制动器中英文翻译.pdf

The Brake BibleBrakes - what do they do?The simple answer: they slow you down.Thecomplexanswer:brakesaredesignedtoslowdownyourvehicle but probably not by the means that you think. Thecommon misconception is that brakes squeeze against adrum or disc, and the pressure of the squeezing action iswhatslowsyoudown.Thisinfactisonlypartoftheequation.Brakesareessentially amechanismtochangeenergytypes.When youre travelling at speed, your vehicle has kineticenergy. When you apply the brakes, the pads or shoes thatpress against the brake drum or rotor convert that energyinto thermal energy via friction. The cooling of the brakesdissipates theheat and thevehicle slows down. Its the FirstLaw of Thermodynamics, sometimes known as the law ofconservation of energy. This states that energy cannot becreated nor destroyed, it can only be converted from oneform to another. In the case of brakes, it is converted fromkinetic energy to thermal energy.Angular force. Because of the configuration of the brakepads and rotor in a disc brake, the location of the point ofcontact where the friction is generated also provides amechanicalmomenttoresisttheturningmotionoftherotor.Thermodynamics, brake fade and drilledrotors.1If you ride a motorbike or drive a race car, youreprobably familiar with the term brake fade, used todescribe what happens to brakes when they get too hot.A good example is coming down a mountain pass usingyour brakes rather than your engine to slow you down.As you start to come down the pass, the brakes on yourvehicleheatup,slowingyoudown.Butifyoukeepusingthem, the rotors or drums stay hot and get no chance tocool off. At some point they cant absorb any more heatso the brake pads heat up instead. In every brake padthere is the friction material that is held together withsomesortofresinandoncethisstartstogettoohot,theresinstartstovapourise,formingagas.Becausethegascant stay between the pad and the rotor, it forms a thinlayer between the two whilst trying to escape. The padslose contact with the rotor, reducing the amount offriction and voila. Complete brake fade.Thetypicalremedyforthiswouldbetogetthevehicletoa stop and wait for a few minutes. As the brakecomponents cool down, their ability to absorb heatreturnsandthenexttimeyouusethebrakes,theyseemto work just fine. This type of brake fade was morecommon in older vehicles. Newer vehicles tend to haveless outgassing from the brake pad compounds but theystill suffer brake fade. So why? Its still to do with thepads getting too hot. With newer brake pad compounds,the pads transfer heat into the calipers once the rotorsare too hot, and the brake fluid starts to boil formingbubbles in it. Because air is compressible (brake fluidisnt) when you step on the brakes, the air bubblescompress instead of the fluid transferring the motion tothe brake calipers. Voila. Modern brake fade.2So how do the engineers design brakes to reduce oreliminate brake fade? For older vehicles, you give thatvapourised gas somewhere to go. For newer vehicles,youfindsomewaytocooltherotorsoffmoreeffectively.Either way you end up with cross-drilled or groovedbrakerotors.Whilegroovingthesurfacemay reducethespecific heat capacity of the rotor, its effect is negligiblein the grand scheme of things. However, under heavybraking once everything is hot and the resin isvapourising, the grooves give the gas somewhere to go,so the pad can continue to contact the rotor, allowingyou to stop.The whole understanding of the conversion of energy iscritical in understanding how and why brakes do whatthey do, and why they are designed the way they are. Ifyouve ever watched Formula 1 racing, youll see thefront wheels have huge scoops inside the wheel pointingtothefront(see thepicture above).This is toductairtothe brake components to help them cool off because inF1 racing, the brakes are used viciously every fewseconds and spend a lot of their time trying to stay hot.Without some form of cooling assistance, the brakeswould be fine for the first few corners but then wouldfade and become near useless by half way around thetrack.Rotor technology.Ifabrakerotorwas asinglecastchunkofsteel,itwouldhave terrible heat dissipation properties and leavenowhere for the vapourised gas to go. Because of this,brake rotors are typically modified with all manner ofextra design features to help them cool down as quicklyas possible as well as dissapate any gas from betweenthe pads and rotors. The diagram here shows someexamples of rotor types with the various modificationthat can be done to them to help them create morefriction, disperse more heat more quickly, and ventilategas. From left to right.31:Basicbrakerotor.2:Groovedrotor-thegroovesgivemore bite and thus more friction as they pass betweenthebrakepadsTheyalsoallowgastoventfrombetweenthe pads and the rotor. 