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遼寧工程技術大學畢業(yè)設計（論文）
附錄A譯文
隨著汽車電子技術的迅猛發(fā)展,人們對汽車轉(zhuǎn)向操縱性能的要求也日益提高。汽車轉(zhuǎn)向系統(tǒng)已從傳統(tǒng)機械轉(zhuǎn)向、液壓助力轉(zhuǎn)向(Hydraulic Power Steering ,簡稱HPS) 、電控液壓助力轉(zhuǎn)向( Elect ric Hydraulic PowerSteering , 簡稱EHPS) , 發(fā)展到電動助力轉(zhuǎn)向系統(tǒng)(Elect ric Power Steering ,簡稱EPS) ,最終還將過渡到線控轉(zhuǎn)向系統(tǒng)(Steer By Wire ,簡稱SBW)。
機械轉(zhuǎn)向系統(tǒng)是指以駕駛員的體力作為轉(zhuǎn)向能源,其中所有傳力件都是機械的,汽車的轉(zhuǎn)向運動是由駕駛員操縱方向盤,通過轉(zhuǎn)向器和一系列的桿件傳遞到轉(zhuǎn)向車輪而實現(xiàn)的。機械轉(zhuǎn)向系由轉(zhuǎn)向操縱機構、轉(zhuǎn)向器和轉(zhuǎn)向傳動機械3大部分組成。
通常根據(jù)機械式轉(zhuǎn)向器形式可以分為:齒輪齒條式、循環(huán)球式、蝸桿滾輪式、蝸桿指銷式。應用最廣的兩種是齒輪齒條式和循環(huán)球式(用于需要較大的轉(zhuǎn)向力時) 。在循環(huán)球式轉(zhuǎn)向器中,輸入轉(zhuǎn)向圈與輸出的轉(zhuǎn)向搖臂擺角是成正比的;在齒輪齒條式轉(zhuǎn)向器中,輸入轉(zhuǎn)向圈數(shù)與輸出的齒條位移是成正比的。循環(huán)球式轉(zhuǎn)向器由于是滾動摩擦形式,因而正傳動效率很高,操作方便且使用壽命長,而且承載能力強,故廣泛應用于載貨汽車上。齒輪齒條式轉(zhuǎn)向器與循環(huán)球式相比,最大特點是剛性大,結(jié)構緊湊重量輕,且成本低。由于這種方式容易由車輪將反作用力傳至轉(zhuǎn)向盤,所以具有對路面狀態(tài)反應靈敏的優(yōu)點,但同時也容易產(chǎn)生打手和擺振等現(xiàn)象,且其承載效率相對較弱,故主要應用于小汽車及輕型貨車上,目前大部分低端轎車采用的就是齒輪齒條式機械轉(zhuǎn)向系統(tǒng)。
隨著車輛載重的增加以及人們對車輛操縱性能要求的提高,簡單的機械式轉(zhuǎn)向系統(tǒng)已經(jīng)無法滿足需要,動力轉(zhuǎn)向系統(tǒng)應運而生,它能在駕駛員轉(zhuǎn)動方向盤的同時提供助力,動力轉(zhuǎn)向系統(tǒng)分為液壓轉(zhuǎn)向系統(tǒng)和電動轉(zhuǎn)向系統(tǒng)2 種。其中液壓轉(zhuǎn)向系統(tǒng)是目前使用最為廣泛的轉(zhuǎn)向系統(tǒng)。
液壓轉(zhuǎn)向系統(tǒng)在機械系統(tǒng)的基礎上增加了液壓系統(tǒng),包括液壓泵、V 形帶輪、油管、供油裝置、助力裝置和控制閥。它借助于汽車發(fā)動機的動力驅(qū)動液壓泵、空氣壓縮機和發(fā)電機等,以液力、氣力或電力增大駕駛員操縱前輪轉(zhuǎn)向的力量,使駕駛員可以輕便靈活地操縱汽車轉(zhuǎn)向,減輕了勞動強度,提高了行駛安全性。
液壓助力轉(zhuǎn)向系統(tǒng)從發(fā)明到現(xiàn)在已經(jīng)有了大約半個世紀的歷史,可以說是一種較為完善的系統(tǒng),由于其工作可靠、技術成熟至今仍被廣泛應用。它由液壓泵作為動力源,經(jīng)油管道控制閥向動力液壓缸供油,通過活塞桿帶動轉(zhuǎn)向機構動作,可通過改變缸徑及油壓的大小來改變助力的大小,由此達到轉(zhuǎn)向助力的作用。傳統(tǒng)液壓式動力轉(zhuǎn)向系統(tǒng)一般按液流的形式可以分為:常流式和常壓式2 種類型,也可根據(jù)控制閥形式分為轉(zhuǎn)閥式和滑閥式。
隨著液壓動力轉(zhuǎn)向系統(tǒng)在汽車上的日益普及,人們對操作時的輕便性和路感的要求也日益提高,然而液壓動力轉(zhuǎn)向系統(tǒng)卻存在許多的缺點: ①由于其本身的結(jié)構決定了其無法保證車輛在任何工況下轉(zhuǎn)動轉(zhuǎn)向盤時,都有較理想的操縱穩(wěn)定性,即無法同時保證低速時的轉(zhuǎn)向輕便性和高速時的操縱穩(wěn)定性; ②汽車的轉(zhuǎn)向特性受駕駛員駕駛技術的影響嚴重; ③轉(zhuǎn)向傳動比固定,使汽車轉(zhuǎn)向響應特性隨車速、側(cè)向加速度等變化而變化,駕駛員必須提前針對汽車轉(zhuǎn)向特性幅值和相位的變化進行一定的操作補償,從而控制汽車按其意愿行駛。這樣增加了駕駛員的操縱負擔,也使汽車轉(zhuǎn)向行駛中存在不安全隱患;而此后出現(xiàn)了電控液壓助力系統(tǒng),它在傳統(tǒng)的液壓動力轉(zhuǎn)向系統(tǒng)的基礎上增加速度傳感器,使汽車能夠隨著車速的變化自動調(diào)節(jié)操縱力的大小,在一定程度上緩和了傳統(tǒng)的液壓轉(zhuǎn)向系統(tǒng)存在的問題。
