英语翻译Planetary gears are used mainly in final drives,rear-mou

英语翻译Planetary gears are used mainly in final drives,rear-mounted splitter boxes and,above all,in automatic transmission.The simplest form of planetary gear (the planetary-gear train) consists of sun gear,ring gear and arm with planet gears.Each of the three elements may act as drive,output or may be atrest.Therefore,gear variation is possible as regards the transmission ratio,ranging as far as a reversal of the direction of rotation.The parallel meshing of several gears under load permits a compact construction.The gear unit has no free bearing forces,allows high torques and power branching,has good efficiency,and input and output are coaxial.Power shifting is possible,since friction-type shifting devices (clutches and brake bands ) can be used.3.6 DifferentialWhen a vehicle is cornered the inner wheel moves through a shorter distance than the outer wheel.This means that the inner wheel must slow down and the outer wheel must speed up.During this period it is desirable that each driving wheel maintains its driving action.The differential performs these two tasks.The principle of the bevel type differential can be seen if the unit its considered as two discs and a lever (Fig.3-19).When the vehicle is travelling straight,the lever will divide the driving force equally and both discs will move the same amount.When the vehicle corners,the driving force will still be divided equally but the inner disc will now move through a smaller distance; this will cause the lever to pivot about its center which will prize forward the outer disc to give it a greater movement.This action shows that the torque applied to each driving wheel is always equal-hence the differential is sometimes called a torque equalizer.3.8 Front-Wheel Drive (FWD)前轮驱动Many modern cars have front-engine front-drive layouts (Fig.3-20).Here the engine still mounts at the front of the vehicle.However,the parts do not align exactly as they do in conventional drive lines.The transmission and final drive transmit engine power and torque to the front wheels.Thus,the front wheels“pull”the vehicle along the road.In conventional systems,the rear wheels“push”the vehicle along.Front-wheel drive has advantages over rear-wheel drive.Because the engine and transmission components in a FWD vehicle are packaged compactly at the front of the car,the use of interior space is more efficient.Thus,a relatively small car can have a reasonable amount of room inside for passengers and luggage.In addition,the weight of the FWD powertrain (engine and transmission) is directly over the driving wheels.This positioning helps to increase traction on slippery surfaces.