Guide to ISO Tolerances for Hole and Shaft Fits

In mechanical design, ensuring precise fits between components directly impacts equipment performance, longevity, and reliability. The ISO tolerance system, as an internationally recognized technical standard, provides clear dimensional deviations and tolerance grades for hole and shaft fits, serving as the foundation for interchangeable manufacturing and quality assurance.

The ISO tolerance system is based on standard tolerance grades (IT grades) and fundamental deviation codes, specifying allowable dimensional variations for components. This system ensures parts manufactured by different producers achieve intended fit characteristics during assembly, including clearance fits, transition fits, or interference fits. ISO 286-2 specifically details hole and shaft tolerances, making it an essential reference in mechanical design.

Hole tolerances consist of basic size, tolerance zone designation, and tolerance grade. The tolerance zone designation indicates the zone's position relative to the basic size, while the tolerance grade determines the zone's magnitude. Common fundamental deviation codes for holes include G, H, J, K, M, and N, each representing different deviation directions and values.

• G: Positive lower deviation for holes, suitable for fits requiring larger clearances.
• H: Zero lower deviation, serving as the common reference for hole-basis fits.
• J: Negative lower deviation, appropriate for transition fits.
• K: Negative lower deviation, used for tighter transition fits.
• M: Both upper and lower deviations negative, designed for interference fits.
• N: Both deviations negative, intended for stronger interference fits.

ISO tolerance grades (IT grades) serve as critical indicators of dimensional precision, with smaller numbers representing higher accuracy. Common hole tolerance grades include IT6, IT7, IT8, and IT9. Selection requires balancing functional requirements, manufacturing costs, and assembly considerations.

Limit deviations represent maximum permissible variations from the basic size, determined by fundamental deviations and tolerance values. Engineers must select appropriate tolerance zone designations and grades to ensure actual dimensions remain within specifications.

The following table presents limit deviation values (in μm) for holes across various designations and grades:

The shaft tolerance system mirrors the hole system, comprising basic size, tolerance zone designation, and grade. Common shaft deviation codes include e, f, g, h, j, k, m, n, p, and r, each defining specific deviation characteristics.

• e: Negative upper deviation, for large-clearance fits.
• f: Negative upper deviation, for clearance fits.
• g: Negative upper deviation, for small-clearance fits.
• h: Zero upper deviation, the shaft-basis reference.
• j: Positive upper deviation, for transition fits.
• k: Positive upper deviation, for tight transition fits.
• m: Both deviations positive, for interference fits.
• n: Both deviations positive, for strong interference fits.
• p: Both deviations positive, for heavier interference.
• r: Both deviations positive, for maximum interference.

Proper fit selection is paramount for mechanical performance. Three primary fit categories exist, each serving distinct applications.

Characterized by hole dimensions exceeding shaft dimensions, creating clearance. Ideal for moving assemblies like bearings and rotating shafts, requiring consideration of lubrication and motion precision.

Where hole dimensions may be larger or smaller than shaft dimensions, permitting either clearance or interference. Used for precision positioning with disassembly capability, such as locating pins and gears.

Featuring shaft dimensions exceeding hole dimensions, creating compression. Essential for torque transmission in pressed bearings and couplings, requiring stress analysis.

Key parameters include maximum/minimum clearance (or interference) and fit tolerance, calculated as:

• Maximum clearance = Maximum hole size - Minimum shaft size
• Minimum clearance = Minimum hole size - Maximum shaft size
• Maximum interference = Maximum shaft size - Minimum hole size
• Minimum interference = Minimum shaft size - Maximum hole size
• Fit tolerance = Hole tolerance + Shaft tolerance

Two principal fitting systems govern manufacturing approaches.

Maintains fixed hole tolerances (typically H7) while varying shaft tolerances to achieve desired fits. Benefits include simplified hole machining and standardized production.

Maintains fixed shaft tolerances (typically h6) while varying hole tolerances. Advantages include reduced shaft variety and simplified inventory management.

Beyond ISO standards, multiple variables influence fit quality.

Precision processes like grinding and honing achieve superior dimensional accuracy and surface finish.

Elastic modulus and thermal expansion coefficients affect deformation and stress under load.

Dimensional changes from temperature fluctuations require compensation in extreme environments.

Roughness impacts friction and contact area, particularly critical for high-precision applications.

The ISO tolerance system provides indispensable technical specifications for mechanical design, establishing clear dimensional standards for hole and shaft fits. Through mastery of these principles and practical application, engineers can develop fits meeting diverse functional requirements, ultimately enhancing product performance, durability, and reliability. Successful implementation requires holistic consideration of manufacturing processes, material properties, environmental conditions, and surface characteristics to achieve design objectives.