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附录AHow Car Steering Worksby Karim NiceYou know that when you turn the steering wheel in your car, the wheels turn. Cause and effect, right? But a lot of interesting stuff goes on between the steering wheel and the tires to make this happen.Up Next Car Suspension Quiz How Car Suspensions Work D: Rocket Shock Absorbers In this article, well see how the two most common types of car steering systems work: rack-and-pinion and recirculating-ball steering. Then well examine power steering and find out about some interesting future developments in steering systems, driven mostly by the need to increase the fuel efficiency of cars. But first, lets see what you have to do turn a car. Its not quite as simple as you might think!When it comes to crucial automotive systems, steering is right up there with the engine and the brakes. The inner workings of this important component are pretty cool.Turning the CarYou might be surprised to learn that when you turn your car, your front wheels are not pointing in the same direction. For a car to turn smoothly, each wheel must follow a different circle. Since the inside wheel is following a circle with a smaller radius, it is actually making a tighter turn than the outside wheel. If you draw a line perpendicular to each wheel, the lines will intersect at the center point of the turn. The geometry of the steering linkage makes the inside wheel turn more than the outside wheel. There are a couple different types of steering gears. The most common are rack-and-pinion and recirculating ball. Rack-and-pinion SteeringRack-and-pinion steering is quickly becoming the most common type of steering on cars, small trucks and SUVs. It is actually a pretty simple mechanism. A rack-and-pinion gearset is enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack.The pinion gear is attached to the steering shaft. When you turn the steering wheel, the gear spins, moving the rack. The tie rod at each end of the rack connects to the steering arm on the spindle (see diagram above). The rack-and-pinion gearset does two things: It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels. It provides a gear reduction, making it easier to turn the wheels. On most cars, it takes three to four complete revolutions of the steering wheel to make the wheels turn from lock to lock (from far left to far right). The steering ratio is the ratio of how far you turn the steering wheel to how far the wheels turn. For instance, if one complete revolution (360 degrees) of the steering wheel results in the wheels of the car turning 20 degrees, then the steering ratio is 360 divided by 20, or 18:1. A higher ratio means that you have to turn the steering wheel more to get the wheels to turn a given distance. However, less effort is required because of the higher gear ratio. Generally, lighter, sportier cars have lower steering ratios than larger cars and trucks. The lower ratio gives the steering a quicker response - you dont have to turn the steering wheel as much to get the wheels to turn a given distance - which is a desirable trait in sports cars. These smaller cars are light enough that even with the lower ratio, the effort required to turn the steering wheel is not excessive. Some cars have variable-ratio steering, which uses a rack-and-pinion gearset that has a different tooth pitch (number of teeth per inch) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheels turning limits. Power Rack-and-pinionWhen the rack-and-pinion is in a power-steering system, the rack has a slightly different design. Part of the rack contains a cylinder with a piston in the middle. The piston is connected to the rack. There are two fluid ports, one on either side of the piston. Supplying higher-pressure fluid to one side of the piston forces the piston to move, which in turn moves the rack, providing the power assist. Well check out the components that provide the high-pressure fluid, as well as decide which side of the rack to supply it to, later in the article. First, lets take a look at another type of steering. Recirculating-ball SteeringRecirculating-ball steering is used on many trucks and SUVs today. The linkage that turns the wheels is slightly different