Lesson Notes By Weeks and Term v4 - SHS 3

Lesson Video

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Performance objectives

• Explain the concepts of limits and fits.
• Identify different types of fits (clearance, interference, and transition).
• Calculate tolerances and limits for given dimensions.
• Analyze a production drawing to identify tolerance specifications.
• Discuss the importance of limits and fits for product quality.

Lesson summary

In the world of manufacturing, making parts that fit together perfectly is crucial. Imagine a car engine where the piston is too loose in the cylinder or a bearing that is too tight on a shaft. The engine would fail! It is impossible and too expensive to make every single part to an *exact* size. Instead, we specify an acceptable *range* of sizes. This lesson explores the system of Limits and Fits, which is the language engineers use on production drawings to tell manufacturers precisely how a part should be made so that it assembles and functions correctly every time.

Lesson notes

2.1 Foundational Terms

To understand limits and fits, we must first learn the vocabulary. Let's use the example of a 20 mm diameter shaft that needs to fit into a hole. Nominal Size: The size used for general identification. In our example, the nominal size is 20 mm. Basic Size: The theoretical exact size from which all limits are calculated. For both the hole and the shaft, the basic size is 20.000 mm. Limits of Size: The maximum and minimum permissible sizes for a part. Upper Limit: The largest acceptable size (e.g., 20.021 mm). Lower Limit: The smallest acceptable size (e.g., 20.000 mm). Tolerance: The total variation permitted in the size of a part. It is the difference between the upper and lower limits. Formula: Tolerance = Upper Limit – Lower Limit *Example:* For a hole with an upper limit of 20.021 mm and a lower limit of 20.000 mm, the tolerance is 20.021 - 20.000 = 0.021 mm. A smaller tolerance means higher precision and usually higher manufacturing cost. Deviation: The difference between a size (actual, upper, or lower) and the basic size. Upper Deviation (ES for hole, es for shaft): Upper Limit – Basic Size. Lower Deviation (EI for hole, ei for shaft): Lower Limit – Basic Size. Zero Line: In diagrams, the basic size is represented as a "zero line". Deviations are measured from this line. Allowance: The intentional difference between the sizes of two mating parts (e.g., hole and shaft) to create a specific type of fit. Minimum Allowance (or Minimum Clearance): Lower Limit of Hole – Upper Limit of Shaft. Maximum Allowance (or Maximum Clearance): Upper Limit of Hole – Lower Limit of Shaft. 2.2 Types of Fits

The allowance determines how two parts will fit together. There are three main categories: Clearance Fit: There is always a gap between the hole and the shaft, even at their tightest possible sizes. The shaft can rotate or slide freely. *Condition:* The largest possible shaft is smaller than the smallest possible hole. *Application:* A pulley rotating on a shaft, a piston in a cylinder. Interference Fit: The parts must be forced together. The shaft is always larger than the hole. This creates a strong, permanent, or semi-permanent joint. *Condition:* The smallest possible shaft is larger than the largest possible hole. *Application:* Pressing a bearing into a housing, fixing a gear onto a motor shaft. Transition Fit: The fit could result in either a small clearance or a small interference, depending on the actual sizes of the manufactured parts. *Condition:* The tolerance zones of the hole and shaft overlap. *Application:* Used for parts that need accurate location but can be assembled and disassembled, like a coupling or a locating pin. 2.3 Hole-Basis and Shaft-Basis Systems

To standardize the thousands of possible fits, engineers use two systems. The Hole-Basis System is the most widely used. Hole-Basis System (H-System): The size of the hole is kept constant as the reference. The lower deviation of the hole is zero (EI = 0), meaning its smallest size is the basic size. Different fits (clearance, transition, interference) are achieved by changing the size of the shaft. The hole is often designated with a capital letter, 'H' (e.g., H7, H8). *Why is it preferred?* It is generally easier and cheaper to accurately machine a shaft (e.g., by grinding) to a specific size than it is to produce a precise hole (which requires tools like drills and reamers of fixed sizes). Shaft-Basis System (h-System): The size of the shaft is the reference. The upper deviation of the shaft is zero (es = 0). Different fits are achieved by changing the size of the hole. The shaft is designated with a small letter, 'h' (e.g., h6, h7). This system is less common and used for special applications, like when multiple parts must fit onto a long, standard-sized shaft. 2.4 The ISO System of Limits and Fits

Evaluation guide

• Explain the application of limits and fits in production drawing .
• Experiential Learning: Show learners a typical production drawing in class and lead them to
• identify limits and fits as used in the production drawing.
• Collaborative Learning: Learners in mixed -ability groups research on the application of limit