JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III ENGINEERING METROLOGY Previous Question Paper (November/December - 2020 - R16)

A fit is the relationship resulting from the difference between the sizes of two mating parts, such as a hole and a shaft, before assembly. It is essential in engineering metrology and manufacturing to ensure the interchangeability of components, control the assembly process, and determine the functional performance of mechanical systems. Proper fit selection prevents premature wear, ensures smooth operation, and maintains the required precision in assemblies.

Fits are classified into three primary categories based on the relative clearance or interference between the mating parts:

1. Clearance Fit: In this type, the hole is always larger than the shaft, ensuring a positive clearance. This allows for relative movement between the parts. It is further subdivided into:
   • Loose fit: Used for parts that require large tolerances and freedom of movement.
   • Running fit: Designed for parts with relative motion, requiring lubrication.
   • Slide fit: Used for parts that require high precision and sliding motion without significant clearance.
2. Interference Fit: In this type, the shaft is always larger than the hole, resulting in a negative clearance. This fit is used when parts must be permanently joined or held together by friction. It includes:
   • Shrink fit: Parts are joined by heating the hole or cooling the shaft.
   • Force fit: Requires significant pressure to assemble the parts.
   • Drive fit: Requires moderate force, often using a hammer or press.
3. Transition Fit: This fit lies between clearance and interference. Depending on the actual dimensions within the tolerance zone, the assembly may result in either a slight clearance or a slight interference. It is used for parts that require accurate location and alignment, such as dowel pins or keys in keyways.

The selection of these fits is governed by the ISO system of limits and fits, which provides standardized tolerance grades and fundamental deviations to ensure consistent manufacturing quality across different industries.

The Indian Standard (IS) system of limits and fits, based on the ISO system, provides a systematic approach to ensure interchangeability and functional performance of mating parts. Its principal features include:

1. Fundamental Tolerance (Grade): The system defines 18 grades of tolerance, denoted as IT01, IT0, and IT1 through IT16. These grades represent the magnitude of the tolerance zone, where a lower number indicates a smaller tolerance (higher precision).

2. Fundamental Deviation: This defines the position of the tolerance zone relative to the zero line. It is represented by letters: capital letters (A to ZC) for holes and lowercase letters (a to zc) for shafts. The letter 'H' for holes and 'h' for shafts represent the fundamental deviation where the lower/upper limit coincides with the zero line.

3. Hole and Shaft Basis Systems: The system allows for two methods of achieving fits:

• Hole Basis System: The hole size is kept constant (H), and different fits are obtained by varying the shaft size. This is preferred in manufacturing as it is easier to produce holes of standard sizes using reamers.
• Shaft Basis System: The shaft size is kept constant (h), and different fits are obtained by varying the hole size. This is used when multiple parts are fitted onto a single standard-sized shaft.

4. Types of Fits: The system categorizes the relationship between mating parts into three primary classes:

• Clearance Fit: The tolerance zones of the hole and shaft are positioned such that a clearance always exists between them, allowing for relative motion (e.g., sliding or running fits).
• Transition Fit: The tolerance zones overlap, resulting in either a clearance or an interference depending on the actual sizes within the tolerance range (e.g., push fits).
• Interference Fit: The tolerance zones are positioned such that the shaft is always larger than the hole, requiring force for assembly and creating a rigid connection (e.g., shrink or press fits).

5. Standardized Notation: A fit is specified by the basic size followed by the hole tolerance symbol and the shaft tolerance symbol (e.g., 50H7/g6). This notation clearly communicates the design intent, manufacturing precision, and the type of assembly required.