Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define a linkage and identify its basic components.
• Describe different types of linkages and their functions.
• Explain how link length affects movement.
• Analyze simple linkage mechanisms to predict motion.
• Design a simple linkage to achieve a specific motion.

Lesson summary

This week, we delve into the fascinating world of mechanical systems and, specifically, linkages. Linkages are fundamental building blocks found in countless devices around us, from simple toys to complex machinery. Understanding how linkages work allows us to design and build our own machines, solve practical problems, and appreciate the ingenuity of engineering solutions. In South Africa, understanding mechanical systems is crucial, as it empowers us to develop and maintain essential technologies in agriculture, manufacturing, and transportation.

Lesson notes

A mechanical system is a group of two or more machine elements arranged to transmit motion and force. A linkage is a type of mechanical system consisting of rigid bars (links) connected together by joints (pivots), usually allowing rotational movement. The purpose of a linkage is to transform an input motion into a desired output motion.

Basic Components: Links: These are the rigid bars that make up the linkage. They can be straight, curved, or any other shape, depending on the desired function. The length of each link is a critical design parameter.

Joints/Pivots: These are the connections between the links, allowing them to rotate relative to each other. Joints are often pins or hinges.

Types of Linkages: Four-Bar Linkage: This is the most fundamental and versatile type of linkage. It consists of four links connected in a closed loop. One link is usually fixed (the ground), while another is driven (the input), and a third acts as a coupler, connecting the input to the output. The fourth link produces the output motion. The four bar linkage is widely used in mechanisms such as bicycle suspensions, car suspensions, and many other industrial machines. The lengths of the links in a four-bar linkage determine the type of motion it produces.

Parallel Linkage: This type of linkage ensures that two links remain parallel to each other throughout their motion. It's commonly used in applications requiring precise linear movement, such as parallel rulers or some types of drawing equipment. Two opposite links in a parallel linkage must have the same length.

Slider-Crank Mechanism: This linkage converts rotary motion into linear motion, or vice-versa. It consists of a rotating crank, a connecting rod, and a sliding piston. A common example is the engine of a car.

Bell Crank: This linkage changes the direction of motion. It typically consists of two links joined at a right angle. A common application is in door latches, where pushing a handle causes the latch to retract.

How Link Lengths Affect Motion: The lengths of the links in a linkage are crucial in determining the type and range of motion it produces. For example, in a four-bar linkage: If one link is much longer than the others, the motion may be primarily rotary. If two links are equal in length, it may produce reciprocating motion (back and forth). Changing the relative lengths of the links will change the output motion's speed, range, and even type (rotary, linear, oscillating).

Worked example

Example 1: Simple Lever (First-Class Lever)

Imagine a seesaw in a South African schoolyard. This is a basic example of a first-class lever, which is a type of linkage. The pivot point (fulcrum) is in the middle, with the effort (force applied by one child) on one side and the load (weight of the other child) on the other side.

Why it matters:* Levers make it easier to lift heavy objects.

How it works:* By positioning the fulcrum closer to the load, you can use less effort to lift the load.

Example 2: Bicycle Brakes (Linkage System)