固体火箭发动机原理两相流动力学跟踪研究正文大学论文.doc

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1、Drum brakeA drum brake with the drum removed as used on the rear wheel of a car or truck. Note that in this installation, a cable-operated parking brake uses the service shoes.A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against a rotating drum-shaped

2、part called a brake drum.The term drum brake usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disk brake, it is sometime

3、s called a pinch drum brake, although such brakes are relatively rare. A related type of brake uses a flexible belt or band wrapping around the outside of a drum, called a band brake.HistoryA drum brake at the rear wheel of a motorbikeKawasaki W800The modern automobile drum brake was invented in 190

4、2 by Louis Renault, though a less-sophisticated drum brake had been used by Maybach a year earlier. In the first drum brakes, the shoes were mechanically operated with levers and rods or cables. From the mid-1930s the shoes were operated with oil pressure in a small wheel cylinder and pistons (as in

5、 the picture), though some vehicles continued with purely-mechanical systems for decades. Some designs have two wheel cylinders.The shoes in drum brakes are subject to wear and the brakes needed to be adjusted regularly until the introduction of self-adjusting drum brakes in the 1950s. In the 1960s

6、and 1970s brake drums on the front wheels of cars were gradually replaced with disc brakes and now practically all cars use disc brakes on the front wheels, with many offering disc brakes on all wheels. However, drum brakes are still often used for handbrakes as it has proven very difficult to desig

7、n a disc brake suitable for holding a car when it is not in use. Moreover, it is very easy to fit a drum handbrake inside a disc brake so that one unit serves as both service brake and handbrake.Early type brake shoes contained asbestos. When working on brake systems of older cars, care must be take

8、n not to inhale any dust present in the brake assembly. The United States Federal Government began to regulate asbestos production, and brake manufacturers had to switch to non-asbestos linings. Owners initially complained of poor braking with the replacements; however, technology eventually advance

9、d to compensate. A majority of daily-driven older vehicles have been fitted with asbestos-free linings. Many other countries also limit the use of asbestos in brakes. Early automotive brake systems, after the era of hand levers of course, used a drum design at all four wheels. They were called drum

10、brakes because the components were housed in a round drum that rotated along with the wheel. Inside was a set of shoes that, when the brake pedal was pressed, would force the shoes against the drum and slow the wheel. Fluid was used to transfer the movement of the brake pedal into the movement of th

11、e brake shoes, while the shoes themselves were made of a heat-resistant friction material similar to that used on clutch plates. This basic design proved capable under most circumstances, but it had one major flaw. Under high braking conditions, like descending a steep hill with a heavy load or repe

12、ated high-speed slow downs, drum brakes would often fade and lose effectiveness. Usually this fading was the result of too much heat build-up within the drum. Remember that the principle of braking involves turning kinetic energy (wheel movement) into thermal energy (heat). For this reason, drum bra

13、kes can only operate as long as they can absorb the heat generated by slowing a vehicles wheels. Once the brake components themselves become saturated with heat, they lose the ability to halt a vehicle, which can be somewhat disconcerting to the vehicles operator.Self-applying characteristicDrum bra

14、kes have a natural self-applying characteristic.1 The rotation of the drum can drag either or both of the shoes into the friction surface, causing the brakes to bite harder, which increases the force holding them together. This increases the stopping power without any additional effort being expende

15、d by the driver, but it does make it harder for the driver to modulate the brakes sensitivity. It also makes the brake more sensitive to brake fade, as a decrease in brake friction also reduces the amount of brake assist.Disc brakes exhibit no self-applying effect because the hydraulic pressure acti

16、ng on the pads is perpendicular to the direction of rotation of the disc. Disc brake systems usually have servo assistance (Brake Booster) to lessen the drivers pedal effort, but some disc braked cars (notably race cars) and smaller brakes for motorcycles, etc., do not need to use servos.Note: In mo

