Internal clearance is the total gap between the rolling elements and the raceways before the bearing carries any load. It is invisible in the part number, easy to get wrong at the fit and temperature stage, and its effect is not obvious until the bearing is already overheating. That combination makes it the single most common cause of premature bearing failure.
This guide covers the ISO 5753 groups from C2 to C5, gives concrete micrometre ranges for two common bores, shows how to compute the operating clearance from the fit loss and the thermal differential, and ends with a selection sequence for real applications.
- Clearance is a range, not a single number. Every group C2 to C5 is a band, and two bearings in the same group can sit at opposite ends of it.
- The number that matters is the operating clearance. Measurement clearance minus fit loss minus thermal loss is what the bearing actually runs with.
- Too little clearance is worse than too much. A bearing in preload runs hot, and the heat lowers clearance further, so the failure accelerates.
- Never pick a group by copying a spare. C3 in one machine may be correct and in another may be an overheat waiting to happen.
What internal clearance is and why it decides life
Clearance exists because a bearing must run with a small gap so that load is shared over a sensible number of rolling elements. With too much clearance, load concentrates on one or two elements and the raceway fatigues early; vibration and noise rise. With too little clearance, the bearing runs in preload, friction and temperature climb, and the heat that results reduces clearance further, so the condition feeds itself.
The correct target is a small positive operating clearance under working conditions, or a controlled light preload in positions that are designed for it. Everything in this guide is aimed at hitting that target rather than at the clearance printed on the box.
Clearance is also what links the fit, the temperature and the load zone together. A preloaded bearing spreads its load over a wider arc of the raceway, so it is stiffer and quieter; a bearing with excessive clearance loads a narrow arc, so the contact stress rises and the life falls. Between those two extremes sits the design target: enough clearance to avoid heat and skidding, and little enough to keep the load zone broad and the stress low. Reaching it is arithmetic rather than guesswork, and the arithmetic uses three numbers - the bench clearance, the fit loss and the thermal loss.
The ISO 5753 clearance groups
ISO 5753-1 defines the radial internal clearance groups for radial bearings. The groups are C2 (less than Normal), CN or Normal, C3, C4 and C5 (progressively greater). They are bands, and the band widens as the bore grows.
| Group | 25 mm bore (6205 class) | 40 mm bore (6308 class) | Typical use |
|---|---|---|---|
| C2 | 0 to 10 µm | 0 to 12 µm | Precision spindles, preloaded sets |
| CN (Normal) | 6 to 20 µm | 8 to 28 µm | General machinery, normal fits |
| C3 | 13 to 28 µm | 18 to 38 µm | Heavier fits, moderate temperature rise |
| C4 | 20 to 36 µm | 30 to 52 µm | Hot running, interference fits |
| C5 | 28 to 48 µm | 45 to 70 µm | Kilns, fans, very hot positions |
Typical ISO 5753 radial internal clearance ranges. Exact limits depend on the series and the manufacturer; confirm against the catalogue for the specific bearing.
C2 and CN
C2 is deliberately tighter than Normal and is used where the bearing will run cool, with a light fit, in a position that needs low vibration or a defined preload. CN is the catalogue default and suits a normal fit at normal temperature.
C3 and C4
C3 and C4 are the workhorse groups for anything that runs warm or is mounted with a real interference fit. The extra clearance is there to be consumed by the fit and the heat, which is why a bearing that measures at the top of C3 can still end up with the right operating clearance in a hot machine.
C5 and larger
C5 is reserved for genuinely hot applications - kilns, high-temperature fans, some gearbox output positions and rail bearings - where the thermal loss alone can approach the whole Normal clearance band.
From measured clearance to operating clearance
The clearance on the bench is only the starting point. Three effects change it once the bearing is installed and running, and they subtract.
The mounting fit loss
An interference fit on the inner ring expands the ring and removes clearance. As the shaft and housing fits guide explains, roughly 70 to 80 % of the diametral interference converts to clearance loss, so a 20 µm interference removes about 15 µm.
The thermal loss and a worked example
When the inner ring runs hotter than the outer ring, it grows more and the clearance falls. The change is roughly proportional to the temperature difference and the bore. Take a 60 mm bore bearing with a 40 °C difference between inner and outer ring at steady state. Using a coefficient of thermal expansion of about 12 × 10-6 per °C for steel, the diametral growth is approximately 12 × 10-6 × 60 mm × 40 °C = 29 µm. Only part of that appears as radial clearance loss, but even half of it, around 14 µm, is a large fraction of a Normal clearance band.
Putting it together
Suppose a 6308 class bearing measures 30 µm (mid C3) on the bench, the shaft fit removes 12 µm and the thermal differential removes 14 µm. The operating clearance is 30 - 12 - 14 = 4 µm. That is still positive and the choice of C3 was correct. Had the same bearing been ordered in CN at 18 µm, the operating clearance would be 18 - 12 - 14 = -8 µm, a preloaded bearing running hot in a machine that was designed for clearance.
Clearance versus preload
Preload is negative clearance: the rolling elements are pressed against both raceways before any external load arrives. It is not simply "too tight" - in the right position it stiffens the shaft, reduces noise and controls runout, which is exactly why precision spindle sets are preloaded deliberately. The difference between correct preload and damaging preload is control. A set that is preloaded by design has matched rings, a defined preload value and a mounting method that achieves it; a set that ends up preloaded by accident because the fit or the temperature was misjudged has none of those. The preload in bearing sets guide covers the deliberate case.
