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					   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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posted:2/14/2012
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