Lesson Notes By Weeks and Term v5 - Grade 10
Basic mechanical joining methods and fasteners – Week 1 focus
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Basic mechanical joining methods and fasteners – Week 1 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: 1
Theme: General lesson support
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Welcome, Grade 10 Mechanical Technology learners! This week, we embark on a crucial area of mechanical engineering: joining methods and fasteners.
Think about it: almost everything around you, from your desk to the taxi you take to school, relies on secure and reliable joining techniques. Understanding these methods and fasteners is fundamental to any career in engineering, manufacturing, and even maintenance. Imagine building a shack safely for a family in need, repairing a vital piece of agricultural machinery, or designing more durable products for the South African market. These skills are not only valuable but can directly contribute to improving lives and the economy.
2.1 Mechanical Joining Methods: An Overview Mechanical joining involves connecting two or more components using physical force or interlocking mechanisms.
These methods are broadly classified into: Permanent Joining: The components cannot be easily separated without damaging or destroying them. Examples include welding, riveting (some types), and adhesive bonding. We will touch briefly on permanent methods for context, but our main focus is temporary and semi-permanent methods.
Temporary Joining: The components can be easily separated and rejoined without damage. Examples include using bolts, nuts, screws, and quick-release fasteners.
Semi-Permanent Joining: The components can be separated, but it may require special tools or cause some minor damage. Examples include some types of riveting, set screws, and interference fits. 2.2 Common Fasteners Let's explore some of the most common fasteners used in mechanical engineering: a)
Bolts and Nuts: Definition: A bolt is a cylindrical metal rod with a head at one end and a helical thread along its length. A nut is a fastener with a threaded hole designed to mate with a bolt.
Application: Used extensively in structural applications, machine assembly, and general-purpose fastening. They are easily removable and reusable, making them suitable for applications requiring maintenance or disassembly.
Types: Hex bolts, carriage bolts, eye bolts, flange bolts. Nuts come in various forms too, such as hex nuts, Nyloc nuts (with a nylon insert for vibration resistance), wing nuts, and castle nuts (used with cotter pins for locking).
How they work: The bolt is inserted through aligned holes in the components being joined. The nut is then tightened onto the bolt, creating clamping force that holds the components together.
Important Considerations: Bolt strength (grade markings), thread type (metric or imperial), and appropriate torque (tightness) are crucial for a secure and reliable joint. Using the wrong grade bolt can lead to failure.
Example: Securing the wheels of a bakkie (pickup truck) using wheel bolts and lug nuts. These are high-strength bolts and nuts designed to withstand the dynamic forces of driving. b)
Screws: Definition: Similar to bolts but are typically designed to create their own thread or mate with an existing thread in one of the components being joined.
Application: Used in a wide range of applications, from securing small components in electronic devices to fastening wood or metal.
Types: Machine screws (for metal), wood screws, self-tapping screws, set screws.
How they work: Screws are driven into a material, creating a threaded connection. Self-tapping screws have a sharp point that cuts a thread as they are driven in.
Important Considerations: Screw type should match the material being fastened (e.g., wood screws for wood, machine screws for metal). Pilot holes are often required to prevent splitting or damaging the material. Screw length and head type are also crucial factors.
Example: Attaching metal sheeting to a steel frame using self-tapping screws. In the construction of a shack, self-tapping screws are often used to fasten corrugated iron sheets to the wooden or metal frame. c)
Rivets: Definition: A cylindrical metal shaft with a head at one end.
Application: Primarily used for permanent joining of metal sheets or plates.
Types: Solid rivets, blind rivets (pop rivets), tubular rivets. Blind rivets are commonly used when access is only available from one side of the joint.
How they work: The rivet is inserted through aligned holes in the components being joined. The headless end of the rivet is then deformed (e.g., by hammering or using a rivet gun) to create a second head, clamping the components together.
Important Considerations: Rivet material should be compatible with the materials being joined. Rivet diameter and length should be appropriate for the thickness of the materials.
Example: Joining metal panels in the construction of a bus or a train. Blind rivets (pop rivets) are often used to secure license plates to vehicles. d)
Washers: Definition: A flat, thin ring of metal or plastic.
Application: Used to distribute the load of a fastener, prevent damage to the surface being fastened, and provide a smooth bearing surface.
Types: Flat washers, lock washers (e.g., spring washers, split washers), fender washers (large diameter).
How they work: The washer is placed under the head of a bolt or nut, providing a larger contact area and reducing stress on the components being joined. Lock washers help to prevent loosening due to vibration.
Important Considerations: Washer material should be compatible with the fastener and the materials being joined. Washer size should be appropriate for the fastener.
Example: Using a flat washer under a bolt when fastening a wooden board to prevent the bolt head from sinking into the wood. e)
Pins: Definition: A cylindrical or tapered rod used to align or secure components.