Bearing Clearance C2, CN, C3, C4, C5 — ISO 5753 Tables & Selection Guide

Bearing clearance (radial internal clearance — RIC) is the total radial distance the inner ring can move freely relative to the outer ring when a bearing is unmounted and unloaded — measured in micrometres (μm), directly determining load distribution, operating temperature, noise level, and service life.

Selecting the wrong clearance group is one of the most common causes of premature bearing failure in the field. A 6308 bearing mounted on an electric motor shaft with k6 interference fit, operating at 80°C — if fitted with CN (normal) clearance, the operational clearance can go negative, overloading the rolling elements and destroying the bearing within a few thousand hours instead of tens of thousands. Conversely, using C3 or C4 on a precision shaft with sliding fit produces excessive vibration, noise, and uneven wear. ISO 5753-1:2009 defines five clearance groups — C2, CN, C3, C4, C5 — and this article provides detailed analysis of each group: lookup tables, operational clearance formulas, worked calculation examples, and guidance for selecting the right clearance group by application. All data based on ISO 5753, SKF, FAG TPI 200, NSK technical reference, and ZVL catalogs.

Radial Internal Clearance Definition

Initial clearance, fitted clearance, and operational clearance

Radial internal clearance is the total distance the inner ring can shift radially relative to the outer ring when the bearing is in free state — not mounted on a shaft, not installed in a housing, and carrying no load. Measurement is performed by holding one ring fixed (typically the outer ring), positioning the other ring at the upper extreme, then shifting it to the lower extreme — the displacement is the radial clearance. Unit: micrometre (μm), equivalent to 0.001 mm.

Three clearance states must be distinguished:

1. Initial clearance — the clearance measured when the bearing is new and unmounted. This is the value listed in catalogs and ISO 5753 tables.

2. Fitted clearance — the clearance after mounting the bearing on the shaft and into the housing. Interference fit of the inner ring on the shaft causes the inner ring to expand, reducing clearance. Interference fit of the outer ring in the housing causes the outer ring to contract, further reducing clearance. Fitted clearance = initial clearance − shaft fit reduction − housing fit reduction.

3. Operational clearance — the actual clearance when the bearing is rotating under load and heat. The inner ring is typically hotter than the outer ring (due to contact with the rotating shaft), expanding more, further reducing clearance. Operational clearance = fitted clearance − thermal reduction.

Operational clearance is the value that determines bearing performance. The initial clearance shown in catalogs is merely the starting point — engineers must calculate through two reduction steps (fit reduction + thermal reduction) to determine actual operational clearance. The operational clearance calculation section of this article presents formulas and a detailed worked example.

Radial clearance vs. axial clearance

Beyond radial clearance, bearings also have axial internal clearance — the axial distance one ring can shift relative to the other. For deep groove ball bearings, axial clearance relates to radial clearance through the contact angle and groove geometry. The approximate formula for single-row ball bearings:

S_a ≈ (d_w / (2 × f_i × d_w − S_r)) × √(4 × f_i × d_w × S_r − S_r²)

Where S_a is axial clearance, S_r is radial clearance, d_w is ball diameter, and f_i is inner ring groove ratio (typically 0.52–0.53). In practice, axial clearance for ball bearings is approximately 7–13 times larger than radial clearance depending on size. For cylindrical roller bearings, axial clearance is essentially zero (line contact rollers with no contact angle).

This article focuses on radial clearance — since this is the parameter specified in ISO 5753, listed in catalogs, and the basis for selecting clearance groups C2/CN/C3/C4/C5.

