Six manufacturers make most of the rolling bearings installed in plants today: SKF (Swedish), FAG / Schaeffler (German), Timken (American), NSK (Japanese), NTN (Japanese), and Koyo / JTEKT (Japanese). Each one has its own designation rules for prefixes, suffixes, and special construction codes. The common ground is the ISO 15 boundary dimensions standard: a 6205 deep groove ball bearing from any of these makers has the same 25 mm bore, 52 mm outside diameter, and 15 mm width. The diverging part is the suffix vocabulary: a contact rubber seal on both sides is 2RS1 or 2RSH in SKF, 2RSR in FAG, DDU in NSK, LLU in NTN, 2RS or 2RU in Koyo, and 2RS in Timken. Same intent, different raw token.
This guide walks the anatomy of a bearing designation, the universal bore-code rule, the suffix categories that every maker groups codes into, the cross-vendor translation problem, the C2 through C5 clearance classes from ISO 5753-1, and the legacy brand aliases (Fafnir, MRC, Bower, RHP) that are still sitting on equipment installed 20 to 50 years ago. The companion Bearing Decoder does the parsing automatically and prefills the six ToolGrit bearing calculators with the decoded geometry. In V1, deep-groove-ball clearance microns are shown; roller-bearing clearance suffixes are decoded but micron values are suppressed because cylindrical and spherical roller families use materially different tables.
Why Bearing Decoding Matters
Bearings are universal: every motor, every pump, every gearbox, every fan, every conveyor, every wheel, every spindle has at least one. They are also where most rotating-equipment failures end. A maintenance planner replacing a bearing has three questions: what exactly is the failed bearing, what is the right replacement, and is the substitute equivalent. Each question requires reading the part number stamped on the failed unit and understanding what every character controls.
The part number that comes off the failed bearing might be SKF, might be FAG, might be NSK, might be Fafnir from 1985 that is no longer made under that name. The replacement on the parts shelf is whichever brand the distributor stocks today. Knowing that SKF 6205-2RS1/C3, NSK 6205-DDU/C3, NTN 6205-LLU/C3, and Timken 6205-2RS/C3 carry the same geometry and decoded closure / clearance intent is the difference between getting the right replacement on the first phone call and ordering the wrong one twice. The cross-vendor translation is what the decoder exists for.
Cross-vendor translation is also where the maker configurators stop being useful. Every maker's configurator builds new orders against that maker's current catalog. None of them cross-references to a competitor, and most of them cannot decode their own legacy designations from 20 years ago. That gap is where the ToolGrit decoder sits.
Anatomy of a Bearing Designation
A typical bearing designation has three layers: optional prefix, required basic designation, optional suffix tokens. The prefix carries construction modifiers like W for stainless steel (SKF) or 4T for case-carburized tapered roller (NTN). The basic designation identifies the bearing type, the dimension series, and the bore. The suffix tokens identify everything else: internal design, cage, sealing, clearance, precision, matched-set arrangement, lubricant, and other modifiers.
For deep groove ball bearings, the basic designation is four digits: type code (6 for deep groove ball, single row), dimension series (00 / 02 / 03 / 04 for the four common width-and-diameter combinations), and bore code (the last two digits). So 6205 reads as: type 6 (deep groove ball), dimension series 2 (light), bore code 05 (25 mm bore). For angular contact ball, type code is 7 (single row) or 32 / 33 (double row). For cylindrical roller, the type is a letter prefix: NU, NJ, NUP, or N, followed by the dimension series and bore code, giving NU305, NJ205, NUP310. For spherical roller, the basic designation is five digits: 222xx (light dimension series) and 223xx (medium dimension series) are the most common, with 50 mm through 150 mm bores covering the plant-floor majority. For tapered roller, the metric form is 302xx, 303xx, 313xx, 320xx, 322xx, 323xx, 330xx (single row) or 322xx, 323xx (double row at five digits). Timken inch-series tapered roller (HM, LM, JLM, JM) is a flat catalog with no formula.
[Prefix] [Basic designation] [Suffix tokens...]
