15-06-2013, 02:38 PM
Cam Based IVT
Cam Based IVT[.docx (Size: 2.24 MB / Downloads: 44)
Abstract
There are several categories of CVTs in existence today, including traction, belt, and ratcheting types. Drives of these types, their attributes, and associated design challenges are discussed to frame the development of the Cam-based IVT. The operation of this transmission is kinematically similar to a planetary gear set, and therefore, its operation is described with that in mind including a description of the six major components of the transmission, those being the cam, followers, carriers, planet gears, sun gears, and one way clutches. The kinematic equation describing its motion is derived based on the similarities it shares with a planetary gear set. Additionally, the equations for the cam design are developed here as the operation of the CVT is highly dependent on the shape of the cam. There are six simple inversions of this device and each inversion has special characteristics and limitations, for example, the available gear range. A method was developed to select the most suitable inversion, gearing, and follower velocity for a given application.
Introduction:
A specific type of CVT is the infinitely variable transmission (IVT), which has an infinite range of input/output ratios in addition to its infinite number of possible ratios; this qualification for the IVT implies that its range of ratios includes a zero output/input ratio that can be continuously approached from a defined "higher" ratio. A zero output implies an infinite input, which can be continuously approached from a given finite input value with an IVT. Low gears are a reference to low ratios of output/input which have high input/output ratios that are taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit. Most continuously variable transmissions are not infinitely variable.Most (if not all) IVTs result from the combination of a CVT with an epicyclic gear system (which is also known as a planetary gear system) that facilitates the subtraction of one speed from another speed within the set of input and planetary gear rotations. This subtraction only needs to result in a continuous range of values that includes a zero output; the maximum output/input ratio can be arbitrarily chosen from infinite practical possibilities through selection of extraneous input or output gear or sprocket sizes without affecting the zero output or the continuity of the whole system. Importantly, the IVT is distinguished as being "infinite" in its ratio of high gear to low gear within its range; high gear is infinite times higher than low gear. The IVT is always engaged, even during its zero output adjustment.
Background:
As mentioned previously, there is a substantial amount of prior work in the field of continuously variable transmissions. In general, these drives can be separated into 4 different categories traction, belt, ratcheting, and hydrostatic drives. Additionally, sometimes electrical couplings (generator/inverter/motor) are considered to be CVTs. Of these, only the hydrostatic and electrical drives depend on power transmission devices which are not mechanically linked and therefore less attention will be given to these.
Traction drives will be addressed first. Their single unifying characteristic is the transmission of power through rolling contact. As one can imagine then, much of their success depends upon the careful design of the contact patch geometry and kinematics through the gear range. The several most common and successful types of traction drives are the Full or Half Toroidal, the Kopp Variator, the Milner CVT and the Beier Variator .
Operation:
Attention is now given to the operation of the Cam-based
IVT, specifically, how the major components detailed above interact to generate a smooth output motion at infinitely many transmission ratios. There are six different inversions of the Cam-based IVT, and while the majority of this work is focused on one particular inversion, the operation of the transmission is better first illustrated with a dynamically simpler one. Therefore the configuration first presented here uses the cam as the input and the sun gear shaft as the output.