17、st designs, the self applying effect only occurs on one shoe. While this shoe is further forced into the drum surface by a moment due to friction, the opposite effect is happening on the other shoe. The friction force is trying to rotate it away from the drum. The forces are different on each brake

18、shoe resulting in one shoe wearing faster. It is possible to design a two-shoe drum brake where both shoes are self-applying (having separate actuators and pivoted at opposite ends), but these are very uncommon in practice.Drum brake designsRendering of a drum brakeDrum brakes are typically describe

19、d as either leading/trailing or twin leading.1Rear drum brakes are typically of a leading/trailing design(For Non Servo Systems), or Primary/Secondary (For Duo Servo Systems) the shoes being moved by a single double-acting hydraulic cylinder and hinged at the same point.1 In this design, one of the

20、brake shoes will always experience the self-applying effect, irrespective of whether the vehicle is moving forwards or backwards.1 This is particularly useful on the rear brakes, where the footbrake (handbrake or parking brake) must exert enough force to stop the vehicle from travelling backwards an

21、d hold it on a slope. Provided the contact area of the brake shoes is large enough, which isnt always the case, the self-applying effect can securely hold a vehicle when the weight is transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A further advantage

22、 of using a single hydraulic cylinder on the rear is that the opposite pivot may be made in the form of a double lobed cam that is rotated by the action of the parking brake system.Front drum brakes may be of either design in practice, but the twin leading design is more effective.1 This design uses

23、 two actuating cylinders arranged so that both shoes will utilize the self-applying characteristic when the vehicle is moving forwards.1 The brake shoes pivot at opposite points to each other.1 This gives the maximum possible braking when moving forwards, but is not so effective when the vehicle is

24、traveling in reverse.1The optimum arrangement of twin leading front brakes with leading/trailing brakes on the rear allows for more braking force to be deployed at the front of the vehicle when it is moving forwards, with less at the rear. This helps to prevent the rear wheels locking up, but still

25、provides adequate braking at the rear when it is needed.1The brake drum itself is frequently made of cast iron, although some vehicles have used aluminum drums, particularly for front-wheel applications. Aluminum conducts heat better than cast iron, which improves heat dissipation and reduces fade.

26、Aluminum drums are also lighter than iron drums, which reduces unsprung weight. Because aluminum wears more easily than iron, aluminum drums will frequently have an iron or steel liner on the inner surface of the drum, bonded or riveted to the aluminum outer shell.AdvantagesDrum brakes are used in m

27、ost heavy duty trucks, some medium and light duty trucks, and few cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels since most of the stopping force is generated by the front brakes of the vehicle and therefore the heat generated in the rear is significantly less. Drum bra

28、kes allow simple incorporation of a parking brake. Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the rear wheels use disk brakes as the main brakes. In this situation, a small drum is usually fitted within or as part of the brake disk also known as a banksia

29、 brake.In hybrid vehicle applications, wear on braking systems is greatly reduced by energy recovering motor-generators (see regenerative braking), so some hybrid vehicles such as the GMC Yukon hybrid and Toyota Prius (except the third generation) use drum brakes.Disc brakes rely on pliability of ca

30、liper seals and slight runout to release pads, leading to drag, fuel mileage loss, and disc scoring. Drum brake return springs give more positive action and, adjusted correctly, often have less drag when released.Certain heavier duty drum brake systems compensate for load when determining wheel cyli

31、nder pressure; a feature unavailable when disks are employed. One such vehicle is the Jeep Comanche. The Comanche can automatically send more pressure to the rear drums depending on the size of the load, whereas this would not be possible with disks.Due to the fact that a drum brakes friction contac

32、t area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake. The increased friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and

33、 braking force. Drum brakes retain heat and are more complex than disc brakes but are often times the more economical and powerful brake type to use in rear brake applications due to the low heat generation of rear brakes, a drum brakes self applying nature, large friction surface contact area, and

