1. Introduction
In manufacturing, it is impossible to produce components with exact dimensions due to machine errors, tool wear, and environmental factors. Therefore, a permissible variation is allowed. This concept is called tolerance.
The system of limits, fits, and tolerances ensures interchangeability, proper assembly, and smooth functioning of machine parts. It is standardized globally by organizations such as the International Organization for Standardization.
2. Basic Terminology
2.1 Nominal Size
The general size used to describe a part.
Example:
A shaft of 50 mm.
2.2 Actual Size
The measured size of the part after manufacturing.
2.3 Basic Size
The theoretical size from which limits are determined.
2.4 Limit
The two extreme permissible sizes between which the actual size lies.
✔ Maximum limit
✔ Minimum limit
2.5 Tolerance
The difference between maximum and minimum limits.
Formula:
Tolerance = Maximum limit – Minimum limit
Tolerance is always a positive value.
2.6 Allowance
The intentional difference between the sizes of mating parts.
It determines the type of fit.
3. Need for Limits and Tolerances
- To allow for manufacturing variations.
- To reduce production cost.
- To ensure interchangeability.
- To facilitate mass production.
- To avoid rejection of parts.
- To maintain proper functioning of machine parts.
4. Types of Tolerances
4.1 Unilateral Tolerance
Variation is allowed only on one side of the basic size.
Example:
50 +0.02 / 0.00 mm.
Advantages:
- Easy inspection.
- Suitable for automated production.
4.2 Bilateral Tolerance
Variation is allowed on both sides of the basic size.
Example:
50 ±0.01 mm.
Advantages:
- Uniform distribution of error.
5. System of Fits
Fit is the relationship between two mating parts such as a shaft and a hole.
It determines whether the parts will be loose, tight, or intermediate.
6. Types of Fits
6.1 Clearance Fit
In this fit, the hole is always larger than the shaft.
✔ There is always clearance.
✔ Easy assembly and disassembly.
Applications:
- Sliding and rotating parts.
- Bearings and shafts.
Examples:
Machine tools.
6.2 Interference Fit
The shaft is always larger than the hole.
✔ Requires force or heating/cooling for assembly.
✔ No relative motion.
Applications:
- Permanent joints.
- Railway wheels.
6.3 Transition Fit
May result in either clearance or interference.
✔ Sometimes tight, sometimes loose.
Applications:
- Precision assemblies.
7. Hole Basis and Shaft Basis System
7.1 Hole Basis System
The hole size is kept constant, and the shaft is varied.
✔ Most commonly used.
Reasons:
- Standard tools are available for holes.
- Easier manufacturing.
7.2 Shaft Basis System
The shaft size is constant, and the hole is varied.
Used when:
- Standard shaft is required.
8. Limits and Fits According to ISO System
The ISO system uses letters and numbers.
Example:
50 H7/g6
✔ Capital letters → Hole
✔ Small letters → Shaft
✔ Number → Tolerance grade.
9. Tolerance Grades
Tolerance grades are designated as IT01, IT0, IT1 to IT16.
✔ Lower number → Higher precision
✔ Higher number → Lower precision.
Example:
- IT5 → Precision engineering
- IT12 → General engineering.
10. Advantages of Limit System
✔ Ensures interchangeability.
✔ Reduces scrap and wastage.
✔ Improves product quality.
✔ Reduces manufacturing cost.
✔ Facilitates mass production.
11. Applications
✔ Automobile industries
✔ Aerospace
✔ Machine tools
✔ Bearings
✔ Gear assemblies
✔ Construction equipment.