Unit 1, Lesson 1.4, Exercise E ≤1.10
Lecture 5 OK. So let’s consider mechanical science first. What are the basic principles of mechanical science? There are seven main areas.
Firstly, dynamics – that’s D-Y-N-A-M-I-C-S – which
is the relationship between forces and motion. So, for example, in order to make a gate open and close automatically, you need to understand dynamics. Vibration is another important area of dynamics, because vibration can destroy a machine.
Secondly, there’s automatic control. Mechanical engineering is often concerned with automating something and controlling it automatically. For example, we can control the gear ratio of a car automatically.
Thirdly, there’s thermodynamics – that’s T-H-E-R- M-O-dynamics. I expect you all know that thermo comes from a Greek word for heat, so thermodynamics is concerned with the relationship between heat, energy and power.
Next, we have fluid flow. When does a mechanical engineer have to worry about fluid flow? Well, there are fluids in most machines – fuel, cooling fluids, lubricants. There are basic principles which tell you how fluid will behave under different circumstances. For example, the Venturi effect dictates that fluid will flow faster through a constricted space. We can see this in a river, for instance, where it narrows.
Fluids are often involved in the next area, too – heat transfer. How do we cool a machine, for example, or heat a room?
I mentioned lubricants just now. Lubrication is the next area. When machines are working, different parts are rubbing together, and these moving surfaces must be lubricated in some way, otherwise the machine will overheat and the part will seize – that means, stick together.
Finally, but perhaps the most important area of
all, mechanical engineers must know the properties of different materials. They must know if a particular material is hard or soft, brittle or malleable, a conductor or an insulator.
Unit 3, Lesson 3.2, Exercise B ≤1.11
Part 1 OK. Is everybody here? Right, let’s start. What I’m going to talk about today is materials in engineering. As you know, when a machine is
116
designed, one of the most critical factors that mechanical engineers have to decide on is the materials to be used. Different materials – steel, aluminium, copper, etc., – have different physical characteristics. For example, aluminium may be the best for the body of a plane, but impossible for the engines. If the wrong material is chosen, the machine will be very expensive due to overengineering, or it could fracture due to underengineering. So we’re going to look at the way that different materials react to stress. Every component in a machine undergoes stress of one kind or another. So, choosing the right materials for the components is fundamental to good design. But before we look at the reactions of materials, let’s talk about the features of stress.
Unit 3, Lesson 3.2, Exercise C ≤1.12
Part 2 When an external force acts on a machine component, internal forces in the component react against it. The rule says that for any force there is an equal and opposite reaction. This is Newton’s third law of motion. When you press against a wall, your hand doesn’t go through the wall because the wall is pushing it back. Equal and opposite reaction. When the opposite forces are equal, the component is said to be in equilibrium, and this is the ideal state. This internal reaction is called stress.
You may have experienced this in driving. If you take a corner fast, although you’re turning right, the car feels as if it wants to go left. All is well, however, unless you have misjudged your speed and overstressed your car, then the car does indeed go left.
Similarly, in a machine, as the load increases, the component rearranges itself to support the load. It is deformed – that is, it changes its shape. A simple example: if you sit on a beach ball, it changes its shape. It flattens. When you stand up, it becomes spherical again. If you pull a rubber band, it gets thinner until you release it. This is what happens inside any material under stress – it is deformed. Sometimes we can’t see it, but it is happening.
Unit 3, Lesson 3.2, Exercise D ≤1.13
Part 3 OK – we need a few definitions now … The deformation we talked about is known as strain. Strain is normal in all materials at work, and, of course, when the load is removed, the stress and
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134
