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More Information
What are Limits and Fits?
Limits and fits are a system of tolerances used to define the dimensional relationship between mating parts. This system ensures parts can be assembled correctly while maintaining the desired functional characteristics.
Clearance Fits
The shaft is always smaller than the hole, creating a clearance gap.
- • Easy assembly
- • Allows relative motion
- • Used for running fits, sliding fits
- • Examples: H7/f6, H7/g6
Transition Fits
Can be either clearance or interference depending on actual dimensions.
- • Precise location
- • May require light press
- • Used for location fits
- • Examples: H7/k6
Interference Fits
The shaft is always larger than the hole, requiring force to assemble.
- • Permanent assembly
- • Requires press force
- • Used for press fits
- • Examples: H7/n6, H7/p6
ISO 286 Standard
The ISO 286 standard (ISO 286-1 and ISO 286-2) defines the system of limits and fits for cylindrical parts. It provides:
- IT (International Tolerance) grades from IT01 to IT18
- Fundamental deviations for holes (A-Z) and shafts (a-z)
- Size ranges from 0 to 500mm
- Standard fit combinations for common applications
Manufacturing Considerations
Tolerance Grades
Tighter tolerances (lower IT grades) require more precise manufacturing methods:
- IT6-IT7: Standard fits, turning/grinding
- IT4-IT5: Precision fits, grinding/honing
- IT1-IT3: Ultra-precision, lapping/superfinishing
Cost Impact
Tighter tolerances significantly increase manufacturing costs. Always specify the loosest tolerance that meets functional requirements.
Common Industry Practices
Hole Basis System: The hole size is kept constant (typically H7), and the shaft is varied to achieve different fit types. This is the most common system.
Common Fits: H7/f6 (loose running), H7/g6 (sliding), H7/h6 (location clearance), H7/k6 (location transition), H7/n6 (press fit) are among the most frequently used.
Bearing Fits: Inner race typically uses interference fits (H7/p6 or tighter), while outer race uses clearance fits (H7/f6 or similar).
Best Practices
- Always consider the manufacturing capabilities when selecting tolerances
- Use standard fit combinations when possible to reduce costs
- Consider thermal expansion for high-temperature applications
- Account for surface finish requirements in tolerance selection
- Document fit selections clearly on technical drawings