Bearing Internal Clearance Codes

Anti-friction bearings are manufactured with a specific radial clearance between the raceways and the rolling elements. That clearance is designed for normal operating temperatures and application conditions; increased clearance options exist for high temperature and high speed applications, to allow for thermal expansion, and reduced clearance is an option for mounted bearings under high shock load and vibration, where it distributes the load over more rolling elements. The clearance class is printed on the bearing number after the size, in the same position as the shield, seal, cage and precision codes. This guide explains what each class means, gives the clearance ranges by bore size, and separates the clearance code from the codes that are often confused with it.

What do C2, CN, C3 and C4 mean on a bearing?

They are radial internal clearance classes. Standard clearance, also called normal, is written CN or C0 and is usually not marked on the bearing at all; C2 is less than standard; C3 is greater than standard; C4 is greater than C3. The classes are defined by ISO 5753, and the clearance each one stands for depends on the bore diameter, so the tables below give the ranges in micrometers.

The clearance tables below list C5 for both deep groove ball bearings and spherical roller bearings.

A stock bearing with standard clearance carries no special marking; C2, C3 and C4 are marked on the inner ring face following the basic bearing number. A 6205 ZZ EMQ is a 25 mm bore shielded bearing with normal clearance, and the same bearing made to the next class is a 6205 ZZ C3 EMQ. Both are in the deep groove ball bearings category, and most of those product pages state the radial clearance as a specification, so the two can usually be told apart without reading the number. Spherical roller bearings are listed in the wider set of classes C1 through C5, and the spherical roller bearings category holds standard, C2, C3 and C4 stock.

Deep groove ball bearings, cylindrical bore

Spherical roller bearings, cylindrical bore

Tapered bore bearings use their own tables, and a tapered bore spherical roller bearing has a somewhat looser range in each class than the cylindrical bore bearing, because it must accommodate the tighter fit of the tapered mounting.

Is C3 wider than CN?

Yes. C3 is greater than standard clearance, and C0 and CN are two spellings of the same standard class: CN is the heading in the clearance tables, C0 is the spelling in some bearing nomenclature and on some product pages, and a bearing number with no C code at all is standard. On a 25 mm bore deep groove ball bearing the standard range is 5 to 20 micrometers and the C3 range is 13 to 28 micrometers; on a 60 mm bore spherical roller bearing such as the 22212 GMEX C3 W33 the standard range is 40 to 65 micrometers and the C3 range is 65 to 90.

The ranges of adjacent classes meet or overlap: on the deep groove table the C3 range begins inside the top of the CN range, and on the spherical roller table the top of CN is the bottom of C3. A C3 bearing is therefore not always looser than every CN bearing of the same size, but its range as a whole sits one step above. The same relationship holds for self-aligning ball bearings, which are listed in C0 to C3, and for cylindrical roller bearings, which are listed in C2 to C4 in the single row design and C1 to C5 in the double row.

Which clearance classes do we stock?

We hold C2, C3 and C4 stock alongside the standard clearance most bearings are built to. The table counts what we have in each class by bearing type, and every count opens that category's listing.

A bearing is counted under the class its own specification states, never under one read out of its number. 2,265 of the 3,240 published bearings in these categories state a radial clearance; the rest do not carry the specification at all and are counted in no class here.

Where a class runs to 60bearings or fewer, every number in it is listed below rather than summarised, and each one opens that bearing's product page. Read the number as separate slots: the class sits after the series and bore, with the cage, groove and seal codes around it.

C3 is stocked in 888 bearings, more than this page prints, so its counts above open the listings instead of a part of the list. The classes are what we hold now rather than the whole range the manufacturer builds; a class absent from the table is one nothing in stock states today.

What does W33 mean on a spherical roller bearing?

W33 is a lubrication feature, not a clearance code. It means the outer ring has an annular groove around its outside diameter with lubrication holes drilled through it, so lubricant supplied at the outside of the outer ring is carried around the groove and through the holes into the bearing: a Rollway spherical roller bearing has the groove and three lubrication holes, and a McGill SPHERE-ROL bearing has the groove feeding two oil holes. On SPHERE-ROL bearings the W33 feature is standard, and every bearing is supplied with it unless otherwise specified.

A spherical roller bearing number stacks several separate codes, and W33 is the last of them. In a McGill SB 22210 C3 W33, SB is the SPHERE-ROL prefix, 222 is the series, 10 is the bore in units of 5 mm (a 50 mm bore), C3 is the clearance and W33 is the groove and holes; the product page lists the lubrication type as a groove and hole and gives the groove width. The full McGill nomenclature also carries K for a tapered bore, placed before the clearance; E for the expansion type, placed after it; a seal code after W33, where S or SS is a Nylaplate seal on one or both sides, TS or TSS the high temperature seal and YS or YSS the Lambda seal; and VA for high temperature grease at the end. The Rollway number reads the same way with the cage stated by its own letters: in a 22212 GMEX C3 W33, 22212 is a 60 mm bore in the 222 series, GMEX is a single piece brass cage guided on the rollers, C3 is the clearance and W33 is the groove and holes. A K suffix is a tapered bore of 1 in 12 on the diameter, and K30 is the 1 in 30 taper used on the 240 and 241 series.

