Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Identify and describe different types of linkages.
• Explain how linkages change motion and force.
• Design a simple mechanical system using a linkage.
• Analyse linkages in existing mechanical systems.

Lesson summary

This week, we delve into the fascinating world of mechanical systems and linkages. Mechanical systems are everywhere around us, from the simple scissors we use to cut paper to the complex suspension systems in our minibus taxis. Linkages, in particular, are essential components within these systems, enabling movement and force transfer in controlled ways. Understanding how these systems work not only equips you with valuable technological knowledge but also helps you appreciate the ingenuity behind everyday objects and solve practical problems in your community.

Lesson notes

What is a Mechanical System? A mechanical system is a collection of components that work together to perform a specific task using mechanical energy. This involves the transfer of force and motion. Think of a simple hand drill. You apply force to the handle, which rotates a gear system (another mechanical system in itself), ultimately causing the drill bit to spin and bore a hole. What is a Linkage? A linkage is a system of rigid bars (or links) connected by joints (usually pivots). These joints allow the links to move relative to each other. When one link moves, it causes the others to move in a predictable way. Linkages are used to transmit force and motion, change the direction of motion, increase or decrease force, and perform many other useful functions.

Types of Linkages and Their Functions: Bell Crank Linkage: This linkage changes the direction of motion by 90 degrees. It usually consists of two arms at right angles to each other, pivoted at their intersection. Think of the brake lever on a bicycle. You pull the lever vertically, and it pulls the brake cable horizontally.

Application: Bicycle brakes, door latches, steering mechanisms in older cars.

Push-Pull Linkage: This linkage transmits a force in a straight line, either pushing or pulling. It essentially allows you to apply force at a distance.

Application: Opening and closing windows in older buses, some types of hand pumps, control rods in engines.

Reverse Motion Linkage: This linkage changes the direction of motion by 180 degrees. If you push on one end, the other end moves in the opposite direction.

Application: Some types of hinges, old-fashioned clothes pegs, some toy mechanisms.

Parallel Motion Linkage: This linkage keeps two links moving parallel to each other.

Application: Pantograph (used for copying drawings), certain types of suspension systems.

Four-Bar Linkage: This is a fundamental linkage consisting of four bars connected by four pivots. By varying the lengths of the bars and the location of the pivots, you can create a wide range of motions. While complex, understanding the basic principle is important.

Application: Suspension systems in cars, folding mechanisms, various types of machinery. Examples and Worked

Examples: Bell Crank

Example: Imagine a security gate with a latch. To open the gate, you lift a handle vertically. This handle is connected to a bell crank linkage. One arm of the bell crank is connected to the handle, and the other arm is connected to a rod that moves horizontally to unlatch the gate.

Explanation: The vertical motion of the handle is converted into horizontal motion to release the latch. The bell crank changes the direction of the force. Push-Pull

Example: Consider a traditional hand water pump. When you move the handle up and down, you are using a push-pull linkage connected to a piston inside the pump.

Explanation: The push-pull action of the handle creates a vacuum inside the cylinder, drawing water up from the well. Reverse Motion

Example: A simple pair of tongs. When you squeeze the handles together, the jaws open. This is because the handles are connected to the jaws via a reverse motion linkage.

Explanation: The force applied inwards on the handles is translated into an outward force on the jaws, allowing you to grip objects. Guided Practice (With Solutions)

Question 1: Identify the type of linkage used in a bicycle brake system that connects the brake lever on the handlebar to the brake cable. What is its primary function?

Solution: The linkage used is a bell crank linkage. Its primary function is to change the direction of motion from vertical (pulling the lever) to horizontal (pulling the brake cable). This allows you to apply force conveniently from the handlebar to activate the brakes.

Question 2: Describe a situation where a push-pull linkage would be beneficial in a garden. Explain why this type of linkage is suitable for that situation.

Solution: A push-pull linkage could be used in a garden sprayer. The handle of the sprayer is connected to a piston inside the sprayer’s cylinder using a push-pull linkage. This type of linkage is suitable because it allows you to apply force repeatedly and linearly to create pressure inside the cylinder, which then forces the liquid out of the nozzle.

Question 3: Sketch a simple diagram illustrating how a reverse motion linkage could be used to operate a simple toy claw machine. Label the input and output forces and the direction of movement.

Solution: (Diagram would ideally be included here showing two levers connected by a pivot. Applying force to one lever causes the other to move in the opposite direction, opening/closing the claw.)

Input Force: Applied to one lever.

Output Force: Exerted by the other lever (controlling the claw).

Direction of Movement: Opposite directions for the two levers. This is crucial because squeezing the handles opens the claw, ready to grab something.