ISO Bearing Fit & Shaft Tolerance Reference Chart
Recommended ISO 286 shaft and housing fit tolerances (g6, h6, j6, k5, m5, n6, H7, J7, K7) for metric ball and roller bearing assemblies. Practical engineering limits, radial clearance guidelines (C2, Normal, C3, C4), and workshop machining standards for factory maintenance across Kenya.
An ISO bearing fit defines the microscopic dimensional clearance or interference between a precision bearing ring and its mating shaft or housing bore conforming to ISO 286. In standard industrial machinery where the inner ring rotates relative to a constant radial load, the shaft must be machined to an interference tolerance (typically k5 for normal loads or m5 for heavy shock loads) to prevent the inner ring from slipping and fretting. The stationary housing bore is machined to a clearance or light transition fit (typically H7 or J7) to permit smooth assembly and accommodate thermal axial expansion.
1. The Fundamental Principles of Bearing Seat Tolerances
Rolling bearings are precision machine elements manufactured to extreme dimensional accuracy conforming to ISO 492 (Normal tolerance class, P6, P5, or P4). The inner ring bore and outer ring outside diameter are ground at the factory within micrometer-level bands that cannot be altered in the field. Consequently, the mechanical behavior of the assembly depends entirely on the accuracy with which the machinist turns or grinds the shaft journal and bores the housing seat.
Under the international ISO 286 limits and fits system, shaft tolerances are designated by lowercase letters (such as g6, h6, k5, m5) and housing tolerances by uppercase letters (such as H7, J7, K7). The letter defines the fundamental deviation (position relative to the nominal dimension), while the number defines the tolerance grade (the permissible width of the tolerance band).
Clearance Fit (e.g. g6, H7)
The mating part is intentionally smaller than the bore (or the bore is larger than the outer ring). Components slide together freely. Essential for non-locating bearings that must float axially during shaft thermal growth.
Transition Fit (e.g. h6, J7)
The tolerance bands overlap. Depending on actual manufactured dimensions, the fit may produce a microscopic clearance or slight interference. Suitable for light loads requiring easy dismantling.
Interference Fit (e.g. k5, m5, M7)
The shaft is larger than the bearing bore (or the housing bore is smaller than the bearing outer diameter). Requires thermal heating or hydraulic mounting. Prevents ring rotation under rotating loads.
The Golden Rule of Bearing Rotation
The bearing ring that rotates relative to the direction of load must always have an interference fit. In 90% of plant machinery—electric motors, centrifugal pumps, conveyor pulleys, hammer mills—the shaft turns while the load direction remains stationary. Under this condition, the inner ring experiences circumferential rotating load. If installed with a clearance fit, the inner ring will creep around the shaft, grinding metal away through fretting corrosion until the journal is ruined.
2. Master ISO 286 Shaft Fit Tolerance Table
Dimensional deviations in micrometers (µm) and decimal millimeters (mm) for solid steel shafts.
Use this table when machining or inspecting shaft journals for deep groove ball bearings, spherical roller bearings, and cylindrical roller bearings. Positive values indicate dimensions larger than nominal (interference); negative values indicate dimensions smaller than nominal (clearance).