than on a rack-and-pinion system. The recirculating-ball steering gear contains a worm gear. You can image the gear in two parts. The first part is a block of metal with a threaded hole in it. This block has gear teeth cut into the outside of it, which engage a gear that moves the pitman arm (see diagram above). The steering wheel connects to a threaded rod, similar to a bolt, that sticks into the hole in the block. When the steering wheel turns, it turns the bolt. Instead of twisting further into the block the way a regular bolt would, this bolt is held fixed so that when it spins, it moves the block, which moves the gear that turns the wheels. Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball bearings that recirculate through the gear as it turns. The balls actually serve two purposes: First, they reduce friction and wear in the gear; second, they reduce slop in the gear. Slop would be felt when you change the direction of the steering wheel - without the balls in the steering gear, the teeth would come out of contact with each other for a moment, making the steering wheel feel loose. Power steering in a recirculating-ball system works similarly to a rack-and-pinion system. Assist is provided by supplying higher-pressure fluid to one side of the block. Now lets take a look at the other components that make up a power-steering system. Power SteeringThere are a couple of key components in power steering in addition to the rack-and-pinion or recirculating-ball mechanism. PumpThe hydraulic power for the steering is provided by a rotary-vane pump (see diagram below). This pump is driven by the cars engine via a belt and pulley. It contains a set of retractable vanes that spin inside an oval chamber. As the vanes spin, they pull hydraulic fluid from the return line at low pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the cars engine speed. The pump must be designed to provide adequate flow when the engine is idling. As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds. The pump contains a pressure-relief valve to make sure that the pressure does not get too high, especially at high engine speeds when so much fluid is being pumped. 附录B汽车转向系统工作原路介绍 Karim Nice 著显而易见，当你坐在车里转动方向盘时。车轮会跟着运动。像一对因果关系，是吧？但是其中是有很多令人感兴趣的部件在方向盘和轮胎之间运动才构成了我们以上看的原因和结果。在这篇文章中，我们将看到两种最常见的转向系统是如何工作的：齿轮齿条式转向器和循环球式转向器。然后我们会研究一下动力转向并发现些令人欣喜的转向系统未来发展动向，其中大部分是由提高汽车燃油效率驱使产生的。但是首先，让我们看看你想让一辆汽车转向都需要做些什么。其中的过程可能并不像你想象的那么简单。当说到汽车行驶的关键系统时，转向系统当然地成为和发动机系统，制动系统处于同样重要的地位。这个至关重要的部分内部的工作过程也是相当激动人心的。汽车的转向当你控制汽车转弯的时候你车的前轮并没有只指向同一个方向，在了解到这些的时候你可能会很奇怪。要想让汽车转向平顺，每个车轮就必须沿着不同的轨迹运动。因为内侧轮胎是沿着较小半径的圆周轨迹运动的。如果沿着每个车轮做一条垂直线，这些线会相交于转向轨迹的中心点。转向系统联动装置的几何学特性使得内侧车轮转向角度比外侧车轮大些。通常是有好几种不同类型的转向齿轮。最常见的就是齿轮齿条式和循环球式。齿轮齿条式转向器齿轮齿条式转向器迅速成为轿车，小型卡车以及多功能越野车转向器中最普遍的型式。它确实是一种比较简单的机构。一套出轮齿条啮合装置被封装在一根金属管子里，齿条分别从管子末端深处。有根干，叫做转向拉杆，分别连在管架的末端。齿轮齿条转向器的齿轮是连在转向轴上的。当转动方向盘时，齿轮转动推动齿条移动。齿条末端的横拉杆连接于转向节上的转向臂上。齿轮齿条转系机构做完成两件事：它将方向盘的转动转化成转动车轮所需要的直线运动。在大多数汽车上，一般需要转动三到四圈方向盘才能使车轮从左止点到右止点。转向系传动比是指转动方向盘角度和车轮转动角度的比率。具体说就是，如果转动方向盘一周车轮随之转动二十度，实际上转向传动比是360除以20，也就是18：1。跟高的转向传动比意味着你需要更大的方向盘转角才能达到同样的车轮转角。当然，高传动比也意味着更小的力量。大体说来，质量小，更为运动型的汽车相比大型轿车和卡车拥有更小的转向比。小传动比意味着更快的转向反应-你无需再费力的转动方向盘才能达到指定的车轮转角这就是跑车所要求的理想特性。这些小型汽车可以用更小的转向比，因为在质量上足够轻，转动车轮所需的转向力并没超过要求。一部分汽车使用可变转向比，它使用一种在中间和两边具有不同的齿间距的齿轮齿条啮合装置。这使得汽车在刚开始转弯后能迅速做出反应（齿条在中间位置附近），同时也降低了转向力限制位置时的转向力。动力齿轮齿条转向系统当齿轮齿条在动力转向系统中时，齿条的设计略有不同。齿条中间位置包含有一个气缸与活塞。活塞连接到齿条上。在活塞两端各有一个液压缸。在活塞的一端提供高压油液以推动活塞移动，继而推动齿条移动，提供转向助力。在接下来的段落里，我们将详细了解一下提供高压油液的组件，然后决定向齿条的哪一方提供高压油液。首先，让我们来看看另一种类型的转向器。循环球转向现在循许多卡车和SUV使用的是循环球转向器。它使车轮转动的联动装置与齿轮齿条转向系统略有不同。循环球转向机构内包含有一个蜗轮。您可以把这个齿轮想象成两部分。第一部分是一块带有内螺纹孔的金属块。这个金属块外侧有切好的齿形，齿形是专门用来啮合一个使转向拉杆移动的齿轮。方向盘连接到螺纹杆上，类似于一个连接到金属块上的螺杆。当方向盘转动时它推动螺杆运动。与一般的螺杆随着旋入螺母的加深不同，这种螺杆在旋转时是固定不动的，并推动螺母移动，螺母使啮合的齿轮转动最终转动车轮。与螺杆直接啮合转向螺母不同，所有啮合螺纹都充满了滚珠球轴承环绕着，齿轮啮合副转动时能绕着螺纹圆周转动的钢球。钢球实际上两个功能：首先，它们减少齿轮啮合副的摩擦和磨损；第二，它们减小齿间间隙。当改变向方向盘转动方向的时候你就会感觉间隙，转向时好像感觉不到钢球，齿型将脱离彼此接触了一会儿，使方向盘感觉松旷。 动力转向的循环球转向系统的运动方式类似于齿轮齿条系统。所提供的助力是高压力液体推动转向螺母的一侧产生的。现在让我们来看看动力转向系统中的其他组成部分。动力转向无论循环球转向器还是齿轮齿条转向器的动力转向系统中都有几个重要组成部分。泵液压动力转向是由旋转叶片泵提供的（如下图） 。这种泵的动力是汽车的发动机通过皮带和带轮驱动的。它包含了一套可移动的叶片，附带一个椭圆形的内腔。随着叶片旋转，叶片从回油道中吸进低压油并将其变成高压油挤压出去，并迫使它变成出口高压。泵所提供的油液总量取决于轿车的引擎转速。该泵的设计必须使发动机空转时也能提供充足的液体。因此，在发动机以更高的转速运行时该泵产生的高压油液超过正常需要。泵包含一个压力安全阀，以确保压力不会太高，尤其是在发动机转速高时，产生大量的高压油液。
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