Lecture 1 – B.Tech Mechanical (6th Sem Automobile Engineering), Mewar University (2013-2014)

Lecture 1 – B.Tech Mechanical (6th Sem Automobile Engineering), Mewar University (2013-2014)


judge when the truck goes around the corner the riders on the outside of the turn have to adjust their speed to keep even with the riders on the inside the man on the outside has to ride a lot farther and a lot faster in order to keep up with the parade the outside wheels must spin faster than the wheels on the inside because they have a greater distance to travel in the same length of time when a wagon turns a corner the wheels can travel at different speeds because each one can turn freely on the axles and in the early automobiles the rear wheels turn separately and only one wheel was connected to the engine but when only one wheel was driven by the engine it had to do all the work and it couldn’t get a good enough grip on the road to do its job properly so the one wheel drive was soon out of date but if two wheels are locked on an axle so that they are not free to turn separately one or the other has to slide so engineers had to find a way to connect both rear wheels to the engine without sliding and slipping on turns the device which makes this possible is a part of the rear axle it is called the differential because it can drive the rear wheels at different speeds the differential looks complicated but once we understand its principle it is amazingly simple these two wheels are mounted on separate axles and supported by a frame so that they can revolve freely at different speeds let’s fasten a spoke on the inner end of each axle so that by turning the spokes we can turn each wheel separately with a bar or crosspiece we can turn both wheels in the same direction at the same rate of speed let’s get something to hold this bar in place so that it will press against the spokes notice that this support is not locked to the axle it turns freely now we can spin the wheels by rotating the support this is fine as long as both wheels are able to turn at the same speed but let’s see what happens when we go around the corner with this arrangement we cannot drive one wheel faster than the other and if we stop one wheel the other wheel won’t budge let’s put this bar on a pivot so that it can swing in either direction now the bar can still turn both wheels at the same speed and because it pivots it lets one wheel turn even when the other is stopped but if turned too far the bar will swing around until it won’t drive the spokes that turn we need another crossbar and more spokes to carry on the job when we stop one wheel the crossbars will continue to push the spokes of the freewheel around as long as both wheels are free to turn the bars do not swing on their pivot and the wheels move at the same speed now we have the working principles of a differential to adapt the model for use in an automobile we will have to make a few changes in order to reduce the jerky action caused by wide spaces between the spokes we will put in more spokes further filling in the spaces between the spokes gives steadier more continuous action and changing the shape gives firm constant contact now we can make the gears thicker and stronger and we have differential gears the edges are cut so that they will fit together more smoothly and silently and another gear is added to share the work of driving the axles the principle is the same in order to turn the support and drive the wheels we can fasten a large gear here connected by a smaller gear to a source of power notice that the power is connected to the differential at the center line we can make our model more compact by moving the gears closer together when we put our differential in an automobile we have to leave room for the driveshaft which carries the power from the engine we may build the floor of the car above the driveshaft but if we do we won’t have much room inside unless we make the top of the car high – of course we could lower the floor and ceiling but the driveshaft would be higher than the floor this would have disadvantages our shaft in the middle of the floor of an automobile would be inconvenient for passengers and would be awkward for carrying luggage today engineers have found a way to make the car roomier and closer to the road without a clumsy shaft above the floor the drive shaft from the engine to the differential is lowered out of the way and the drive shaft is connected to the rear axle at the bottom the new low Center Drive makes the rear axle quieter stronger and more durable because it gives better smoother contact between the gears the automobile of today with the low Center Drive is stronger and more rugged every part of the rear axle has been built to withstand strains far greater than it will ever meet on the straightaway more around the corner you you you we’re going to explore what a differential is and how various differential designs operate when a vehicle is turning each of its wheels travels along a different path because each path is a different length each wheel has to have the ability to rotate at a different speed in order to allow the wheels to turn at different speeds a differential is used power is supplied from the engine via the transmission to a driveshaft which connects to the differential opinion gear is attached to the end of the driveshaft this turns the ring gear the ring gear surrounds a set of differential gears that enable the two rear axles to turn at different speeds when the vehicle is traveling straight the left and right axles turn at the same speed when the vehicle is turning the difference in rotating speed between the two axles is compensated for by the differential gears notice that the pinion gear turns at a different speed than the ring gear the ratio between these two gears is called the axle ratio in this example the ring gear turns one time for every four revolutions of the pinion gear this axle would be said to have a four to one ratio by changing the axle ratio the torque multiplication and top speed is changed either increasing or decreasing them axle ratio is an important component of the transmission gear ratio all the gear ratios between the engine and drive wheels determine how much power is actually applied to the wheels this is why some vehicles like trucks will have different axle ratios available the vehicle can be optimized for either heavy hauling and towing or normal driving and highway cruising but not typically both you may also hear this described as final drive ratio you this example is known as an open differential open differentials are inexpensive lightweight and require little maintenance however the major disadvantage of an open differential is that the engines power follows the path of least resistance this means that in cases where one wheel has less traction than the other wheel the engines power will go to the wheel with the least traction a way to limit this loss of traction is to limit the amount of independence between the two axles this can be accomplished via a number of mechanical electronic or hydraulic systems which are known as limited slip differentials or LSDs the benefit of a limited slip differential is that the amount of power sent to the wheel with the least traction is limited so that the wheel with the most traction receives continuous power in order to get the vehicle moving the disadvantage of a limited slip differential is the amount of power that can be transmitted to the wheel with traction is also limited the next type of differential is known as a locking differential locking differentials have mechanisms that can literally lock the two rear axles together when needed with a locking differential both axles receive equal power so that the wheel with the most traction will always have power locking differentials are often used for heavy-duty off-road or other low traction situations the disadvantage is that they have to be unlocked to allow differential wheel speeds during turning if the axles are locked during turning handling is negatively affected an alternative to differential based LSDs and locking differentials are brake activated LSDs such as Toyota’s Auto LSD system brake activated LSD s use an open differential but selectively apply the brakes to the wheel with the least traction this takes advantage of the path of least resistance to transfer power to the wheel that isn’t having the brake applied the auto LSD system is designed primarily to improve traction at times when the vehicle is likely to get stuck the systems electronic can trol gives it better torque transferring abilities than other types of differentials other benefits of these systems are that they require fewer parts and are less complex than mechanical systems this reduces maintenance and weight you


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