ISO 5753 Clearance Groups: C2, CN, C3, C4, C5

ISO 5753-1:2009 defines five radial internal clearance groups for rolling bearings, from smallest to largest:

Clearance group designation on bearing codes — see details at how to read bearing codes:

• No suffix = CN (normal) — example: 6205 = CN clearance
• C2 — example: 6205 C2
• C3 — example: 6205 C3
• C4 — example: 6308 C4
• C5 — example: 22220 EK/C5

ZVL, SKF, FAG, NSK, and NTN all use the same designation per ISO standards. When ordering, the clearance suffix is mandatory if other than CN — if omitted, the manufacturer defaults to CN. Comparing two bearings with the same designation but different clearance — 6205 CN vs. 6205 C3 — the difference lies entirely in the gap between rolling elements and raceways, not in outer dimensions or material.

Ball Bearing Clearance Table — ISO 5753-1

The table below is extracted from ISO 5753-1:2009, applying to single-row deep groove ball bearings — the most common bearing type, accounting for over 70% of global production. Values in μm, each cell shows min–max clearance.

How to read the table

Example: bearing 6206 has bore diameter d = 30 mm. Look up the row "over 24 to 30":

• C2: 1 – 11 μm (less than normal)
• CN: 5 – 20 μm (normal)
• C3: 13 – 28 μm (greater than normal)
• C4: 23 – 41 μm (much greater)
• C5: 30 – 53 μm (special)

The min–max values represent manufacturing tolerance. A batch of 6206 CN bearings manufactured to standard will have clearance within 5–20 μm — any bearing below 5 μm or above 20 μm is rejected. In practice, most bearings in a batch cluster near the mean (12.5 μm for 6206 CN).

The gap between adjacent clearance groups is significant. At d = 40 mm: CN mean is 13 μm, C3 mean is 25.5 μm — nearly double. One clearance group in the wrong direction can shift a bearing from optimal operation to overload or excessive looseness.

Cylindrical Roller Bearing Clearance Table — ISO 5753-2

Cylindrical roller bearings (CRB) have larger clearance than ball bearings of the same size, because line contact between rollers and raceways (versus point contact in ball bearings) demands more clearance to avoid edge stress under slight shaft deflection or thermal expansion. The table below is extracted from ISO 5753-2, applying to single-row cylindrical roller bearings (NU, NJ, NUP series).

Ball bearing vs. cylindrical roller bearing clearance

At d = 50 mm, CN clearance: ball bearing 6–23 μm (mean 14.5 μm), cylindrical roller bearing 20–40 μm (mean 30 μm) — roller bearings require approximately double the clearance. Reason: cylindrical rollers make line contact across the full roller length, requiring larger clearance to prevent edge stress concentration under slight shaft deflection or thermal growth.

Practical note: NU 2210 C3 (d = 50 mm) has clearance 35–55 μm — approximately 1.5 times 6210 C3 (18–36 μm). When substituting a ball bearing with a cylindrical roller bearing (or vice versa) in the same application, operational clearance must be recalculated using the correct table. Do not apply ball bearing clearance tables to roller bearings.

Clearance Reduction: Fit Interference and Thermal Expansion

Clearance reduction from interference fit

When the inner ring is mounted on a shaft with interference fit, the shaft forces the inner ring to expand radially. The inner raceway diameter increases, narrowing the gap between rolling elements and raceway — clearance decreases. The reduction is approximately:

Δδ_fit ≈ 0.85 × Δd_i

The 0.85 factor (rather than 1.0) accounts for the inner ring being a thick-walled cylinder — the bore surface expands more than the outer surface (which contacts the rolling elements). If the outer ring also has an interference fit in the housing:

Δδ_fit ≈ 0.85 × Δd_i + 0.75 × Δd_o

The 0.75 factor for the outer ring reflects that the housing compresses the outer ring inward, with less effect on clearance than inner ring expansion.

Fit reduction table

Values in μm, for single-row deep groove ball bearings (reference: SKF catalog and ISO 286).

Practical rule: clearance reduction from interference fit is approximately 70–90% of the actual interference. A k6 shaft creating 12 μm interference reduces clearance by approximately 9–11 μm. An m6 shaft creating 18 μm interference reduces clearance by approximately 14–16 μm.

