Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define friction and explain its causes and effects.
• Describe the different types of friction.
• Explain the principles and types of lubrication.
• Discuss methods of applying lubricants.
• Identify advantages and disadvantages of different methods.

Lesson summary

Friction and lubrication are fundamental concepts in mechanical technology. Friction, the force that opposes motion between surfaces in contact, is present in virtually every mechanical system. While sometimes desirable (e.g., in brakes), it often leads to energy loss through heat generation and wear of components, reducing efficiency and lifespan. Lubrication, the process of introducing a substance (lubricant) between surfaces to reduce friction, is crucial for minimizing these negative effects.

Lesson notes

2.1 Friction: Friction is a force that opposes motion between two surfaces in contact. It arises from the microscopic irregularities on the surfaces, which interlock and resist movement. The magnitude of friction depends on the nature of the surfaces (their roughness) and the force pressing them together (the normal force).

Types of Friction: Static Friction (Fs): The force that prevents an object from starting to move. It is a variable force, increasing until it reaches its maximum value (Fs(max)), at which point the object begins to move. Fs ≤ μsN, where μs is the coefficient of static friction and N is the normal force.

Kinetic Friction (Fk): The force that opposes the motion of an object that is already moving. It is generally less than static friction. Fk = μkN, where μk is the coefficient of kinetic friction and N is the normal force.

Rolling Friction (Fr): The force that opposes the rolling motion of an object over a surface. It is usually much smaller than static or kinetic friction, especially when using ball bearings or roller bearings. Coefficient of Friction (μ): The coefficient of friction is a dimensionless number that represents the ratio of the frictional force to the normal force. It indicates the relative "stickiness" between two surfaces. A higher coefficient of friction means a greater force is required to overcome the friction.

Formulae: Fs ≤ μsN Fk = μkN where: Fs = Static Friction Fk = Kinetic Friction μs = Coefficient of Static Friction μk = Coefficient of Kinetic Friction N = Normal Force (the force pressing the surfaces together) 2.2 Lubrication: Lubrication is the process of reducing friction between moving surfaces by introducing a lubricant. This reduces wear, heat generation, and energy loss.

Principles of Lubrication: The basic principle is to create a thin film of lubricant between the surfaces, preventing direct contact.

This film can be: Fluid Film Lubrication: A thick film of lubricant completely separates the surfaces. This is the ideal scenario, resulting in very low friction and wear. Examples include oil-lubricated bearings in engines.

Boundary Lubrication: The film of lubricant is very thin, and some direct contact between the surfaces occurs. This happens at high loads or low speeds. Additives in the lubricant provide some protection against wear.

Mixed Film Lubrication: A combination of fluid film and boundary lubrication.

Types of Lubricants: Oils: Most common lubricant, available in various viscosities and with different additives. Mineral oils are derived from petroleum, while synthetic oils are chemically manufactured. In South Africa, locally produced mineral oils are often more cost-effective.

Greases: Semi-solid lubricants consisting of oil thickened with a soap or other thickener. Used where oil would leak out or for infrequent lubrication.

Solid Lubricants: Solid materials like graphite, molybdenum disulfide, and Teflon. Used in extreme conditions (high temperature, high pressure) where liquid lubricants are unsuitable. Examples include graphite lubricants used in some mining applications.

Gases: Air or other gases used as lubricants in specialized applications.

Methods of Applying Lubricants: Manual Lubrication: Applying lubricant by hand using an oil can or grease gun. Common in older or simpler machinery.

Drip Feed Lubrication: Oil is dripped onto the moving parts at a controlled rate.

Splash Lubrication: The moving parts dip into a reservoir of oil, splashing it around to lubricate other components. Common in gearboxes.

Pressure Feed Lubrication: Oil is pumped under pressure to the moving parts, ensuring a constant supply of lubricant. Used in engines and other critical components.

Oil Bath Lubrication: Components are submerged in an oil bath.

Grease Fittings (Zerk fittings): Allow grease to be injected into bearings using a grease gun. Advantages and Disadvantages of Lubrication Methods: | Method | Advantages | Disadvantages | | ---------------- | ------------------------------------------ | ----------------------------------------- | | Manual | Simple, low cost | Inconsistent, requires frequent attention | | Drip Feed | Controlled application | Can be messy, susceptible to contamination | | Splash | Simple, self-regulating | Not suitable for high speeds or loads | | Pressure Feed | Reliable, constant supply, good cooling | More complex, higher cost | | Oil Bath | Excellent lubrication and cooling | Bulky, potential for leaks | | Grease Fittings | Convenient, protects against contamination | Requires regular maintenance | Example 1: Calculating Friction Force A steel crate weighing 500 N rests on a concrete floor. The coefficient of static friction between steel and concrete is 0.6, and the coefficient of kinetic friction is 0.4. a) What force is required to start moving the crate? b) What force is required to keep the crate moving at a constant speed?

Solution: a) To start the crate moving, we need to overcome static friction.