Load Ratings Explained: What C, C0, Cr and C90 Really Mean

Two ratings, two questions

A bearing catalogue quotes a load rating because engineers keep asking two different questions. How much load can this bearing carry without permanent damage? and how much load can it carry while still reaching the life I need? Those are the basic static load rating and the basic dynamic load rating, and they are not interchangeable numbers on the same scale.

This matters when you comparison-shop across brands, because the two quantities get printed with half a dozen different symbols — and one of Timken’s ratings is defined on a completely different life basis from everyone else’s. Reading the number without reading the symbol is how mismatched comparisons start.

ISO defines the quantity, not the symbol

The ISO standards define what a load rating means and how it is calculated. They do not mandate the letter you print in a table.

• ISO 76 covers the basic static load rating. It corresponds to a calculated contact stress at the most heavily loaded rolling element / raceway contact. Practically, a static rating answers “how much load before the raceway takes a permanent dent?” It is a load, not a life.
• ISO 281 covers the basic dynamic load rating and rating life. The basic dynamic load rating is defined against a rating life of one million revolutions for a group of apparently identical bearings. The same standard introduces the fatigue load limit — a stress below which the steel is assumed to have effectively unlimited life.
• ISO 15312 covers the thermal speed rating: the surface speed at which the bearing can run and shed heat without overheating. It is a speed, not a load, and it depends on lubrication and cooling, but it is frequently printed beside the load ratings and mistaken for them.

Because ISO fixes the definition and leaves the notation to the manufacturer, two catalogues can print the same physical quantity with different glyphs — and different manufacturers genuinely do.

The symbol table, brand by brand

The ratings below are faithful to what each manufacturer actually prints. The right-hand column is the key: some brands use one pair everywhere, others switch symbol by bearing type.

Brand	Dynamic	Static	Notes / type-specific wording
SKF	C	C0	Printed plain for every rolling-bearing type in scope.
FAG	Cr	C0r	Adds Cur for the fatigue load limit.
NSK	Cr	C0r	Radially. Four-point contact tables print Ca / C0a.
NTN	Cr	C0r	Radially. Four-point contact tables print Ca / C0a.
Timken — deep groove ball	Cr	C0r	ISO-style pair, printed with a subscript zero.
Timken — angular contact	Ce	Co	“Extended Dynamic Load Rating” — a different quantity from the plain dynamic rating.
Timken — cylindrical roller	C1	Co	The C1 column carries the 10⁶-revolution basic rating.
Timken — spherical roller	C	Co	Also prints a geometry factor Cg.
Timken — tapered roller	C1	C0	Adds C90 / Ca90 / Cg — see below.

Three observations fall straight out of that table.

First, the four-point contact case is not a typo. A four-point contact bearing is designed to take axial load in both directions, so NSK and NTN print the axial symbols Ca / C0a for the same kind of quantity the other brands print radially. Same physical idea, different axis.

Second, Timken does not use one notation across its own catalogue family. You will find Cr / C0r on its deep groove ball pages, Ce / Co on its angular contact pages, and C1 / Co or C1 / C0 on the roller pages. If you have ever “corrected” a Timken angular contact figure to Cr, check which catalogue it came from — you may have changed the meaning, not just the spelling.

Third, the fatigue load limit appears under its own symbol where the catalogue publishes it: FAG prints Cur, SKF prints Pu. It is a third quantity — neither the dynamic nor the static rating — and it is the one that tells you when a lightly loaded bearing is no longer fatigue-limited.

C90 and Ca90: the same bearing, a different reference life

The standard dynamic load rating is anchored to one million revolutions. Timken’s tapered roller catalogue prints an additional pair of ratings anchored to a different life:

• C90 — the basic dynamic radial load rating of a single-row bearing for an L10 of 90 million revolutions.
• Ca90 — the corresponding dynamic thrust rating, for an L10 of 90 million revolutions or 3000 hours at 500 r/min.
• Cg — the dimensionless geometry factor used in the a3l life adjustment equation.

