Fit tolerance is a fundamental design element that determines how tightly or loosely parts fit together. Improper settings can lead to issues such as abnormal noise, seizure, and increased manufacturing costs. Therefore, it is essential to understand fit tolerances from both theoretical and practical perspectives.
This article explains:
• Basic types of fits
• Advantages of the hole-basis system
• Practical tips for selecting tolerances in real-world applications
What You’ll Learn
• The three main types of fits and their applications
• Why tolerances are often defined starting from the hole
• How to read drawing symbols and IT grades
• Practical selection tips considering thermal expansion and material properties
What Is Fit Tolerance?
Fit tolerance refers to a combination of dimensional tolerances between a shaft and a hole, usually expressed as symbols such as:
H7/g6
It defines the degree of clearance or interference between mating parts.
Since perfectly exact machining is impossible, designers specify allowable ranges (tolerances) to ensure proper functionality.
Why Fit Tolerance Matters
If tolerances are incorrectly set:
• Parts may not be securely fixed
• Noise and vibration may occur
• Excessive friction may cause seizure
• Product reliability may decrease
Proper tolerance design is essential for both performance and cost efficiency.
Three Basic Types of Fits
Different applications require different fit types depending on motion and fixation requirements.
| Fit Type | Characteristics | Function | Typical Combination |
| Clearance Fit | Always has clearance | Rotation, sliding | H7/g6, H8/f7 |
| Transition Fit | Clearance or slight interference | Precise positioning | H7/js6、H7/k6 |
| Interference Fit | Always interference | Strong fixation | H7/p6、H7/s6 |
1.Clearance Fit
A clearance fit means:
• The shaft is always smaller than the hole
• A gap exists after assembly
Suitable for:
• Rotating shafts
• Sliding mechanisms
Ensures smooth movement and reduced friction.
2.Interference Fit
An interference fit means:
• The shaft is larger than the hole
Assembly requires:
• Press fitting
• Shrink fitting (thermal expansion)
Suitable for:
• Gear mounting
• Bearing fixation
Careful design is required to avoid material damage.
3.Transition Fit
A transition fit:
• May result in slight clearance or slight interference
Suitable for:
• Precise positioning
• Alignment components (e.g., dowel pins)
Understanding Fit Symbols and IT Grades
Example:
40H7/g6
• 40 → Nominal diameter (mm)
• H → Hole tolerance zone
• g → Shaft tolerance zone
• 7, 6 → IT (International Tolerance) grade
Example:
40H7/g6
• 40 → Nominal diameter (mm)
• H → Hole tolerance zone
• g → Shaft tolerance zone
• 7, 6 → IT (International Tolerance) grade
IT Grade Basics
• Lower numbers = higher precision
• Higher precision = higher manufacturing cost
Engineers must balance:
• Required function
• Manufacturing cost
How to Select Fit Tolerances in Practice
1. Use the Hole-Basis System
The most common approach:
• Fix the hole tolerance (e.g., H7)
• Adjust the shaft accordingly
Why:
• Hole machining tools (drills, reamers) have fixed sizes
• Shaft machining (turning) allows flexible adjustment
Result: Lower cost and easier manufacturing
2. Select Based on Functional Requirements
Choose fit type depending on function:
• Rotation → Clearance fit
• Positioning → Transition fit
• Torque transmission → Interference fit
Also consider:
• Thermal expansion
• Lubrication conditions
3. Balance Cost and Accuracy (IT Grade)
Improving precision increases cost significantly.
Example:
• IT8 → drilling
• IT7 → reaming
• IT6 → grinding
General rule:
Select the loosest tolerance that still satisfies functionality.
How to Calculate Fit Tolerance Clearance Fit
• Maximum Clearance
= Hole Max − Shaft Min
• Minimum Clearance
= Hole Min − Shaft Max
Interference Fit
• Maximum Interference
= Shaft Max − Hole Min
• Minimum Interference
= Shaft Min − Hole Max
Why Max and Min Values Matter
Design must consider worst-case conditions:
Clearance Fit Example
• Maximum clearance → loosest condition
• Minimum clearance → tightest condition
Interference Fit Example
• Maximum interference → highest stress
• Minimum interference → loosest condition
This ensures:
• Proper function
• Structural safety
Importance of Fit Tolerance in Product Design
Fit tolerance is not just a dimensional specification—it directly affects:
• Product reliability
• Assembly performance
• Manufacturing cost
Proper tolerance design ensures:
• Smooth assembly
• Stable long-term operation
Data-Driven Quality: Intelligent Die Set Technology
Even with precise design, real conditions such as heat and load affect manufacturing.
Nichidai addresses this with:
• Intelligent Die Sets equipped with sensors (e.g., AE sensors)
• Real-time monitoring of load and displacement
Benefits:
• Early detection of abnormalities
• Data-driven quality control
• Reduced reliance on experience-based judgment
FAQ
What does “H7/g6” represent?
A commonly used clearance fit:
• Ensures smooth motion
• Minimizes looseness
• Widely used for rotating shafts
Recommended fit for bearings?
Typical setup:
• Hole: H7
• Shaft: k6 or m6
Must consider load direction
(rotating ring requires interference fit)
How much does higher precision increase cost?
Cost varies, but:
• Moving from IT8 to IT7 adds processes
• IT6 often requires grinding
Costs increase significantly depending on:
• Size
• Material
• Production method
Conclusion
Fit tolerance is a core element of mechanical design that directly impacts:
• Performance
• Reliability
• Cost
Success requires:
• Proper fit selection
• Balanced precision and cost
• Consideration of real operating conditions