| Nominal Shaft Dia. (mm) | g6 (Sliding / Loose) | h6 (Line-to-Line) | j6 (Light Transition) | k5 (Normal Interference) | m5 (Heavy Interference) | n6 (Severe Shock) |
|---|---|---|---|---|---|---|
| > 10 to 18 mm | -6 / -17 µm (-0.006 / -0.017 mm) | 0 / -11 µm (0 / -0.011 mm) | +8 / -3 µm (+0.008 / -0.003 mm) | +9 / +1 µm (+0.009 / +0.001 mm) | +15 / +7 µm (+0.015 / +0.007 mm) | +23 / +12 µm (+0.023 / +0.012 mm) |
| > 18 to 30 mm | -7 / -20 µm (-0.007 / -0.020 mm) | 0 / -13 µm (0 / -0.013 mm) | +9 / -4 µm (+0.009 / -0.004 mm) | +11 / +2 µm (+0.011 / +0.002 mm) | +17 / +8 µm (+0.017 / +0.008 mm) | +28 / +15 µm (+0.028 / +0.015 mm) |
| > 30 to 50 mm | -9 / -25 µm (-0.009 / -0.025 mm) | 0 / -16 µm (0 / -0.016 mm) | +11 / -5 µm (+0.011 / -0.005 mm) | +13 / +2 µm (+0.013 / +0.002 mm) | +20 / +9 µm (+0.020 / +0.009 mm) | +33 / +17 µm (+0.033 / +0.017 mm) |
| > 50 to 80 mm | -10 / -29 µm (-0.010 / -0.029 mm) | 0 / -19 µm (0 / -0.019 mm) | +12 / -7 µm (+0.012 / -0.007 mm) | +15 / +2 µm (+0.015 / +0.002 mm) | +24 / +11 µm (+0.024 / +0.011 mm) | +39 / +20 µm (+0.039 / +0.020 mm) |
| > 80 to 120 mm | -12 / -34 µm (-0.012 / -0.034 mm) | 0 / -22 µm (0 / -0.022 mm) | +13 / -9 µm (+0.013 / -0.009 mm) | +18 / +3 µm (+0.018 / +0.003 mm) | +28 / +13 µm (+0.028 / +0.013 mm) | +45 / +23 µm (+0.045 / +0.023 mm) |
| > 120 to 180 mm | -14 / -39 µm (-0.014 / -0.039 mm) | 0 / -25 µm (0 / -0.025 mm) | +14 / -11 µm (+0.014 / -0.011 mm) | +21 / +3 µm (+0.021 / +0.003 mm) | +33 / +15 µm (+0.033 / +0.015 mm) | +52 / +27 µm (+0.052 / +0.027 mm) |
* Highlighting indicates the primary engineering fits for industrial plant rotating inner rings (k5 and m5).
* For deep groove ball bearings on 4-pole electric motors up to 22 kW, k5 is the global manufacturer standard.
3. Master ISO 286 Housing Bore Tolerance Table
Dimensional limits for cast iron, ductile iron, and steel bearing housing bores.
Housing tolerances determine how the bearing outer ring sits in end-shields, gearboxes, and plummer blocks. For stationary outer rings, positive numbers represent bore dimensions larger than nominal (clearance), enabling ease of assembly and axial movement.
| Nominal Bore Dia. (mm) | H7 (Standard Clearance) | J7 (Transition / Light Hold) | K7 (Light Press Fit) | M7 (Firm Press Fit) | N7 (Heavy Press in Alloy) |
|---|---|---|---|---|---|
| > 18 to 30 mm | +21 / 0 µm (+0.021 / 0 mm) | +12 / -9 µm (+0.012 / -0.009 mm) | +2 / -19 µm (+0.002 / -0.019 mm) | -4 / -25 µm (-0.004 / -0.025 mm) | -11 / -32 µm (-0.011 / -0.032 mm) |
| > 30 to 50 mm | +25 / 0 µm (+0.025 / 0 mm) | +14 / -11 µm (+0.014 / -0.011 mm) | +3 / -22 µm (+0.003 / -0.022 mm) | -4 / -29 µm (-0.004 / -0.029 mm) | -12 / -37 µm (-0.012 / -0.037 mm) |
| > 50 to 80 mm | +30 / 0 µm (+0.030 / 0 mm) | +18 / -12 µm (+0.018 / -0.012 mm) | +4 / -26 µm (+0.004 / -0.026 mm) | -5 / -35 µm (-0.005 / -0.035 mm) | -14 / -44 µm (-0.014 / -0.044 mm) |
| > 80 to 120 mm | +35 / 0 µm (+0.035 / 0 mm) | +22 / -13 µm (+0.022 / -0.013 mm) | +4 / -31 µm (+0.004 / -0.031 mm) | -6 / -41 µm (-0.006 / -0.041 mm) | -16 / -51 µm (-0.016 / -0.051 mm) |
| > 120 to 180 mm | +40 / 0 µm (+0.040 / 0 mm) | +26 / -14 µm (+0.026 / -0.014 mm) | +4 / -36 µm (+0.004 / -0.036 mm) | -8 / -48 µm (-0.008 / -0.048 mm) | -20 / -60 µm (-0.020 / -0.060 mm) |
