ISO 286 – Limits, Fits & Tolerances
What is ISO 286?
ISO 286 is the international standard that defines the system of limits, tolerances, and fits used for cylindrical mating components such as shafts and holes. It establishes standardized tolerance grades and deviation zones, allowing engineers to specify assemblies that provide the required clearance, transition, or interference fit without custom calculations.
Why ISO 286 Matters
Choosing the correct fit directly affects manufacturing quality, assembly performance, and product reliability.
Benefits include:
Standardized hole and shaft fits
Improved interchangeability
Reduced assembly errors
Lower manufacturing costs
Better inspection consistency
Global design compatibility
Understanding the ISO Fit System
The ISO fit system provides a standardized method for defining the dimensional relationship between mating components, such as holes and shafts. Instead of requiring exact dimensions, which are impractical to achieve in manufacturing, the system specifies acceptable tolerance ranges that ensure consistent assembly, interchangeability, and performance.
Each fit is determined by a combination of the nominal size, tolerance grade, and deviation position. Together, these elements define whether assembled components will have clearance, interference, or a transition fit.
Basic Size
The basic size is the nominal dimension from which the limits of size for both the hole and shaft are established. It serves as the common reference value for determining deviations and tolerances.
For example, if a shaft and hole are both specified as 50 mm, then 50 mm is the basic size. The actual manufactured dimensions may vary within the specified tolerance limits, but the basic size remains the reference point for the fit.
Upper and Lower Deviations
Upper deviation and lower deviation define the allowable variation from the basic size.
Upper deviation is the maximum permitted difference above the basic size.
Lower deviation is the maximum permitted difference below the basic size.
These deviations establish the acceptable dimensional limits for manufacturing while ensuring components remain functional and interchangeable.
For example, a shaft with a nominal size of 25 mm may be allowed to vary within a small range above or below that value depending on its specified tolerance zone.
Tolerance Zone
A tolerance zone represents the total allowable dimensional variation between the upper and lower deviations. Its position relative to the basic size determines whether the feature is larger, smaller, or centered around the nominal dimension.
Each tolerance zone is identified by a letter and a number:
The letter indicates the position of the tolerance zone relative to the basic size.
The number indicates the tolerance grade or precision level.
For example:
H7 specifies a hole tolerance zone.
g6 specifies a shaft tolerance zone.
When combined, these designations define the final engineering fit.
IT Grades
International Tolerance (IT) Grades define the manufacturing precision of a feature. Lower IT numbers represent tighter tolerances and greater precision, while higher numbers allow larger dimensional variation.
Examples include:
IT5–IT7 – High-precision components used in measuring instruments, bearings, and precision machinery.
IT8–IT11 – General engineering applications and standard mechanical assemblies.
IT12 and above – Components with less demanding dimensional accuracy, such as fabricated or cast parts.
Selecting the appropriate IT grade helps balance manufacturing cost with functional performance.
Hole Basis System
The Hole Basis System is the most commonly used fitting system in manufacturing. In this approach, the hole remains at a standard H tolerance, while the shaft tolerance is adjusted to achieve different types of fits.
For example:
H7/g6 – Clearance fit
H7/k6 – Transition fit
H7/p6 – Interference fit
Keeping the hole size constant simplifies tooling, machining, and inspection, making this system the preferred choice for most engineering applications.
Shaft Basis System
The Shaft Basis System keeps the shaft at a standard h tolerance while varying the hole tolerance to achieve the required fit.
Although used less frequently than the Hole Basis System, it is beneficial when the shaft size cannot easily be changed, such as when using standard bar stock, pre-machined shafts, or commercially available components.
This approach allows engineers to maintain a fixed shaft dimension while modifying the hole to obtain the desired clearance or interference.
Bringing It All Together
The ISO fit system combines basic size, deviations, tolerance zones, and IT grades to create standardized fit combinations for engineering assemblies. By selecting the appropriate hole and shaft tolerances, engineers can ensure components assemble correctly, perform reliably, and remain interchangeable across different manufacturers and production environments.
Types of Engineering Fits
Clearance Fit
A clearance fit always provides space between the shaft and hole after assembly. The shaft is smaller than the hole, allowing free movement.
Common Applications
Sliding mechanisms
Guide rods
Linear bearings
Rotating shafts
Precision positioning systems
Common Examples
H7/f6
H7/g6
Transition Fit
A transition fit may produce either a slight clearance or a slight interference depending on manufacturing variation. These fits prioritize accurate positioning while remaining serviceable.
Common Applications
Gear hubs
Couplings
Bearing seats
Locating components
Common Examples
H7/k6
H7/m6
H7/n6
Interference Fit
An interference fit creates a press fit between components. The shaft is intentionally larger than the hole, producing a rigid mechanical connection.
Common Applications
Bearings
Bushings
Flywheels
Permanent gear mounting
Heavy-duty couplings
Common Examples
H7/p6
H7/r6
H7/s6
Comparison of Engineering Fit Types
Feature
Clearance Fit
Transition Fit
Interference Fit
Assembly
Easy hand assembly
Light force may be required
Press or thermal assembly
Relative Movement
Free movement
Minimal or no movement
No movement
Holding Strength
Low
Moderate
High
Ease of Disassembly
Easy
Moderate
Difficult
Typical Applications
Sliding and rotating parts
Precision alignment
Permanent mechanical joints
High-Cost Errors
Over-Specifying Tight Tolerances
Specifying tighter fits than the application requires increases machining costs, inspection time, and manufacturing complexity without improving product performance.
Selecting the Wrong Fit for the Application
Using a clearance fit where rigidity is required or an interference fit where movement is needed can lead to premature wear, assembly failures, vibration, or reduced component life.
How to Choose the Right Fit
Does the assembly require movement?
Is precise positioning required?
Will the assembly be disassembled?
How much load must the joint transmit?
What manufacturing process will be used?
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Comparison & Related Standards
Engineering Fits
Controls the size relationship between mating holes and shafts.
Ensures the desired clearance, transition, or interference after assembly.
Primarily defined by ISO 286.
Used for selecting hole and shaft fit combinations.
Geometric Tolerancing
Controls the shape, orientation, location, and profile of features.
Ensures features meet functional and manufacturing requirements regardless of size.
Primarily defined by ISO 1101.
Used for controlling feature geometry and inspection criteria.
ISO 286
Defines the ISO system of limits, tolerances, and fits for holes and shafts, providing the foundation for selecting clearance, transition, and interference fits.
ISO 1101
Specifies the symbols and rules used to define geometric tolerances such as flatness, position, parallelism, and profile on engineering drawings.