Lesson Notes By Weeks and Term v5 - Grade 10
Basic mechanical joining methods and fasteners – Week 4 focus
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Basic mechanical joining methods and fasteners – Week 4 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Mechanical Technology
Class: Grade 10
Term: 2nd Term
Week: 4
Theme: General lesson support
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Mechanical joining methods and fasteners are fundamental to nearly every manufactured product, structure, and machine we encounter. From assembling furniture to constructing buildings and maintaining vehicles, understanding how things are held together is essential. In the South African context, these skills are crucial for careers in manufacturing, construction, automotive repair, and even agriculture, where machinery is vital. A strong grasp of mechanical joining principles allows for effective repairs, maintenance, and even the design and construction of innovative solutions for local challenges. This topic provides a foundation for more advanced engineering studies.
Mechanical Joining Methods: These are techniques used to connect two or more components together using mechanical force, often with the aid of fasteners. Unlike welding or gluing, mechanical joining generally allows for disassembly.
Fasteners: These are hardware devices that mechanically join or affix two or more objects together.
Types of Fasteners: Bolts and Nuts: These are perhaps the most common type of fastener. A bolt is a threaded rod with a head, designed to be used with a nut. The nut is a threaded component that screws onto the bolt, clamping the materials together. Different types of bolt heads exist (e.g., hexagon, socket head, countersunk). The strength of a bolt is indicated by grade markings on the bolt head. Metric bolts are typically identified with numbers such as 4.8, 8.8, 10.9 etc, while imperial bolts are typically indicated with lines on the head.
Example Application:* Securing the engine to the chassis of a car.
Important Considerations:* Bolt size (diameter and length), thread type (metric vs. imperial, coarse vs. fine), bolt grade (strength), and tightening torque.
Screws: Similar to bolts, but typically designed to tap their own threads into the material being joined. Screws often have pointed ends to assist in this process. Types include wood screws, self-tapping screws, machine screws (designed to be used with pre-tapped holes), and drywall screws.
Example Application:* Attaching wooden planks to form a table top, securing electronic components inside a device.
Important Considerations:* Screw type, length, head type (e.g., flat head, pan head), pilot hole size (for wood screws).
Rivets: These are permanent fasteners that are inserted through pre-drilled holes and then deformed (usually by hammering or using a rivet gun) to create a secure joint. They are particularly useful for joining thin sheets of metal.
Example Application:* Joining panels in aircraft construction, attaching number plates to cars, securing the joints in roofing sheets.
Important Considerations:* Rivet material (e.g., aluminum, steel), rivet diameter, grip length (the thickness of the materials being joined).
Pins: These are used to align or secure two parts together. Types include dowel pins (solid, precise pins used for alignment), cotter pins (inserted through a hole in a bolt or shaft to prevent a nut from loosening), and clevis pins (used with a clevis to create a pivoting joint).
Example Application:* Securing a wheel to an axle using a cotter pin, aligning two halves of a machine casing using dowel pins.
Important Considerations:* Pin diameter, pin length, material strength, corrosion resistance.
Keys: These are used to transmit torque from a shaft to a pulley, gear, or other rotating component. The key fits into a keyway (a slot) cut into both the shaft and the component. Common types include square keys, gib-head keys, and Woodruff keys.
Example Application:* Connecting a motor shaft to a gearbox.
Important Considerations:* Key size, key material, keyway dimensions, torque rating.
Washers: These are flat, circular discs placed under bolt heads or nuts.
Washers serve several purposes: distributing the load over a larger area, preventing damage to the surface being fastened, and providing a smoother bearing surface for the nut or bolt to turn against. Types include flat washers, spring washers (to prevent loosening), and lock washers (designed to prevent loosening due to vibration).
Example Application:* All bolted joints.
Important Considerations:* Washer size (inner and outer diameter), washer material, washer type (flat, spring, lock).
Factors Influencing Fastener Selection: Load Requirements: The amount of force the joint must withstand (tension, shear, bending, torsion). Higher loads require stronger fasteners and materials.
Material Type: The materials being joined influence the choice of fastener. Softer materials may require larger bearing surfaces to prevent crushing. Some materials may react negatively to certain metals, causing corrosion.
Environmental Conditions: Exposure to moisture, chemicals, or high temperatures can affect fastener performance. Corrosion-resistant materials (e.g., stainless steel, brass) may be necessary.
Accessibility: The ease with which the fastener can be installed and removed.
Cost: The cost of the fastener and the labor required for installation.
Appearance: The desired aesthetic of the joint.
Example 1: Choosing a bolt for a trailer hitch: A trailer hitch needs to withstand a tensile force of 5000 N. You are using steel plates that are 10mm thick each. What size and grade of bolt would you select?
Solution:*
Step 1: Determine the required bolt length. The bolt needs to pass through both plates (2 x 10mm = 20mm) plus allow for the nut and washers (estimate 10mm).
Total length: 30mm. We should select a bolt that is at least 30mm long to ensure adequate thread engagement.
Step 2: Determine the required bolt diameter and grade. This requires knowledge of material properties and safety factors which are beyond the scope of the Grade 10 curriculum.
However, for this example, we will specify an M10 grade 8.8 bolt. M10 indicates a 10mm diameter bolt.
Step 3: Verify the bolt's tensile strength. A grade 8.8 M10 bolt has an approximate tensile strength of 50,000 N. This is much more than the required 5000N, providing a sufficient safety factor.
Therefore, select an M10 x 30mm grade 8.8 bolt.
Example 2: Selecting a rivet for joining aluminum sheets: You need to join two 2mm thick aluminum sheets. What type and size of rivet would be appropriate?
Solution:*
Step 1: Determine the required grip length. The grip length is the total thickness of the materials being joined: 2mm + 2mm = 4mm.
Step 2: Select the rivet diameter. A 3.2mm diameter rivet would be suitable for this application.
Step 3: Select the rivet material. Aluminum rivets are the best choice for joining aluminum sheets to avoid corrosion caused by dissimilar metals.
Therefore, select a 3.2mm diameter aluminum rivet with a grip length of 4mm.
Guided Practice (With Solutions)
Question 1: You are building a wooden bookshelf and need to join two pieces of pine wood together. What type of fastener would be most suitable, and why?
Solution:* Wood screws would be the most suitable fastener. Wood screws are designed to create their own threads in wood, providing a strong and reliable connection. They are also relatively easy to install with basic hand tools.
Question 2: A mechanic is replacing a wheel on a car. After tightening the wheel nuts, what additional fastener should be used to prevent the nuts from loosening due to vibration?