Understanding the Hole Basis System in Manufacturing

The question asks about the fundamental principle of the hole basis system in engineering fits. It specifically states that the hole is kept constant and asks what component's size is varied to achieve different fits.

In manufacturing and mechanical engineering, achieving specific fits between mating parts like a hole and a shaft is crucial for proper function. There are two main systems for defining tolerances and fits: the hole basis system and the shaft basis system.

What is the Hole Basis System?

The hole basis system is one of the standard methods used to define the tolerances and deviations for holes and shafts that are intended to be assembled together. The core idea of this system is to simplify manufacturing by keeping the hole's dimensions constant and varying the shaft's dimensions to achieve the desired type of fit.

• In this system, the lower deviation of the basic size for the hole is typically set to zero. This means the smallest permissible size of the hole is equal to the basic size.
• Manufacturers can use standard tools like drills or reamers to produce holes within this constant tolerance range relatively easily.
• Different types of fits (like clearance, transition, or interference fits) are then achieved by manufacturing shafts with varying upper and lower deviations relative to the basic size.

Analyzing the Options

Let's look at the provided options in the context of the hole basis system:

• Tolerance: Tolerance is the permissible variation in a dimension. While tolerances are applied to both the hole and the shaft, tolerance itself is not the component whose size is varied.
• Shaft: As explained in the description of the hole basis system, the size of the shaft is intentionally varied (by changing its tolerance zone relative to the basic size) to create different types of fits with the constant hole.
• Hole: The question and the definition of the hole basis system explicitly state that the hole is kept constant. Its size and tolerance zone are fixed, typically with the lower deviation at zero.
• Fit: Fit describes the relationship between the assembled hole and shaft (e.g., clearance, interference). It is the result of the dimensions and tolerances, not a component whose size is varied.

Based on the definition and analysis, in the hole basis system where the hole is constant, the size of the shaft is the variable component used to achieve different fits.

How Varying Shaft Size Creates Different Fits

Assuming a constant hole with a specific tolerance (e.g., H7), varying the shaft tolerance zone (e.g., h6, js6, p6) results in:

This table illustrates how the dimensions of the shaft are adjusted relative to the constant hole to produce different assembly conditions, or fits.

Conclusion

The question directly asks what component's size is varied when the hole is kept constant in the hole basis system to get the desired fit. As established, this is the shaft. The options were Tolerance, Shaft, Hole, and Fit. The component that is varied is the Shaft.

Therefore, the correct answer is Shaft.

Revision Table: Key Concepts in Hole Basis System

Additional Information: Comparing Fit Systems and Types

Shaft Basis System

The other main system is the shaft basis system. In this system, the shaft's size and tolerance are kept constant, typically with the upper deviation at zero. The size of the hole is then varied to achieve different fits.

The choice between the hole basis and shaft basis system often depends on manufacturing considerations, such as available tooling (standard reamers for holes vs. standard bar stock for shafts) or cost.

Types of Fits

There are generally three categories of fits:

• Clearance Fit: The shaft is always smaller than the hole, allowing relative movement. There is always a gap or "clearance" between the mating surfaces. Example: H7/h6.
• Transition Fit: The tolerance zones of the hole and shaft overlap. This means the assembly can result in either a small clearance or a small interference. Example: H7/js6.
• Interference Fit (Press Fit or Shrink Fit): The shaft is always larger than the hole. Assembly requires force (pressing) or temperature difference (shrinking/expanding). This creates a tight joint without relative movement. Example: H7/p6.

These fits are standardized internationally (e.g., ISO system of limits and fits) using specific tolerance grades (like IT7 for hole H7 or shaft h6) and deviation codes (like H, h, js, p).