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黑龍江工程學(xué)院本科生畢業(yè)設(shè)計
附錄A
How Car Steering Works
by Karim Nice
You 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.
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In this article, we'll see how the two most common types of c ar steering systems work: rack-and-pinion and recirculating-ball steering. Then we'll 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, let's see what you have to do turn a car. It's 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 Car
You 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 Steering
Rack-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 don't 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 wheel's turning limits.
Power Rack-and-pinion
When 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.
We'll 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, let's take a look at another type of steering.
Recirculating-ball Steering
Recirculating-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 let's take a look at the other components that make up a power-steering system.
Power Steering
There are a couple of key components in power steering in addition to the rack-and-pinion or recirculating-ball mechanism.
Pump
The hydraulic power for the steering is provided by a rotary-vane pump (see diagram below). This pump is driven by the car's 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 car's 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.
附錄B
汽車轉(zhuǎn)向系統(tǒng)工作原路介紹
Karim Nice 著
顯而易見，當(dāng)你坐在車?yán)镛D(zhuǎn)動方向盤時。車輪會跟著運動。像一對因果關(guān)系，是吧？但是其中是有很多令人感興趣的部件在方向盤和輪胎之間運動才構(gòu)成了我們以上看的原因和結(jié)果。
在這篇文章中，我們將看到兩種最常見的轉(zhuǎn)向系統(tǒng)是如何工作的：齒輪齒條式轉(zhuǎn)向器和循環(huán)球式轉(zhuǎn)向器。然后我們會研究一下動力轉(zhuǎn)向并發(fā)現(xiàn)些令人欣喜的轉(zhuǎn)向系統(tǒng)未來發(fā)展動向，其中大部分是由提高汽車燃油效率驅(qū)使產(chǎn)生的。但是首先，讓我們看看你想讓一輛汽車轉(zhuǎn)向都需要做些什么。其中的過程可能并不像你想象的那么簡單。
當(dāng)說到汽車行駛的關(guān)鍵系統(tǒng)時，轉(zhuǎn)向系統(tǒng)當(dāng)然地成為和發(fā)動機系統(tǒng)，制動系統(tǒng)處于同樣重要的地位。這個至關(guān)重要的部分內(nèi)部的工作過程也是相當(dāng)激動人心的。
汽車的轉(zhuǎn)向
當(dāng)你控制汽車轉(zhuǎn)彎的時候你車的前輪并沒有只指向同一個方向，在了解到這些的時候你可能會很奇怪。
要想讓汽車轉(zhuǎn)向平順，每個車輪就必須沿著不同的軌跡運動。因為內(nèi)側(cè)輪胎是沿著較小半徑的圓周軌跡運動的。如果沿著每個車輪做一條垂直線，這些線會相交于轉(zhuǎn)向軌跡的中心點。轉(zhuǎn)向系統(tǒng)聯(lián)動裝置的幾何學(xué)特性使得內(nèi)側(cè)車輪轉(zhuǎn)向角度比外側(cè)車輪大些。
通常是有好幾種不同類型的轉(zhuǎn)向齒輪。最常見的就是齒輪齒條式和循環(huán)球式。