Clearance reduction from temperature differential

When the bearing is operating, the inner ring runs hotter than the outer ring because it contacts the rotating shaft directly. The inner ring expands more, increasing the inner raceway diameter and reducing clearance further.

Δδ_thermal = α × d_m × ΔT

Where:

• α = coefficient of thermal expansion for steel = 12.5 × 10⁻⁶ /°C
• d_m = pitch diameter (diameter through ball centers) ≈ (d + D) / 2, mm
• ΔT = temperature difference of inner ring relative to outer ring, °C

Example: bearing 6310, d = 50 mm, D = 110 mm → d_m = 80 mm. If ΔT = 20°C:

Δδ_thermal = 12.5 × 10⁻⁶ × 80 × 20 = 0.020 mm = 20 μm

This 20 μm exceeds the entire CN clearance range for bearing 6310 (6–23 μm). This is why electric motors — where ΔT is typically 15–30°C — require C3.

Combined effect: two reduction steps

Actual operational clearance:

δ_op = δ_initial − Δδ_fit − Δδ_thermal

Both reduction steps are cumulative. An electric motor with k6 shaft at 80°C: fit reduction 8–12 μm, thermal reduction 12–20 μm → total reduction 20–32 μm. Using CN (mean 14.5 μm for d = 40 mm), operational clearance is certainly negative. C3 (mean 25.5 μm) allows operational clearance near zero — the optimal zone.

Worked Example: 6308 C3 on Motor Shaft

Problem statement

Bearing 6308 C3 mounted on a 15 kW electric motor shaft. Parameters:

• Bearing: 6308, d = 40 mm, D = 90 mm
• C3 clearance (from table, row "over 30 to 40"): 15–33 μm → mean δ_initial = 24 μm
• Shaft tolerance: k6 → actual interference Δd_i = 13 μm
• Housing tolerance: H7 → sliding fit, Δd_o = 0
• Inner ring temperature: 85°C, outer ring: 65°C → ΔT = 20°C

Step 1 — Calculate fit reduction

Δδ_fit = 0.85 × 13 = 11 μm

Step 2 — Calculate thermal reduction

d_m = (40 + 90) / 2 = 65 mm

Δδ_thermal = 12.5 × 10⁻⁶ × 65 × 20 = 0.01625 mm ≈ 16 μm

Step 3 — Calculate operational clearance

δ_op = 24 − 11 − 16 = −3 μm

Operational clearance is negative 3 μm — meaning the bearing runs under light preload. For ball bearings, light preload (−3 to −5 μm) is acceptable — in practice many electric motors operate successfully with operational clearance near zero or slightly negative.

Comparison if CN were used instead of C3

δ_initial CN (mean) = (6 + 23) / 2 = 14.5 μm

δ_op = 14.5 − 11 − 16 = −12.5 μm

Preload of −12.5 μm is excessive — rolling elements are heavily compressed, heat generation spikes, increased temperature further reduces clearance (thermal spiral), and the bearing fails rapidly. This is precisely why electric motors require C3.

Additional example: 22220 EK/C3 on hot gas fan shaft

Self-aligning roller bearing 22220 EK/C3 — d = 100 mm, D = 180 mm, mounted on a cement kiln exhaust fan shaft. Shaft k6, housing H7. Inner ring 120°C, outer ring 70°C → ΔT = 50°C.

• δ_initial C3 (d = 100 mm, self-aligning roller bearing): 50–75 μm → mean 62.5 μm
• Δδ_fit = 0.85 × 17 (k6 interference at d = 100 mm) = 14.5 μm
• d_m = (100 + 180) / 2 = 140 mm
• Δδ_thermal = 12.5 × 10⁻⁶ × 140 × 50 = 0.0875 mm = 87.5 μm
• δ_op = 62.5 − 14.5 − 87.5 = −39.5 μm → SEVERELY NEGATIVE

At ΔT = 50°C, even C3 is insufficient. Solution: switch to 22220 EK/C4 (mean clearance 90 μm) → δ_op = 90 − 14.5 − 87.5 = −12 μm — still negative. Additional housing cooling (water or air) needed to reduce ΔT to 30°C → Δδ_thermal = 52.5 μm → δ_op = 90 − 14.5 − 52.5 = +23 μm — positive, safe.

