简单机械和刀具

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

第二章

课程A  什么是简单机械？

    简单机械是那些能够把能量从一个地方迁移到另一个地方的设备，这些机械使我们的生活变得更加容易。

    在许多人看来，"机械"是指诸如拖拉机、电钻、推土机、缝纫机或自行车这样的东西。这些都是机械，但它们实际上是由许多简单机械构成的复杂设备。简单机械只有为数不多的几种，如杠杆、轮轴、斜面、齿轮、滑轮和液压传送装置等。

    简单机械能够做下列工作：

    能够把能量从一个地方迁移到另一个需要它的地方；

    能够改变力的大小和方向，某些机械使我们能做到用很小的力产生一个很大的力，即所谓力增益；

    能够改变运动物体的距离和速度，这被称作产生距离增益或速度增益。

    简单机械的典型例子就是杠杆。它在我们的生产实践中得到极其广泛的应用。像跷跷板、斧子、手推车、剪刀和锤子等设备都是简单机械，它们都是杠杆应用的具体实例。运用这些设备，使得那些原本难以完成的工作变得较为容易。

    杠杆是一根刚性棒，图1中的撬棍是一个简单杠杆的例子。所有杠杆都由以下部分组成：

    支点，是杠杆可围绕其转动的固定点；

    作用力，施加在杠杆上的力。有时称为输入力，或简称为力；

   力臂，从支点到作用力施加点之间的距离；

    重力，移动载荷的力。它是杠杆输出力，有时简称为载荷；

    重力臂，是从支点到载荷施加点之间的距离。

    借助手推车，我们只需用相当小的力就可以举起一个很重的东西，手推车可以画成如图1所示的一根刚性棒，轮轴即支点作用。可以清楚地看出，力臂比重力臂长，因此产生了力增益，因为重力大于所施加的作用力。但是，作用力运动的距离明显比重力要长。

    如果支点的位置使得重力臂比力臂长，那么就需要用很大的力去移动一个很小的载荷，但是却能移动得很远，这就是所谓的速度增益。钓鱼竿就利用了速度增益原理，钓鱼者必须用较大的力才能钓起很轻的载荷（鱼），但是他却可以以很快的速度把鱼拉上来。

    简单机械常采用双杠杆原理，如剪刀、老虎钳、胡桃钳、镊子等。

课程B  工具与机械

    每一个制造业的部门都必须使用工具来完成自己的任务。一般来说，工具和机器都用于加工材料或信息。在生产部门工作的工人们使用工具将材料加工成成品；财务部门使用计算器和计算机记载公司的财务状况。市场营销人员通过录像和录音设备（机器）将产品信息传达给消费者。因此，从事制造业的人员必须了解如何使用其本行业的工具。

工具和机器的定义

    工具扩展了人们加工（改变）材料或信息的能力。我们无法赤手空拳将钉子钉入木板，但用一种工具——锤子却很容易做到这一点。我们可以在头脑中计算数学问题（求解数学问题），但电子计算器却能算得更快、更准确（精确）。锤子和计算器都是扩展人的能力的工具。

    一般来说，工具可以描述为手动工具、电动手用工具，或机器。手动工具是最简单形式的工具。使用者手持工具，挥动工具来完成工作。使用者给手动工具施加动力，手据、螺丝刀、手动刨等都是手动工具的例子。电动工具是改进型的手动工具。使用者将这种工具握在手中使其移动（运动）来进行工作，但加工动力则来自外部能源，如一台电机。电动圆盘锯，电动螺丝刀，电刨等都是电动（手工）工具。在加工过程中，机器保持禁止，而使用一个安装在机器上带外部能源的工具来实行实际的加工。台式锯、钻床、刨床等都是这样的机器。

    另一类制造工具是设备。设备包括那些无法定义为机器、电动工具或手动工具的用具。设备在加工过程中稳定地固定在一个结构上，使用人力或热力完成对材料的加工。如人力剪切金属的人力电动削剪，烤炉和熔化材料的炉子等都属于设备。

六种基本机械

    我们经常用六种基本的机械来描述工具的工作原理。这六种基本机械（如图所示）分别为轮子、杠杆、滑轮、斜面、楔子和螺丝。这些基本机械的用途是用来在做工时获得机械增益。机械增益是指力的增加。力的机械增益缩写为MAF。往木头里钉钉子就是一个简单的例子。没有锤子，你就无法把钉子钉进去。把锤子拿在手里，你就能形成一个杠杆，使你得到一个对钉子的机械增益。钉子本身就利用自己的楔形尖打进木头中去。

    另一个例子是，看一下实验室里经常用到的压钻。钻头上的楔形尖是供钻孔使用的。斜面将钻头固定在卡盘上，有螺丝将压钻固定紧。滑轮将能量从电机上传送到钻头上。压钻的手柄就起着一个与轮和轴相连的杠杆的作用。这六种基本机械中的任何一种每一次用作工具或机械使用时都能实现机械增益。你还可到你的实验室去认识一下这六种基本机械在其他工具或机械中的体现。