TL;DR
- Internal clearance is an assembly result, not a letter on the box. It is the gap between the rolling element and the raceway at the tightest point with the bearing unloaded, and it is produced by the internal geometry and by the manufacturing process: heat treatment, grinding, assembly.
- C2, C3, C4 and C5 are clearance groups from the tolerance standard, and they run in one direction only. C2 is closer to the basic clearance, C5 is the loosest. The published tolerance standard for radial bearings is ISO 492, and the terminology is set in ISO 5593.
- Clearance decides whether the balls roll or skid. Too loose and the rolling elements slide instead of rolling, which generates heat and wear and shortens life; too tight for the housing and the assembly runs with preload, which generates its own heat and noise.
- Rated life is only valid for the clearance it was calculated for. ISO 281 computes rating life under defined load, geometry and clearance conditions, so a different clearance is not a small variation on the same number.
The suffix on a bearing designation carries more than the brand usually explains, and clearance is the one people most often get wrong in both directions. A buyer who treats C2 as “the premium quiet one” and another who treats C5 as “the robust one” are both working from a slogan rather than a measurement. What the code describes is how much gap the rolling elements have in the unloaded state, and that decides whether the balls roll the way the catalogue assumes.
This guide is built from our own published product data, starting with the deep groove ball bearing 6301 ZZ specification, where clearance, tolerance class, vibration grade and seal type are all listed as separate options on the same product, and the bearing range we supply to distributors. For anything that has to be confirmed against a drawing or a sample, our sales engineers answer technical enquiries directly.
What internal clearance physically is
Radial internal clearance is the gap between the rolling element and the raceway at the point where the two are closest, measured with the bearing unloaded and with no interference from the shaft or the housing. It is defined in ISO 5593, and the groups into which radial bearings are sorted for clearance are given in ISO 492.
Two features of that definition matter here. The first is that it is an unloaded measurement: once the bearing is on a shaft in a housing, the running clearance is the manufactured clearance minus whatever the fit has taken out of it. The second is that it is manufactured rather than specified on paper: the raceway is ground, the rings heat treated, the balls sized and matched, the assembly set. Two bearings with the same designation are close but not identical, and the tolerance class controls how close.
The codes run one way. C2 is a modest step above the basic clearance group, C3 is a further step, C4 and C5 progressively looser again, and each step is a defined range of measured values rather than a loose description. Because the groups are defined ranges and not a scale of “better”, so C4 is not a better bearing than C3, it is a different working condition, and the right group is the one that matches how the shaft, the housing and the temperature behave together.
What our published product pages give you, and what they do not
Our 6301 ZZ specification lists, on one product, the full set of codes a buyer has to combine. Writing them out makes the point better than any single explanation can.
| Published option | What it controls | Why it cannot stand alone |
|---|---|---|
| Internal clearance C2, C3, C4, C5 | The unloaded gap between rolling element and raceway | Only meaningful together with the housing fit and the operating temperature |
| Tolerance class ABEC-1, ABEC-3, ABEC-5 | How closely the manufactured dimensions and running clearance are held | A precise clearance in a loose tolerance class is still a loose bearing |
| Vibration level V3, V2, V1 | The cleanliness of the finished surfaces | A separate quality axis; it does not substitute for clearance |
| Shield or closure Open, Z, ZZ, RS, 2RS, 2RZ | Whether the bearing is open, shielded or sealed | Decides lubrication retention and contamination protection, not the gap |
| Cage: stamped steel, or solid brass | How the rolling elements are retained | Stamped steel is used on our range up to 400 mm outside diameter |
| Material: chrome steel, carbon steel, stainless steel, ceramic | Load capacity, corrosion behaviour, speed | Sets the duty the clearance group has to work in |
Our published range also lists the series that carry these codes, from the 6000 and 6200 families through 6300, 6400, 6800, 6900, 16000, 62200 and 62300 to the NR series, with extra-large bearings from 180 mm to 6300 mm bore and specials made to a customer’s drawing or sample. The practical point is the one in the table: a clearance code on its own is half a specification, and the half that decides whether the bearing works is usually the other half.
How clearance changes life
The mechanism is short. A ball that rolls does not slide; a ball that skids does, and sliding friction is roughly an order of magnitude worse than rolling friction at the contact. With more clearance than the geometry intends, the balls can migrate across the raceway instead of staying in the groove, and every migration is a place where heat is generated and material is worn.
That is why rated life is not a property of the part alone. ISO 281 computes rating life for a defined operating condition: a defined load, geometry and internal clearance. Run the same bearing with materially more clearance than the calculation assumed and the balls start to skid, the lubricant film thins, the temperature rises and life follows the temperature rather than the catalogue. A wrong clearance does not invalidate the datasheet figure; it stops the figure describing what the bearing is doing.
Too little clearance is not the safe version of this. Once mounted, the running clearance is the manufactured clearance less the interference taken by the fit, and a fit that is too aggressive for the clearance group removes the gap entirely. The bearing then runs with preload, generating heat, and in the worst case binding in the seat. Heating the bearing for assembly, commonly to 70 or 80 degrees Celsius, is standard practice precisely because it allows a tight fit without the fit consuming the clearance.
