|The cam-based transmission has six primary components. Click to enlarge.|
Engineers at the Virginia Polytechnic Institute and State University have developed a continuously variable transmission based on a cam and a planetary gearset that might increase fuel efficiency 3-5%.
Much work has gone into the field of continuously variable transmissions, which can be categorized as traction, belt, ratcheting and hydrostatic drives. The new cam-based system is a ratcheting drive type. Ratcheting drives take rotational input, convert it to an oscillating motion of varying amplitude, and then rectify this motion through a number of one-way clutches to a rotational output.
ne of the distinctive attributes of the new transmission developed by Derek Lahr and Dennis Hong compared to other ratcheting drive transmissions is that it not only produces uniform output given uniform input, but its output can be matched to nearly any periodic waveform.
As a result, the developers believe that the IVT would be ideally suited for application in wind turbines as well as in automotive and other industrial applications.
|The cam. Click to enlarge.|
The core of the transmission is the centrally located cam. A number of cam followers are rotably mounted to a carrier plate; each of the followers is fixed to a planet gear. An equal number of sun gears—each meshed to one of the planet gears.—connect to the output shaft.
By stacking an infinite series of profiles along the length of the cam to make a three dimensional camoid, an infinite number of transmission ratios can be selected. By varying the position of the cam followers in relation to the cam, the particular profile they follow can be changed. This affects the magnitude of the follower’s oscillations and therefore the transmission output.
|The transmission. Note the quarter-circle pully (purple) at the mid-right.|
Instead of gears with teeth, the transmission uses quarter-circle pullies and kevlar cord—this does away with the friction loss of one gear on another. (See diagram at right.) A flexible nylon ring to move the followers across the 3-D cam to change gears.
The developers have built an initial prototype as a test bed for design concepts such as the shifting mechanism, and to verify the underlying the kinematic equations.
A second prototype is in design, and will be tested for efficiency, durability, stress levels, and overall operability. The second design will also incorporate several improvements to increase the available gear range and torque capacity.
Among those improvements is a major change in cam implementation. The next prototype will have a barrel cam with the cam shape on the inside of a cylinder. The followers thus will be inside the cam, and held against the cam surface by centrifugal force. Sprag clutches are used so energy loss with the ratcheting is nearly zero.