Decision table by application
| Application | Typical group | Why |
|---|---|---|
| Electric motor, normal fit, cool | CN | Modest fit loss, low temperature rise |
| Precision spindle, preloaded pair | C2 or matched set | Low vibration, defined preload |
| Hot pump, interference fit | C3 | Fit loss plus a real temperature rise |
| Gearbox output, heavy fit | C3 or C4 | Heavy interference and warm housing |
| Kiln, high-temperature fan | C4 or C5 | Large inner-ring to outer-ring differential |
| Rail axlebox | C4 | Heavy fits and sustained running temperature |
How to choose a clearance group
- Estimate the operating temperature difference between the inner and outer ring at steady state, not the ambient.
- Convert it to a clearance loss using the coefficient of thermal expansion and the bore, as in the worked example above.
- Estimate the fit loss. Take 70 to 80 % of the diametral interference on the rotating ring.
- Add the two losses to get the total reduction from the bench clearance.
- Choose a group whose lower limit is still comfortably above the total loss. The operating clearance should stay positive for clearance positions.
- Check the speed. High-speed positions need enough clearance to avoid skidding and heat; the thermal reference speed guide and the speed limits guide both matter here.
- Confirm the vibration requirement. Where noise or EMQ grade is specified, the clearance band and the running accuracy interact, as described in the noise and vibration guide.
- Record both the group and the intended operating clearance on the drawing so a future substitute can be checked against the same target.
Measuring clearance and the axial relationship
Radial internal clearance is measured by holding one ring and displacing the other radially under a small defined load, reading the movement with a dial gauge or a dedicated clearance gauge. Axial clearance is measured the same way in the axial direction. The two are related but not equal: for a deep groove ball bearing the axial clearance is generally larger than the radial clearance, and the exact ratio depends on the groove geometry, so the two values must never be treated as interchangeable in a specification. Measure the parameter the specification actually names.
| Quantity | Symbol | How it is obtained |
|---|---|---|
| Radial internal clearance | delta r | Radial displacement of one ring under a small defined load |
| Axial internal clearance | delta a | Axial displacement of one ring under a small defined load |
| Operating clearance | - | Bench value minus fit loss minus thermal loss |
Two practical points follow. First, clearance must be measured with the bearing at a known temperature, because a ring that has just come off a warm press or a heater will read differently from a cold one. Second, the gauge load must be small enough not to compress the contact, or the reading will include elastic deflection that is not clearance at all. The measuring bearing accuracy guide follows the gauging conditions through the thermal case.
Common mistakes
- Ordering C3 because the spares were C3. The group is a function of the fit and the running temperature, both of which can differ from the last machine.
- Forgetting the fit loss. A bearing ordered at the bottom of CN in a tight fit arrives preloaded.
- Mixing clearance groups in a pair. Two bearings of different groups in one set do not share load as designed.
- Measuring clearance on the bench and stopping there. Bench clearance is not operating clearance; the thermal and fit losses can consume the whole band.
- Reading axial clearance against a radial specification. The two numbers are different quantities.
- Assuming a hot position can be fixed with more clearance alone. Fit loss and thermal loss must both be budgeted, and the housing and shaft fit still have to be right.
- Measuring clearance hot and comparing it with a cold specification. A warm ring reads differently, so the number is not comparable to the catalogue band.
- Substituting a larger group to hide a noise problem. If the vibration comes from the fit or the seat geometry, more clearance moves the problem rather than solving it.
Our deep groove ball bearings, the self-aligning roller range and the cylindrical roller range are all selected with this clearance-first logic, and the groups are cross-checked against the NSK and SKF catalogue ranges. The related temperature and clearance guide takes the thermal side further, the combined load guide explains how axial load changes the load zone, the terms are defined in the glossary, and the selector filters by clearance group across the catalogue.
Frequently asked questions
What is the difference between CN and C3 clearance?
CN is the Normal group and C3 is one band larger. For a 40 mm bore bearing, CN is roughly 8 to 28 micrometres of radial internal clearance and C3 roughly 18 to 38 micrometres. C3 exists because interference fits and hot running conditions both remove clearance once the bearing is installed and warm, so a larger starting band ends up at a similar operating clearance.
What does C3 mean on a bearing?
C3 is a radial internal clearance group defined by ISO 5753-1, one step above the Normal, or CN, group. It is specified for positions where the fit removes clearance or where the bearing runs warm. It does not mean the bearing is loose or defective; it means the starting clearance is deliberately larger so that a correct operating clearance remains at working temperature.
How do I calculate operating clearance?
Start with the measured bench clearance, then subtract the fit loss and the thermal loss. The fit loss is roughly 70 to 80 percent of the diametral interference on the rotating ring. The thermal loss comes from the inner ring running hotter than the outer ring, and scales with the difference in temperature and the bore. What remains is the operating clearance.
Is negative clearance the same as preload?
Yes. Preload is negative clearance, meaning the rolling elements are pressed against both raceways before any external load is applied. Deliberate preload, as in a matched spindle set, stiffens the shaft and reduces noise. Accidental preload, caused by a fit or a temperature that was not budgeted, produces the same tightness without the control, and the bearing overheats.
When should I use a C2 clearance bearing?
Use C2 where the bearing runs cool, is mounted with a light fit, and needs low vibration or a defined preload, such as a precision spindle or a measuring instrument. Because C2 is tighter than Normal, it leaves little margin for fit or thermal loss, so it is only safe where both have been checked and are small.
What clearance does a hot application need?
Hot applications need C4 or C5, because the inner ring expands more than the outer ring and that difference consumes clearance. Estimate the steady-state temperature difference, convert it to a clearance loss using the coefficient of thermal expansion and the bore, and size the group so that a positive operating clearance remains. Kilns, high-temperature fans and rail axleboxes are typical C4 positions.