Description of Pulley-Based Continuously Variable Transmissions By: Julanda Al Riyami Introduction Mechanical machines consist of a power feed and a power transmission system, which provides controlled application of power distribution. A transmission system is a collection of parts, including speed-controlling gears and a propeller shaft in which power is transmitted from an engine to an axle. In automobiles, often, a transmission has multiple gear ratios with the ability to switch between them as the speed of the wheels change. Continuously variable transmission (CVT) is a transmission technology that facilitates infinite variability within a finite range of gear ratios, the highest and the lowest gear ratio, without needing to shift gears. However, the word “gear” is still used when speaking of CVTs because, broadly speaking, gear refers to the ratio of engine shaft speed to driveshaft speed. The transmission system receives power through the crankshaft; however, unlike traditional automatic transmissions, CVTs don't have a gearbox with a set number of gears. A pulley-based CVT operates on an innovative pulley system that allows an infinite variability between highest and lowest gears without separate shifts. The pulleys comprise of cones, adjusted by hydraulic pressure cylinders, and a metal belt to turn the pulleys. Variable Pulley System Essentially, a CVT transmission operates by varying the effective diameters of the two main pulleys in the transmission. Most CVTs only have three key components: a variable input "driving" pulley, a variable output "driven" pulley, and a high-power metal belt. Power Distribution Through the Transmission Variable diameter pulleys always come in pairs. One of the pulleys, known as the driving pulley, is connected to the crankshaft of the engine. The driving pulley is also called the input pulley because it is where the power from the engine enters the transmission. Power is simultaneously transferred through the metal belt and onto the second pulley, the driven pulley. The second pulley is also known as the output pulley, which transfers the acquired power to the driveshaft. Pulley Cone Design The variable diameter pulley design is the core of a CVT, such that each pulley is made of two 20-degree plates, or cones, facing one another. A metal belt rides in the groove between the two cones. When the two cones of the pulley are far apart the belt lies low in the groove, and the radius of the belt loop cycling around the pulley is small. When the cones are close together the belt sits higher in the groove, and the radius of the belt loop cycling around the pulley is larger. The distance between the contact surface of the groove and the centers of the pulleys is known as the transfer pitch radius. The working gear is determined by the ratio of the pitch radius on the driving pulley to the pitch radius on the driven pulley. The figure on the right illustrates an overview of the CVT system with the drive and driven plates, along with the drive metal belt. Image from < http://freeze4you.blogspot.com/2010_10_03_archive.html> Metal V-Belt One of the most significant attributes of a CVT is the complex design of the metal belt connecting the pulleys. These flexible belts are composed of several thin bands of steel that hold together high strength pieces of metal. The steel belt typically consists of 280 hardened steel V-blocks engineered to accustom the sloped design of the pulley cones. The V-blocks remain in contact by two steel bands, each made of nine to twelve sealed strips that fit closely together in a very strong and flexible ring. The figure below clarifies the composition of the steel belt. Image from <http://www.insightcentral.net/_images/cvt-belt.jpg> The blocks are guided by the bands but are not attached to them in order for a push process to take place to transfer power. Drive is transmitted to the driven pulley by the compression of individual blocks rather than relying on tension forces in the steel bands. Each block leaving the driving pulley pushes the blocks ahead of it, to the driven pulley where the bands keep the blocks in contact with the pulley faces. The blocks are in compression on the drive side (driving pulley to driven pulley), and after the power is transmitted to the output pulley they float loosely along the bands on the return side (driven pulley to driving pulley). The large number of blocks in contact with the pulleys keeps contact surface pressures low, allowing high torque to be transmitted. Metal belts are highly durable and don't slip due to the design of the cones, enabling CVTs to handle exceptionally high engine torque. Achieving Infinite Gear Ratios When one pulley increases its radius, the other decreases its radius to keep the belt tight. For each of the pulleys, one side is fixed and the other side is moveable, activated by a hydraulic pressure cylinder. As the cylinder is actuated, it can increase or decrease the amount of space between the two cones. The two pulleys change their radii relative to one another, creating an infinite number of gear ratios – from low to high and everything in between. For example, when the pitch radius is small on the driving pulley and large on the driven pulley, then the rotational speed of the driven pulley decreases, resulting in a lower gear. When the pitch radius is large on the driving pulley and small on the driven pulley, then the rotational speed of the driven pulley increases, resulting in a higher gear. Thus, in theory, a CVT has an infinite number of gears that it can utilize at any time, at any engine or vehicle speed. The figure below demonstrates the variable pulley diameters of low and high gear ratios. Image from <http://wikicars.org/en/Image:Cvt_list01.gif> Conclusion Automobiles with CVT facilitate more power and give a better fuel economy as a result of the unique working mechanism of the system. The continuous variable transmission system has two pulleys and a belt attached as its principal components. While the input pulley is attached to the engine through a crankshaft, it receives power to rotate the pulley. This power is then transferred to the V-blocks of the metal belt by compressing them along the drive side. As the blocks come into contacts with the output pulley, power is transmitted to the driveshaft then further to the wheels. Although a driver controls the acceleration and speed of a CVT-integrated automobile with the gas pedal, the variable diameters of the two pulleys perform the mechanics and adjust to the desired gear ratios. Because of the step-less nature of its design, a CVT can work to keep the engine at its optimum power range, hence increasing efficiency and gas mileage. CVT cars assure a smooth driving experience supported by optimal engine and driveshaft speeds and a decent fuel economy.
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