英语翻译DisadvantagesDrum brakes are designed to convert kinetic

英语翻译
Disadvantages
Drum brakes are designed to convert kinetic energy into heat energy via the process of friction.This heat is intended to be further transferred to atmosphere,but can just as easily transfer into other components of the braking system.
Brake drums have to be substantial pieces of steel or cast-iron to cope with the forces that are involved which can retain a lot of heat.Heat transfer to atmosphere can be aided by incorporating fins into the design of the drum (see heat sink).However,excessive heating can occur due to heavy or repeated braking which can cause the drum to distort,leading to vibration under braking.
The other consequence of overheating is brake fade.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 increases slightly due to thermal expansion of the material,this means the brakes shoes have to move farther and the brake pedal has to be depressed more.
2.The properties of the friction material can change if heated,creating less friction.This is usually only temporary and the material regains its efficiency when cooled,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,which reduces the hydraulic pressure being applied to the brake shoes.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 occurs sooner.
Brake 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.The water tends to stay there until it is heated sufficiently to vapourise,at which point braking efficiency is fully restored.
Disc brakes are not immune to any of these processes,but they deal with heat and water more effectively than drums.