2 Simple Machines and Tool
A What Do the Simple Machines Mean?
Simple machines are devices which allow energy to be transferred from one place to another. With the help of machines our lives are made much easier.
To many people the word "machine" means things like a tractor, an electric drill, a bulldozer, a sewing machine or a bicycle. These are machines, but they are really very complicated ones, such as ones made up of many simple machines. There are only a few kinds of simple machines. They are the lever, the wheel and axle, the inclined plane, gears, pulleys and hydraulics.
Simple machines can do the following:
They allow energy to be transferred from the place where it is available to the place where it is used.
They can change the size and direction of force. Certain types of machines allow us to apply a very large force to something by using a small force. This is called a force advantage.
They can change the distance and speed with which something is moving. This is called giving a distance or speed advantage.
A typical example of simple machines is the lever, which has found extremely wide use in our production practice. Some other simple machines can be seen like a seesaw, an axe, a wheelbarrow, a pair of scissors and a hammer. These are examples of levers. By using these, tasks which would be difficult for you to do can be done more easily.
A lever is a rigid bar. The crowbar in Figure 1 is an example of a simple lever. All levers have the following parts:
1. The fulcrum is the fixed point around which the lever can turn.
2. The effort force is the force applied to the lever. It is sometimes called the input force or simply the effort.
3. The effort arm is the distance between the fulcrum and the point where the effort force is applied.
4. The load force is the force moving the load. It is the output force of the lever and is sometimes simply called the load.
5. The load arm is the distance from the fulcrum to the position of the load.
A wheelbarrow allows us to lift a heavy load by using a fairly small force. The wheelbarrow can be draw as a rigid bar as is shown in figure 2. the wheel axle acts as the fulcrum. It can easily be seen that the effort arm is longer than the load arm. This gives a force advantage because the load force is greater than the effort force that is applied. However, the effort force has to be moved much further than the load.
Fig.1 A crow-bar Fig.2 A wheelbarrow
If the fulcrum is placed so that the load arm is longer than the effort arm, a large force is needed to move a small load, but it moves the load a long way. This gives a speed advantage. This idea can be seen in the fishing rod. The large effort force applied by the fisherman moves only a small load, the fish. However itdoes allow the fisherman to drag the fish in quickly.
Often simple machines are made of double levers. Scissors, pliers, nutcrackers and tinsnips are all double levers.
double levers 双重杠杆
effort arm 力臂
effort force 作用力
force advantage 力增益
in a more convenient way 以某种较为方便的方式
inclined plane 斜面
load arm 重力臂
load force 荷载力（重力）
rigid bar 刚性杆
speed advantage 速度增益
input force 输入力 文中为作用力
output force 输出力 文中为载荷力或阻力
B Tools and Machines
Each department in manufacturing uses tools to do its job. In general, tools and machines process (change) materials or information. Production department workers use tools to change materials into finished products. The finance department uses calculators and computers to keep track of the company's finances. Marketing workers send product information to consumers through advertisements made with video and audio recording machines. Workers in manufacturing must know how to use the tools of their trade.
Defining tools and machines
Tools extend human abilities in doing the work of processing (changing) materials or information. So, strictly speaking, machines are also tools. Tools extend human abilities by increasing the power, speed, efficiency, accuracy, and productivity of work. We cannot drive nails in boards with our bare hands, but we can drive nails with a tool – the hammer. We can do math problems in our head, but an electronic calculator is faster and more accurate. Both the hammer and the calculator are tools that extend our abilities.
Generally, tools can be described as hand tools, power hand tools, or machines. A hand tool is the simplest form. The user holds it in the hand and moves it to perform work. It is powered only by the user. Hand saws, screwdrivers, and hand planes are examples of hand tools. Power hand tools are improved hand tools. The user holds one in the hand and moves it to perform work, but the processing power comes from an external source, such as an electric motor. Power circular saws, electric screwdrivers, and power planes are some power hand tools. Machines stay still during processing and use an externally powered tool that is fastened to the machine to do the actual processing. Table saws, drill presses, and planers are all machines.
Another category of manufacturing tools is equipment. Equipment covers devices that cannot be defined as machine, power hand tools, or hand tools. Equipment stays still on a structure during processing and uses human or thermal (heat) power too process materials. Examples include the human-powered squaring shears for shearing metal and ovens, and furnaces used to melt materials.
All tools, machines, and equipment extend human abilities by increasing the power, speed, efficiency, accuracy, and productivity of processing materials or information.
The six basic machines
We base the principles that describe how tools work on the basic machines – wheels, levers, pulleys, inclined planes, wedges, and screws, (see the fig.). The purpose of these basic machines is to gain a mechanical advantage in doing work. A mechanical advantage is an increase in a force. Mechanical advantage of force is abbreviated MAF. A simple example is driving nails in wood. Without a hammer, you would not be able to drive the nails. By placing the hammer in your hand, you create a lever that gives you a mechanical advantage of force over the nail. The nail itself uses wedge action to cut into the wood.
For anther example, look at the frill press, often found in labs. The drill bit uses a wedge for its cutting action. Inclined planes hold the drill bit in the chuck. Screw threads hold the drill press together. Pulleys transfer power from the motor to the drill bit. The handle on the drill press acts as a lever attached to a wheel and axle. Every time one of the six basic machines is used in a tool or machine, mechanical advantage is realized. Identify the six basic machines in other tools and machines in your lab.
drill bit 钻头
drill press 钻床
electric screwdriver 电动螺丝刀
finance department 财务部
marketing worker 销售人员
power circular saw 电动圆盘锯
power hand tool 电动工具
power plane 电刨
mechanical advantage 机械增益