CN — Normal Applications

When CN is the right choice

CN (Normal clearance) is the default clearance group per ISO 5753. When no clearance suffix appears on a bearing designation, the manufacturer supplies CN. This group is appropriate when three conditions are met simultaneously:

1. Standard fits: inner ring with light interference (j5, j6) or sliding fit (h5, h6, g6) — fit reduction is small (below 5 μm for small-to-medium bearings)
2. Normal operating temperature: below 70°C, inner-to-outer ring temperature differential below 10°C
3. No special requirements: no preload needed, no extreme low-vibration requirements, no thermal shock

Typical CN applications: light-duty conveyors, household fans, office equipment, washing machines (light-load spin axis), bicycles, skateboards, and consumer applications where loads are light, speeds are low-to-moderate, and temperatures stay near ambient.

When CN is the wrong choice

CN is incorrect when:

• Interference fit (k5, k6, m5, m6, n6, p6) — reduces clearance by 5–15 μm, potentially eliminating all CN clearance in small bearings
• Operating temperature above 70°C — thermal differential further reduces clearance
• Combined interference fit + heat — the most common scenario causing bearing failure due to choosing CN instead of C3

A very common real-world mistake: a technician orders bearing 6308 (d = 40 mm, CN = 6–23 μm) for a 15 kW electric motor, shaft tolerance k6 (clearance reduction 8–12 μm), operating at 80°C (additional reduction 5–8 μm). Total reduction: 13–20 μm. Mean initial CN clearance approximately 14.5 μm — after both reductions, operational clearance can go negative. The bearing fails after 3,000–5,000 hours instead of the designed 20,000+ hours. Correct solution: use C3 (18–36 μm).

Relationship between operational clearance and bearing life

Operational clearance directly affects load distribution across the rolling elements, and load distribution determines fatigue life — per the L₁₀ life formula.

According to SKF calculations (Rolling Bearings catalog, 2018), single-row deep groove ball bearings achieve maximum L₁₀ life when operational clearance is zero to slightly positive (0 to +5 μm for medium-sized bearings). Negative clearance of −10 μm reduces life by approximately 30–50%. Excessive positive clearance of +30 μm reduces life by approximately 15–25%. Selecting the right clearance group is not about choosing the largest or smallest clearance — it is about selecting the initial clearance group so that operational clearance falls within the optimal range after subtracting fit reduction and thermal reduction.

C3 — Motors, Pumps, and Industrial Applications

Why C3 is the most common industrial clearance group

C3 clearance is approximately 50–80% larger than CN, and is the most widely used clearance group in industrial applications. SKF, FAG, NSK, and ZVL all recommend C3 as standard for electric motors, pumps, industrial fans, compressors, gearboxes, and most industrial machinery — for two reasons:

1. Standard industrial fits: motor shafts and pump shafts typically use tolerance k5 or k6 (sometimes m6 for heavy loads), creating interference fits that significantly reduce clearance
2. Operating temperatures above ambient: motors, pumps, and fans running continuously at 60–100°C — the inner ring runs 10–30°C hotter than the outer ring

C3 provides sufficient initial clearance so that after subtracting fit reduction and thermal reduction, operational clearance remains positive — ensuring the bearing is not overloaded.

Applications requiring C3

• Electric motors (IEC/NEMA): 0.75 kW and above — most motor manufacturers (ABB, Siemens, WEG, Teco) install C3 bearings at the factory
• Centrifugal pumps: shaft tolerance k6 or m6, fluid temperature transmitted through the bearing
• Industrial fans: especially hot gas exhaust fans above 60°C
• Air/gas compressors: interference fit + compression heat
• Industrial gearboxes: shaft tolerance k5/k6, continuous loading
• Industrial conveyors: interference fit shafts, continuous 24/7 operation

In practice, approximately 70% of bearings sold for industrial applications are C3. ZVL supplies the full size range of 6200–6300 series, 6800–6900 series, and NU/NJ series with C3 clearance readily available from stock.

