Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Identify at least five common types of mechanical fasteners.
• Explain the pros and cons of different joining methods.
• Select the correct size and type of fastener for a task.
• Demonstrate correct use of hand tools for fasteners.
• Distinguish between different thread types.

Lesson summary

This week, we'll be diving into the world of mechanical joining methods and fasteners. These are the essential building blocks of almost everything around us – from the furniture in your house to the cars on the road and the infrastructure that makes modern life possible.

Think about it: every structure, every machine, every device is held together by some form of joining method. In a country like South Africa, where manufacturing, construction, and maintenance are vital sectors of the economy, understanding these principles is crucial for skilled artisans, engineers, and technicians.

Lesson notes

2. 1. Introduction to Mechanical Fasteners Mechanical fasteners are components used to mechanically join two or more parts together. Unlike welding or adhesives, they allow for disassembly, making maintenance and repairs easier. The choice of fastener depends on several factors, including the materials being joined, the loads the joint will experience, environmental conditions, and the desired lifespan of the assembly. 2.

2. Types of Fasteners Bolts: Bolts are externally threaded fasteners designed to be inserted through holes in assembled parts and tightened with a nut. They provide strong clamping force.

Types of Bolts: Hex bolts (most common), carriage bolts (smooth head, used in wood), eye bolts (with a loop for attaching cables).

Material: Steel (various grades), stainless steel, brass. Steel bolts are often coated with zinc for corrosion resistance.

Screws: Screws are similar to bolts but are designed to be inserted into a pre-tapped hole or to create their own thread as they are tightened.

Types of Screws: Machine screws (for metal), wood screws (for wood), self-tapping screws (create their own thread).

Head Types: Flat head (flush mounting), pan head, round head, countersunk head.

Nuts: Nuts are internally threaded fasteners used to tighten bolts.

Types of Nuts: Hex nuts (most common), lock nuts (prevent loosening), wing nuts (for hand tightening).

Washers: Washers are thin, flat rings used under nuts and bolt heads.

They serve several purposes: Distributing load: Prevents damage to the joined material.

Providing a smooth bearing surface: Reduces friction during tightening.

Locking: Some washers are designed to prevent loosening.

Sealing: Some washers provide a watertight seal.

Rivets: Rivets are permanent fasteners used to join two or more pieces of material. They consist of a smooth cylindrical shaft with a head on one end. They are installed by deforming the shaft end to create a second head, clamping the materials together.

Types of Rivets: Solid rivets (strongest), blind rivets (pop rivets – used when access is limited to one side). 2.

3. Joining Methods Bolting: Using bolts and nuts to clamp materials together. Offers high strength and ease of disassembly.

Advantages: Strong, reusable, easy to inspect.

Disadvantages: Requires access to both sides, can loosen over time if not properly tightened.

Screwing: Using screws to fasten materials together. Suitable for lighter loads and applications where disassembly is required.

Advantages: Simple, inexpensive, readily available.

Disadvantages: Lower strength than bolting, can strip threads if over-tightened.

Riveting: Using rivets to create a permanent joint. Suitable for applications where vibration and repeated stress are present.

Advantages: Strong, resistant to vibration, tamper-proof.

Disadvantages: Permanent, requires special tools for installation and removal. 2.

4. Threads The threads on bolts and screws are helical ridges used to create a clamping force.

Metric Threads (M): The most common thread system worldwide, including South Africa. Designated by the letter "M" followed by the nominal diameter in millimeters (e.g., M8). Pitch (distance between threads) is also specified (e.g., M8 x 1.25). Imperial Threads (Unified National Coarse - UNC/ Unified National Fine - UNF): Commonly used in older equipment and the USA. Specified in inches and threads per inch (e.g., 1/4"-20 UNC). 2.

5. Torque Torque is a rotational force that is applied to tighten a fastener. Applying the correct torque is crucial for achieving the desired clamping force and preventing joint failure. Over-tightening can strip threads or damage the fastener, while under-tightening can lead to loosening and joint failure. Torque wrenches are used to apply a specific amount of torque.

Example 1: Bolt Selection A structural steel component needs to be bolted to a concrete foundation. The component will experience a tensile load of 15k

N. Select a suitable bolt, assuming a safety factor of

3. Step 1: Calculate the required bolt strength: Required Strength = Load x Safety Factor = 15kN x 3 = 45kN Step 2: Consult a bolt strength chart: Steel bolts are typically graded according to their tensile strength. An 8.8 grade bolt has a tensile strength of approximately 800 MPa.

Step 3: Calculate the required bolt area: Area = Force / Tensile Strength = 45,000 N / 800 MPa = 56.25 mm².

Step 4: Select a bolt size: Referring to a bolt size chart, an M10 bolt has a tensile stress area of approximately 58 mm².

Therefore, an M10 bolt of grade 8.8 would be suitable.

Step 5: Consider the nut and washer: Choose a matching M10 nut of grade 8 or higher, and a suitable washer to distribute the load on the steel component.

Example 2: Rivet Selection Two sheets of aluminum, each 3mm thick, need to be riveted together in an application where shear stress is the primary concern.