专业英语复习范围

UNIT 10 THE DIFFERENTIAL P99Most cars have a standard, or unlimited-slip, differential. (A limited-slip differential costs more and is not necessary in most normal driving conditions.) As mentioned, a differential must split power unequally when a car goes around a corner. Like the transmission, the differential has many gears and parts.10.1 The Main GearsSuppose a shaft spins in one direction. You want that shaft to drive another shaft at an angle of 90 to the first shaft. You can do this by meshing two gears, each having teeth set at a 45 angle. There would be one gear on each shaft. Together, the two gears produce a 90 change in direction. Figure 10-1. Gears with teeth at such an angle are called bevel gears because they are beveled, or cut on an angle.The bevel gears in Figure 10-1 work well when one shaft must drive another. But in a final drive assembly, two axle shafts must be driven. (One shaft goes to each drive wheel.) Then one of the beveled gears must have a hole in its center through which the shaft extends. The large circular gear on the axle shafts is called the ring gear. Figure 10-1.The final drive assembly also provides gear reduction so that the drive wheels spin more slowly than the drive shaft. The gear reduction varies, depending on engine size and power, engine torque, and vehicle size and weight. In general most final drives produce gear reduction ratios of about 2.50: 1 to 3.50: 1. Thus, the drive wheels turn at about one-third the speed of the drive shaft. (If the drive shaft spins at 3,000 rpm, the drive wheels turn at about 1,000 rpm.) To give the necessary reduction, the driving gear in the final drive must be smaller than the driven gear, which is the ring gear. Thus, the driving gear must make several turns for every single turn of the ring gear. The driving gear is called the pinion gear or pinion. Figure 10-1. The drive shaft turns the pinion. The pinion rotates the ring gear and axle shafts. If the teeth of the beveled gears are curved, the pinion meshes more easily and quietly with the larger ring gear. Now we can refer to the gears as spiral bevel gears. Figure 10-2 shows a set of spiral bevel gears.10.2 The Differential SystemThe final drive works fine as long as both drive wheels turn at the same speed. However, this system cannot provide different rates of speed for different wheels. As noted earlier, a differential system is needed to allow for speed differences. This gear system is the heart of the final drive assembly.The differential system has a differential case. Figure 10-3. This case houses the differential gears. The ring gear bolts to a flange on one side of the case. When the pinion drives the ring gear, the differential case also turns. The case has circular holes through which the axle shafts fit. Thus, the inside ends of the axle shafts are inside the case. On the end of each axle shaft, there is a bevel gear called a side gear or differential gear. In fact, the side gears are splined to the axle shafts. Whenever the side gears turn, the axle shafts turn.10.3 How the Differential System WorksRefer to Figure 10-5. In normal, straight-ahead driving, the drive shaft turns the drive pinion, which turns the ring gear. The ring gear turns the entire differential case because it is bolted solidly to the case. Since the pinion shaft is bolted to the case, it revolves with the case. The differential pinions also turn end-over-end with the pinion shaft. The differential pinions press against the side gears, and the side gears rotate. Since the axle shafts are splined to the side gears the shafts also rotate.When a car moves straight ahead, the drive wheels have an equal amount of traction. The forces on the differential pinions are equal, and the differential pinions will not turn about the pinion shaft. Instead, the differential pinions act as if they were locked to the pinion shaft. Now both the side gears and axle shafts move at the same speed. Power flows through the drive pinion, ring gear, case, and pinion shaft to the differential pinions. Then power splits equally and goes to the side gears, axle shafts, and drive wheels.When a car goes around a corner, power can no longer be divided evenly between the two side gears when it reaches the differential pinions. The inside wheel turns more slowly and lowers the speed of the inside axle shaft. Figure 10-6. As a result, the differential pinions begin to rotate about the pinion shaft. The rotation of the differential pinions makes the outside gear speed up. As a result, the outside wheel turns faster than the inside wheel.Reading material: Disk Brake P115Disc brakes, like many automotive innovations, were originally developed for auto racing, but are now standard equipment on virtually every car made. On most cars, the front brakes are of the disc type, and the rear brakes are of the drum type. Drum brakes use two semi-circular shoes to press outward against the inner surfaces of a steel drum. Older cars often had drum brakes on all four wheels, and many new cars now have 4-wheel disc brakes.Though disc brakes rely on the same basic principles to slow a vehicle (friction and heat), their design is far superior to that of drum brakes. Because disc brakes can fling off water more easily than drum brakes, they work much better in wet conditions. This is not to say that water does not affect them, it definitely does. If you splash through a puddle and then try to apply the brakes, your brakes may not work at all for a few seconds! Disc brakes also allow better airflow cooling, which also increases their effectiveness. Some high performance disc brakes have drilled or slotted holes through the face of the rotor, which helps to prevent the pads from glazing (becoming hardened due to heat). Disc brakes were introduced as standard equipment on most cars in the early seventies.The main components of a disk brake (Figure 11-5) are the Brake Pads, Rotor (Disc), Caliper and Caliper Support.Reading material: Wheel Alignment P126The purpose of proper wheel alignment is to provide maximum safety, ease of handling, stability, and directional control of the vehicle. This requires that each of the steering angles (steering geometry) be adjusted to the specifications recommended by the vehicles require different settings. Follow the specific shop manual for each vehicle. The wheels must also be in proper dynamic and static balance to achieve these purposes.Steering geometry refers to the angels formed by the steering and suspension parts in relationship to the frame and body of the vehicle. These angles include camber, caster, steering axis inclination, toe in (toe out), and toe out on turn (turning radius). Ideally, the vehicle center line, geometric center line, and the thrust line would all be identical and the car would form a perfect 90 rectangle. Because