Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define friction and differentiate between its types.
• Explain the causes of friction.
• Describe different types of lubricants and their uses.
• Explain the principles and regimes of lubrication.
• Identify methods to reduce friction.

Lesson summary

Friction and lubrication are fundamental concepts in Mechanical Technology. Understanding them is crucial for designing, maintaining, and operating machines and mechanical systems efficiently and reliably. In South Africa, industries like mining, manufacturing, transportation, and agriculture rely heavily on machinery. Effective lubrication and friction management translate directly into cost savings through reduced wear and tear, less downtime, and improved energy efficiency, contributing to economic growth and job creation.

Furthermore, proper understanding reduces environmental impact through reduced material waste and energy consumption.

Lesson notes

2.1 What is Friction? Friction is a force that opposes motion between two surfaces in contact. It's a resistive force that converts kinetic energy into heat. Friction is present in almost all mechanical systems and significantly impacts their performance and lifespan. Without friction, we wouldn't be able to walk, drive a car, or even hold a pen!

Static Friction (Fs): The force that prevents an object from starting to move. It's a variable force that increases up to a maximum value before the object begins to slide. The maximum static friction force is given by: Fs(max) = μs N, where μs is the coefficient of static friction and N is the normal force (the force pressing the two surfaces together).

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

The kinetic friction force is given by: Fk = μk N, where μk is the coefficient of kinetic friction and N is the normal force. 2.2 Causes of Friction: At a microscopic level, even seemingly smooth surfaces have irregularities (asperities). When two surfaces are pressed together, these asperities interlock, creating resistance to motion. The interlocking of asperities is the primary cause of friction. Adhesive forces (molecular attraction) between the surfaces also contribute, especially with clean and smooth surfaces.

The amount of friction depends on: Type of materials: Different materials have different coefficients of friction. For example, rubber on asphalt has a high coefficient of friction, while steel on ice has a low coefficient.

Surface finish: Rougher surfaces have higher friction than smoother surfaces.

Normal force: Increasing the force pressing the surfaces together increases friction.

Temperature: Higher temperatures can sometimes reduce friction (e.g., in some polymers).

Presence of lubricants: Lubricants drastically reduce friction by separating the surfaces. 2.3 Coefficient of Friction (μ): The coefficient of friction (μ) is a dimensionless number that represents the ratio of the frictional force to the normal force. It is a measure of the "stickiness" between two surfaces. μs is the coefficient of static friction, and μk is the coefficient of kinetic friction. 2.4 Types of Lubricants: Lubricants are substances used to reduce friction between moving surfaces. They form a thin film between the surfaces, preventing direct contact and reducing wear and tear.

Oils: Liquid lubricants. Commonly petroleum-based, but synthetic oils are also available. Oils are good for high-speed applications and can provide cooling.

Examples: Engine oil, hydraulic oil, cutting oil.

Advantages:* Good cooling properties, readily available, relatively inexpensive.

Disadvantages:* Can leak, may be affected by temperature changes, require careful selection for specific applications.

Greases: Semi-solid lubricants. Oils thickened with a soap or other thickening agent. Greases are better for slow-speed, high-load applications where leakage is a concern.

Examples: Wheel bearing grease, chassis grease.

Advantages:* Stays in place, seals out contaminants, long-lasting.

Disadvantages:* Poorer cooling properties than oils, more difficult to apply and remove.

Solid Lubricants: Solid materials used as lubricants. Good for extreme temperatures or pressures where liquids cannot be used.

Examples: Graphite, molybdenum disulfide (MoS2), PTFE (Teflon).

Advantages:* Can withstand extreme conditions, doesn't attract dirt, suitable for dry applications.

Disadvantages:* Lower load-carrying capacity compared to oils or greases, can be expensive. 2.5 Lubrication Regimes: The effectiveness of lubrication depends on the lubrication regime, which describes the extent to which the lubricant separates the surfaces.

Boundary Lubrication: The surfaces are only partially separated by a thin layer of lubricant. Significant asperity contact occurs. This happens at startup, shutdown, or under high loads and low speeds.

Mixed Lubrication: A combination of boundary lubrication and hydrodynamic lubrication. Some asperity contact occurs, but a partial fluid film provides some separation.

Hydrodynamic Lubrication: The surfaces are completely separated by a thick film of lubricant. The pressure in the fluid film is generated by the relative motion of the surfaces. This is ideal for minimizing wear.

Elasto-hydrodynamic Lubrication (EHL): Similar to hydrodynamic lubrication, but considers the elastic deformation of the surfaces under high pressure. Important for gears and rolling element bearings. 2.6 Methods to Reduce Friction: Lubrication: The most common method. Using oil, grease, or solid lubricants to create a film between moving surfaces.

Material Selection: Choosing materials with lower coefficients of friction. For example, using bronze bushings instead of steel bushings.

Surface Finish: Polishing or grinding surfaces to reduce roughness.