目前我國生產(chǎn)的商用車和轎車上采用的大多是電控液壓助力轉(zhuǎn)向系統(tǒng),它是比較成熟和應用廣泛的轉(zhuǎn)向系統(tǒng)。盡管電控液壓助力裝置從一定程度上緩解了傳統(tǒng)的液壓轉(zhuǎn)向中輕便性和路感之間的矛盾,然而它還是沒有從根本上解決HPS 系統(tǒng)存在的不足,隨著汽車微電子技術的發(fā)展,汽車燃油節(jié)能的要求以及全球性倡導環(huán)保,其在布置、安裝、密封性、操縱靈敏度、能量消耗、磨損與噪聲等方面的不足已越來越明顯,轉(zhuǎn)向系統(tǒng)向著電動助力轉(zhuǎn)向系統(tǒng)發(fā)展。
電動助力轉(zhuǎn)向系統(tǒng)是現(xiàn)在汽車轉(zhuǎn)向系統(tǒng)的發(fā)展方向,其工作原理是:EPS 系統(tǒng)的ECU 對來自轉(zhuǎn)向盤轉(zhuǎn)矩傳感器和車速傳感器的信號進行分析處理后,控制電機產(chǎn)生適當?shù)闹D(zhuǎn)矩,協(xié)助駕駛員完成轉(zhuǎn)向操作。近幾年來,隨著電子技術的發(fā)展,大幅度降低EPS的成本已成為可能,日本的大發(fā)汽車公司、三菱汽車公司、本田汽車公司、美國的Delphi 汽車系統(tǒng)公司、TRW公司及德國的ZF 公司都相繼研制出EPS。Mercedes2Benz 和Siemens Automotive 兩大公司共同投資6500萬英鎊用于開發(fā)EPS ,目標是到2002 年裝車,年產(chǎn)300 萬套,成為全球EPS 制造商。到目前為止,EPS 系統(tǒng)在輕微型轎車、廂式車上得到廣泛的應用,并且每年以300 萬臺的速度發(fā)展。
Steering is the term applied to the collection of components, linkages, etc. which allow for a vessel ( ship , boat ) or vehicle ( car ) to follow the desired course.轉(zhuǎn)向是一個專業(yè)術語,適用于采集部件,聯(lián)系等,其中允許一艘(艦船)或汽車(轎車)按照預期的方向行駛. An exception is the case of rail transport by which rail tracks combined together with railroad switches provide the steering function.一個例外的情況是鐵路運輸由路軌組合在一起鐵路道岔提供轉(zhuǎn)向功能。
Many modern cars use steering mechanisms, where the steering wheel turns the pinion gear; the pinion moves the rack, which is a sort of linear gear which meshes with the pinion, from side to side.許多現(xiàn)代轎車使用齒輪齒條式轉(zhuǎn)向器,在方向盤末端有轉(zhuǎn)動齒輪;該齒輪帶動齒條移動,它是一種線性的齒輪緊密配合,從一邊到一邊。這種運動把轉(zhuǎn)矩通過轉(zhuǎn)向橫拉桿和一種叫做轉(zhuǎn)向節(jié)臂的短形臂傳遞給轉(zhuǎn)向輪的主銷。
mechanism, which is still found on trucks and utility vehicles.以前的設計往往采用循環(huán)球式轉(zhuǎn)向器,而這種轉(zhuǎn)向器仍然應用在卡車和多用途車輛。This is a variation on the older and thus steers the wheels.這是一種老式的螺母和齒扇設計，該轉(zhuǎn)向管柱轉(zhuǎn)動大螺絲("蝸輪"),它與一個齒扇齒輪嚙合,當蝸輪轉(zhuǎn)動時，齒扇也隨之轉(zhuǎn)動，一個安裝在齒扇軸上且與轉(zhuǎn)向聯(lián)動有關的搖臂帶動轉(zhuǎn)向節(jié)臂 ,從而使車輪轉(zhuǎn)動. The recirculating ball version of this apparatus reduces the considerable friction by placing large ball bearings between the teeth of the worm and those of the screw; at either end of the apparatus the balls exit from between the two pieces into a channel internal to the box which connects them with the other end of the apparatus, thus they are "recirculated".循環(huán)球式轉(zhuǎn)向器通過安裝滾珠減少螺母和螺桿之間的摩擦;兩根導管和螺母內(nèi)的螺旋管狀通道組合成兩條各自獨立的封閉的鋼球“流到”。
