Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Identify four main types of linkages.
• Explain how linkages convert motion.
• Design a simple mechanical system with a linkage.
• Analyze the pros and cons of different linkages.

Lesson summary

This week, we delve into the fascinating world of mechanical systems and linkages. Mechanical systems are everywhere, from the simple tools we use daily to complex machinery in industries. Understanding how these systems work allows us to design and improve them, making our lives easier and more efficient. Linkages are essential components of many mechanical systems, converting motion and force in predictable ways. In South Africa, a good understanding of mechanical systems and linkages is valuable in various sectors, including agriculture (tractors, irrigation systems), manufacturing (factory machinery), mining (excavators, conveyors), and transportation (vehicle mechanics).

Lesson notes

What is a Mechanical System? A mechanical system is a collection of components that work together to perform a specific task. These components can include gears, levers, pulleys, and, importantly, linkages. Mechanical systems transfer forces and motion to achieve a desired outcome. Think of a bicycle – the pedals, chain, gears, and wheels form a system that converts the rider's leg power into forward motion. What are Linkages? Linkages are a series of rigid bars or links connected by joints (pivots or hinges). These linkages transmit and transform motion and force. They are crucial for converting one type of motion into another, changing the direction of force, and amplifying or reducing force.

Types of Linkages: Lever Linkage: A lever linkage uses a rigid bar (the lever) that pivots around a fixed point called a fulcrum. Applying force to one end of the lever causes movement at the other end.

Think of a see-saw: the fulcrum is in the middle, and when one person goes down, the other goes up. A crowbar is another example - it allows you to apply a small force over a large distance to move a heavy object a small distance. There are three classes of levers, depending on the placement of the fulcrum, load, and effort.

Class 1: Fulcrum between load and effort (See-saw, crowbar)

Class 2: Load between fulcrum and effort (Wheelbarrow, bottle opener)

Class 3: Effort between fulcrum and load (Tweezers, fishing rod)

Crank Linkage: A crank linkage converts rotary motion into linear motion, or vice versa. It typically consists of a rotating disc (the crank) connected to a connecting rod. As the crank rotates, the connecting rod moves back and forth in a linear motion. A common example is the engine in a car. The rotating crankshaft (driven by the pistons) is a crank linkage that ultimately turns the wheels.

Push-Pull Linkage: A push-pull linkage transmits motion in a straight line. It consists of a rigid rod that is pushed or pulled to move an object. A simple example is the steering linkage in a car. When you turn the steering wheel, a push-pull linkage transmits that motion to the wheels, causing them to turn.

Reverse Motion Linkage: A reverse motion linkage changes the direction of motion. If one part of the linkage moves forward, another part moves backward. Scissors are a good example. When you close the handles (moving them towards each other), the blades move together in the opposite direction, cutting the material.

Worked example

Example 1: Designing a simple lever system

Problem: A farmer in Limpopo wants to lift a heavy rock to clear his field. He has a long wooden plank and a smaller rock to use as a fulcrum. How can he use these to create a lever system?

Solution: The farmer can create a Class 1 lever. He should place the smaller rock (fulcrum) closer to the heavy rock (load) and then apply force (effort) to the other end of the wooden plank. By placing the fulcrum closer to the load, he increases the mechanical advantage, meaning he can lift the heavy rock with less effort.

Explanation: Placing the fulcrum closer to the load increases the distance between the fulcrum and the point where effort is applied. This longer distance multiplies the force applied. This principle is essential for tasks requiring high force and minimal effort, especially in agricultural contexts.