Real-world case: textile mill in Nam Dinh province

At a textile mill in Nam Dinh province, a 15 kW loom motor was fitted with 6308 C4 bearings (the only stock available). C4 clearance at d = 40 mm is 30–51 μm — approximately 15 μm larger than C3 (18–36 μm). Result: motor vibration increased from 2.8 mm/s to 4.5 mm/s (per ISO 10816), noise rose 8 dB, and woven fabric showed uneven patterns from vibration transmitted through the shaft. Switching back to C3 after 2 weeks resolved the issue completely. C3 is not "the safest option" — C3 is the optimal clearance group for motor operating conditions.

C4/C5 — Kilns, Dryers, and High-Temperature Applications

When C3 is not enough

C4 clearance is approximately 40–60% larger than C3, intended for applications where operating temperature exceeds 100°C or the inner-to-outer ring temperature differential exceeds 30°C. Under these conditions, thermal reduction is so large that even C3 cannot compensate.

Specific example: bearing 6310 (d = 50 mm) in a furnace exhaust fan at 180°C:

• Initial C3 clearance: 18–36 μm (mean 27 μm)
• Fit reduction with k6: approximately 10 μm
• Thermal reduction (inner ring 150°C, outer ring 80°C, ΔT = 70°C): approximately 20–25 μm
• Operational clearance: 27 − 10 − 22 = −5 μm → NEGATIVE → bearing seizure

With C4 (30–51 μm, mean 40 μm): 40 − 10 − 22 = +8 μm → positive, safe.

Typical C4 applications

C5 — special applications

C5 has the largest clearance of all five ISO 5753 groups, approximately 1.5–2 times C4. Used only when C4 remains insufficient — cement rotary kilns with shafts exposed to 200–400°C radiant heat, metallurgical furnaces near molten zones, large industrial rotary dryers with 200–350°C hot gas directly on the shaft.

SKF, FAG, and ZVL do not mass-produce C5 for all sizes. Most C5 bearings are made to order or available only in select large sizes (bore diameter above 80 mm). Lead time is typically 4–12 weeks.

C4 and C5 considerations

C4/C5 has larger clearance, so the load zone is narrower than CN and C3 — fewer rolling elements carry load, each rolling element carries more load. This reduces calculated life (per the L₁₀ life formula) by approximately 5–15% compared to the same bearing at optimal clearance. However, this is a necessary trade-off: a C4 bearing with slightly positive operational clearance will far outlast a C3 bearing with negative operational clearance.

When operating above 120°C continuously, combine C4/C5 with stabilized heat treatment bearings (suffix S0, S1, S2 on SKF; S0, S1 on FAG) to ensure dimensional stability — see bearing materials for details. Do not use standard bearings (no S suffix) above 120°C continuous operation.

Alternatives to C5 when sourcing is difficult: cool the housing with water or air, install thermal insulation between housing and heat source, or use self-aligning roller bearings (SRBs) with C4 — SRBs have larger clearance than ball bearings at the same clearance group.

Real-world case: cement plant in Ha Nam province

At a cement plant in Ha Nam province, a vibrating screen used SRB 22316 EK CN (Normal clearance) instead of C4. The 4G vibrating screen at 900 rpm generated significant friction heat. Normal clearance was eliminated after interference fit plus thermal expansion — the bearing became overloaded and failed after 2,200 hours instead of the designed 12,000 hours. Switching to 22316 EK/C4 — residual clearance 25–30 μm at 95°C — achieved 14,000 hours service life. C4 bearings cost less than 10% more than CN, but service life increased 6 times.

