Shaft Tolerance and Fit Guide: ISO Standards and Practical Applications
Complete guide to shaft tolerance and fit: ISO 286 standards, clearance fit, transition fit, interference fit, and how to select the right fit for your application
2026-08-117 min
1. Introduction to Tolerance and Fit
Tolerance and fit are fundamental concepts in mechanical engineering that ensure parts assemble correctly and function as intended. Understanding these concepts is crucial for shaft design and manufacturing.
2. Basic Tolerance Concepts
- Nominal Size: The theoretical exact size
- Actual Size: The measured size of the manufactured part
- Tolerance: The allowable variation from nominal size
- Upper Deviation: Maximum limit - nominal size
- Lower Deviation: Minimum limit - nominal size
3. ISO 286 Tolerance System
The ISO 286 standard defines international tolerance (IT) grades:
| IT Grade | Application | Tolerance Range |
|---|---|---|
| IT01-IT4 | Gauge blocks, precision instruments | 0.3-4 μm |
| IT5-IT7 | Precision fits, bearings | 4-25 μm |
| IT8-IT11 | General engineering | 25-160 μm |
| IT12-IT16 | Rough machining, castings | 160-1600 μm |
4. Types of Fits
Clearance Fit
Always provides clearance between shaft and hole:
- H11/c11: Loose running fit
- H9/d9: Free running fit
- H8/f7: Close running fit
- H7/g6: Sliding fit
- H7/h6: Location clearance fit
Transition Fit
May provide either clearance or interference:
- H7/k6: Location transition fit
- H7/n6: Close transition fit
Interference Fit
Always provides interference (press fit):
- H7/p6: Light press fit
- H7/s6: Medium press fit
- H7/u6: Heavy press fit
5. Selecting the Right Fit
Consider these factors when selecting fits:
- Function: Does the part need to rotate, slide, or stay fixed?
- Load: What forces will the joint experience?
- Temperature: Will thermal expansion affect the fit?
- Assembly: How will the parts be assembled?
- Disassembly: Will the joint need to be separated?
6. Common Shaft Applications and Fits
| Application | Recommended Fit | Type |
|---|---|---|
| Ball bearing inner ring | k5, k6, m5, m6 | Transition/Interference |
| Gear on shaft | k6, m6, n6 | Transition/Interference |
| Pulley on shaft | H7/h6, H7/g6 | Clearance |
| Coupling hub | H7/k6, H7/m6 | Transition |
| Keyway fit | H9/h9 | Clearance |
7. Tolerance Stack-up Analysis
When multiple parts assemble, tolerances accumulate:
- Worst Case: All parts at extreme limits
- Statistical: Based on normal distribution
- Monte Carlo: Simulation-based analysis
8. Measuring Shaft Tolerances
- Micrometer: ±0.001mm accuracy
- Vernier Caliper: ±0.01mm accuracy
- CMM: ±0.0005mm accuracy
- Air Gauge: High-precision production measurement
- Go/No-Go Gauge: Quick pass/fail inspection
9. Corgi Capacity Network Precision Machining
Corgi Capacity Network delivers precision shafts with tight tolerances:
- IT5-IT7 tolerance capability
- Manufacturing Processexpertise
- AI Smart Quote - Get quotes for precision shaft projects