Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define friction and lubrication and their effects.
• Describe different types of friction (static, kinetic, rolling).
• Explain the principles and types of lubrication.
• Analyse lubrication methods.
• Select appropriate lubricants for specific applications.

Lesson summary

Friction and lubrication are fundamental concepts in Mechanical Technology, impacting everything from the efficiency of a bicycle to the performance of heavy machinery used in South African industries like mining and agriculture. Understanding these principles allows us to design, maintain, and improve mechanical systems, saving energy, reducing wear and tear, and ultimately contributing to a more sustainable and productive society. Imagine a taxi’s engine seizing due to poor lubrication, costing the driver a day's income. Or a malfunctioning combine harvester during maize season, impacting food security. This topic empowers you to understand these issues and contribute to solving them.

Lesson notes

2.1 Friction: Friction is a force that opposes motion between surfaces in contact. It's a resistive force arising from the microscopic irregularities on the surfaces that interlock and create resistance as they try to slide past each other. It's not always undesirable; we rely on friction for walking, driving, and holding objects.

However, in mechanical systems, excessive friction leads to energy loss (as heat), wear, and reduced efficiency.

Types of Friction: Static Friction (Fs): The force that prevents an object from starting to move when a force is applied. Static friction increases as the applied force increases, up to a maximum value (F s max ). Once the applied force exceeds F s max , the object begins to move.

Kinetic Friction (Fk): The force that opposes the motion of an object that is already moving. Kinetic friction is generally less than static friction.

Rolling Friction (Fr): The resistance to motion when a round object (like a wheel or ball bearing) rolls over a surface. Rolling friction is generally much less than static or kinetic friction, which is why wheels are so effective. Coefficient of Friction (μ): A dimensionless number that represents the ratio of the frictional force to the normal force (the force pressing the surfaces together). It indicates how much friction exists between two surfaces. μ s = Coefficient of static friction μ k = Coefficient of kinetic friction μ r = Coefficient of rolling friction The formulas for frictional force are: F s max = μ s N (where N is the normal force) F k = μ k N F r = μ r N Example 1 (Static Friction): A toolbox weighing 50N is resting on a workshop floor. The coefficient of static friction between the toolbox and the floor is 0.

4. What is the maximum force you can apply horizontally to the toolbox before it starts to move?

Solution: Identify the knowns: Weight (W) = 50

N. This is also the normal force (N) since the floor is horizontal. N = 50N μ s = 0.4 Apply the formula: F s max = μ s N F s max = 0.4 50 N F s max = 20 N Answer: You can apply a maximum horizontal force of 20 N before the toolbox starts to move.

Example 2 (Kinetic Friction): A steel block weighing 200 N is sliding across a steel surface. The coefficient of kinetic friction between the steel surfaces is 0.

2. Calculate the force of kinetic friction acting on the block.

Solution: Identify the knowns: Weight (W) = 200 N. This is also the normal force (N) since the surface is horizontal. N = 200N μ k = 0.2 Apply the formula: F k = μ k N F k = 0.2 200 N F k = 40 N Answer: The force of kinetic friction acting on the block is 40 N. 2.2 Lubrication: Lubrication is the process of reducing friction between moving surfaces by introducing a lubricant. Lubricants create a thin film between the surfaces, preventing direct contact and reducing friction, wear, and heat generation.

Principles of Lubrication: Fluid Film Lubrication (Hydrodynamic Lubrication): A thick film of lubricant completely separates the moving surfaces. This requires sufficient lubricant supply, appropriate viscosity, and relative motion to create a pressure that supports the load.

Example: Engine crankshaft bearings.

Boundary Lubrication: Occurs when the lubricant film is very thin, and some direct contact between the surfaces occurs. This usually happens at start-up, low speeds, or high loads. Special additives in the lubricant (extreme pressure additives) help prevent seizing.

Example: Gears under heavy load.

Elastohydrodynamic Lubrication (EHL): Occurs in highly stressed contacts like gears and ball bearings. The high pressure causes the lubricant and the contacting surfaces to deform elastically, creating a thicker lubricant film than would otherwise be expected.

Types of Lubricants: Oils: Commonly used for engine lubrication, hydraulic systems, and general-purpose lubrication. They offer good cooling properties and can be formulated with various additives to improve performance. Mineral oils are derived from crude oil, while synthetic oils are manufactured.

Greases: Semi-solid lubricants consisting of oil thickened with a soap or other thickener. They are useful where oil would drip or leak, and they provide good sealing properties.

Example: Wheel bearings.

Solid Lubricants: Used in extreme conditions where oils and greases are not suitable (e.g., high temperatures, high vacuum). Examples include graphite, molybdenum disulfide (MoS2), and PTFE (Teflon).

Example: Some dry bearings.

Example 3 (Lubricant Selection): A large industrial gear system in a sugar mill operates at a high load and relatively low speed. It is also exposed to dusty conditions. Which type of lubricant would be most suitable, and why?

Solution: A grease would be the most suitable lubricant. Oils would tend to drip and not stay in place under high load. The dusty environment is a problem because dust contaminates the oil, and the lubricant film needs to be sealed from foreign contaminants.