO — International Organization for Standardization	The global reference for boundary dimensions, tolerances and clearances (the ISO 15, 281, 492 series).	Worldwide; the basis most others align to.
DIN — Deutsches Institut für Normung	German standards (e.g. DIN 620 for tolerances); closely matches ISO.	Europe.
JIS — Japanese Industrial Standards	Japanese standards (e.g. JIS B 1514 tolerances); aligned to ISO.	Japan / Japanese makers.
ANSI / ABMA — American Bearing Manufacturers Association	US standards; the ABEC precision grades come from here (ABMA Std 20).	North America.
BIS — Bureau of Indian Standards	Indian Standards (IS) published by the Bearings committee, largely harmonised with ISO — e.g. IS 5934 corresponds to ISO 582 (chamfers); IS 7461 / IS 14691 cover tapered-roller boundary dimensions.	India.

In plain termsBecause all the big standards agree on the core dimensions, a bearing's size and basic specification are universal. The differences between bodies mostly show up in how precision is graded — which is why you will see both ISO "P" codes and American "ABEC" codes on the same shelf.

How a bearing is sized — boundary dimensions

A rolling bearing is defined first by three external (boundary) dimensions, almost always written in millimetres as d × D × B:

Symbol	Dimension	Meaning
`d`	Bore diameter	The inside diameter — the shaft size the bearing fits.
`D`	Outside diameter	The outer diameter — the housing bore it seats into.
`B` (or `T`, `H`)	Width / height	The axial width of the bearing (T for tapered roller, H for thrust).

ISO 15 fixes a general plan of boundary dimensions: for any given bore, a small set of standard outer diameters and widths is allowed. These are grouped into dimension series — combinations of a diameter series (how large the OD is) and a width series (how wide the bearing is) for the same bore. This is why, for a 25 mm shaft, you can get a light 6205, a medium 6305 or a heavy 6405: same bore, progressively larger section and load capacity.

For buyersIf a customer gives you only "25 mm bore," that is not enough to quote — confirm the series (e.g. 6205 vs 6305). They share a bore but have different ODs, widths, prices and load ratings, and are not interchangeable in the housing.

Reading a bearing number

Most metric bearing designations follow the same logic: an optional prefix, a basic number (type + dimension series + bore), and one or more suffixes. The exact suffix letters vary slightly between SKF, FAG, NSK, NTN and others, but the basic number is common.

Step 1 — Type (first character/s of the basic number)

Code	Bearing type
1	Self-aligning ball bearing
2	Spherical roller bearing
3	Tapered roller bearing (also some double-row angular contact)
4	Double-row deep-groove ball bearing
5	Thrust ball bearing
6	Single-row deep-groove ball bearing
7	Single-row angular-contact ball bearing
8	Cylindrical roller thrust bearing
N, NU, NJ, NUP, NF	Cylindrical roller bearings (the letters describe the flange/rib arrangement)
QJ	Four-point-contact ball bearing

Conventions vary slightly by manufacturer; the above is the common ISO-based scheme.

Step 2 — Dimension (size) series

The digit(s) after the type code give the dimension series — the relative cross-section. For the same bore, a higher series means a larger OD, wider section and higher load capacity. Common diameter series in increasing size: 8, 9, 0, 1, 2, 3, 4. So the 6200 series is lighter than the 6300, which is lighter than the 6400.

Step 3 — Bore code (the last two digits)

Bore code	Bore (mm)	Rule
00 / 01 / 02 / 03	10 / 12 / 15 / 17	Special fixed codes.
04 to 96	20 to 480	Multiply the code by 5 (e.g. 05 → 25 mm, 12 → 60 mm).
under 10 mm / over 480 mm	—	Stated by special rules (often the bore in mm appears directly, sometimes after a "/").

Worked examples

Designation	Breakdown
`6205-2RS C3`	6 deep-groove ball · 2 diameter series 2 · 05 bore 05×5 = 25 mm · 2RS two contact rubber seals · C3 internal clearance greater than normal.
`7206 B`	7 angular-contact ball · 2 diameter series 2 · 06 bore = 30 mm · B 40° contact angle.
`NU 309`	NU cylindrical roller (two ribs on the outer ring, none on the inner — allows axial float) · 3 diameter series 3 · 09 bore 09×5 = 45 mm.

In plain termsThe number tells you the type, the size and the fit. Once you can read it, you can look at any bearing and know roughly what it is and what shaft it belongs on — without a catalogue.

Precision & tolerance classes (ISO 492 / ABEC)

Tolerance class controls how accurately the bearing is made — its dimensional accuracy and running accuracy (run-out). ISO 492 defines the classes; the American ABEC scale (ABMA Std 20) is the common cross-reference. Watch the direction: in the ISO scale Normal is the loosest and P2 the tightest, while in the ABEC scale a higher number means tighter — so the two scales run in opposite directions.

