component. Don’t forget to do a bit of research on that before you come. Thanks. See you soon.
Unit 3, Lesson 3.4, Exercise B ≤1.18
Part 1 In the last lecture we talked about the stress in a mechanical component due to the forces applied to it. The amount of stress in a component depends on five factors, including the way the force is applied, and, of course, the material itself.
Let’s concentrate today on the way the force is applied. The directions of forces all break down into five types: tension, compression, shear, torque and bending.
Unit 3, Lesson 3.4, Exercise C ≤1.19
Part 2 Tension is the pulling force which acts along the length of the component. In other words, the movement is outwards or away from the load. The cable of a crane lifting a load is an example of this. Compression, on the other hand, is the opposite. It is a pushing force. The movement is inwards, towards the load. Think of press, as in a table vice. Both these forces are uniaxial – that is, the action and reaction act along the same axis, as shown in slides 1 and 6.
When a component is under shear, however, although there are still two equal and opposite forces, they are parallel, not uniaxial. Think of a pair of wire cutters. Another good example of a shearing force is a rivet joining two plates, as in slide 2. The rivet is being pulled two ways and, as a result, is the weakest point of the component. Failure at the rivets is common.
Torque is the twisting force. The simplest example
of this is a key being turned in a lock. Or a screwdriver turning in the slot of a screw. An example of a large component under torque is the driveshaft of a machine.
Bending occurs when the load action and reaction are further apart. We've already talked about a crane. Look again at the jib – that is, the arm of the crane in slide 1. One end is supported by the structure. A load is suspended from the other end. The natural effect of this is for the jib to bend – to curve down. And it is only the reaction in the structure and the strength of the material which prevents it. This kind of structure – that is, held at one end only – is called a cantilever.
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Many sports stadiums and car parks are covered by a cantilever roof.
Alternatively, take a beam held at either end, but supporting a load halfway along – for example, a truck parked halfway across a small bridge. The deformation caused by this force would be a downward curve. We can see this more clearly in slide 5. When a component is under a bending force, the outside of the curve is under tension, but the inside is under compression.
Unit 3, Lesson 3.4, Exercise D ≤1.20
Part 3 Of course, these different forces don’t act alone. A machine will normally have several forces acting in different ways, but as long as it is well designed, it will be in equilibrium and will work efficiently. Let’s look at one very simple example from the workshop. The table vice. You know how a table vice works – let’s have a look at slide 6 now. You turn the handle and the screw thread pulls one set of jaws in to grip the work – a piece of wood, for example. When you turn the handle, you are applying a bending force, but the handle is applying torque to the screw, which in turn applies compression to the work in the vice. If you then saw a piece of the work off, this is a shearing force.
Unit 3, Lesson 3.4, Exercise E ≤1.21
Part 4 So, to recap, we are looking at stress in components. So far, we have seen the different ways that force can be applied. In industry today, there is much research into methods of processing metals, composite materials and alloys to produce materials which react well to different types of force, so that the mechanical engineer can find the right material for the job in hand.
Unit 5, Lesson 5.2, Exercise B ≤1.22
Part 1 Good afternoon, everyone. In this series of lectures, we’re going to be looking at two topics: micro-electromechanical systems, called MEMS for short, and nanotechnology. MEMS and nanotechnology are popular topics for research at present. In today’s lecture, I’ll give you a quick overview of both topics. Of course, you’ll be going over everything in more detail in your assignments and project exercises.
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