3: Grooved, drilled rotor - thedrilled holes again give more bite, but also allow aircurrents (eddies) to blow through the brake disc toassist cooling and ventilating gas. 4: Dual ventilatedrotors - same as before but now with two rotors insteadof one, and with vanes in between them to generate avortex which will cool the rotors even further whilsttrying to actually suck any gas away from the pads.Animportantnoteaboutdrilledrotors:Drilledrotorsaretypically only found (and to be used on) race cars. Thedrilling weakens the rotors and typically results inmicrofractures to the rotor. On race cars this isnt aproblem - the brakes are changed after each race orweekend. But on a road car, this can eventually lead tobrake rotor failure - not what you want. I only mentionthisbecauseofalotofperformancesuppliers will supplyyouwithdrilledrotorsforstreetcarswithoutmentioningthis little fact.Big rotors.How does all this apply to bigger brake rotors - acommon sports car upgrade? Sports cars and race bikestypically have much bigger discs or rotors than youraverage family car. A bigger rotor has more material init so it can absorb more heat. More material also meansa larger surface area for the pads to generate frictionwith, and better heat dissipation. Larger rotors also putthe point of contact with the pads further away from theaxle of rotation. This provides a larger mechanicaladvantage to resist the turning of the rotor itself. Tobest illustrate how this works, imagine a spinning steeldisc on an axle in front of you. If you clamped yourthumbs either side of the disc close to the middle, your4thumbs would heat up very quickly and youd need topush pretty hard to generate the friction required toslow the disc down. Now imagine doing the same thingbut clamping your thumbs together close to the outerrim of the disc. The disc will stop spinning much morequickly and your thumbs wont get as hot. That, in anutshell explains the whole principle behind why biggerrotors = better stopping power.The different types of brake.All brakes work by friction. Friction causes heat which ispart of the kinetic energy conversion process. How theycreate friction is down to the various designs.Bicycle wheel brakesIthoughtIdcoverthesebecausetheyreaboutthemostbasic type of functioning brake that you can see, watchworking, and understand. The construction is verysimple and out-in-the-open. A pair of rubber blocks areattached to a pair of calipers which are pivoted on theframe. When you pull the brake cable, the pads arepressed against the side or inner edge of the bicyclewheel rim. The rubber creates friction, which createsheat, which is the transfer of kinetic energy that slowsyou down. Theres only really two types of bicycle brake- those on which each brake shoe shares the same pivotpoint,andthosewithtwo pivot points.Ifyoucanlook ata bicycle brake and not understand whats going on, therest of this page is going to cause you a bit of aheadache.5Drum brakes - single leading edgeThe next, more complicated type of brake is a drumbrake. The concept here is simple. Two semicircularbrake shoes sit inside aspinning drum which is attachedto the wheel. When you apply the brakes, the shoes areexpanded outwards to press against the inside of thedrum. This creates friction, which creates heat, whichtransfers kinetic energy, which slows you down. Theexample below shows a simple model. The actuator inthis case is the blue elliptical object. As that is twisted,itforcesagainstthebrakeshoes andinturnforces themto expand outwards. The return spring is what pulls theshoes back away from the surface of the brake drumwhen the brakes are released. See the later section formore information on actuator types.The single leading edge refers to the number of partsof the brake shoe which actually contact the spinningdrum. Because the brake shoe pivots at one end, simplegeometry means that the entire brake pad cannotcontact the brake drum. The leading edge is the termgiventothepartofthebrakepadwhich does contactthedrum, and in the case of a single leading edge system,its the part of the pad closest to the actuator. Thisdiagram (right) shows what happens as the brakes areapplied. The shoes are pressed outwards and the part ofthe brake pad which first contacts the drum is theleading edge. The action of the drum spinning actuallyhelpstodrawthebrakepadoutwardsbecauseoffriction,which causes the brakes to bite. The trailing edge ofthebrake shoe makes virtually no contact with the drumat all. This simple geometry explains why its reallydifficult to stop a vehicle rolling backwards if itsequipped only with single leading edge drum brakes. As6the drum spins backwards, the leading edge of the shoebecomes the trailing edge and thus doesnt bite.Drum brakes - double leading edgeThe drawbacks of the single leading edge style of drumbrake can be eliminated by adding a second returnspringandturningthepivotpointintoasecondactuator.Now when the brakes are applied, the shoes are pressedoutwards at two points. So each brake pad now has oneleading and one trailing edge. Because there are twobrake shoes, there are two brake pads, which meansthere are two leading edges. Hence the name doubleleading edge.Disc brakesSome background. Disc