Surfaces may look smooth, but on a microscopic scale, they are rough. When two surfaces are pressed together, contact is made at the peaks of the roughness or asperities. The real area of contact can be much less than the apparent or nominal area. At the points of intimate contact, adhesion or even local welding can take place. If we want to slide one surface over the other then we have to apply a force to break those junctions. (470)
Task 2. Answer the question: Is friction desirable or undesirable in life?
The Force of Friction
Friction force is the resistance encountered when one body moves relative to another body with which it is in contact. Static friction force is how hard something must be pushed to make it move, whilst dynamic friction force is how hard it must be pushed to maintainmotion. The ratio of the frictional force F to the normal force P is called the coefficient of friction.
Usually we want low friction (in a car engine, for example) not to waste excessive energy getting it moving. But in some cases we need high friction, e.g. in brakes. Friction is also important for car tires to grip the road and between shoes and the ground in walking (640)
Task 3. Write a summary in two sentences:
Keeping the Surfaces Apart - Lubrication
If we put a layer of oil between two surfaces, then we can separate them and easily slide one over the other with reduced friction and wear. Mineral oils are the most common lubricants, but other low shear strength materials are also used, such as graphite, teflon, and soft metals like lead or gold.
The selection of the best lubricant and understanding the mechanism by which it acts to separate surfaces in a bearing or other machine components is a major issue for study in tribology. (520)
Task 4. Write an abstract in Russian (3-4 sentences):
When Things Wear Out
If one surface slides over another, the asperities come into contact and there is a possibility that wear can occur. The breaking of all these little junctions can cause material removal (adhesive wear), or the asperities of a hard surface can plough grooves in a soft surface (abrasive wear).
Wear is usually unwelcome; it leads to increased clearances between moving components, increased mechanical load and even fatigue. But in grinding and polishing processes the high wear is desirable.
In addition to adhesive and abrasive wear, there are other mechanisms whereby material can be removed from a surface. Erosive wear occurs when particles (or even water droplets) strike a surface and break off a bit of the material. Hard particles can become trapped in contact surfaces areas and cause the material to be removed from one or both of the surfaces. One of the main reasons for frequent change of the car engine oil is that it becomes contaminated withhard debris particles that can wear out the engine components. (1040)
Home task:
1. To prepare for a dictation.
2. To write in English the definitions of 10 terms: damage; destruction; disruption; asperities; area of close contact; friction force; to maintain; most common lubricants; fatigue; hard debris particles
WEEK 5
Dictation - 25 min. (+10 min. for check)
Text 4
LUBRICATION (Part 1)