of factory tolerances and the unitized construction common to todays cars, this is rarely, if ever, the case.On all vehicles it is important to remember that the rear axle dictates the position of the front wheels. On frame-type vehicles, two-wheel alignments are taken from the frame and the rear axle is assumed to be in correct alignment. On unitized vehicles with four wheel independent suspension, there is no frame to work with, so we can no longer assume that rear wheels are in correct alignment; therefore, four-wheel alignment is necessary to give proper steering and handling.Tracking and wheelbaseFor proper tracking, all four wheels must be parallel to the frame. This requires that the wheelbase be equal on both sides of the vehicle. The four wheels should be positioned to form a rectangle. CamberCamber is the inward or outward tilt of the wheel at the top. Inward tilt is negative camber and outward tilt is positive camber. The tilt of the wheel (camber) is measured in degrees and is adjustable on many vehicles.CasterCaster is the forward or backward tilt of the spindle or steering knuckle at the top when viewed from the side. Forward tilt is negative caster and backward tilt is positive caster. Caster is measured in the number of degrees that it is forward or backward from true vertical and is adjustable on many vehicles.Steering Axis inclinationSteering axis inclination is the inward tilt of the steering knuckle at the top. Steering axis inclination is measure in degrees and is not adjustable. If incorrect, suspension parts are at fault and must be replaced.Toe-in (Toe-Out)Toe-in occurs when the front wheels are slightly closer together at the front wheels than at the rear. Toe-in is measured in inches, millimeters, or degrees. A limited amount of toe-in or toe-out is needed to allow for the fact that the wheels spread apart or come together slightly at the front when driving down the road, depending on vehicle design. This provides a zero running toe and no tire scuffing.Incorrect toe-in or toe-out is the most frequent cause of rapid tire-tread wearing. Toe setting is the last adjustment to be made when performing a wheel alignment. On most front-wheel-drive vehicles, toe-out setting is required to provide a zero running toe. This is because the driving front wheels are trying to go around the steering axis inclination pivot point with a negative scrub radius. Rear wheels are designed with zero to slight toe-in, depending on the vehicle. This provides straight running as driving forces tend to push back the rear spindles. Correct toe is important for increasing tire life.Center Point SteeringAlthough not technically an alignment angle, this causes more customer complaints than any other condition. Customers generally know little about alignment and to them, when the steering wheel is not centered, the car is not aligned correctly.Wheel Alignment ProcedureCustomer and vehicle safety depend on the technician ability to follow proper procedures and specifications. To achieve this, the following factors should be included.1. Perform all pre-alignment checks properly to determine extent of repairs required.2. The vehicle s steering and suspension system, including tires, should be in good condition before attempting alignment.3. Use all alignment equipment as recommended by manufacturer.4. Tighten all fasteners to specified torque.5. Install cotter pins wherever required.6. Observe all safety precautions when positioning the vehicle on the alignment machine.Reading material: An Introduction to Active Suspension Systems P157Background Traditionally automotive suspension designs have been a compromise between the three conflicting criteria of road holding, load carrying and passenger comfort.The suspension system must support the vehicle, provide directional control during handling manoeuvres and provide effective isolation of passengers/payload from road disturbances Wright 84. Good ride comfort requires a soft suspension, whereas insensitivity to applied loads requires stiff suspension. Good handling requires a suspension setting somewhere between the two.Due to these conflicting demands, suspension design has had to be something of a compromise, largely determined by the type of use for which the vehicle was designed. Active suspensions are considered to be a way of increasing the freedom one has to specify independently the characteristics of load carrying, handling and ride quality.A passive suspension system has the ability to store energy via a spring and to dissipate it via a damper. Its parameters are generally fixed, being chosen to achieve a certain level of compromise between road holding, load carrying and comfort.An active suspension system has the ability to store, dissipate and to introduce energy to the system. It may vary its parameters depending upon operating conditions and can have knowledge other than the strut deflection the passive system is limited to.Reading material: Air Bags P157For years, the trusty seat belt provided the sole form of passive restraint in our cars. There were debates about their safety, especially relating to children, but over time, much of the country adopted mandatory seat-belt laws. Statistics have shown that the use of seat belts has saved thousands of lives that might have been lost in collisions.Air bags have been under development for many years. The attraction of a soft pillow to land against in a crash must be very strong. In the 1980s, the first commercial air bags appeared in automobiles .Since model year 1998, all new cars have been required to have air bags on both driver and passenger sides. (Light trucks came under the rule in 1999. ) To date, statistics show that air bags reduce the risk of dying in a direct frontal crash by about 30 percent. Some experts say that within the next few years, our cars will go from having dual air bags to having six or even eight airWhat an air bag wants to do is to slow the passenger s speed to zero with little or no damage. The constraints that it has to work within are huge. The air bag has the space between the passenger and the steering wheel or dash board and a fraction of a second to work with. Even that tiny amount of space and time is valuable, however, if the system can slow the passenger evenly rather than forcing an abrupt halt to his or her motion.The inflation system (Figure 16-4, 5) is not unlike a solid rocket booster. The air bag system ignites a solid propellant, which burns extremely rapidly to create a large volume of gas to inflate the bag. The bag then literally bursts from its storage site at up to 200 mph (322 kph)faster than the blink of an eye! A second later, the gas quickly dissipates through tiny holes in the bag, thus deflating the bag so you can move.The Future of Air BagsActivities aimed at maintaining and improving the lif