| > 180 to 250 mm | +46 / 0 µm (+0.046 / 0 mm) | +30 / -16 µm (+0.030 / -0.016 mm) | +5 / -41 µm (+0.005 / -0.041 mm) | -8 / -54 µm (-0.008 / -0.054 mm) | -22 / -68 µm (-0.022 / -0.068 mm) |
4. Radial Internal Clearance (RIC): Why Shaft Fits Squeeze Bearings
One of the most expensive misunderstandings in East African factory workshops is treating radial internal clearance as "slop" or "play." When you unbox a new SKF, NSK, or FAG bearing, the rolling elements have a microscopic gap between them and the raceways. This is called initial radial clearance.
As soon as you mount the bearing onto an interference shaft fit (such as k5 or m5), the inner ring expands outward. Empirical engineering measurements show that 75% to 85% of the shaft interference is directly transferred into radial expansion of the inner ring raceway.
The Mathematics of Clearance Reduction
Consider a standard 6309 deep groove ball bearing (45 mm bore, 100 mm OD) mounted on a 45 mm shaft:
- Normal clearance (CN) initial unmounted gap: 8 to 28 µm.
- Shaft machined to upper m5 limit: +0.020 mm (20 µm interference).
- Inner ring expansion (80% of 20 µm): 16 µm reduction in clearance.
- Remaining cold operating clearance: -8 µm to +12 µm.
If that bearing is installed in a motor running at 75°C, the shaft heats up and expands faster than the cast iron housing. The remaining 12 µm of clearance completely vanishes. The balls are pinched tight against both raceways with zero radial freedom. Friction skyrockets, the grease burns into black carbon within 48 hours, and the bearing seizes solid.
C2 (Reduced)
Clearance less than Normal. For machine tool spindles, dental equipment, and applications demanding extreme rotational stiffness without vibration.
Normal / CN
Standard clearance for light transition fits (j6, h6) and light loads operating below 50°C. Standard hand-tools and agricultural implements.
C3 (Greater than Normal)
Essential for electric motors, centrifugal pumps, boiler draft fans, and any machine where shaft heat or k5/m5 fits consume radial clearance.
C4 (Extra Large)
For extreme heat and heavy vibration: quarry vibrating screens in Athi River, cement kiln trunnion rollers, and steel rolling mills.
5. Two Catastrophic Workshop Failure Modes Observed in Kenya
Diagnostic insights gathered from factory maintenance teams along Enterprise Road and industrial zones across East Africa.
Inner Ring Slippage, Fretting Corrosion & Journal Scoring
The Scenario: A lathe operator turns a pump shaft seat 0.03 mm undersized so the bearing "slides on easily by hand."
What Happens: Under rotating radial load, the microscopic gap allows the inner ring to roll around the shaft journal. The contact creates microscopic shear tears in the steel.
The Symptoms: Within weeks, a fine reddish-brown powder (cocoa powder / fretting rust) bleeds out from the bearing seat. The shaft develops deep grooves, runout exceeds 0.15 mm, and the mechanical seal begins leaking profusely.
Negative Internal Clearance, Ball Pinching & Raceway Brinelling
The Scenario: A shaft is welded and turned oversize by 0.05 mm. The technician takes a heavy pipe and a 5 kg sledgehammer and beats the bearing into place.
