Lesson Notes By Weeks and Term v5 - Grade 10
Basic mechanical joining methods and fasteners – Week 2 focus
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Basic mechanical joining methods and fasteners – Week 2 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: 2
Theme: General lesson support
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Welcome to Week 2 of Mechanical Technology! This week, we delve deeper into the fascinating world of mechanical joining methods and fasteners. Understanding how different parts are joined together is crucial for any aspiring technician, engineer, or even a DIY enthusiast. From the simple screws holding together furniture to the complex rivets securing aircraft components, mechanical joining is fundamental to modern technology. In South Africa, this knowledge is incredibly valuable for industries like manufacturing, construction, automotive repair, and even small-scale entrepreneurial ventures.
2.1 Rivets: Rivets are permanent mechanical fasteners. They consist of a smooth cylindrical shaft with a head on one end. Installation involves inserting the shaft through pre-drilled holes in the materials being joined and then deforming the other end (the "tail") to create a second head. This deformation locks the rivet in place, clamping the materials together.
Types of Rivets: Solid Rivets: These are the strongest type of rivet, requiring access to both sides of the joint for installation. They are commonly used in structural applications like aircraft construction and bridge building. Solid rivets need a bucking bar on one side and a rivet gun on the other for proper setting.
Tubular Rivets: These rivets have a hollow shank. They are easier to set than solid rivets and are often used in applications where strength is not critical, such as leather goods, clothing, and some sheet metal assemblies. They are set using a riveting machine or special hand tools.
Blind Rivets (Pop Rivets): These rivets can be installed from only one side of the joint. They consist of a rivet body (a hollow cylinder) and a setting mandrel (a pin that passes through the body). A rivet gun is used to pull the mandrel, which expands the rivet body and creates a head on the blind side. Once the rivet is set, the mandrel breaks off. Blind rivets are used extensively in applications where access is limited, such as automotive repair, appliance manufacturing, and DIY projects. Blind rivets are incredibly popular in South Africa due to their ease of use.
Drive Rivets: Another type of blind rivet, drive rivets have a pin that is driven into the rivet body to expand it.
Materials: Rivets are made from various materials, including aluminum, steel, copper, and stainless steel, depending on the application and the materials being joined.
Rivet Selection: The choice of rivet type and material depends on the strength requirements, the materials being joined, the accessibility of the joint, and the environmental conditions.
Advantages of Riveting: Permanent Joint: Creates a strong, permanent connection.
Relatively Low Cost: Rivets are generally inexpensive.
Suitable for Dissimilar Metals: Can be used to join different metals without causing galvanic corrosion (unlike some welding processes).
Ease of Inspection: Riveted joints are relatively easy to visually inspect for defects.
Disadvantages of Riveting: Permanent Joint: Difficult to disassemble without damaging the materials.
Requires Pre-Drilled Holes: Can weaken the materials being joined.
Potential for Corrosion: Corrosion can occur around the rivet if not properly sealed.
Appearance: Riveted joints may not be as aesthetically pleasing as other joining methods. 2.2 Keys and Keyways: Keys and keyways are used to transmit torque between a shaft and a hub (e.g., a gear, pulley, or sprocket). The key is a small piece of metal (usually steel) that fits into matching grooves called keyways, one in the shaft and one in the hub. When the shaft rotates, the key transmits the rotational force (torque) to the hub, causing it to rotate as well.
Types of Keys: Square Key: A simple and common type of key with a square cross-section. Easy to manufacture and install.
Rectangular Key: Similar to a square key but with a rectangular cross-section. Often used when a larger bearing area is required.
Woodruff Key: A semi-circular key that fits into a matching semi-circular keyway in the shaft. Woodruff keys are self-aligning and are often used in tapered shafts. Useful for light-duty applications.
Gib-Head Key: A rectangular key with a head (the "gib") that allows for easy removal using a hammer and chisel. This makes it easy to disassemble components.
Feather Key: A key that is fixed to either the shaft or the hub, allowing the other component to slide along the shaft. Used when a sliding fit is required.
Keyway: The groove cut into the shaft and hub to accommodate the key. The keyway must be precisely machined to ensure a good fit and proper torque transmission.
Torque Transmission: The amount of torque a key can transmit depends on the key's size, material, and the shear strength of the key material. A larger key made of stronger material will be able to transmit more torque.
Key Selection: The key size should be chosen based on the shaft diameter and the amount of torque to be transmitted. Standards such as ISO and BS provide guidelines for key selection.
Advantages of Keys and Keyways: Simple and Inexpensive: Keys and keyways are relatively simple and inexpensive to manufacture and install.
Easy to Replace: Damaged keys can be easily replaced.
Reliable Torque Transmission: Provides a reliable method for transmitting torque.
Disadvantages of Keys and Keyways: Stress Concentration: Keyways can create stress concentrations in the shaft, potentially leading to fatigue failure.
Limited Torque Capacity: The torque capacity is limited by the shear strength of the key.