Tolerance & Fit Selection for Self-Lubricating Bearings
ISO s6/H7–Based Interference Design for Reliable Installation
1. Engineering Context
For self-lubricating bearings—whether composite bushings, metal-backed plain bearings, or polymer sleeves—material performance alone does not determine service life. Installation accuracy plays a decisive role.
An improperly selected fit can lead to:
- Micro-movement (fretting or creeping)
- Loss of thermal stability
- Edge loading and accelerated wear
A controlled interference fit ensures:
- Stable positioning under dynamic load
- Effective heat transfer to the housing
- Uniform stress distribution across the bearing wall
This guide outlines how to define housing bore tolerances and calculate interference ranges using ISO s6/H7 conventions.
2. Fit Classification in Bearing Assembly
The interaction between the bearing outer diameter (OD) and housing bore determines the functional fit:
Interference Fit (Press Fit)
- OD > Bore in all tolerance conditions
- Requires press force or thermal assembly
- Generates radial contact pressure
- Suitable for:
- Oscillating motion
- High-load applications
- Vibration environments
Transition Fit
- Overlapping tolerance zones
- May result in slight clearance or interference
- Limited industrial relevance without selective assembly
Clearance Fit
- OD < Bore
- Allows relative motion between bearing and housing
- Not acceptable for bushing outer diameter (risk of spinning failure)
- Only applicable to shaft–bearing ID interface
3. ISO s6/H7 Fit System (Industrial Baseline)
The s6 (shaft/bushing OD) + H7 (housing bore) pairing is widely adopted for press-fit bushings due to its predictable interference envelope.
Tolerance Distribution (Unit: μm)
| Nominal Diameter (mm) |
H7 Bore (μm) |
s6 OD (μm) |
Fit Type |
| 1 – 3 |
0 ~ +10 |
+14 ~ +20 |
Interference |
| 3 – 6 |
0 ~ +12 |
+19 ~ +27 |
Interference |
| 6 – 10 |
0 ~ +15 |
+23 ~ +32 |
Interference |
| 10 – 18 |
0 ~ +18 |
+28 ~ +39 |
Interference |
| 18 – 30 |
0 ~ +21 |
+35 ~ +48 |
Interference |
| 30 – 50 |
0 ~ +25 |
+43 ~ +59 |
Interference |
| 50 – 80 |
0 ~ +30 |
+53 ~ +72 |
Interference |
| 80 – 120 |
0 ~ +35 |
+62 ~ +83 |
Interference |
4. Interference Calculation Methodology
To validate assembly feasibility and retention force, define the interference window:
- Minimum Interference
Δ_min = OD_min − Bore_max
- Maximum Interference
Δ_max = OD_max − Bore_min
5. Practical Example (Nominal Ø10 mm)
Tolerance Inputs:
- Bore (H7): 10.000 – 10.015 mm
- Bushing OD (s6): 10.023 – 10.032 mm
Calculated Interference:
- Δ_min = 10.023 − 10.015 = 0.008 mm (8 μm)
- Δ_max = 10.032 − 10.000 = 0.032 mm (32 μm)
Interpretation:
- Guaranteed positive interference across all tolerance combinations
- Sufficient retention for general industrial duty
- No risk of loosening under nominal conditions
6. Critical Design Considerations
6.1 Housing Material Modulus
- Steel / cast iron → standard s6/H7 applicable
- Aluminum / polymers → risk of elastic relaxation
- Engineering implication:
- Increase interference
- Or shift to tighter tolerance class (e.g., u6)
6.2 ID Reduction After Press Fit (Close-In Effect)
Press-fitting compresses the bearing wall, reducing internal clearance.
Design impact:
- Shaft tolerance (e.g., h7, f7) must compensate for:
- Radial contraction
- Operating temperature expansion
Failure to account leads to:
- Seizure (if clearance too small)
- Noise and instability (if too large)
6.3 Surface Roughness of Housing Bore
Recommended: Ra 0.8 – 1.6 μm
- Too rough → material shaving during press-fit → reduced interference
- Too smooth → lower friction coefficient → reduced holding force
6.4 Thermal & Load Coupling Effects
- Interference generates contact pressure → affects heat dissipation
- Excessive interference may:
- Distort bearing geometry
- Increase friction torque
7. Implementation Strategy (Engineering Workflow)
- Define load, speed, and duty cycle
- Select bearing material system
- Choose baseline tolerance (s6/H7)
- Calculate interference window
- Evaluate:
- Housing material stiffness
- Thermal expansion mismatch
- Adjust tolerance class if needed
- Validate via:
- Finite element simulation (optional)
- Press-fit force estimation
8. Conclusion
Applying ISO s6/H7 tolerance pairing provides a structured and repeatable method to achieve stable press-fit installation of self-lubricating bearings. Proper interference control:
- Prevents rotational slip
- Enhances load transfer
- Maintains dimensional integrity over time
However, real-world performance depends on multi-variable coupling—material properties, thermal conditions, and load dynamics must all be considered during final validation.
9. Disclaimer
This document provides generalized engineering guidance. Actual performance depends on application-specific variables including load spectra, environmental conditions, lubrication regime, and material behavior. Independent verification—through testing or simulation—is required before final implementation.