Deep groove ball: [Type 6][Dim series 02/03/04][Bore code 2 digits]
Angular contact ball: [Type 7][Dim series][Bore code]
Cylindrical roller: [NU / NJ / NUP / N][Dim series][Bore code]
Spherical roller: [Type 2][Dim series 22 or 23][Bore code]
Tapered roller (metric): [Type 3][Dim series 02 / 03 / 13 / 20 / 22 / 23][Bore code]
Tapered roller (Timken inch): flat lookup, e.g., HM518445 (cone) + HM518410 (cup)
Bearing Part Number Decoder
Decode SKF, FAG, Timken, NSK, NTN, and Koyo bearing designations. Boundary dimensions per ISO 15, clearance microns per ISO 5753-1, cross-vendor equivalents, source-cited per maker catalog. Prefills the six ToolGrit bearing calculators.
The Bore Code Rule (the most-failed bearing question)
The trailing two digits of the basic designation encode the bore in a piecewise rule that is universal across SKF, FAG, Timken, NSK, NTN, MRC, and most other makers for common metric radial families. The exceptions are the first four codes; everything 04 and up is the multiplier.
- 00 = 10 mm (exception)
- 01 = 12 mm (exception)
- 02 = 15 mm (exception)
- 03 = 17 mm (exception)
- 04 = 20 mm (rule: code times 5 begins here)
- 05 = 25 mm
- 10 = 50 mm
- 20 = 100 mm
- 96 = 480 mm (largest common code; bigger bores use slash-bypass notation)
So 6204 means 20 mm bore, NOT 4 mm. 6205 means 25 mm, NOT 5 mm. 6206 means 30 mm. The most common bearing-decoding mistake on the internet is to read 6204 as "4 mm bore" because the brain pattern-matches the last two digits as the bore. They are not; they are an encoded bore code, and the rule has a piecewise jump at 04.
For bearings whose bore does not fit the code system (e.g., a custom 22 mm bore, or a very large 500 mm bearing), the maker uses a slash-bypass notation: 60/22 means type 6, dimension series 0, bore literally 22 mm. The decoder handles slash-bypass explicitly and reads "60/22" as a 22 mm bore deep groove ball, not as a parsed code.
Suffix Categories (the same nine groups, different vocabularies)
SKF groups suffixes into a published order: internal design, external design, cage, materials and heat treatment, tolerance / clearance / preload / quiet running, bearing sets, stabilization, lubrication, other variants. The other makers follow a similar grouping with maker-specific code letters. Reading a suffix block goes faster once you know the categories.
Internal design covers letters like E or EC (enhanced internal design, higher load capacity), K (tapered bore 1:12, for adapter-sleeve mount), K30 (tapered bore 1:30, for large spherical rollers on adapter sleeves), B / C / AC (contact angle for angular contact ball: typically 40 deg / 15 deg / 25 deg). FAG marks its enhanced design with E1 (X-life). Schaeffler also uses TVH / TVP / TVPB for polyamide cages that came in with the enhanced design.
Cage covers J (pressed steel), M / MA / MB / MP (machined brass or steel), TN9 / TVH / TVP / T / TR (polyamide). Polyamide cages have a continuous-temperature ceiling around 120 C and a hard limit around 150 C. For high-temperature service, choose a machined-brass cage instead.
Sealing / shielding is the highest-collision category across makers. The intent layer normalizes raw tokens to one of these: open (no closure), one_shield, two_shields, one_contact_seal, two_contact_seals, one_non_contact_seal, two_non_contact_seals, one_low_friction_seal, two_low_friction_seals. The maker tokens for two contact seals: SKF 2RS1 or 2RSH, FAG 2RSR, Timken 2RS, NSK DDU, NTN LLU, Koyo 2RU or 2RS. Two metal shields: SKF 2Z, FAG 2Z, Timken ZZ, NSK ZZ, NTN ZZ, Koyo ZZ.
Clearance is universal across makers and conforms to ISO 5753-1: C2 less than Normal, CN Normal (often omitted), C3 greater than Normal (common in TEFC motor bearings), C4 greater than C3 (hot service or heavy interference fit), C5 greater than C4 (rare). Timken occasionally writes C0 in legacy contexts; it is the same as CN.
Precision is also universal: P0 / P6 / P5 / P4 / P2 per ISO 492. Better tolerance toward lower numbers (P2 is highest). The American ABEC scale is related: ABEC-1 = P0, ABEC-3 = P6, ABEC-5 = P5, ABEC-7 = P4, ABEC-9 = P2. Some makers use both notations in the same catalog.
