The TransmissionCo.docx
- 1. The Transmission Contents Abstract 3 Introduction 3 Discussion 3 (Automatic-transmission) 3 (Manual Transmission) 5 (Type of Gears) 6 (Gear Ratios) 7 (Transmission and Shaft work) 8 Relative-Motion (Analysis: Velocity) 9
- 2. Conclusion 10 References 11 Abstract The aim of this report is to introduce the types of transmissions and how they operate. A description of the internal parts and external parts are included to guide the reader better understand the system. Since the report is based on a thermodynamic perspective, some basic equations will also be presented. The relationship between thermodynamics and transmission system is mainly in the shaft work.Introduction To overcome inertia, the automobile engine develops power that is transmitted as torque from the engine crankshaft to the wheels. In order to achieve a smooth and gradual transfer of power, we use a clutch friction unit to engage and disengage the power flow. The use of clutches are actually found in both transmissions, however, the location is different. The transmission is used to best choose the gear ratio according to the power and speed. It is also present to provide the proper gear ratio for the car in different conditions, such as when the car is accelerating, stopping, maintaining speed, or reversing. The basic components necessary to deliver power to the drive wheels are the clutches, control linkages, flywheel (only in manual) or flexplate (only in automatic), torque converter (automatic), and the transmission. The relationship between thermodynamics and transmission system is mainly in the shaft
- 3. work.Discussion (Automatic-transmission) To begin, it is important to understand that all transmissions are different. There is the automatic, semi-automatic, and manual (standard) transmission. The one that will first be introduced is the more common type— the automatic transmission. The automatic transmission is called “automatic” because the gear ratios are changed automatically without the driver’s required input, typically using hydraulics o select ears depending on the pressure exerted by the fluid within the transmission assembly.. It is said that the clutch is replaced by a mechanism called the torque converter. This torque converter is located in front of the transmission housing as shown. The torque converter is a mechanical device that transfers power from engine to the transmission. (Note that the torque converter is only found in automatic transmissions). Inside the torque converter there is an impeller (pump), stator, and a turbine as shown. The impeller is connected directly to the engine. The impeller is responsible for moving the turbine. The movement of the impeller accompanied with the transmission fluid forces the turbine to rotate. This rotation causes the kinetic energy to be transferred to the input shaft, and later to the inside of the transmission. Lastly, there is small mechanism that is called the stator. This component is in mounted in the center of the impeller and turbine. Its usage is to prevent the transmission fluid from being redirected to the impeller. This is caused because the stator moves in the opposite direction, causing the “fluid to change directions as it hits the blades.”(Howstuffworks.com). Figure 3: Planetary Gear Figure 2 Torque Converter Diagram Figure 1: Transmission and Torque Converter The automatic transmission contains a special kind of gear set
- 4. called the planetary gear set. It consists of the sun gear, planet gear, and ring gear. The sun gear is the gear that is at the center of the ring gear. It is surrounded by other smaller gears like the solar system. Unlike the manual transmission, where gears are always shifting into different gears, the gears of the automatic transmission are always in contact with the same gears. Any gear change made will be accomplished by locking and unlocking the gears. (Manual Transmission) It is said that the simplest transmission is the manual transmission mainly because the driver has to change the gear ratios on his own. The manual transmission, unlike the automatic, works by manually changing gears using the stick shift (lever). The online article found on howrah.org states that in order for a smooth gear shift, “a clutch is provided to disengage the engine from the transmission”. The commonly used dry single disk clutch has a steel disk with a friction lining that is sandwiched between a flywheel on the engine shaft and a pressure plate on the transmission input shaft. To understand the clutch we must first dissect it. The clutch is made up of three components; the pressure plate, the clutch disk, and throw out bearing. The clutch disk is inside the pressure plate. The pressure plate is required, because with the use of the throw out bearing, it will disengage or engage the clutch. For example, when you depress the clutch pedal, the throttle bearing will push on the pressure plate. When the teeth of the pressure plate are pressed, pressure plate will actually move in the opposite direction of the force applied, causing the pressure plate to momentarily be disengaged from clutch. This means that when you press the clutch pedal you disconnect the engine from the wheels, allowing you to change the gear ratios. For a novice driver, it is important to note, that whenever a change in gear is to be made in a manual transmission car, one must first press the clutch pedal. This suggestion was made clear in the article How to Drive a Manual.