C2 — Low Vibration, High Precision

When less-than-normal clearance is needed

C2 has clearance smaller than CN — the minimum value can be zero or very close to zero. Purpose: increase the number of rolling elements carrying load (widen the load zone), reduce vibration, lower noise, and improve radial stiffness of the bearing arrangement.

Mandatory conditions for using C2:

1. Sliding or very light interference fit: shaft tolerance h5, h6, g6, or j5 — interference near zero, fit reduction is minimal
2. Small temperature differential: operating temperature near ambient, inner-to-outer ring ΔT below 10°C
3. No thermal shock: stable temperature, no sudden hot-cold cycles

If any condition is not met, operational clearance can go excessively negative — the bearing runs in excessive preload, heat generation increases rapidly, and failure occurs quickly.

Typical C2 applications

• Automotive gearboxes: ball bearings in transmissions need light preload to reduce vibration and noise — C2 provides small clearance that creates natural preload when mounted
• Machine tool spindles: angular contact bearings typically use preload via spacers, but support ball bearings may use C2 for increased stiffness
• Ceiling fans, household ventilation fans: low noise required, very light loads, low temperature
• Medical equipment: centrifuges, infusion pumps — ultra-low vibration required
• Encoders, speed sensors: high precision and minimal play needed

C2 is often combined with precision grades P6 or P5. P6/C2 or P5/C2 bearings control both clearance and geometric tolerance — suitable for applications demanding high rotational accuracy.

Common Mistakes in Clearance Selection

Mistake 1: Using CN for electric motors

This is the single most common mistake. Many bearing suppliers sell CN (default) bearings for electric motors because they are "cheaper" or "in stock." Electric motors from 0.75 kW upward almost always require C3 — unless loads are very light, speeds are very low, and temperature stays below 50°C (rare in industrial settings). Consequence: bearing fails after 3,000–8,000 hours instead of the designed 20,000–40,000 hours. C3 bearings cost approximately 5–10% more than CN — but service life increases 3–5 times.

Mistake 2: Using C3 for precision shafts and gearboxes

The opposite of mistake 1: using C3 when C2 or CN is needed. CNC machine tool spindles or automotive gearboxes require small clearance or light preload to minimize vibration, noise, and maximize stiffness. Using C3 in these applications creates excessive positive operational clearance — causing vibration, noise, reduced rotational accuracy, and uneven wear.

Mistake 3: Ignoring temperature when selecting clearance

Choosing C3 because "motors use C3" without calculating actual temperature differential. A small 1.5 kW motor in an air-conditioned room at 25°C has a completely different thermal profile than a 90 kW motor in a steel rolling mill at 45°C ambient. Same C3, but vastly different operational clearance. High-power motors with heavy loads in hot environments may need C4 instead of C3.

Mistake 4: Confusing initial clearance with operational clearance

Many technicians measure initial clearance with a feeler gauge, see "clearance exists," and conclude the bearing is fine. But initial clearance is not operational clearance. A 6310 CN bearing measured at 25 μm initial clearance (within tolerance), but after m6 fit reduces 15 μm and 30°C thermal differential reduces another 25 μm → operational clearance = −15 μm → failure. Calculation is required, not measurement alone.

Mistake 5: Mixing clearance groups in a bearing pair

On a shaft with two bearing supports, both bearings should use the same clearance group (or compatible groups per design specification). Installing C3 at one end and CN at the other creates uneven load distribution between the two supports — the CN bearing carries more load, fails first, then the C3 bearing takes the full load and also fails.

Clearance selection summary by application

ZVL supplies bearings with C2, CN, C3, and C4 clearance groups fully compliant with ISO 5753 — ensuring clearance values equivalent to SKF, FAG, and NSK at identical sizes and clearance groups. When selecting a bearing and substituting ZVL for another brand, simply maintain the same basic designation and clearance group — for example: SKF 6308 C3 → ZVL 6308 C3, no recalculation needed. See how to read bearing codes for cross-referencing designations between manufacturers.