Matched-set arrangement applies mainly to angular contact and tapered roller: DB (back-to-back), DF (face-to-face), DT (tandem). Critical for spindle service and where bidirectional thrust matters.
Cross-Vendor Translation: Same Bearing, Different Designation
ISO 15 fixes the boundary dimensions for radial bearings, so any maker's 6205 has the same 25 mm bore, 52 mm OD, 15 mm width. ISO 5753-1 fixes radial clearance classes, but the micron ranges depend on bearing family and bore size. V1 shows the deep-groove-ball table and suppresses roller-bearing microns because the matching roller tables are different enough to mislead: at a 50 mm bore, SKF lists C3 as 18 to 36 um for deep groove ball bearings, 50 to 80 um for cylindrical roller bearings, and 55 to 75 um for spherical roller bearings. Beyond those anchors, every maker has its own vocabulary for seals, shields, cages, lubricants, internal-design markers, and material codes.
Cross-vendor translation works in two layers. The first layer is geometric: two designations are at the same boundary dimensions if their basic designation maps to the same ISO 15 row (most metric basic designations literally share digits across makers). The second layer is intent: the closure, clearance, and cage intents from the source designation are projected into the target maker's vocabulary. A decoded SKF 6205-2RS1/C3 produces FAG 6205-2RSR/C3, Timken 6205-2RS/C3, NSK 6205-DDU/C3, NTN 6205-LLU/C3, and Koyo 6205-2RS/C3 in the current tool output. All of these will fit on the same shaft and pillow block. Whether they are functionally identical depends on the cage design, the seal material (NBR is the assumption unless the suffix indicates fluororubber FKM or another compound), and any internal-design refinements that the maker has not surfaced in the suffix.
For routine motor and general-machine service, the cross-vendor substitution is typically fine and the maintenance department will not see the difference. For spindle service, food-grade service, hot service, or precision-application service, verify the maker's full specification before treating the substitute as equivalent. The decoder labels the cross-vendor equivalence at five levels (Catalog match, Same size class, Likely equivalent, Maker-listed, Unverified) so the user is not blind to the gradient.
The C3 Clearance Class and the Over-Spec Trap
C3 (radial internal clearance greater than Normal) is the clearance class that most TEFC motor manufacturers spec on their bearings. The reason is thermal: the inner ring runs hotter than the outer ring on a TEFC motor (heat from the windings flows down the rotor and out through the shaft), the inner ring grows thermally, and the operating clearance shrinks compared to the unmounted clearance. C3 starts with more clearance so the operating clearance ends up where Normal-class operating clearance would have ended up if the inner ring had not heated up.
The mistake comes from generalizing this rule: tradespeople default to C3 when replacing a bearing thinking "looser is safer." It is not. For cool-running, lightly loaded service, C3 means too much clearance, which means more vibration and reduced fatigue life. CN (Normal) is the right call for most general-purpose service. C3 is right when the inner ring runs hot, the load is heavy, or the interference fit is large. C4 and C5 are for even hotter service or for very large heated shafts (mining, mill drives).
For a 25 mm bore bearing (the standard 6205 size), ISO 5753-1 gives the unmounted radial clearance ranges per class: C2 is 1 to 11 microns, CN is 5 to 20 microns, C3 is 13 to 28 microns, C4 is 23 to 41 microns, C5 is 30 to 53 microns. The bearing decoder surfaces these microns when a clearance suffix is detected so the planner can see the actual range, not just the class letter.
Legacy Brand Aliases: Fafnir, MRC, Bower, RHP
Equipment installed in the 1980s, 1990s, and earlier often carries brand names that no longer exist as standalone makers. Fafnir was acquired by Torrington, which was acquired by Timken in 2003. MRC was acquired by SKF (via TRW Bearings) in 1987. Bower was acquired by Timken. RHP was acquired by NSK in 1990. The decoder handles source-backed aliases as one-way lookups: paste an MRC 205S and the tool maps it to its modern equivalent (SKF 6205), then runs the standard cross-vendor + prefill flow on the modern equivalent. Fafnir and RHP starter mappings remain low confidence until better legacy source pages are added. Bower is recognized as a brand name, but V1 does not ship a Bower part-number cross-reference.
