Engineering tolerances - Fits (ISO)

Summary

This video explains the concept of fits in engineering drawing, focusing on the relationship between a hole and shaft in mechanical assemblies. It covers the three main types of fits: clearance fit, interference fit, and transition fit, detailing how they are defined, calculated, and applied. The video also explores fundamental terminology, such as nominal size, actual size, and tolerance classes, and provides guidance on selecting the appropriate fit based on ISO standards. Additionally, it emphasizes the importance of hands-on experience and collaboration with manufacturing teams to better understand and apply fit specifications in real-world design.

Takeaways

• 😀 Fits in engineering drawings define the relationship between the size of external and internal components, ensuring precise manufacturing for interchangeability.
• 😀 There are three main types of fits: clearance fit, interference fit, and transition fit, each serving different purposes in component assembly.
• 😀 Clearance fit is used when the shaft must rotate or slide freely within the hole, with the clearance between shaft and hole being zero or greater.
• 😀 Interference fit is used when a strong hold is required between the shaft and hole, often needing high force to assemble and disassemble without damaging the components.
• 😀 Transition fit is a balance between clearance and interference fit, allowing for secure assembly but also disassembly without damage.
• 😀 Tolerance classes, such as 'H7' for holes and 'G7' for shafts, help define the limits for the size of the components to achieve the correct fit.
• 😀 The fundamental tolerance is the difference between the upper and lower limits of a component's size, determining the fit between the parts.
• 😀 ISO 286-12010 and ISO 286-22010 provide the standards for defining and calculating fits, with detailed tables and equations available for accurate fit selection.
• 😀 The two primary systems for selecting fits are the hole basis system (where the hole tolerance is fixed) and the shaft basis system (where the shaft tolerance is fixed).
• 😀 Choosing the proper fit requires considering factors like economic needs, assembly requirements, and manufacturing precision, as well as understanding the potential impact on cost.
• 😀 For best results, engineers are encouraged to physically interact with assembled components to better understand the fit and the forces involved in assembly and disassembly.

Q & A

What is a 'fit' in mechanical design?

In mechanical design, a 'fit' refers to the relationship between two components, such as a shaft and a hole, when they are assembled. It defines how tightly or loosely the components interact, which could involve sliding, pressing, or securing them in place.

What are the three general categories of fits?

The three general categories of fits are clearance fit, interference fit, and transition fit. A clearance fit allows components to move freely, an interference fit ensures a tight, secure fit, and a transition fit is a compromise between the two, allowing either ease of assembly or secure holding.

How is a clearance fit defined?

A clearance fit is defined when the shaft's size is equal to or smaller than the hole's size, ensuring that there is always space (clearance) between the two components. This type of fit is used when parts need to slide or rotate freely within each other.

What is the difference between minimum and maximum clearance in a clearance fit?

The minimum clearance occurs when the shaft is manufactured at the upper limit of its size, and the hole is created at its smallest size. The maximum clearance occurs when the shaft is at its smallest size and the hole at its largest size.

What is an interference fit, and when is it used?

An interference fit is used when a tight, secure fit is required, and components need to be held firmly in place. The shaft is slightly larger than the hole, and force is needed to assemble the parts. This type of fit is often used in applications where components should not come apart easily.

How does a transition fit differ from a clearance and interference fit?

A transition fit is a compromise between a clearance fit and an interference fit. It allows the components to either slide or fit tightly, but still be disassembled without causing damage. The tolerance is chosen based on whether a tight or loose fit is preferred.

What is the role of tolerance in defining fits?

Tolerance defines the permissible variation from the nominal size of a component. It determines the allowable size range for parts, ensuring that the components fit together properly according to the required fit category (clearance, interference, or transition).

What are the two common systems used to select a fit, and how do they differ?

The two common systems are the hole basis and shaft basis systems. In the hole basis system, the hole’s fundamental tolerance is fixed, and the shaft’s tolerance is adjusted. In the shaft basis system, the shaft’s tolerance is fixed, and the hole’s tolerance is adjusted. The hole basis is typically used for general purposes, while the shaft basis is used when parts with holes are mounted on a shaft.

How do you calculate the tolerance class for a fit?

The tolerance class for a fit is determined using the fundamental deviation and standard tolerance grade numbers specified in the ISO 286 standards. This includes the upper and lower deviation limits for both the hole and shaft, which can be found in tolerance tables or calculated using specific formulas.

Why is it important for engineers to understand the implications of fits in manufacturing?

Understanding fits is crucial because they directly affect the ease of assembly, the function of the parts, and the cost of manufacturing. An incorrect fit can lead to problems like difficulty in assembly, poor performance, or increased manufacturing costs due to additional precision requirements.