The rack and pinion design has the advantages of a large degree of feedback and direct steering "feel"; it also does not normally have any , or slack.齒輪齒條式轉(zhuǎn)向器設計具有很大程度的反饋和直接轉(zhuǎn)向"路感";它也通常不會有任何反彈,或呆滯。A disadvantage is that it is not adjustable, so that when it does wear and develop lash, the only cure is replacement.缺點是,它是不可調(diào)的,因此當它磨損唯一的解決辦法更換。
The recirculating ball mechanism has the advantage of a much greater , however, this is no longer an important advantage, leading to the increasing use of rack and pinion on newer cars.循環(huán)球式轉(zhuǎn)向器的優(yōu)點是機械優(yōu)勢,因此,它被使用在較大較重的車輛,而齒輪齒條式原本僅限于較小和較輕;由于幾乎普遍采用動力轉(zhuǎn)向系統(tǒng),不過,這已不再是一個重要的優(yōu)勢,導致越來越多地在新型汽車應用齒輪齒條式轉(zhuǎn)向器。 to account for wear, but it cannot be entirely eliminated or the mechanism begins to wear very rapidly.循環(huán)球式轉(zhuǎn)向器設計在中心也有明顯的沖擊,或"死點"。凡一分鐘交替方向盤出不來并不移動轉(zhuǎn)向機構;這是很容易可調(diào)螺桿的端部來減少磨損,但它并不能完全消除或機制開始磨損很快。 This design is still in use in trucks and other large vehicles, where rapidity of steering and direct feel are less important than robustness, maintainability, and mechanical advantage.這項設計目前仍在使用中,在卡車和其他大型車輛,也應用于迅速轉(zhuǎn)向,路感與穩(wěn)健性,可維護性,和機械的優(yōu)勢相比不太重要的場合。 The much smaller degree of feedback with this design can also sometimes be an advantage; drivers of vehicles with rack and pinion steering can have their thumbs broken when a front wheel hits a bump, causing the steering wheel to kick to one side suddenly (leading to driving instructors telling students to keep their thumbs on the front of the steering wheel, rather than wrapping around the inside of the rim).較小程度的反饋,這樣的設計也有時是一種優(yōu)點;當前輪碰撞時，使用齒輪齒條轉(zhuǎn)向的司機只有自己的大拇指受傷,造成方向盤揭開一邊突然(因為駕駛教練告訴學生把自己的大拇指在前面的方向盤,而非放在左右的內(nèi)邊緣). This effect is even stronger with a heavy vehicle like a truck; recirculating ball steering prevents this degree of feedback, just as it prevents desirable feedback under normal circumstances.這種效果在像卡車一樣的重型汽車更為明顯;循環(huán)球式轉(zhuǎn)向防止這種程度的反饋,只是因為它可以在正常情況下防止可取反饋。
