Lesson Notes By Weeks and Term v3 - Senior Secondary 1
Classification of fasteners
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Classification of fasteners
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Subject: Welding & Fabrication
Class: Senior Secondary 1
Term: 2nd Term
Week: 2
Theme: Classification Of Fasteners
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This topic introduces students to the fundamental concept of fasteners, which are essential components in all aspects of welding and fabrication, as well as in everyday life. Fasteners are mechanical devices used to join two or more objects together. Their proper identification and selection are crucial for ensuring the structural integrity, safety, and functionality of assembled products, from simple household items to complex industrial machinery. Understanding the classification of fasteners allows for informed decision-making in design, assembly, maintenance, and repair, which are vital skills for future welders and fabricators in the Nigerian industrial landscape.
Classification Of Fasteners Classification of fasteners Term: 2nd Term Week: 19 ---
1. Overview and Learning Objectives This topic introduces students to the fundamental concept of fasteners, which are essential components in all aspects of welding and fabrication, as well as in everyday life. Fasteners are mechanical devices used to join two or more objects together. Their proper identification and selection are crucial for ensuring the structural integrity, safety, and functionality of assembled products, from simple household items to complex industrial machinery. Understanding the classification of fasteners allows for informed decision-making in design, assembly, maintenance, and repair, which are vital skills for future welders and fabricators in the Nigerian industrial landscape.
Performance Objectives: By the end of this lesson, students will be able to: Identify and categorize different types of fasteners based on their characteristics. Distinguish between threaded and non-threaded fasteners. Recognize common fasteners used in various applications, such as in vehicles, furniture, and construction within Nigeria.
2. Key Concepts and Explanations Definition of Fasteners: Fasteners are mechanical elements used to temporarily or permanently join two or more parts together. In welding and fabrication, while welding creates permanent joints, fasteners offer advantages such as ease of assembly, disassembly for maintenance, and joining dissimilar materials that cannot be welded.
Broad Classification of Fasteners: Fasteners can be broadly classified based on whether they rely on threads for their joining action.
1. Threaded Fasteners: These fasteners incorporate helical ridges (threads) that engage with corresponding threads in another component or create their own threads in the material. They are generally designed for temporary or removable joints, allowing for disassembly without damaging the components.
Examples: Bolts, Screws, Nuts, Studs.
2. Non-Threaded Fasteners: These fasteners do not rely on threads for their primary joining mechanism. They often depend on friction, deformation, or interference fits to secure components. Some non-threaded fasteners create permanent joints, while others are removable.
Examples: Rivets, Pins, Washers, Keys, Retaining Rings (Circlips). Detailed Classification of Common Fasteners:
A. Threaded Fasteners:
1. Bolts: Definition: An externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.
Characteristics: Typically have a head (for turning or applying torque) and a shank, part of which is threaded. Types and Nigerian Context
Examples: Hex Head Bolt: Most common, hexagonal head, used widely in machinery, automotive (e.g., securing engine components in vehicles like 'Okada' or 'Keke Napep'), and structural steelwork.
Square Head Bolt: Older type, less common now, but still found in some heavy machinery or historical structures. Carriage Bolt (Round Head Square Neck Bolt): Has a smooth, domed head and a square section under the head that 'seats' into a square hole in wood or soft material, preventing rotation while the nut is tightened. Common in furniture assembly (e.g., benches in public spaces, wooden market stalls).
Lag Bolt/Screw (Lag Screw): A large wood screw with an external hex head. Used for heavy-duty timber construction (e.g., joining heavy wooden beams, securing railings).
Eye Bolt: A bolt with a loop at one end, used to attach a cable or chain for lifting or securing. Common in lifting operations in construction sites or workshops.
U-Bolt: A 'U' shaped bolt with threads on both ends, typically used to fasten pipes or round bars to a support. Seen securing vehicle exhaust systems or antennae to poles.
2. Screws: Definition: An externally threaded fastener capable of being inserted into holes in assembled parts, mating with pre-formed internal threads or forming its own thread, and being tightened by turning its head. Unlike bolts, screws are typically driven directly into the material or a pre-tapped hole without a nut.