Common legacy designations the decoder handles:
- Fafnir 205P => Timken 6205 (open deep groove ball, 25 mm bore)
- Fafnir 205PP => Timken 6205-2RS (contact rubber seals both sides)
- Fafnir 205PP3 => Timken 6205-2RS with C3 clearance hint
- Fafnir 305K => Timken 6305 (deep groove ball, 25 mm bore, medium series)
- MRC 205S => SKF 6205 (open deep groove ball, 25 mm bore)
- MRC 205SZZ => SKF 6205-2RSH (contact seals both sides)
- RHP 6205-2RS => NSK 6205DDU
The decoder shows the legacy origin in a callout, surfaces the function summary (e.g., "Deep groove ball, 25 mm bore, contact rubber seals both sides, C3 clearance"), and resolves the modern equivalent so the maintenance planner can order the replacement from a current distributor. MRC rows are source-verified from the MRC handbook. Fafnir and RHP rows are retained as low-confidence starter mappings. Legacy designations route to modern; modern designations do not list legacy aliases in the cross-vendor table.
Worked Example: Reading SKF 6205-2RS1/C3 End-to-End
Consider the most common motor bearing: SKF 6205-2RS1/C3.
- Brand prefix:
SKFsets the manufacturer namespace. - Type code:
6= deep groove ball, single row. - Dimension series:
2= light (the workhorse series). - Bore code:
05= 25 mm bore (per the times-5 rule). - Boundary dimensions: 25 mm bore, 52 mm OD, 15 mm width (per ISO 15 and SKF Rolling Bearings 17000 EN).
- Dynamic load rating C: 14.8 kN (SKF baseline). Enhanced E variants run higher; Timken / NSK / NTN values are within 10 percent.
- SKF speed values: 28,000 RPM reference speed and 18,000 RPM limiting speed for the open 6205. Sealed variants run lower and should be checked against the maker table.
- Suffix
2RS1: Contact rubber seals, both sides, NBR (nitrile). Category: sealing / shielding. Intent: two_contact_seals. Source: SKF Rolling Bearings 17000 EN p. 35. - Suffix
C3: Radial internal clearance greater than Normal. Category: clearance. Intent: C3. Source: SKF designation system. - ISO 5753-1 clearance microns at 25 mm bore: 13 to 28 um for class C3.
- Cross-vendor equivalents: FAG 6205-2RSR/C3, Timken 6205-2RS/C3, NSK 6205-DDU/C3, NTN 6205-LLU/C3, Koyo 6205-2RS/C3. All at the same boundary dimensions, same two-contact-seal intent, same C3 clearance class.
- Field note (fires because of C3): "Most TEFC motor manufacturers spec C3 to compensate for thermal expansion of the inner ring on a hot shaft. If you are replacing a motor bearing, check the original tag. If it was C3, the replacement should be C3 too. Do not downgrade to CN thinking tighter is safer."
- Prefill links: Bearing Speed Limit Calculator (bore 25 mm, OD 52 mm, deep_groove_ball preset, 1750 RPM placeholder), Bearing Life Calculator (dynamic load rating C = 14.8 kN, ball exponent), Bearing Grease Calculator (bore 25 mm, OD 52 mm, RPM placeholder), and the other three.
Plain English: SKF 6205, deep groove ball, single row, 25 mm bore, 52 mm OD, 15 mm width, contact rubber seals both sides (NBR), C3 clearance (greater than Normal, 13 to 28 microns at this bore size). Compatible with FAG 6205-2RSR/C3, Timken 6205-2RS/C3, NSK 6205-DDU/C3, NTN 6205-LLU/C3, Koyo 6205-2RS/C3 on geometry and decoded intents.
When the Decoder Says "Could not decode"
The decoder is honest. When a suffix token is not in the V1 dictionary for the matched manufacturer, it is reported as "Could not decode" rather than guessed. The likely causes are: the token is from a deprecated revision of the maker catalog, the token is a lubricant or vibration-class code (lowest-confidence category, highest churn across revisions), the token is a customer-specific or special-order code, or the input has a typo. V1 also does not yet decode tapered roller (metric or inch), thrust bearings, mounted units, super-precision bearings, or four-point contact codes; if your input is from those families, the decoder flags the family hint and asks you to verify against the maker catalog.
The decoder is conservative on this on purpose. Silently guessing a suffix meaning propagates a wrong answer into a maintenance plan or a replacement order. Flagging the token as "could not decode" forces the planner to verify against the source catalog, which is what should happen anyway.