The primary function of a transmission is to transmit mechanical power from a power source to some form of useful output device. Since the invention of the internal combustion engine, it has been the goal of transmission designers to develop more efficient methods of coupling the output of an engine to a load while allowing the engine to operate in its most efficient or highest power range. Conventional transmissions allow for the selection of discrete gear ratios, thus limiting the engine to providing maximum power or efficiency for limited ranges of output speed. Because the engine is forced to modulate its speed to provide continuously variable output from the transmission to the load, it operates much of the time in low power and low efficiency regimes. A continuously variable transmission (CVT) is a type of transmission, however, that allows an infinitely variable ratio change within a finite range, thereby allowing the engine to continuously operate in its most efficient or highest performance range, while the transmission provides a continuously variable output to the load. The development of modern CVTs has gene-rally focused on friction driven devices, such as those commonly used in off-road recreational vehicles, and recently in some automobiles. While these devices allow for the selection of a continuous range of transmission ratios, they are inherently inefficient. The reliance on friction to transmit power from the power source to the load is a source of power loss because some slipping is possible. This slipping is also a major contributor to wear, which occurs in these devices. To overcome the limitations inherent in the current CVT embodiments employing friction, a conceptual, continuously variable, positive engagement embodiment has been proposed for investigation at Brigham Young University. This concept proposes utilizing constantly engaged gears which transmit power without relying on friction. Because the proposed embodiment is new, no engineering analysis has yet been performed to determine its kinematic and meshing characteristics, an understanding of which are necessary to validate the pro-posed concept as a viable embodiment. This research will investigate both the kinematic and meshing characteristics of this and related concepts.
Continuously variable transmissions have been in use for many years. Near the beginning of the twentieth century, cars like the Sturtevant, Cartercar, and Lambert featured friction dependent CVTs (Puttré, 1991). These friction drive CVTs were common in automotive use until engines capable of producing higher torques became common and necessitated the move to geared, fixed-ratio transmissions capable of high torque transfer and having better wear characteristics than friction dependent CVTs. Only in the past few years, with the advent of advanced materials and technology, have friction dependent CVTs returned to commercial application in the automotive industry. To provide a foundation and motivation for the research presented, this chapter first presents a definition of a continuously variable transmission. For background purposes, a review of the current literature on CVTs is included. The families in which various embodiments can be classified are presented, along with a description of the operating principles in each family. A new family of embodiments of the positive engagement classification is also presented, along with the principles governing this new classification. This research focuses most heavily on embodiments in the final classification.
DEFINITION AND TERMINOLOGY
A transmission is a device which allows the transmission of power from a rotating power source to a rotating load. Conventional transmissions allow for the selection of discrete gear ratios, thus limiting the engine to providing maximum power or efficiency for limited ranges of transmission output...