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 the direction and magnitude of a force.
• Sketch basic linkage diagrams and represent their movement.
• Construct simple linkage models.
• Apply mechanical advantage to analyze linkages.

Lesson summary

This week, we delve into the fascinating world of mechanical systems and, more specifically, linkages. Linkages are essential components in many of the machines and devices we use every day, from simple hand tools to complex industrial robots. Understanding how linkages work allows us to design and build our own solutions to real-world problems. In South Africa, where resourcefulness and innovation are highly valued, understanding mechanical systems empowers us to create and maintain essential equipment, contributing to local economies and improving everyday life.

Lesson notes

What is a Mechanical System? A mechanical system is a collection of parts that work together to perform a specific task. These parts include things like gears, pulleys, levers, and, of course, linkages. The system takes an input force or motion, modifies it, and produces an output force or motion. In simpler terms, it's a machine that does work. What are Linkages? Linkages are mechanical systems made up of rigid bars (links) connected by joints (pivots or hinges). These links are arranged in a way that allows motion to be transferred and transformed. They are incredibly versatile and can be used to perform a wide variety of tasks. Linkages don't use gears or belts to transfer motion; instead, they rely on the geometric arrangement of the links and pivots.

Types of Linkages: Bell Crank Linkage: A bell crank linkage is a lever bent at an angle (often 90 degrees). It changes the direction of motion. Think of a bicycle brake lever – when you pull the lever towards the handlebars (one direction), it pulls the brake cable downwards (another direction). Another example could be the steering mechanism of older machinery.

Push-Pull Linkage (or Slider-Crank): This linkage converts rotary motion into linear (back-and-forth) motion, or vice versa. A common example is the piston and crankshaft system in a car engine. The rotating crankshaft drives the piston up and down, and the piston's motion turns the crankshaft. It's also used in reciprocating saws and older hand-operated water pumps.

Four-Bar Linkage: This is one of the most fundamental and versatile linkages. It consists of four bars connected by four joints. One of the bars is usually fixed (the frame), and the other three bars are free to move. The relative lengths of the bars determine the type of motion produced. Four-bar linkages are found in everything from windshield wipers and excavator arms to sewing machines and folding chairs.

How Linkages Change Force and Motion: Linkages can change the direction, magnitude (size), and type of motion.

Changing Direction: A bell crank linkage is a perfect example of changing direction. The input force acts in one direction, and the output force acts in a different direction.

Changing Magnitude (Mechanical Advantage): Linkages can increase or decrease the force applied. This is related to the concept of mechanical advantage. Mechanical advantage (MA) is the ratio of the output force to the input force. MA = Output Force / Input Force If MA > 1, the linkage amplifies the force (but reduces the distance moved). If MA < 1, the linkage reduces the force (but increases the distance moved). Imagine using a long lever to lift a heavy rock. The lever (a type of linkage) allows you to apply a smaller force over a longer distance to move the rock. This is mechanical advantage at work.

Changing Type of Motion: As seen in the push-pull linkage, rotary motion can be converted to linear motion, and vice versa. Other linkages can create more complex and specialized movements.