Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define 'mechanical system', 'input', 'output', and 'linkage' with examples.
• Identify and describe different types of simple linkages in everyday objects.
• Explain how linkages change the direction or magnitude of a force.
• Design and sketch a simple linkage system to perform a specific task.

Lesson summary

Welcome to the exciting world of Mechanical Systems and Linkages! This week, we'll be diving into how machines use levers, linkages, and other clever designs to make work easier.

Think about it: from the humble pliers used to fix a leaking tap in Alexandra Township to the massive cranes building infrastructure in Gauteng, mechanical systems are all around us. Understanding these systems allows you to not only appreciate the technology we use every day but also to design and improve them. This knowledge is crucial for future engineers, technicians, and anyone interested in problem-solving in the real world.

Lesson notes

What is a Mechanical System? A mechanical system is a combination of parts working together to perform a specific task. It usually involves an input, a process, and an output.

Think of it like this: you input energy (force) into a system, and the system processes it to produce a different kind of energy, force, or movement as the output.

Input: The force, energy, or motion applied to the system. For example, pushing down on a bicycle pedal is an input.

Process: What happens inside the system to transform the input. In a bicycle, the pedal motion turns gears and chains.

Output: The result of the process. The output of a bicycle is the movement of the wheels and the bicycle forward. What is a Linkage? A linkage is a series of rigid bars (or links) connected together by joints. These joints allow the links to move relative to each other. Linkages are used to transmit and transform motion and force. They can change the direction of a force, change the amount of force (making it stronger or weaker), or change the type of motion (e.g., rotary to linear).

Types of Linkages and Their Functions: Bell Crank Linkage: This linkage changes the direction of motion by 90 degrees. Imagine a gate latch. When you lift the handle (vertical motion), the latch moves horizontally to release the gate. This is a bell crank linkage at work.

Example:* Gate latches, old-fashioned doorbells, some types of car suspensions.

Push-Pull Linkage: This linkage transmits motion in a straight line. One link pushes or pulls another link.

Example:* Brakes on a bicycle. When you squeeze the brake lever, a cable pulls on a mechanism at the wheel, applying the brakes. The bicycle's braking system uses push-pull linkages.

Four-Bar Linkage: This is a fundamental linkage consisting of four links connected by four joints. The motion of one link determines the motion of the other links. By changing the lengths of the links, you can create different types of motion.

Example:* Car suspensions (complex four-bar linkages), folding chairs, some types of robotic arms. Think about the suspension of a taxi - a four-bar linkage helps provide a smoother ride when going over the many potholes in South African roads.

Reverse Motion Linkage: A reverse motion linkage transforms a motion in one direction to an opposite direction.

Example:* A see-saw in a playground, or even some types of paper guillotine.

Parallel Motion Linkage: Used to keep the object it is attached to, always level.

Example:* Drawing boards, adjustable lamps, and some pantograph mechanisms.

How Linkages Change Force and Direction: Linkages act as force multipliers or force reducers. The principle of moments (also known as torque) governs this. The moment is the force multiplied by the distance from the pivot point (fulcrum).

Increasing Force (Mechanical Advantage): If the input force is applied at a longer distance from the pivot than the output force, the output force will be greater than the input force. This is like using a long spanner (wrench) to loosen a tight bolt. Decreasing Force (Increasing Speed/Distance): If the input force is applied at a shorter distance from the pivot than the output force, the output force will be smaller than the input force, but the distance the output moves will be greater. Think of a catapult - a little force and movement applied on the catapult can send a small object a large distance and with a relatively high speed.