Why is C3 chosen instead of normal clearance?

Because the clearance a bearing is made with is not the clearance it runs with. A tight fit takes clearance away when the bearing is mounted, and a hotter inner ring takes more away as the parts expand, so a bearing that is press fitted or runs hot is ordered in a larger class to keep a positive running clearance. Increased clearance options exist specifically for high temperature and high speed applications, to allow for thermal expansion.

The tapered bore spherical roller bearing shows the effect in figures. Driving the bearing up its taper reduces the radial clearance, and the mounting data states the reduction by bore size: for a 50 to 65 mm bore the reduction is 0.03 to 0.04 mm, against a C3 clearance before mounting of 0.075 to 0.095 mm and a standard clearance before mounting of 0.055 to 0.075 mm. The smallest radial clearance permitted after mounting for that bore is 0.025 mm in CN, 0.035 mm in C3 and 0.055 mm in C4. That is why the tapered bore ranges are made looser than the cylindrical bore ranges in every class, and why a tighter press fit in a housing calls for a review of whether the standard internal clearance is sufficient or a modified clearance is needed to prevent reduction or elimination of the clearance.

Heat works the same way, which is why the high temperature insert bearings are built with increased internal clearance to accommodate thermal expansion of the components. Where the fit is heavy and the heat is high at the same time, the next class up is used: a 6206 ZZ C4 is the C4 version of a 30 mm bore shielded bearing. In the other direction, reduced clearance is an option for mounted bearings exposed to high shock load and vibration, and for oscillating applications, because the load is carried over more rolling elements and the stress on the raceways falls.

Is ZZ the same as C3?

No. ZZ is a shield code and C3 is a clearance code; they sit in different positions on the bearing number, and one bearing can carry both. ZZ means two steel shields, filled with lithium base grease for a peak operating temperature of 120 C; 2RS means two NBR rubber contact seals, filled with lithium base grease for a peak of 100 C; a number with neither is an open bearing. None of these says anything about clearance, and none of them changes it.

The confusion comes from the codes sitting next to each other at the end of the number. Read them as separate slots, closure first and clearance second: a 6205 ZZ EMQ is shielded with normal clearance, a 6205 ZZ C3 EMQ is shielded with C3 clearance, and a 6206 2RS C3 EMQ is sealed with C3 clearance. The two closures also behave differently. Shields do not contact the inner ring, so the speed limit is the same as the open version, but small dust particles or moisture can still enter, and shields suit a dry environment with low or medium dust contamination. Contact seals generate heat at the lip, which reduces the speed limit by 20 percent compared with an open or shielded bearing. The EMQ suffix on all three is electric motor quality, a low noise precision bearing for electric motors; a ZZ EMQ bearing is filled with polyurea grease rather than lithium.

What are precision classes P0 to P6?

P0 to P6 are precision classes, set by ISO 492, and a precision class is a separate code from the clearance class: a bearing number can carry one of each. P63 is the combined suffix for precision class P6 with radial clearance class C3, which shows the two axes are independent. Deep groove, self-aligning, angular contact and spherical roller bearings are listed in P0 to P6, and double row cylindrical roller bearings in P0 to P4.

P0 is the class the stock bearings are made to: where a spherical roller bearing product page states a precision rating it is P0, and the mounted ball bearing inserts that state one list ABEC 1. A precision class does not change the clearance selected; when a print calls for P5 or P6, send the details through the bearing selection form and we will confirm the class and the clearance together.

How is clearance stated on a mounted or insert bearing?

Mounted ball bearings and ER insert bearings list their internal clearance as Standard or Increased rather than as a C class. Increased is the high temperature bearing: the ERX-TREME HI insert and the high temperature housed units are designed with a larger internal clearance to accommodate thermal expansion of the components, and the other inserts in the insert bearings category that state a clearance list Standard.

The housing fit matters here as well. The published bore for a cylindrical OD insert in a stationary housing is a light load conveyor roll fit; a tighter press fit needs a review of whether the standard internal clearance is sufficient, because a press fit on the outer ring reduces the clearance in the same way a tight shaft fit does. How the insert is built, locked and numbered is covered in the ER bearings guide, and the rest of a housed unit number in the guide to mounted bearing nomenclature. Where a mounted bearing product page states an internal clearance, it states it in words rather than as a C class.