User:Chela5808/My sandbox Sam5
|Before Styling||Sample 1||Sample 2||Sample 3||Sample 4||Sample 5||Sample 6|
|After Styling||Sample 1||Sample 2||Sample 3||Sample 4||Sample 5||Sample 6|
OUTPUT (Before any styling by hand)
Created from PDF via Acrobat SaveAsXMLMapping table version: 28-February-20030.2 The wheel
Wheels, in one form or another, have also been in use for many thousands of years. In this book, we met them ﬁrst in thinking about pushing and pulling things, using some kind of cart: without the cart, and its wheels, you’d have to drag everything you wanted to move – so the invention of the wheel was an enormous step forward. Now we know about friction it’s clear that the wheel makes it easier to move things by getting rid (almost completely) of the forces called into play when things rub together: if you try to drag a heavy box over rough ground it may be impossible, but if you put wheels on it it will run smoothly. Until a few hundred years ago this was one of the most important uses of the wheel; another one being that it made it easier also to rotate heavy objects, like a heap of wet clay, by putting them onto a horizontal wheel, or table, with the axis pointing vertically upwards. The “potter’s wheel” has been in use for probably two or three thousand years in the production of urns and pots of all kinds.
The next big advance was in the use of wheels for moving water in countries where it hardly ever rains and water is very precious. Nothing can grow without water; and, even if there is a short rainy season, the water soon runs away unless you can get it out of the river and up onto the land, where it’s needed for growing crops. How to do it is the problem of irrigation. And if you have to move water you need energy. One way of solving both problems came with the development of the water wheel.
The ﬁrst wheels of this kind probably came from Egypt, where they were in use over a thousand years ago: they came to be known as “noria” wheels and in some places they can still be found. At Hama, for example, in Syria there are some giant wheels (20 m or more in diameter!) which have been running continuously for hundreds of years, using water power to lift water from the River Euphrates and supply it to aqueducts, which carry it a long way to irrigate the ﬁelds.
First, let’s look at a simple water wheel (Fig.43) of the kind that was widely used in some countries during the Industrial Revolution – when people started working with machines, in factories, instead of depending on their own muscles. Fig.43 shows a wheel of the type
used in driving a heavy ‘millstone’ for milling grain to make ﬂour for bread. They were also once used for driving mechanical hammers in the steel industry (but more of that later).
The ‘water supply’ comes from a point ‘upstream’ on the river, where the water level is higher; it comes to the mill wheel along an open pipe or channel (at the top in the Figure) and falls onto the specially made ‘boxes’ around the edge of the wheel. As the boxes ﬁll with water, their weight produces the torque that turns the wheel. But as they go down the water spills out; and ﬁnally it goes back into the river as ‘waste water’ – having done its work.
The giant noria wheels at Hama are very similar in design, but do exactly the opposite job: they take water from the river (at low level), by scooping it up in the ‘buckets’ ﬁxed around the rim of the wheel, and then emptying them into the aqueduct (at high level) when they reach the top. A wheel of this kind is shown Fig.44, where you see it from the edge, which lies in the vertical plane with the axle horizontal. The river, shown in blue, is ﬂowing away from you and the top edge of the big wheel is coming down towards you.
Because a lot of energy is needed to lift all that water, the wheel needs power to drive it; but, if there’s plenty of fast-ﬂowing water in the river, some of it can be used to turn a much smaller wheel like the one in Fig.44 and this can provide the power. How to get the energy from one wheel to the other is a problem in power transmission, which we’ll think about next.
Suppose we have two wheels, one big and one small, and want to make one drive the other. The simplest way of doing it is to put them side by side in the same plane, each with its own axle, which supports the wheel and provides the axis around which it can turn; and then to tie them together with a loop of rope or a ‘belt’ – as in Fig.45 (below). Each axle must be carried by a pair of ‘bearings’, to hold it in the right place, and the belt must be kept tight so that it doesn’t slip when the wheels are turning.
(a) (b) Figure 45
Note that the wheels don’t have to be close together; but if they are (Fig 45a) then they mustn’t touch. That’s because (as the red arrows show) the two wheels turn in the same sense (clockwise or anticlockwise), so the parts that come closest are going in opposite directions; and if they were rubbing together the friction would slow them down – or even stop them.