Characteristics: Varies greatly in head type, thread type, and tip. Types and Nigerian Context
Examples: Wood Screw: Tapered shank and sharp point, coarse threads for gripping wood. Used extensively in carpentry, furniture making (e.g., assembling chairs, tables, cabinet doors in local workshops).
Machine Screw: Fully threaded, designed to be used with a pre-tapped hole or a nut. Found in small assemblies, electronics (e.g., radios, fans, small appliances).
Self-Tapping Screw: secure components, or act as shear pins. Types and Nigerian Context
Examples: Dowel Pin: Precision-ground cylindrical pin, used for accurate alignment of parts (e.g., aligning engine blocks during assembly, jigs and fixtures in workshops).
Cotter Pin (Split Pin): A metal pin with two tines that are bent apart after insertion. Used to secure other fasteners (like castellated nuts) or to prevent rotation/movement of a component (e.g., in vehicle suspension systems to secure ball joints, agricultural machinery).
Clevis Pin: Has a head and a hole at the other end for a cotter pin. Used to connect a clevis to another component, allowing pivoting motion (e.g., linkages in farming equipment).
3. Rivets: Definition: A permanent mechanical fastener consisting of a smooth cylindrical shaft with a head on one end. The other end is deformed (bucked or upset) after insertion to form a second head, creating a permanent joint.
Characteristics: Create permanent joints; removal requires destruction of the rivet. Types and Nigerian Context
Examples: Solid Rivet: Requires access to both sides for installation (one side is hammered). Used for heavy-duty structural applications (e.g., older bridges, aircraft, local fabrication of large water tanks or heavy metal structures).
Blind Rivet (Pop Rivet): Can be installed when access is only available from one side. Widely used for joining sheet metal, attaching license plates to vehicles, assembling light kiosks, or general sheet metal work in local fabrication shops.
4. Keys: Definition: Mechanical elements used to prevent relative rotation between a shaft and a hub (e.g., a gear, pulley, or coupling). They transmit torque. Types and Nigerian Context
Examples: Parallel Key (Square or Rectangular Key): Fits into keyways (slots) in both the shaft and the hub. Common in electric motors (e.g., grinding machines, water pumps) to connect the motor shaft to a pulley or fan.
Taper Key: Has a slight taper, providing a tighter fit when hammered in.
Woodruff Key: A semi-circular key, often used for lighter loads and easier assembly.
5. Retaining Rings (Circlips): Definition: A metal ring sprung into a groove on a shaft or inside a bore to prevent axial movement of components.
Types: External Retaining Ring: Fits into a groove on a shaft to hold a component (e.g., bearings, gears).
Internal Retaining Ring: Fits into a groove within a bore to hold a component.
Nigerian Context: Found in various mechanical assemblies, such as securing bearings on shafts in small engines, bicycle hubs, or gearboxes.
3. Teaching and Learning Activities Introduction (10 minutes): Teacher Activity: Begin by displaying a collection of various fasteners (bolts, nuts, screws, washers, rivets, pins) without naming them. Ask students to observe and think about what these items are used for. Lead a brief discussion on the importance of joining materials in everyday life and in the field of welding and fabrication. Ask students to provide examples of where they have seen these items used (e.g., bicycles, chairs, doors, vehicles).
Student Activity: Students observe the displayed fasteners, share their initial thoughts on their uses, and provide examples from their immediate environment.
Presentation of Key Concepts (25 minutes): Teacher Activity: Use charts, projector slides, or actual large samples (if available) to systematically introduce the classification of fasteners.
Start with the broad categories: threaded vs. non-threaded. Then, explain each specific type of fastener (bolts, screws, nuts, washers, pins, rivets, keys, retaining rings) in detail, highlighting their characteristics, specific applications, and providing relevant Nigerian examples for each. Emphasize how to visually identify each type.
Student Activity: Students listen attentively, take notes, ask clarifying questions, and observe the fastener samples or diagrams provided.
Activity 1: Fastener Identification and Grouping (20 minutes): * Teacher Activity: Divide the class into small groups. Provide each group with a mixed tray containing a variety of common fasteners (e.g., hex bolt, wood screw, hex nut, flat washer, split lock washer, pop rivet, cotter pin, machine screw, carriage bolt).
Instruct each group to:
1. Identify each fastener by its specific type.
2. Classify them into the two main categories: Threaded and Non-Threaded.
3. Present their findings and take notes, ask clarifying questions, and observe the fastener samples or diagrams provided.
Activity 1: Fastener Identification and Grouping (20 minutes): Teacher Activity: Divide the class into small groups. Provide each group with a mixed tray containing a variety of common fasteners (e.g., hex bolt, wood screw, hex nut, flat washer, split lock washer, pop rivet, cotter pin, machine screw, carriage bolt).
Instruct each group to:
1. Identify each fastener by its specific type.
2. Classify them into the two main categories: Threaded and Non-Threaded.
3. Present their findings and explain their reasoning.
Student Activity: In groups, students collaboratively examine, identify, and classify the fasteners. They discuss characteristics and justify their classifications to their group members before presenting to the class.
Activity 2: Application Scenario Discussion (15 minutes): Teacher Activity: Present a few real-world scenarios relevant to Nigerian contexts.
For example: "A local carpenter is building wooden school desks. What types of fasteners would be most suitable?" or "A fabricator is joining thin sheet metal panels for a kiosk where access is only from one side. What fastener should they use?" Guide a class discussion on the appropriate fastener choices and the reasons behind them, reinforcing the learned classifications and applications.
Student Activity: Students engage in the discussion, propose fasteners for the given scenarios, and articulate their justifications based on the lesson's content.
Wrap-up and Q&A (10 minutes): Teacher Activity: Summarize the main points of the lesson, reiterating the importance of proper fastener classification. Address any lingering questions and preview the next steps or practical applications.
Student Activity: Students ask final questions and review their understanding of the topic.
4. Guided Practice (With Solutions)
1. Question: Identify the following fasteners and classify them as either 'Threaded' or 'Non-threaded': (a) A long metal rod with threads at one end and a hexagonal head at the other, designed to be used with a nut. (b) A small, cylindrical metal pin with two tines that are bent apart after insertion. (c) A flat, round piece of metal with a hole in the center, placed under a bolt head. (d) A fastener with a pointed tip and coarse threads, designed to grip into wood directly.
Solution: (a)
Hex Bolt: Threaded fastener. (b)
Cotter Pin: Non-threaded fastener. (c)
Flat Washer: Non-threaded fastener. (d)
Wood Screw: Threaded fastener.
Commentary: This question assesses the basic ability to distinguish between threaded and non-threaded fasteners based on their primary characteristics and function, which is fundamental to the classification.
2. Question: A local 'Okada' (motorcycle) mechanic is replacing a worn-out exhaust pipe. He notices a 'U'-shaped metal piece with threads at both ends used to hold the exhaust pipe to the frame. What specific type of fastener is this, and is it a threaded or non-threaded fastener?
Solution: The fastener is a U-Bolt. It is a threaded fastener.
Commentary: This question connects the theoretical classification to a common, practical application in Nigeria, helping students to relate the concept to their real-world experiences. It tests both identification and broad classification.
3. Question: Explain the primary functional difference between a 'Flat Washer' and a 'Split Lock Washer'.
Solution: Flat Washer: Its primary function is to distribute the load of a threaded fastener over a larger surface area, preventing damage to the material being joined and ensuring a more even clamping force. * Split Lock Washer: Its primary function is to prevent fasteners from loosening due to vibration or thermal expansion/contraction by providing tension or friction against the nut or bolt head. It acts as a spring to maintain clamping force.
Commentary: This question requires students to differentiate between two non-threaded fasteners that might look similar at first glance but have distinct functional roles. It encourages understanding beyond mere identification.
4. Question: In a local fabrication workshop, a worker is assembling a metal kiosk. They need to join two thin sheet metal panels, but they can only access one side of the joint. Which type of rivet would be most suitable for this task, and why?
Solution: A Blind Rivet (Pop Rivet) would be most suitable.
Reason: Blind rivets are specifically designed for applications where