What Happens: Driving force applied through the outer ring transmits shock through the rolling elements, indenting the raceways (true brinelling). The massive interference completely swallows the radial internal clearance.
The Symptoms: The motor draws high no-load current from minute one. Within 24 hours, operating temperature spikes past 95°C, grease liquefies, and the raceways spall destructively. The technician falsely claims: "Hii bearing ilikuwa fake."
6. Workshop Machining & Measurement Protocols
To achieve reliable bearing life conforming to ISO standards, maintenance machinists should adopt strict measurement protocols before installing replacement bearings:
Use Calibrated External Micrometers (Never Digital Vernier Calipers)
A digital vernier caliper has a resolution of 0.01 mm (10 µm) and an accuracy of ±0.02 mm. A k5 tolerance band on a 40 mm shaft is only 11 µm wide. Checking a bearing fit with a vernier is pure guesswork. Always measure shaft journals with a calibrated 25–50 mm or 50–75 mm external micrometer reading to 0.001 mm.
Measure at Three Axial Positions and Two Perpendicular Angles
Measure the shaft diameter at both ends and in the middle of the seat. At each position, take two measurements at 90° to check for ovality (out-of-roundness) and taper. Total shaft ovality and taper must not exceed 50% of the total ISO tolerance band.
Control Surface Finish (Ra Value)
A rough-turned shaft will lose its interference fit quickly because the microscopic steel peaks crush flat when the bearing is pressed on. Shaft seats must have a surface roughness of Ra 0.8 µm for ground seats or Ra 1.6 µm for fine-turned seats.
Mount with Controlled Induction Heating (Max 110°C)
Never heat bearings with an open oxy-acetylene torch or submerge them in contaminated waste oil drums. Use an electric induction bearing heater equipped with a magnetic temperature probe set strictly to 110°C (230°F). This thermal expansion allows the bearing to slide effortlessly onto a k5 or m5 shaft seat without mechanical force.
Frequently Asked Questions (FAQ)
For standard electric motor shafts up to 50 mm diameter where the inner ring rotates under normal load, ISO k5 is the standard interference fit. For larger motor shafts between 50 mm and 100 mm with heavier radial belt pull or gear mesh, specify an m5 fit. These interference fits prevent the inner ring from fretting and spinning on the shaft.
Both k5 and m5 provide true interference fits, but m5 delivers greater interference for heavier radial shock loads. On a 40 mm shaft, a k5 tolerance requires the shaft to be machined between +0.002 mm and +0.011 mm above nominal, whereas an m5 tolerance requires +0.009 mm to +0.018 mm above nominal.
Specify C3 radial internal clearance whenever the inner ring has an interference fit (such as k5 or m5), whenever operating temperatures exceed 70°C, or where there is a substantial temperature differential between inner and outer rings (such as electric motors, boiler draft fans, and vibrating screens). An interference fit expands the inner ring, consuming roughly 80% of the initial clearance.
If a shaft seat is machined undersized (such as turning a k5 seat to an h6 or g6 tolerance on a rotating inner ring), the inner ring will slip and spin relative to the shaft. This relative motion causes rapid fretting corrosion, abrasive iron oxide powder generation, shaft neck scoring, and catastrophic machine vibration.
For stationary outer rings under normal unidirectional loads, an ISO H7 housing bore tolerance is standard. It provides a light clearance fit, allowing the outer ring to be pushed into the housing by hand or light arbor press and allowing non-locating bearings to slide axially during thermal expansion of the machine shaft.
Welding and re-turning is common in Kenyan workshops but must be executed with extreme care. Uneven arc heat causes thermal warping and shaft runout. Always pre-heat the shaft, use appropriate low-hydrogen electrodes, relieve stress, and grind or finish-turn between centers with a precision micrometer to achieve proper cylindricity and surface finish (Ra 0.8 µm to 1.6 µm).
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