- 5. Figure 4 Clutch Figure 5: Clutch Pedal (Type of Gears) A gear is a mechanical element responsible for converting the speed of a shaft from one speed to another. The application of the gear is to transmit power from one shaft to the other is usually made up of two or more gears mounted together on a frame so that the teeth of the gears engage. The shafts in may be aligned in parallel or at any skew axis. The unique thing about gears is that when they transfer power, they do it with a very high efficiency (up to 99% in the case of parallel shafts). Gears work by rotating on each other using their teeth. Therefore, the shape and the number of the teeth are very important, since it determines how smooth the transfer of motion is. Gears have different kind; the first one is parallel gear shafts. The parallel gear shafts are like spur gears, helical gears, and herring one gears. This type of gear shaft offer maximum transmission of power and efficiency. Furthermore, they have higher road capacity and make less noise. On the other hand, they are more expensive to manufacture and created axial thrust. These gear shafts are wildly used for manual transmission. Figure 6: Parallel Gear shafts Then there are the intersecting shaft gears which as the name suggests, have intersecting shafts. These include bevel gears, straight bevel gears, zero bevel gears, and spiral gears. The intersecting shaft gears are efficient transmitters of power and motion between intersecting shafts at an angle. We use them instead of the parallel gear shafts because they are cheaper and have high strength. These gears are commonly used in automobiles. Figure 5: Spiral Bevel Gear
- 6. Finally, we have the hypoid gears. The hypoid gears are neither intersecting nor parallel; these include crossed helical gears and worm gears. The hypoid gears require high-ratio speed reduction in a limited space using non-intersecting shafts. Their efficiency can be easily increased by lowering their ratios. Their advantage is that they are the cheapest of all gears, and they are also used in automobiles. A gear train is usually made up of two or more gears mounted together on a frame so that the teeth of the gears engage. The first kind of gear train is the simple gear train which is when there is only one gear on each shaft, these are typically spur gears. The simple gear train consists of two gears the rotate on the same direction, on is called the driver and one is the follower, and the idler gear, which is only function is to change the direction of rotation. (Gear Ratios) Gear ratios are very important. To understand, let us first be introduced to the simplest gear—the spur gear. To understand gear ratios let’s assume we have a gear train example like the one pictured below. Assume the little gear is the “drive gear”, gear in motion. And the “driven gear” the bigger gear. Assume also that the drive gear has twenty teeth and the other gear has thirty gears. The ratio is calculated by dividing . In this case the gear ratio would be 30/20 which is 1.5. This means the drive gear must move one complete revolution plus a half so that the bigger gear can complete one revolution. This ratio is typically written as 1.5 : 1. Figure 6 Gear Ratio The gear ratios are chosen depending on the vehicle weight, expected loaded weight, and the size of the engine. Today’s transmission systems can have four, five or six speeds forward. However, since the engine rotates in only one direction, the
- 7. reversing procedure must be accomplished inside the transmission, and it is usually one speed. The fifth gear in a five-speed transmission is usually an overdrive. This gear is used for higher speed driving where little load is placed on the engine. This gear ratio rests the engine since the RPM is lowered to maintain a specific speed. The input shaft rotates only 0.87 of a turn, while the output shaft rotates one revolution, resulting in the output shaft rotating faster than the input shaft. (Transmission and Shaft work) As mentioned above, the shaft from the engine is connected to the transmission which choses the best gear ratio and transfers the power to the wheels. Transferring energy using rotating shaft is extremely common in engineering. The engine and the gearbox are bolted together, with