34、long life wear characteristics(%life used/kW of braking power).Brake technology, just like suspension technology and fuel-system technology, has come a long way in recent years. What began in the 60s as a serious attempt to provide adequate braking for performance cars has ended in an industry where

35、 brakes range from supremely adequate to downright phenomenal. The introduction of components like carbon fiber, sintered metal and lightweight steel, along with the adoption of ABS, have all contributed to reduced stopping distances and generally safer vehicles (though ABS continues to provide cont

36、roversy). Although drum brakes are often the better choice for rear brake applications in all but the highest performance applications, vehicle manufactures are increasingly installing disc brake system at the rear wheels. This is due to the popularity rise of disc brakes after the introduction fron

37、t ventilated disc brakes. Front ventilated disc brakes performed much better than the front drum brakes they replaced. The difference in front drum and disc brake performance caused car buyers to purchase cars that also had rear disc brakes. Additionally rear disc brakes are often associated with hi

38、gh performance race cars which has increase their popularity in street cars. Rear disc brakes in most applications are not ventilated and offer no performance advantage over drum brakes. Even when rear discs are ventilated, it is likely that the rear brakes will never benefit from the ventilation un

39、less subjected to very high performance racing style driving.DisadvantagesDrum brakes, like most other types, are designed to convert kinetic energy into heat by friction.1 This heat is intended to be further transferred to atmosphere, but can just as easily transfer into other components of the bra

40、king system.Brake drums have to be large to cope with the massive forces that are involved, and they must be able to absorb and dissipate a lot of heat. Heat transfer to atmosphere can be aided by incorporating cooling fins onto the drum. However, excessive heating can occur due to heavy or repeated

41、 braking, which can cause the drum to distort, leading to vibration under braking.The other consequence of overheating is brake fade.1 This is due to one of several processes or more usually an accumulation of all of them.1. When the drums are heated by hard braking, the diameter of the drum increas

42、es slightly due to thermal expansion, this means the brakes shoes have to move farther and the brake pedal has to be depressed further.2. The properties of the friction material can change if heated, resulting in less friction. This is usually only temporary and the material regains its efficiency w

43、hen cooled,1 but if the surface overheats to the point where it becomes glazed the reduction in braking efficiency is more permanent. Surface glazing can be worn away with further use of the brakes, but that takes time.3. Excessive heating of the brake drums can cause the brake fluid to vapourise, w

44、hich reduces the hydraulic pressure being applied to the brake shoes.1 Therefore less retardation is achieved for a given amount of pressure on the pedal. The effect is worsened by poor maintenance. If the brake fluid is old and has absorbed moisture it thus has a lower boiling point and brake fade

45、occurs sooner.1Brake fade is not always due to the effects of overheating. If water gets between the friction surfaces and the drum, it acts as a lubricant and reduces braking efficiency.1 The water tends to stay there until it is heated sufficiently to vapourise, at which point braking efficiency i

46、s fully restored. All friction braking systems have a maximum theoretical rate of energy conversion. Once that rate has been reached, applying greater pedal pressure will not result in a change of this rate, and indeed the effects mentioned can substantially reduce it. Ultimately this is what brake

47、fade is, regardless of the mechanism of its causes.Disc brakes are not immune to any of these processes, but they deal with heat and water more effectively than drums.Drum brakes can be grabby if the drum surface gets light rust or if the brake is cold and damp, giving the pad material greater frict

48、ion. Grabbing can be so severe that the tires skid and continue to skid even when the pedal is released. Grabbiness is the opposite of fade: when the pad friction goes up, the self-assisting nature of the brakes causes application force to go up. If the pad friction and self-amplification are high e

49、nough, the brake will stay on due to self-application even when the external application force is released.Another disadvantage of drum brakes is their complexity. A person must have a general understanding of how drum brakes work and take simple steps to ensure the brakes are reassembled correctly when doing work on drum brakes. Incompetent mechanics should not attempt working on drum brakes.Re-arcingBefore 1984, it was common to re-arc b