Understanding the Basic Shaft System in Limits and Fits

In mechanical engineering, the system of limits and fits is used to define the permissible range of sizes for mating parts, such as shafts and holes, to ensure proper assembly and function. Key concepts include basic size, deviations, tolerances, and fits.

The basic size is the theoretical size from which deviations are measured.

Deviation is the algebraic difference between the actual size and the basic size.

• Upper deviation: The algebraic difference between the maximum limit of size and the basic size.
• Lower deviation: The algebraic difference between the minimum limit of size and the basic size.

The question asks about the characteristics of a basic shaft system.

There are two main systems for defining fits:

1. Basic Hole System: In this system, the hole's lower deviation is zero. This means the minimum size of the hole is equal to the basic size. Different fits are achieved by varying the shaft's size and tolerance.
2. Basic Shaft System: In this system, the shaft's upper deviation is zero. This means the maximum size of the shaft is equal to the basic size. Different fits are achieved by varying the hole's size and tolerance.

Based on this definition, in a basic shaft system, the maximum permissible size of the shaft is equal to the basic size. This implies that the difference between the maximum limit of size of the shaft and the basic size is zero. This difference is defined as the upper deviation.

Therefore, a basic shaft system is one whose upper deviation is zero.

Let's examine the given options:

• Option 1: Lower deviation is zero. This describes a basic hole system.
• Option 2: Upper deviation is zero. This correctly describes a basic shaft system.
• Option 3: Minimum clearance is zero. This relates to the fit achieved (e.g., a transition or interference fit) and not the definition of the basic system itself.
• Option 4: Maximum clearance is zero. This relates to an interference fit and not the definition of the basic system itself.

Thus, the characteristic defining a basic shaft system is that its upper deviation is zero.