鼓式制动器外文文献及翻译大学论文Word格式.docx

1、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 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 usu

2、ally called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disk brake, it is sometimes 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

3、 brake.HistoryA drum brake at the rear wheel of a motorbikeKawasaki W800The modern automobile drum brake was invented in 1902 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 a

4、nd rods or cables. From the mid-1930s the shoes were operated with oil pressure in a small wheel cylinder and pistons （as in 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

5、 the brakes needed to be adjusted regularly until the introduction of self-adjusting drum brakes in the 1950s. In the 1960s 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

6、disc brakes on all wheels. However, drum brakes are still often used for handbrakes as it has proven very difficult to design 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 b

7、rake and handbrake.Early type brake shoes contained asbestos. When working on brake systems of older cars, care must be taken 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

8、-asbestos linings. Owners initially complained of poor braking with the replacements; however, technology eventually advanced 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 aut

9、omotive brake systems, after the era of hand levers of course, used a drum design at all four wheels. They were called drum 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 s

10、hoes against the drum and slow the wheel. Fluid was used to transfer the movement of the brake pedal into the movement of the 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 cir

11、cumstances, but it had one major flaw. Under high braking conditions, like descending a steep hill with a heavy load or repeated 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

12、 principle of braking involves turning kinetic energy （wheel movement） into thermal energy （heat）. For this reason, drum brakes 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

13、ability to halt a vehicle, which can be somewhat disconcerting to the vehicles operator.Self-applying characteristicDrum brakes 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, wh

14、ich increases the force holding them together. This increases the stopping power without any additional effort being expended 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 fric

15、tion also reduces the amount of brake assist.Disc brakes exhibit no self-applying effect because the hydraulic pressure acting 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, b

16、ut some disc braked cars （notably race cars） and smaller brakes for motorcycles, etc., do not need to use servos.Note: In most 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 happen

17、ing on the other shoe. The friction force is trying to rotate it away from the drum. The forces are different on each brake 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

18、ends）, but these are very uncommon in practice.Drum brake designsRendering of a drum brakeDrum brakes are typically described 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） t

19、he shoes being moved by a single double-acting hydraulic cylinder and hinged at the same point.1 In this design, one of the 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 brake

20、s, where the footbrake （handbrake or parking brake） must exert enough force to stop the vehicle from travelling backwards and 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 we

21、ight is transferred to the rear brakes due to the incline of a slope or the reverse direction of motion. A further advantage 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 sy

22、stem.Front drum brakes may be of either design in practice, but the twin leading design is more effective.1 This design uses 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 point

23、s to each other.1 This gives the maximum possible braking when moving forwards, but is not so effective when the vehicle is 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

24、 the vehicle when it is moving forwards, with less at the rear. This helps to prevent the rear wheels locking up, but still 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 fo

25、r front-wheel applications. Aluminum conducts heat better than cast iron, which improves heat dissipation and reduces fade. 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 ste

26、el liner on the inner surface of the drum, bonded or riveted to the aluminum outer shell.AdvantagesDrum brakes are used in most 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

27、is generated by the front brakes of the vehicle and therefore the heat generated in the rear is significantly less. Drum brakes 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 th

28、e main brakes. In this situation, a small drum is usually fitted within or as part of the brake disk also known as a banksia 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

29、 as the GMC Yukon hybrid and Toyota Prius （except the third generation） use drum brakes.Disc brakes rely on pliability of caliper 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

30、, often have less drag when released.Certain heavier duty drum brake systems compensate for load when determining wheel cylinder 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 dependi

31、ng on the size of the load, whereas this would not be possible with disks.Due to the fact that a drum brakes friction contact 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 sh

32、oes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and 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