轉(zhuǎn)向連鎖連接轉(zhuǎn)向器和車輪通常符合一個阿克曼轉(zhuǎn)向幾何的變化,它交代了一個事實:當轉(zhuǎn)向是,內(nèi)輪轉(zhuǎn)過的半徑比外輪小得多，因此適合駕駛的直路,是不適合曲折。
As vehicles have become heavier and switched to front wheel drive , the effort to turn the steering wheel manually has increased - often to the point where major physical exertion is required.由于車輛已成為較重而改用前輪驅(qū)動,為了扭轉(zhuǎn)方向盤，通常的,主要的是體力。為了解決這一問題,汽車業(yè)發(fā)展的動力轉(zhuǎn)向系統(tǒng)。 There are two types of power steering systems—hydraulic and electric/electronic.有兩種類型的助力轉(zhuǎn)向系統(tǒng)-液壓和電氣/電子。 There is also a hydraulic-electric hybrid system possible.還有一種液壓-電動混合系統(tǒng)。
A hydraulic power steering (HPS) uses hydraulic pressure supplied by an engine-driven pump to assist the motion of turning the steering wheel.液壓助力轉(zhuǎn)向系統(tǒng)(hps)利用油壓供應的一個發(fā)動機驅(qū)動泵,以協(xié)助將方向盤轉(zhuǎn)轉(zhuǎn)動。 Electric power steering (EPS) is more efficient than the hydraulic power steering, since the electric power steering motor only needs to provide assist when the steering wheel is turned, whereas the hydraulic pump must run constantly.電動助力轉(zhuǎn)向系統(tǒng)(EPS)方式,是較有效率的液壓助力轉(zhuǎn)向系統(tǒng),由于電動助力轉(zhuǎn)向汽車只需要提供協(xié)助時,方向盤被轉(zhuǎn)動,而液壓泵必須不斷運行。 In EPS the assist level is easily tunable to the vehicle type, road speed, and even driver preference.在EPS的幫助下是很容易調(diào)節(jié)車型,最高車速,甚至駕駛的喜好。 An added benefit is the elimination of environmental hazard posed by leakage and disposal of hydraulic power steering fluid.另外一個好處是,通過泄漏和處置動力轉(zhuǎn)向液消除對環(huán)境構成危險 。
An outgrowth of power steering is speed adjustable steering, where the steering is heavily assisted at low speed and lightly assisted at high speed.動力轉(zhuǎn)向的分支是速度可調(diào)轉(zhuǎn)向而轉(zhuǎn)向是大量輔助以低速行駛,稍微協(xié)助高速。 The auto makers perceive that motorists might need to make large steering inputs while manoeuvering for parking, but not while traveling at high speed.汽車制造商認為,當要停車時駕駛?cè)丝赡苄枰龀龃罅哭D(zhuǎn)向投入，但當時高速行駛時則不然。The first vehicle with this feature was the Citro?n SM with its Diravi layout, although rather than altering the amount of assistance as in modern power steering systems, it altered the pressure on a centring cam which made the steering wheel try to "spring" back to the straight-ahead position.第一輛有這特點的汽車,是雪鐵龍與其diravi，雖然改變了現(xiàn)代汽車轉(zhuǎn)向系統(tǒng)資金的投入，但它改變了定心凸輪的壓力，使得方向盤盡力去回到原來的位置。Modern speed-adjustable power steering systems reduce the pressure fed to the ram as the speed increases, giving a more direct feel.現(xiàn)代速度可調(diào)式動力轉(zhuǎn)向系統(tǒng),當速度增長時減少了活塞的壓力 ,給予更直接的感受。This feature is gradually becoming commonplace across all new vehicles.這一特點在所有新車正逐漸成為司空見慣。
四輪轉(zhuǎn)向(或全輪轉(zhuǎn)向)是一種系統(tǒng),當高速行駛時能增加車輛穩(wěn)定型，而在低速行駛時可以減小轉(zhuǎn)彎半徑。
In most four-wheel steering systems, the rear wheels are steered by a computer and actuators.大多數(shù)的四輪轉(zhuǎn)向系統(tǒng),后輪轉(zhuǎn)向通過單片機和驅(qū)動器實現(xiàn)。 The rear wheels generally cannot turn as far as the Alternatively, several systems, including Delphi's Quadrasteer and the system in Honda's Prelude line, allow for the rear wheels to be steered in the opposite direction as the front wheels during low speeds.后輪一般不能反過來,有幾個系統(tǒng),包括Delphi的quadrasteer，該系統(tǒng)在本田的生產(chǎn)前線，當前輪低速時，允許后輪在相反方向轉(zhuǎn)向。This allows the vehicle to turn in a significantly smaller radius — sometimes critical for large trucks or vehicles with trailers.這使得車輛轉(zhuǎn)彎半徑較小,有時應用于大型卡車車輛及掛車。
附錄B外文文獻
Along with automobile electronic technology swift and violent development, the people also day by day enhance to the motor turning handling quality request. The motor turning system changed, the hydraulic pressure boost from the traditional machinery changes (Hydraulic Power Steering, is called HPS), the electrically controlled hydraulic pressure boost changes (Elect ric Hydraulic Power Steering, is called EHPS), develops the electrically operated boost steering system (Elect ric Power Steering, is called EPS), finally also will transit to the line controls the steering system (Steer By Wire, will be called SBW).
The machinery steering system is refers by pilot's physical strength achievement changes the energy, in which all power transmission all is mechanical, the automobile changes the movement is operates the steering wheel by the pilot, transmits through the diverter and a series of members changes the wheel to realize. The mechanical steering system by changes the control mechanism, the diverter and major part changes the gearing 3 to be composed.
Usually may divide into according to the mechanical diverter form: The gear rack type, follows round the world -like, the worm bearing adjuster hoop type, the worm bearing adjuster refers sells the type. Is the gear rack type and follows using the broadest two kinds round the world -like (uses in needing time big steering force).In follows round the world -like in the diverter, the input changes the circle and the output steering arm pivot angle is proportional; In the gear rack type diverter, the input changes the turn and the output rack displacement is proportional. Follows round the world -like the diverter because is the rolling friction form, thus the transmission efficiency is very high, the ease of operation also the service life are long, moreover bearing capacity, therefore widely applies on the truck. The gear rack type diverter with follows round the world -like compares, the most major characteristic is the rigidity is big, the structure compact weight is light, also the cost is low. Because this way passes on easily by the wheel the reacting force to the steering wheel, therefore has to the pavement behavior response keen merit, but simultaneously also easy to have phenomena and so on goon and oscillation, also its load bearing efficiency relative weak, therefore mainly applies on the compact car and the pickup truck, at present the majority of low end passenger vehicle uses is the gear rack type machinery steering system.
Along with the vehicles carrying capacity increase as well as the people to the vehicles handling quality request enhancement, the simple mechanical type steering system were already unable to meet the needs, the power steering system arise at the historic moment, it could rotate the steering wheel while the pilot to provide the boost, the power steering system divides into the hydraulic pressure steering system and the electrically operated steering system 2kinds.Hydraulic pressure steering system is at present uses the most widespread steering system.
The hydraulic pressure steering system increased the hydraulic system in the mechanical system foundation, including hydraulic pump, V shape band pulley, drill tubing, feed installment, boost installment and control valve. It with the aid of in the motor car engine power actuation hydraulic pump, the air compressor and the generator and so on, by the fluid strength, the physical strength or the electric power increases the pilot to operate the strength which the front wheel changes, enables the pilot to be possible nimbly to operate motor turning facilely, reduced the labor intensity, enhanced the travel security.
The hydraulic pressure boost steering system from invented already had about half century history to the present, might say was one kind of more perfect system, because its work reliable, the technology mature still widely is applied until now. It takes the power supply by the hydraulic pump, after oil pipe-line control valves to power hydraulic cylinder feed, through the connecting rod impetus rotation gear movement, may changes the boost through the change cylinder bore and the flowing tubing head pressure size the size, from this achieved changes the boost the function. The traditional hydraulic pressure type power steering system may divide into generally according to the liquid flow form: Ordinary flow type and atmospheric pressure type 2 kind of types, also may divide into according to the control valve form transfers the valve type and the slide-valve type.
Along with hydraulic pressure power steering system on automobile daily popularization, the people to operates when the portability and the road feeling request also day by day enhance, however the hydraulic pressure power steering system has many shortcomings actually: ①Because its itself structure had decided it is unable to guarantee vehicles rotates the steering wheel when any operating mode, all has the ideal operation stability, namely is unable simultaneously to guarantee time the low speed changes the portability and the high speed time operation stability;②The automobile changes the characteristic to drive the pilot technical the influence to be serious; ③The steering ratio is fixed, causes the motor turning response characteristic along with changes and so on vehicle speed, transverse acceleration to change, the pilot must aim at the motor turning characteristic peak-to-peak value and the phase change ahead of time carries on certain operation compensation, thus controls the automobile according to its wish travel. Like this increased pilot's operation burden, also causes in the motor turning travel not to have the security hidden danger; But hereafter appeared the electrically controlled hydraulic booster system, it increases the velocity generator in the traditional hydraulic pressure power steering system foundation, enables the automobile along with the vehicle speed change automatic control force size, has to a certain extent relaxed the traditional hydraulic pressure steering system existence question.
At present our country produces on the commercial vehicle and the passenger vehicle uses mostly is the electrically controlled hydraulic pressure boost steering system, it is quite mature and the application widespread steering system. Although the electrically controlled hydraulic servo alleviated the traditional hydraulic pressure from certain degree to change between the portability and the road feeling contradiction, however it did not have fundamentally to solve the HPS system existence insufficiency, along with automobile microelectronic technology development, automobile fuel oil energy conservation request as well as global initiative environmental protection, it in aspect and so on arrangement, installment, leak-proof quality, control sensitivity, energy consumption, attrition and noise insufficiencies already more and more obvious, the steering system turned towards the electrically operated boost steering system development.
The electrically operated boost steering system is the present motor turning system development direction, its principle of work is: EPS system ECU after comes from the steering wheel torque sensor and the vehicle speed sensor signal carries on analysis processing, controls the electrical machinery to have the suitable boost torque, assists the pilot to complete changes the operation. In the last few years, along with the electronic technology development, reduces EPS the cost to become large scale possibly, Japan sends the car company, Mitsubishi Car company, this field car company, US's Delphi automobile system company, TRW Corporation and Germany's ZF Corporation greatly all one after another develops EPS.Mercedes2Benz 和Siemens Automotive Two big companies invested 65,000,000 pounds to use in developing EPS, the goal are together load a car to 2002, yearly produce 300 ten thousand sets, became the global EPS manufacturer. So far, the EPS system in the slight passenger vehicle, on the theater box type vehicle obtains the widespread application, and every year by 300 ten thousand speed development.
Steering is the term applied to the collection of components, linkages, etc. which allow for a vessel (ship, boat) or vehicle (car) to follow the desired course. An exception is the case of rail transport by which rail tracks combined together with railroad switches provide the steering function.
The most conventional steering arrangement is to turn the front wheels using a hand–operated steering wheel which is positioned in front of the driver, via the steering column, which may contain universal joints to allow it to deviate somewhat from a straight line. Other arrangements are sometimes found on different types of vehicles, for example, a tiller or rear–wheel steering. Tracked vehicles such as tanks usually employ differential steering — that is, the tracks are made to move at different speeds or even in opposite directions to bring about a change of course.
Many modern cars use rack and pinion steering mechanisms, where the steering wheel turns the pinion gear; the pinion moves the rack, which is a sort of linear gear which meshes with the pinion, from side to side. This motion applies steering torque to the kingpins of the steered wheels via tie rods and a short lever arm called the steering arm.
Older designs often use the recirculating ball mechanism, which is still found on trucks and utility vehicles. This is a variation on the older worm and sector design; the steering column turns a large screw (the "worm gear") which meshes with a sector of a gear, causing it to rotate about its axis as the worm gear is turned; an arm attached to the axis of the sector moves the pitman arm, which is connected to the steering linkage and thus steers the wheels. The recirculating ball version of this apparatus reduces the considerable friction by placing large ball bearings between the teeth of the worm and those of the screw; at either end of the apparatus the balls exit from between the two pieces into a channel internal to the box which connects them with the other end of the apparatus, thus they are "recirculated".
The rack and pinion design has the advantages of a large degree of feedback and direct steering "feel"; it also does not normally have any backlash, or slack. A disadvantage is that it is not adjustable, so that when it does wear and develop lash, the only cure is replacement.
The recirculating ball mechanism has the advantage of a much greater mechanical advantage, so that it was found on larger, heavier vehicles while the rack and pinion was originally limited to smaller and lighter ones; due to the almost universal adoption of power steering, however, this is no longer an important advantage, leading to the increasing use of rack and pinion on newer cars. The recirculating ball design also has a perceptible lash, or "dead spot" on center, where a minute turn of the steering wheel in either direction does not move the steering apparatus; this is easily adjustable via a screw on the end of the steering box to account for wear, but it cannot be entirely eliminated or the mechanism begins to wear very rapidly. This design is still in use in trucks and other large vehicles, where rapidity of steering and direct feel are less important than robustness, maintainability, and mechanical advantage. The much smaller degree of feedback with this design can also sometimes be an advantage; drivers of vehicles with rack and pinion steering can have their thumbs broken when a front wheel hits a bump, causing the steering wheel to kick to one side suddenly (leading to driving instruct