the clutch between them to allow the disconnection while switching gears as shown in the figure below. In order to find out how much work is transmitted to the gearbox, we use the equation of the shaft work from thermodynamics. The shaft work is measured in kJ, while the power transmitted via the shaft per unit time is measured in kW. The shaft work that engine provides to the transmission is then adjusted by using the appropriate gear ratios and transferred to the wheel. Obviously, as any mechanical system, energy is being lost during the process as heat during to friction, which can be reduced by reducing the friction. Figure 7 Relative-Motion (Analysis: Velocity) An exercise that is practical for this topic is to determine the angular velocity of a planetary gear. The figure below taken from the textbook titled Dynamics by R. C. Hibbeler, shows a
- 8. planetary gear. The exercise says the ring gear (outer gear) is stationary. The planetary gears and sun gears are implied to be moving. The question is to find the angular velocity of the planetary gear. First, we must draw a free body diagram. In this case, a rough diagram of the velocity vectors has been provided. Since the ring gear is stationary, there is no velocity at point B. Thus, the velocity at Va does exist. We calculate the velocity at point A, by using the expression V=rw, where “r” is the radius and “w” is the angular velocity. The sun gear has a radius of 80 mm and a given angular velocity of 5 rad/s. With this, we can simply plug in the numbers to Va=rw (V=5*80) and obtain 400 mm/s. We have acquired two points A and B where velocities may exist. When this is the case, a special formula Vb=Va +W x rb/a. We know Vb is “0”and we know Va is 400mm/s. The angular velocity W is 5 rad/s (rotating around the k-axis) and is crossed with the relative position vector r b/a. This vector is called “relative” because it takes into account the point of interest B, with respect to point A. So the vector that was omitted in the diagram below travels from point B to point A. After all set and done, we should end up with 0=(-400i)+(wk)x (80 j). This is reduced to w=5 rad/s. Notice that in order for the cross product to work , a consideration of the unit vectors must be taken into account. Conclusion In a standard transmission, in order to transfer power from the engine crankshaft to the transmission, the clutch and pressure plates are locked together by friction. When that happens, the clutch shaft rotates with the engine crankshaft. When the power in transferred to the transmission, the power is routed through different gear ratios to obtain the best speed and power to start and keep vehicle moving. The crankshaft transfers the power to the transmission with the clutch in between. In the transmission, the best gear is chosen to provide the most suitable gear ratio according to the speed of the vehicle. The study of mechanical engineering revolves around the use
- 9. of many different gears. Thus an extensive study of the transmission has provided a clearer understanding of how internal components mechanically interact together. The shaft work is perhaps the most well-known formula for thermodynamics. Hopefully, after completing this report, we can better explain and even recognize the differences between a manual and automatic transmissions. References Hibbeler, R.C. 2012 Mechanics for Engineers: Dynamic. Singapore: Pearson Education Toboldt, William. 2006. Automotive Encyclopedia. USA. GoodHeart William Company The Stator - HowStuffWorks. (n.d.). Retrieved from http://auto.howstuffworks.com/auto-parts/towing/towing- capacity/information/torque-converter3.htm Clutch and Manual Transmission/Transaxle. (n.d.). Retrieved from http://www.procarcare.com/icarumba/resourcecenter/encycloped ia/icar_resourcecenter_encyclopedia_manualtran.asp Performance Transmission: Increase The Power of Your Ride. (n.d.). Retrieved from http://www.howrah.org/transmission.html How to Drive Manual: 15 Steps (with Pictures) - wikiHow. (n.d.). Retrieved April 5, 2015, from http://www.wikihow.com/Drive-Manual "Performance Transmission." : Increase The Power of Your Ride. N.p., n.d. Web. 06 Apr. 2015. "Clutch and Manual Transmission/Transaxle." Clutch and Manual Transmission/Transaxle. N.p., n.d. Web. 06 Apr. 2015.