In practice the groups map to conditions rather than to quality. A tighter group such as C2 is chosen where running accuracy and noise matter and the temperature difference between shaft and housing is well controlled. C3 is the general-purpose choice for most industrial applications. The looser groups such as C4 and C5 exist for cases where the shaft grows more than the housing with temperature, or where the fit cannot be made tight enough, and the extra clearance absorbs the difference. That mapping is industry practice rather than a rule written into the standard, and it is worth confirming against your own duty.
How clearance changes noise
Noise and clearance are linked in two opposite directions, which is why the answer is not simply “more clearance equals more noise” or its opposite.
With too much clearance, the rolling elements can skid and the irregular rumble appears, rising with speed and load. With too little clearance for the fit, the bearing runs with preload, the lubricant film thins, the temperature climbs and the noise of a hot, thin film is worse than that of a skidding bearing. The quietest arrangement is in the middle, where the rolling elements stay in the groove.
Two other codes sit next to clearance in a purchase order and are frequently confused with it. Vibration grade, published on our range as V3, V2 and V1, describes the cleanliness of the finished surfaces. Seal type, published as Open, Z, ZZ, RS, 2RS and 2RZ, decides what the bearing keeps out and what it keeps in. Neither substitutes for a clearance code, and a quiet bearing with the wrong clearance still gets loud under load.
The housing decides whether the clearance choice works
Whatever group is ordered, the working clearance is set on the machine, not in the carton. A loose shaft fit with a tight housing fit opens the clearance up; a tight shaft fit with a loose housing fit closes it down. Shafts expand faster than housings, so the running clearance of a hot machine is smaller than the clearance measured cold, which is the second reason a group has to be chosen with the operating temperature in mind rather than from the catalogue.
This is also the reason the housing fit is part of the enquiry. A bearing supplier can only give a sensible clearance recommendation if they know the shaft material, the housing material, the fit, the mounting temperature and the maximum operating temperature. Because the clearance is the difference between two fits rather than a number in isolation, so asking for a clearance recommendation without the fit information produces an answer that is arithmetically correct and practically useless.
How to write the clearance into a purchase order
A bearing line that will survive a goods-in check needs five things on it, and clearance is the first.
The clearance group code, written as the code itself: C2, C3, C4 or C5, never as small, medium or large, because those words mean nothing to the factory. The tolerance class alongside it, from the ABEC classes we publish, because clearance precision is only as good as the class holding it. The seal or shield code, which decides lubrication retention and contamination protection. The cage and material, where the duty calls for a specific one. And the lubricant, if the bearing is supplied filled, since that is a manufacturing decision.
Written together, a line looks like a bearing number followed by the codes that matter: a 6301 in a sealed configuration at C3 clearance, to a stated tolerance class, with the vibration grade and the cage agreed, in a stated material, under the BMT trademark, industrially packed and shipped under HS 8482800000. Our published pages carry exactly this structure, so a distributor can build the line from our own data.
The last item saves time later: the operating conditions behind the choice, stated once at the top of the order — duty, load, speed, mounting temperature, maximum operating temperature and the housing fit. That is what lets our sales engineers answer a clearance question with a clearance answer instead of a catalogue.
Frequently asked questions
Where does C3 sit against the basic clearance group?
C3 is a clearance group in the radial bearing tolerance standard, looser than the basic group rather than a refinement of it. In general industrial practice it is the most commonly specified group, with C2 used where running accuracy and noise are the priority and the fit is well controlled, and C4 and C5 where the running clearance has to absorb a temperature difference or a loose fit.
How do we read a designation like 6301-2RS/C3?
In three parts: 6301 is the series and size, 2RS the closure on both sides, C3 the internal clearance group. On a full order line you would also expect the tolerance class, vibration grade, cage, material and lubricant, and our published pages list all of those as separate options on the same product, with trademark, packaging type and HS code for the shipment.
When is a looser clearance the wrong answer?
When the application is noise critical and the fit is well controlled, since extra clearance lets the rolling elements migrate and skid. It is also the wrong answer in high-speed applications, where the heat generated by sliding has to be carried away from a smaller surface. The right question is not which group is bigger, but whether the running clearance in the finished assembly still leaves the balls in the groove at the highest operating temperature.
If a bearing runs hot after fitting, whose problem is the clearance?
That is a fit problem before it is a clearance problem, because the running clearance is the manufactured clearance minus the interference of the fit. Too much interference removes the clearance and the bearing runs with preload; too little lets the clearance grow and the balls skid. The way to settle it is to compare the bearing that is running hot with the clearance group that was ordered, and the fit that was used, before either is changed.
Our motor runs in a hot ambient and the shaft expands. C2 or C4?
It is a calculation rather than a rule, because the answer depends on how much the shaft grows relative to the housing, what the mounting temperature was and what the fit is. Send our engineers the shaft and housing material, the fit, the mounting temperature and the maximum operating temperature, and they will work the running clearance back from the duty and the fit rather than from the group name.
Post time: Oct-29-2026