This is the trap worth internalising: 90 million revolutions is 90 times the ISO reference life of one million. A C90 figure and a Cr figure describe load capacity at different lives, so their magnitudes are not comparable. A bearing’s C90 is numerically smaller than its 10⁶-revolution rating because you are demanding far longer life from it. Never line up a C90 against a competitor’s Cr and conclude one bearing is weaker — you would be comparing a 90-million-revolution rating against a one-million-revolution rating, which are different quantities entirely.

Cg, for its part, is not a load at all. It is unitless, and it exists only to feed the life-modification calculation. It should never appear in a “maximum load” column.

Ratings change between editions — use the latest

Manufacturers revise their load ratings between catalogue editions, so the same bearing size can carry a different published figure in two editions of the same catalogue. Always take the number from the latest edition of the catalogue, or from the manufacturer’s current online data, rather than from an older print copy. The figures on this site come from the most recent in-print editions available when the data was compiled.

The C0 versus Co glyph trap

Look closely at the static symbols and you will find both a digit zero and a letter o.

• Timken’s tapered roller table header typesets the static rating as C0 (subscript digit zero), while the same catalogue’s symbol summary and the cylindrical / spherical roller headers typeset it as Co (subscript letter o).
• Timken’s ball catalogue prints C0r — digit zero again.
• SKF prints C0; FAG, NSK and NTN print C0r; Timken’s angular contact and roller pages print Co.

Strictly, these are two different glyphs, not two different quantities, and manufacturers use them inconsistently — sometimes within one document. The practical rule is: treat C0 and Co as the same static rating for reading purposes, but reproduce whichever glyph the source prints when you quote it. Do not “normalise” one to the other in a data pipeline and then trust the result; the normalisation is exactly where an automated reader loses the distinction.

A practical checklist

Before you write a load rating into a comparison, a spreadsheet, or a specification, run through these:

1. Check the unit. Catalogues print kN or N, sometimes both, and sometimes in different columns of the same table. Convert before you compare.
2. Check the life basis. One million revolutions (ISO 281) is the default. Timken’s C90 / Ca90 use 90 million revolutions. Same bearing, different basis, different number.
3. Check “Extended” versus standard. An “Extended Dynamic Load Rating” is not the plain dynamic rating. Match like with like.
4. Only compare the same symbol and the same quantity. Cr against Cr, Ca against Ca, static against static.
5. Never treat a static rating as a dynamic one. C0 answers a no-permanent-damage question; C answers a life question. They are not on the same scale, and a bearing is not “stronger” because its C0 is larger.
6. Do not average across manufacturers. If two sources disagree, keep both, cite both, and explain the divergence.
7. Watch the glyph. C0 (zero) and Co (letter o) may look alike, but the source’s exact printing is part of the evidence.

Sources

The symbol usage and figures above were checked against the manufacturer catalogues this site cites. Page numbers refer to the printed pages of each catalogue.

Catalogue	Pages checked
Timken, Deep Groove Ball Bearings Catalog 10857	pp. 10, 23
Timken, Angular Contact Ball Bearing Catalog 11193	pp. 22–26
Timken, Cylindrical Roller Bearing Catalog 10447	p. 55
Timken, Spherical Roller Bearing Catalog	p. 56
Timken, Tapered Roller Bearing Catalog (“Summary of Symbols”)	pp. 12, 84
SKF rolling bearing catalogue (load-rating table 3.4)	table 3.4
FAG rolling bearings catalogue (HR 1)	rating tables
NSK / NTN four-point contact tables	rating tables

ISO references: ISO 76 (static load rating), ISO 281 (dynamic load rating and rating life, 10⁶ revolutions, fatigue load limit), ISO 15312 (thermal speed rating). Where a catalogue and a standard appear to conflict, assume first that they are describing different quantities.