ISO / DIN class	ABEC (ball)	Precision	Typical use
Normal (P0)	ABEC 1	Standard	General machinery — the vast majority of applications.
Class 6 (P6)	ABEC 3	Higher	Where somewhat better running accuracy is needed.
Class 5 (P5)	ABEC 5	High	Machine-tool spindles, electric motors needing low vibration.
Class 4 (P4)	ABEC 7	Very high	Precision spindles, high speed.
Class 2 (P2)	ABEC 9	Ultra-high	Instruments, aerospace, semiconductor and similar.

For buyersPrice rises sharply with precision — the biggest jumps are from ABEC 3→5 and 5→7. Do not over-specify: most industrial replacements are Normal/ABEC 1. Only quote P5 and above when the application (spindles, high-speed, low-vibration) genuinely calls for it.

Internal radial clearance (C2–C5)

Internal clearance is the small internal gap between the rolling elements and the raceways before mounting. It matters because fitting onto a shaft (interference fit) and the temperature difference between the inner and outer rings in service both reduce that gap. Too little operating clearance pre-loads the bearing and makes it run hot; too much causes noise and uneven load.

Class	Relative to Normal	When it is used
`C2`	Less than Normal	Where running accuracy and quiet running matter and temperature rise is low.
`CN` (Normal)	Standard	General-purpose default; often left unmarked.
`C3`	Greater than Normal	Tight interference fits, or a temperature difference between rings — common factory default for electric motors.
`C4`	Greater than C3	Higher operating temperatures or heavier interference fits.
`C5`	Greater than C4	Very high temperature or very heavy interference (e.g. some traction and heavy-industry duties).

In plain termsThe clearance code is chosen so that after the bearing is fitted and warmed up, the leftover gap is just right. C3 is the most common "non-standard" choice because so many bearings run warm or sit on a tight shaft.

Suffix decoder (common codes)

Suffixes describe seals, cages, clearance and precision. These differ between makers — always confirm against the manufacturer's catalogue — but the most common are:

Suffix	Means
`Z` / `ZZ` (or `2Z`)	One / two metal shields (non-contact).
`RS` / `2RS` (or `RSR`, `DDU`)	One / two contact rubber seals.
`C2 / C3 / C4 / C5`	Internal radial clearance (see above).
`P6 / P5 / P4 / P2`	Tolerance / precision class.
`M`	Machined brass cage.
`TN / TVP / TN9`	Polyamide (nylon) cage.
`E`	Reinforced / optimised internal design (higher capacity).
`B / C / AC` (angular contact)	Contact angle (e.g. B ≈ 40°, AC ≈ 25°).

For buyersThe suffix is where most cross-brand confusion happens. When matching an equivalent, replicate the function (seals, clearance, precision, cage) — not just the letters, since each maker codes them differently. A future Cross-Reference tool will automate this; until then, check the catalogue suffix tables.

Key standards at a glance

Standard	Covers
ISO 281	Dynamic load ratings and basic rating life (the L10 calculation).
ISO 76	Static load ratings (C0).
ISO 15	General plan of boundary dimensions for radial bearings.
ISO 492	Tolerances (precision classes) for radial bearings.
ISO 199	Tolerances for thrust bearings.
ISO 582	Chamfer dimensions — maximum values (BIS: IS 5934).
ISO 5753	Internal clearance.
ISO 15312	Thermal speed rating.
DIN 620	German tolerance standard (aligns with ISO 492).
JIS B 1514	Japanese tolerance standard.
ANSI/ABMA Std 20	ABEC / RBEC precision grades.
BIS (IS series)	Indian Standards published by the Bearings committee, harmonised with ISO (e.g. IS 5934, IS 7461, IS 14691).

Glossary

Term	Meaning
Bore (d)	Inner diameter; the shaft size the bearing fits.
Outside diameter (D)	Outer diameter; the housing bore the bearing seats in.
Raceway	The precision-ground track on each ring that the rolling elements run along.
Radial load	Load acting at right angles to the shaft axis.
Axial (thrust) load	Load acting along the shaft axis.
Dynamic load rating (C)	The load giving a basic rating life of one million revolutions (used in L10).
Static load rating (C0)	The load a stationary bearing can take without harmful permanent deformation.
Basic rating life (L10)	The life, in million revolutions, that 90% of a group of identical bearings will reach or exceed.
Internal clearance	The internal gap between rolling elements and raceways before mounting.
Preload	A deliberate negative clearance applied to increase stiffness and running accuracy.
Contact angle	The angle along which load is transmitted through the bearing (angular-contact and tapered types).
Interference (press) fit	A fit where the ring is slightly larger than its seat, so it grips when pressed on.
Brinelling	Permanent dents in the raceway from static overload or impact during mounting.
False brinelling	Wear marks from small vibratory movement while the bearing is stationary (e.g. in transit).

Bearing Standards Hub