齒輪齒條式轉(zhuǎn)向器
齒輪齒條式轉(zhuǎn)向器迅速成為轎車，小型卡車以及多功能越野車轉(zhuǎn)向器中最普遍的型式。
它確實是一種比較簡單的機構(gòu)。一套出輪齒條嚙合裝置被封裝在一根金屬管子里，齒條分別從管子末端深處。有根干，叫做轉(zhuǎn)向拉桿，分別連在管架的末端。
齒輪齒條轉(zhuǎn)向器的齒輪是連在轉(zhuǎn)向軸上的。當(dāng)轉(zhuǎn)動方向盤時，齒輪轉(zhuǎn)動推動齒條移動。齒條末端的橫拉桿連接于轉(zhuǎn)向節(jié)上的轉(zhuǎn)向臂上。
齒輪齒條轉(zhuǎn)系機構(gòu)做完成兩件事：
它將方向盤的轉(zhuǎn)動轉(zhuǎn)化成轉(zhuǎn)動車輪所需要的直線運動。
在大多數(shù)汽車上，一般需要轉(zhuǎn)動三到四圈方向盤才能使車輪從左止點到右止點。
轉(zhuǎn)向系傳動比是指轉(zhuǎn)動方向盤角度和車輪轉(zhuǎn)動角度的比率。具體說就是，如果轉(zhuǎn)動方向盤一周車輪隨之轉(zhuǎn)動二十度，實際上轉(zhuǎn)向傳動比是360除以20，也就是18：1。跟高的轉(zhuǎn)向傳動比意味著你需要更大的方向盤轉(zhuǎn)角才能達到同樣的車輪轉(zhuǎn)角。當(dāng)然，高傳動比也意味著更小的力量。
大體說來，質(zhì)量小，更為運動型的汽車相比大型轎車和卡車擁有更小的轉(zhuǎn)向比。小傳動比意味著更快的轉(zhuǎn)向反應(yīng)--你無需再費力的轉(zhuǎn)動方向盤才能達到指定的車輪轉(zhuǎn)角—這就是跑車所要求的理想特性。這些小型汽車可以用更小的轉(zhuǎn)向比，因為在質(zhì)量上足夠輕，轉(zhuǎn)動車輪所需的轉(zhuǎn)向力并沒超過要求。
一部分汽車使用可變轉(zhuǎn)向比，它使用一種在中間和兩邊具有不同的齒間距的齒輪齒條嚙合裝置。這使得汽車在剛開始轉(zhuǎn)彎后能迅速做出反應(yīng)（齒條在中間位置附近），同時也降低了轉(zhuǎn)向力限制位置時的轉(zhuǎn)向力。
動力齒輪齒條轉(zhuǎn)向系統(tǒng)
當(dāng)齒輪齒條在動力轉(zhuǎn)向系統(tǒng)中時，齒條的設(shè)計略有不同。
齒條中間位置包含有一個氣缸與活塞?；钊B接到齒條上。在活塞兩端各有一個液壓缸。在活塞的一端提供高壓油液以推動活塞移動，繼而推動齒條移動，提供轉(zhuǎn)向助力。?在接下來的段落里，我們將詳細了解一下提供高壓油液的組件，然后決定向齒條的哪一方提供高壓油液。首先，讓我們來看看另一種類型的轉(zhuǎn)向器。
循環(huán)球轉(zhuǎn)向
現(xiàn)在循許多卡車和SUV使用的是循環(huán)球轉(zhuǎn)向器。它使車輪轉(zhuǎn)動的聯(lián)動裝置與齒輪齒條轉(zhuǎn)向系統(tǒng)略有不同。
循環(huán)球轉(zhuǎn)向機構(gòu)內(nèi)包含有一個蝸輪。您可以把這個齒輪想象成兩部分。第一部分是一塊帶有內(nèi)螺紋孔的金屬塊。這個金屬塊外側(cè)有切好的齒形，齒形是專門用來嚙合一個使轉(zhuǎn)向拉桿移動的齒輪。方向盤連接到螺紋桿上，類似于一個連接到金屬塊上的螺桿。當(dāng)方向盤轉(zhuǎn)動時它推動螺桿運動。與一般的螺桿隨著旋入螺母的加深不同，這種螺桿在旋轉(zhuǎn)時是固定不動的，并推動螺母移動，螺母使嚙合的齒輪轉(zhuǎn)動最終轉(zhuǎn)動車輪。
與螺桿直接嚙合轉(zhuǎn)向螺母不同，所有嚙合螺紋都充滿了滾珠球軸承環(huán)繞著，齒輪嚙合副轉(zhuǎn)動時能繞著螺紋圓周轉(zhuǎn)動的鋼球。鋼球?qū)嶋H上兩個功能：首先，它們減少齒輪嚙合副的摩擦和磨損；第二，它們減小齒間間隙。當(dāng)改變向方向盤轉(zhuǎn)動方向的時候你就會感覺間隙，轉(zhuǎn)向時好像感覺不到鋼球，齒型將脫離彼此接觸了一會兒，使方向盤感覺松曠。?
動力轉(zhuǎn)向的循環(huán)球轉(zhuǎn)向系統(tǒng)的運動方式類似于齒輪齒條系統(tǒng)。所提供的助力是高壓力液體推動轉(zhuǎn)向螺母的一側(cè)產(chǎn)生的。?
現(xiàn)在讓我們來看看動力轉(zhuǎn)向系統(tǒng)中的其他組成部分。
動力轉(zhuǎn)向
無論循環(huán)球轉(zhuǎn)向器還是齒輪齒條轉(zhuǎn)向器的動力轉(zhuǎn)向系統(tǒng)中都有幾個重要組成部分。
泵
液壓動力轉(zhuǎn)向是由旋轉(zhuǎn)葉片泵提供的（如下圖） 。這種泵的動力是汽車的發(fā)動機通過皮帶和帶輪驅(qū)動的。它包含了一套可移動的葉片，附帶一個橢圓形的內(nèi)腔。
隨著葉片旋轉(zhuǎn)，葉片從回油道中吸進低壓油并將其變成高壓油擠壓出去，并迫使它變成出口高壓。泵所提供的油液總量取決于轎車的引擎轉(zhuǎn)速。該泵的設(shè)計必須使發(fā)動機空轉(zhuǎn)時也能提供充足的液體。因此，在發(fā)動機以更高的轉(zhuǎn)速運行時該泵產(chǎn)生的高壓油液超過正常需要。
泵包含一個壓力安全閥，以確保壓力不會太高，尤其是在發(fā)動機轉(zhuǎn)速高時，產(chǎn)生大量的高壓油液。
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