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4.4. Nuts and Bolts
Nuts and bolts are such simple and well-understood things that the term is often used to mean ‘the normal little bits and pieces of the problem.’ And yet, by thinking differently about them, we can challenge our normal mental model of nuts and bolts and rebuild it with more useful detail.
Firstly, when we consider the purpose or function of nuts, bolts or any other type of fastener, we will find that it varies from job to job. Engineers may talk about the purpose of nuts and bolts as being to hold, retain or contain things, a language which suggests keeping things the same. When we examine the situations in more detail we find that the world is a much more dynamic place and that we want our designs to make things do things not keep them the same.
How is this so? If you bolt two parts together so that a liquid or gas cannot escape, then the function is to seal. If you are keeping things together, then you have a joining function. And when the overall structure is solid, you have a strength-creating function. Let us consider each of these in more detail.
Like the screw thread on the bottle, the screw thread between a nut and bolt is a way of delivering pressure, although you might reasonably be asking now, ‘Why do I want the pressure?’
Suppose you want to seal in a gas. So you put two metal plates on to each other and think about how to create a pressure between the plates so that the gas does not leak. If one plate starts off sitting cleanly on the other place, the pressure is evenly spread. You then put in lots of bolts and start to turn them, and suddenly the pressure is not even! Even if you put in a million bolts, as you tighten them up, the pressure will be uneven.
But why are you creating uneven pressure when you wanted even pressure? With a bolt and nut we are pushing locally in one place so we get a reaction in another. To create a better design we should think about what is happening everywhere across the surfaces.
In an injection-moulding machine (Fig. 4.5) a liquid is injected which then hardens. But before it hardens it might leak between the plates. What is needed is to create pressure around the joints so that the liquid is sealed in while hardening takes places. You may well have come across many plastic parts where some leakage still exists despite the attempt to cut it away after manufacture.
Fig. 4.5 Injection sealant leakage
So where do you want the pressure? This is where thinking about molecules comes in useful. The metal molecules around the edge of the shape you are making need to come together at high pressure so that the liquid does not leak. But in many injection-moulding machine the plates are flat. Molecules along the flat plates mostly push other molecules apart. You want pressure only where the plates need to seal. Not at other points. To deliver this you need to consider the shape of the plates and how pushing them together creates the pressure profile you need, as in Fig. 4.6.
Fig. 4.6 Preventing unwanted sealant leakage
Consider the simple problem of bolting a road wheel to a car. The first question again is to ask the question ‘Why?’ to discover the function of this attachment. Most people will first answer that this is to hold the wheel on, but if this were all that were necessary, then we could weld or rivet the wheel on, although this would be a problem when we want to change the tyre.
Going back to basics and thinking about energy, force, matter, space and time, we can reframe the problem as one of time. Sometimes I want the wheel and car to stay together, sometimes to come apart. Time is particularly important for racing cars doing a pit stop, so they use a single nut that allows a wheel to be changed in a few seconds. Thinking about force, a tapered fit and single nut also overcomes the problem of distortion that came up in the previous section on sealing, although it also increases the risk that if the one nut fails, the wheel falls off with potentially fatal consequences. To try and balance these two problems, some manufacturers use three or five nuts.
Thinking differently again, what happens when the car is going around a corner? The forces will tend to slide the car, distort the tyre and apply a shearing force to the wheel connection. The problem now could be viewed as one of preventing the car from sliding sideways. Perhaps we could use the method of joining the wheel to the car to supplement the steering geometry, maybe tilting the wheel to compensate for the lateral forces.
A different view is of how the car is joined to the road, enabling the energy in the engine to be transformed into forward motion. This is prevented sometimes when the wheel lifts from the road. So perhaps we could also combine the joining of the wheel to the car with a suspension function. We might thus design a nut and bolt system which does not transfer small movements only big movements. Can you think of ways to do this?
Other sections in this chapter: