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外國文獻(xiàn)
HYDRAULIC BRAKE BASICS
Air?brakes?get more attention, but?hydraulic?brakes?are installed on more vehicles.?Understanding how they work is the first step to safe, cost-effective diagnosis and repair.
Ever wonder why there can't be just one kind of?brake??It's because air and?hydraulic?brakes?each have operating characteristics that make one or the other ideal for certain applications.
In heavy-duty combination vehicles, air is the clear choice because of the large volume of liquid that would be needed to acadia all the wheel cylinders.?Plus, dealing with gladhands and hoses filled with?hydraulic?fluid would be messy.
But for light and medium-duty straight-truck applications,?hydraulic?brakes?offer advantages including:
· Brake?feel — that is, as the pedal is pressed farther down, effort increases;
· High line pressures, which permit the use of lighter, more compact braking components;
· Less initial expense, due to smaller and fewer components;
· Cleanliness —?hydraulic?brakes?are closed systems;
· Ease of locating leaks, since fluid is visible.
There are many more permutations of?hydraulic?brake?systems than found in air systems, but all have basic similarities.
THE HYDRAULIC SYSTEM
All hydraulic brake systems contain a fluid reservoir, a master cylinder, which produces?hydraulic?pressure,?hydraulic?lines and hoses to carry pressurized fluid to the brakes, and one or more wheel cylinder(s) on each wheel.
The wheel cylinders expand under fluid pressure, and force the?brake?shoes against the insides of the drums.?If disc?brakes?are used, calipers, with integral cylinders, clamp down on the rotors when pressure is applied.
Because a vehicle must be able to stop much more quickly than it can accelerate, a tremendous amount of braking force is needed.?Therefore, the retarding horsepower generated by the?brakes?must be several times that of the engine.
In order to develop the forces required to hold the?brake?linings against the drums or discs, and to achieve controlled deceleration, it is necessary to multiply the original force applied at the?brake?pedal.
When a?hydraulic?system is used, the only mechanical leverage is in the foot pedal linkage.?However, varying the diameter of the wheel cylinders or caliper diameters, in relation to the master cylinder bore diameter, provides an additional increase in ratio.
In a?hydraulic?system, the pressure delivered by the various wheel cylinders is directly affected by the areas of their pistons.?For example, if one wheel-cylinder piston has an area of 2 square inches, and another piston has an area of 1 square inch, and the system pressure is 400 psi, the 2-square-inch piston will push against the brake?shoes with a force of 800 pounds. The 1-square-inch piston will exert a force of 400 pounds.?The ratio between the areas of the master cylinder and the wheel cylinders determine the multiplication of force at the wheel cylinder pistons.
Keep in mind that the larger a wheel cylinder's diameter, the more fluid must be supplied by the master cylinder to fill it.?This translates into a longer master-cylinder stroke.
If the master cylinder bore diameter is increased and the applying force remains the same, less pressure will be developed in the system, but a larger wheel-cylinder piston can be used to achieve the desired pressure at the wheel cylinder.?Obviously, a replacement master cylinder, wheel cylinder or caliper must be of the same design and bore as the original unit.
Hydraulic?brake?systems are split systems, comprising two discreet braking circuits.?One master-cylinder piston and reservoir is used to actuate the?brakes?on one axle, with a separate piston and reservoir actuating the?brakes?on the other axle(s).?Although rare, some light-duty brake systems are split diagonally rather than axle by axle.
The reason for the split system is that if a leak develops in one?hydraulic?circuit, the other will stop the vehicle.?Of course, the vehicle shouldn't be driven any farther than necessary to have the?brake?system repaired.
When one of the?hydraulic?circuits fails, a pressure -differential switch senses unequal pressure between the two circuits.?The switch contains a piston located by a centering spring and electrical contacts at each end.?Fluid pressure from one?hydraulic?circuit is supplied to one end of the pressure
-differential switch, and pressure from the other circuit is supplied to the other end.?As pressure falls in one circuit, the other circuit's normal pressure forces the piston to the inoperative side, closing the contacts and illuminating a dashboard warning light.
POWER ASSIST
Power assist units, or boosters, reduce operator effort at the?brake?pedal.?Vacuum boosters, popular on light-duty vehicles, make use of an engine vacuum on one side of a diaphragm, and atmospheric pressure on the other side.?A valve allows the vacuum to act on the diaphragm in proportion to?brake?pedal travel.?This assists the pedal effort, and allows increased pressure on the?brake?fluid, without an undue increase in pedal effort.
Other types of boosters use?hydraulic?pressure — either from the vehicle's power steering pump or from a separate electric pump, or both — to assist pedal effort. As the?brake?pedal is depressed, a valve increases?hydraulic?pressure in a boost chamber to apply increased pressure to the master cylinder pistons.
Some systems use both vacuum and?hydraulic?assist.?In other systems, air pressure from an onboard compressor is used to generate?hydraulic?system pressure.
VALVING
Valves commonly found in?hydraulic?brake?systems include: Proportioning, or pressure-balance valves.?These restrict a percentage of?hydraulic?pressure to the rear?brakes?when system pressure reaches a preset high value. This improves front/rear?brake?balance during high-speed braking, when some of a vehicle's rear weight is transferred forward, and helps prevent rear-wheel lockup. Some proportioning valves are height-sensing.?That is, they adjust rear-brake?pressure in response to vehicle load.?As a vehicle's load increases (decreasing height) more?hydraulic?pressure to the rear?brakes?is allowed; Metering valves.?These hold off pressure to front disc?brakes?to allow rear drum?brake?shoes to overcome return-spring pressure and make contact with the rear drums.?This prevents locking the front?brakes?on slippery surfaces under light braking applications.?These valves do not come into play during hard braking.
PARKING
The parking function varies greatly among?hydraulic?brake?systems.?Many light-duty vehicles with rear drum?brakes?use a passenger-car type lever-and-cable setup. A ratcheted lever or foot pedal pulls a cable, which, in turn, pulls a lever assembly at each rear wheel end.?The lever forces the?brake?shoes apart, and they are mechanically held against the drums until the ratchet is released.
Other parking systems include spring chambers, like those used on air-brake?systems.?These are spring-engaged, but are disengaged by?hydraulic?pressure instead of air.
ANTILOCK
On many hydraulically braked light-duty trucks, antilock?brakes?are used on the rear wheels to preserve braking stability when these vehicles are lightly loaded.?Front and rear-wheel antilock is usually an option, except for vehicles over 10,000 pounds GVWR, which are required to have steer and drive-axle antilock.
In current?hydraulic?antilock systems, a dump valve releases pressurized?hydraulic?fluid into an accumulator in the event of an impending wheel lockup.
An electronic control box receives speed signal(s) from sensors in the transmission and/or at the wheels.?When the?brakes?are applied, the control box senses the decrease in rear wheel speed, and activates the dump valve(s) if the rate of deceleration exceeds a predetermined limit.
The control box energizes the dump valve with a series of rapid pulses to bleed-off wheel?hydraulic?pressure.?Continuing in antilock mode, the dump valve is pulsed to keep the wheels rotating, while maintaining controlled deceleration.
At the end of such a stop, the valve de-energizes and any fluid in the accumulator is returned to the master cylinder.?Normal?brake?operation resumes.
FOUNDATION BRAKES
Foundation?brakes?in?hydraulic?systems can be either drum or disc.?In many applications, discs are used on the front axle and drums on the rear.
Drum?brakes?are said to be self-energizing.?That's because when the?brake?shoes expand and contact a rotating drum, the leading, or forward,?brake?shoe is pushed against the trailing shoe by the force of the moving drum.?This results in higher lining-to-drum pressure than would be produced by the wheel cylinder alone.
As?brake?linings wear, the shoes periodically must be moved closer to the drums to ensure proper contact during braking.?While some older drum?brake?assemblies are manually adjusted, most are automatic.?These use a star wheel or ratchet assembly, which senses when the wheel cylinder has traveled beyond its normal stroke, and expands the pivot point at the other end of the?brake?shoes.
In addition to being one of the friction elements, the?brake?drum or rotor also acts as a heat sink.?It must rapidly absorb heat during braking, and hold it until it can be dissipated into the air.?The heavier a drum or rotor is, the more heat it can hold.
This is important, since the hotter the?brake?linings get, the more susceptible they are to heat fade.?Heat fade is induced by repeated hard stops and results in reduced lining-to-drum/rotor friction and increased vehicle stopping distance.?As a rule, high-quality linings will display less heat fade than inferior ones.?Also, discbrakes?are far more resistant to heat fade than drum?brakes.
Another type of fade that?brakes?are susceptible to is water fade.?Drum?brakes, with their large surface areas, apply fewer pounds per square inch of force between lining and drum during a stop than disc?brakes.?This, added to the drum's water-retaining shape, promotes hydroplaning between shoe and drum under wet conditions.?The result is greatly increased stopping distance.
Disc?brakes, with their smaller friction surfaces and high clamping forces, do a good job of wiping water from rotors, and display little reduction in stopping capability when wet.
中文翻譯
液壓制動(dòng)基礎(chǔ)
空氣制動(dòng)系統(tǒng)得到更多的關(guān)注，但更多的車輛上安裝液壓制動(dòng)器。了解它們是如何工作的，是安全，具成本效益的診斷和修復(fù)的第一步。
有沒有想過為什么不能只是其中的一種制動(dòng)？這是因?yàn)榭諝夂鸵簤褐苿?dòng)器，使一個(gè)或某些應(yīng)用程序的其他理想的經(jīng)營特色。重型組合的車輛，空氣是明確的選擇，因?yàn)閷⑿枰罅康囊后w阿卡迪亞所有分泵。此外，充滿液壓油與制動(dòng)分泵和軟管的將是混亂的。但對(duì)于輕型和中型卡車直應(yīng)用，液壓制動(dòng)器提供的優(yōu)勢包括：
?制動(dòng)感覺 - 那就是，踏板越往下壓，努力增加;高線壓力，允許使用更輕，更緊湊的制動(dòng)組件;
?更少的初始費(fèi)用，由于用更小和更少的元件;
?衛(wèi)生，液壓制動(dòng)器是封閉的系統(tǒng);
?易于定位泄漏，因?yàn)橐后w是可見的。液壓制動(dòng)系統(tǒng)有更多的排列，比在空氣系統(tǒng)中發(fā)現(xiàn)，但都基本相似。
液壓系統(tǒng)
所有的液壓制動(dòng)系統(tǒng)包含流體水庫，主缸，液壓，液壓管路，對(duì)制動(dòng)器進(jìn)行加壓流體的軟管和一個(gè)或多個(gè)輪缸（S）對(duì)每個(gè)車輪產(chǎn)生。分泵擴(kuò)大流體壓力下，迫使制動(dòng)蹄對(duì)鼓的內(nèi)側(cè)。如果使用盤式制動(dòng)器，卡鉗與不可分割的氣瓶打擊轉(zhuǎn)子時(shí)施加壓力。
因?yàn)檐囕v必須能夠更迅速，它可以加速到停止，需要大量的剎車力。因此，必須減速剎車產(chǎn)生的馬力的發(fā)動(dòng)機(jī)作用多次。為了發(fā)展須持有對(duì)鼓或盤制動(dòng)器襯片的力量，實(shí)現(xiàn)受控減速，這是要乘原始的力量施加在剎車踏板。
當(dāng)使用液壓系統(tǒng)，機(jī)械杠桿是在腳踏板聯(lián)動(dòng)。然而，不同分泵或卡尺直徑的直徑，關(guān)系到主缸內(nèi)徑，提供了一個(gè)額外增加的比率。液壓系統(tǒng)中，各分泵交付的壓力，直接影響由活塞地區(qū)。例如，如果一個(gè)輪缸活塞面積2平方英寸，另一個(gè)活塞面積1平方英寸，系統(tǒng)壓力為400磅，2平方英寸的活塞將針對(duì)制動(dòng)器推一個(gè)迫使800磅。1平方英寸的活塞施加一個(gè)400磅的力量?？偙煤头直玫牡貐^(qū)之間的比例確定在輪缸活塞的力量倍增。為保持在頭腦，直徑較大的輪缸的，更流暢，必須提供由主缸行程較長的碩士轉(zhuǎn)化。請(qǐng)記住，直徑較大的輪缸的，更流暢，必須由主缸提供，以填補(bǔ)它。這意味著進(jìn)入一個(gè)較長的主缸行程。如果主缸孔直徑增加和相同的申請(qǐng)仍然有效，更少的壓力將在系統(tǒng)的開發(fā)，但一個(gè)更大的輪缸活塞可以用來實(shí)現(xiàn)在輪缸所需的壓力。顯然，必須更換主缸，輪缸或卡尺相同的設(shè)計(jì)，并作為原單位承擔(dān)。
液壓系統(tǒng)中，各分泵交付的壓力，直接影響由活塞地區(qū)。例如，如果一個(gè)輪缸活塞面積2平方英寸，另一個(gè)活塞面積1平方英寸，系統(tǒng)壓力為400磅，2平方英寸的活塞將針對(duì)制動(dòng)鞋推一個(gè)迫使800磅。 1平方英寸的活塞施加一個(gè)400磅的力量?？偙煤头直玫牡貐^(qū)之間的比例確定在輪缸活塞的力量倍增。液壓制動(dòng)系統(tǒng)分割的系統(tǒng)，包括兩個(gè)謹(jǐn)慎的制動(dòng)電路。一主缸活塞和水庫是一個(gè)單獨(dú)的活塞及伺服制動(dòng)器上的其他橋（S）的水庫，用來驅(qū)動(dòng)一軸剎車。雖然罕見，一些輕型制動(dòng)系統(tǒng)分裂對(duì)角線而非橋橋。分割系統(tǒng)的原因是，如果一個(gè)液壓回路泄漏的發(fā)展，將停止車輛。當(dāng)然，不應(yīng)該被驅(qū)動(dòng)的車輛遠(yuǎn)超過必要的制動(dòng)系統(tǒng)修復(fù)。當(dāng)液壓回路發(fā)生故障，壓力差開關(guān)感官兩個(gè)電路之間的不平等的壓力。交換機(jī)包含由彈簧片，并在每年年底電觸頭位于活塞。從一個(gè)液壓回路中流體的壓力提供壓力差開關(guān)的一端，并從其他電路的壓力提供給另一端。隨著壓力的一個(gè)電路，其他電路的正常壓力，迫使活塞的失效一邊，關(guān)閉的接觸，并照亮儀表板警示燈。
動(dòng)力輔助
協(xié)助電力單位，或助推器，減少運(yùn)營商的努力，在剎車踏板。真空助力器，輕型汽車的流行，使發(fā)動(dòng)機(jī)真空隔膜一側(cè)，對(duì)對(duì)方的大氣壓力。一個(gè)閥門，使真空作用于剎車踏板的行程中的比例隔膜。這有助于踏板的努力，并增加對(duì)制動(dòng)液的壓力，無需過分增加在踏板努力。其他類型的助推器使用液壓壓力 - 無論是從車輛的動(dòng)力轉(zhuǎn)向泵，或從一個(gè)單獨(dú)的電動(dòng)泵，或兩者兼而有之 - 協(xié)助剎車踏板被踩下踏板作用，閥門液壓升壓室申請(qǐng)?jiān)黾拥膲毫υ谠黾又鞲谆钊Ｓ行┫到y(tǒng)使用真空和液壓助力。在其他系統(tǒng)中，從船上壓縮機(jī)的空氣壓力產(chǎn)生液壓系統(tǒng)的壓力。
閥桿液壓制動(dòng)系統(tǒng)中常見的閥門包括：
配比，或壓力平衡閥門。這些限制液壓比例后輪剎車系統(tǒng)壓力達(dá)到預(yù)設(shè)的高阻值。提高前輪/后輪在高速制動(dòng)的制動(dòng)平衡時(shí)，一些車輛的前后重量轉(zhuǎn)移，并有助于防止后輪配料閥高度傳感器。也就是說，他們調(diào)整后輪制動(dòng)壓力，在車輛荷載的響應(yīng)。隨著車輛的負(fù)載增加（降低高度）液壓后輪剎車是不允許的; ?測光閥門。這些保持了前盤式制動(dòng)器的壓力，讓后輪鼓式制動(dòng)蹄克服返回彈簧的壓力，使接觸后鼓。這可以防止鎖定在濕滑路面上的前剎車燈制動(dòng)應(yīng)用。這些閥門不來硬制動(dòng)過程中發(fā)揮作用。
泊車
停車功能的液壓制動(dòng)系統(tǒng)之間的差別很大。許多輕型車輛使用與后輪鼓式制動(dòng)器桿和電纜相配合，逐步加大桿或腳踏拉電纜，這反過來，拉杠桿總成，每個(gè)后輪結(jié)束的客運(yùn)車類型。杠桿迫使制動(dòng)蹄外，他們對(duì)鼓機(jī)械棘輪被釋放，直到舉起。
其他泊車系統(tǒng)包括彈簧腔，像那些用于空氣制動(dòng)系統(tǒng)。這是彈簧控制，但由液壓脫開而不是空氣。
防抱死
許多輕型卡車液壓制動(dòng)，防抱死制動(dòng)系統(tǒng)上使用的后輪保持輕載時(shí)，這些車輛制動(dòng)穩(wěn)定性。前面和后輪防抱死通常是一個(gè)選項(xiàng)，GVWR超過10,000磅的車輛，這是需要引導(dǎo)和驅(qū)動(dòng)橋防抱死關(guān)閉。在當(dāng)前的液壓防抱死系統(tǒng)，轉(zhuǎn)儲(chǔ)閥釋放壓力到一個(gè)累加器在即將車輪鎖死的情況下液壓油。
電子控制箱接收來自傳感器的傳輸和/或在車輪速度信號(hào)（S）。當(dāng)施加制動(dòng)，控制箱檢測在后輪的速度，減少和激活轉(zhuǎn)儲(chǔ)閥（S），如果減速率超過預(yù)定的限制。
控制箱通電一系列流血輪液壓快速脈沖的單向閥。繼續(xù)轉(zhuǎn)儲(chǔ)閥是脈沖在防抱死模式，以保持車輪轉(zhuǎn)動(dòng)，同時(shí)保持控制的減速。在最后的停止，閥門的激勵(lì)和累加器中的任何液體返回到主缸，恢復(fù)正常的剎車操作。
基礎(chǔ)剎車
在液壓系統(tǒng)的基礎(chǔ)制動(dòng)器可以是鼓或光盤。在許多應(yīng)用中，光盤上使用前軸后方的鼓。鼓式制動(dòng)器說是自激。這是因?yàn)橹苿?dòng)蹄擴(kuò)大和聯(lián)系一個(gè)旋轉(zhuǎn)的滾筒，引導(dǎo)或向前制動(dòng)蹄被推向?qū)x車制動(dòng)箍由移動(dòng)鼓的力量。這個(gè)結(jié)果在更高的襯里鼓比將僅由輪缸產(chǎn)生的壓力。
隨著制動(dòng)器襯片的磨損，必須定期移近鼓，以確保在制動(dòng)過程中適當(dāng)?shù)慕佑|。雖然一些舊的鼓式制動(dòng)器總成，手動(dòng)調(diào)整，大部分都是自動(dòng)。這些使用一個(gè)星輪或棘輪大會(huì)，這感官分泵時(shí)已超出其正常行程前往，并擴(kuò)大在另一端的制動(dòng)蹄的支點(diǎn)。
除了摩擦的元素之一，制動(dòng)鼓或轉(zhuǎn)子也充當(dāng)散熱器。它必須迅速制動(dòng)過程中吸收的熱量，并保持它，直到它可以將空氣中消散。鼓或轉(zhuǎn)子較重的是，它可以容納更多的熱量。這是很重要的，因?yàn)橹苿?dòng)器襯片熱，他們更容易受到熱衰退。熱衰退是誘發(fā)重復(fù)的硬盤停止和結(jié)果的減少鼓形輪子連接的摩擦和增加車輛的制動(dòng)距離。作為一項(xiàng)規(guī)則，高品質(zhì)的襯里，將顯示低于劣質(zhì)的熱褪色。此外碟式剎車比鼓式制動(dòng)器耐熱褪色性能更好。
另一個(gè)褪色的類型，剎車容易褪色水。鼓式制動(dòng)器，其表面積大，在安全范圍內(nèi)比盤式制動(dòng)器每平方英寸之間需要更少的襯力和鼓力。加上鼓的保水的形狀，鞋和鼓之間的潮濕條件下促進(jìn)水面滑行。結(jié)果是制動(dòng)距離大大增加。
盤式制動(dòng)器，具有較小的摩擦表面和高夾緊力，做一個(gè)良好的工作從轉(zhuǎn)子擦水，并顯示在潮濕時(shí)停止能力幾乎沒有減少。
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