A CVT For Every Application

When the subject of CVTs or continuously variable transmissions comes up, the chances are that most readers will think of the various motor vehicles they’ve appeared in. Whether it’s a DAF, a Ford, a FIAT, or a Chevrolet, most major manufacturers have tried one at some point or another with greater or lesser success. The automotive ones inevitably use a variation on a V-belt or metal band between variable separation conical pulleys, but this is by no means the only CVT configuration. Serial tinkerer [Robert Murray-Smith] takes an in-depth look at the subject as part of his ongoing fascination with wind turbines.

What caught our eye about this video isn’t so much the final 3D-printed design he selects for his experiments, but the history and his look at the different CVT designs which have appeared over the years. We see the V-belts, as well as the various cone configurations, the disk transmissions, the hydrostatic ones, and even magnetic versions. His transmission uses two cones with a rubber coating, with of all things a movable golf ball between them. We’re guessing it will appear somewhere in his future videos, so watch out for it.

Meanwhile, this isn’t the first time we’ve seen a CVT, [James Bruton] used a hemisphere to make one on a robot.

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Ball CVT Drives Robot From A Constant Speed Motor

[James Bruton] is experimenting is a series of interesting mechanical mechanisms, the latest being a CVT transmission system which uses a tilting sphere to get a variable speed output from a constant speed input. Video after the break.

In [James]’ proof of concept RC vehicle, a single powered disc is mounted on top, at 90 degree to the wheels. A rotating sphere makes contact with both the driven disc and the wheel. When the rotation axis of the sphere is at 45° between the disc and the wheel, it provides a one 1:1 transmission ratio. As the axis is tilted, the contact points on the sphere shift, changing the relative circumference at the contact points, and therefore changing the transmission ratio. It can also reverse by tilting the sphere in the opposite direction, and disconnected from the output wheel by aligning it with the hole in the bottom of the sphere. [James]’ simple two-wheel RC car concept quite well, driving around his kitchen with the transmission spheres being tilted by servos.

Thanks to the response time, CVT gearboxes are generally not needed for electric motors, but on internal combustion engines that which run best within a certain RPM range they can be very useful. One possible weak point of a design like this is it’s dependence on friction to transfer torque, which makes it vulnerable to wear and slipping.

This build is a spin-off of his spherical omni-wheels and the robot chassis he developed around them. For another interesting robot mechanism, check out is gyroscope balancing system. Continue reading “Ball CVT Drives Robot From A Constant Speed Motor”

Building A Continuously Variable Transmission With Lego

The first continuously variable transmission fitted to an automobile actually has its roots in the late 19th century, though the technology has only become popular in the last two decades or so. While a relatively complex technology in its modern automotive form, it’s still possible to illustrate the basic principles with everyone’s favourite mechanical learning toy – Lego.

The transmission as built isn’t great at high torque delivery, as the belt tends to slip on the smooth plastic of the cones. Increasing friction would help.

The build consists of a Lego motor driving the transmission’s input shaft, upon which a cone is mounted. A similar cone is mounted on the output shaft, and a rubber belt stretched between the two. With the cones mounted in opposing directions, the gear ratio can be continually varied by changing where upon the cones the belt rides. By riding on the small diameter section of the input cone, the belt correspondingly rides on the large diameter section of the output cone, leading to a slower, high torque output. By sliding the belt to the other end of the cone, the ratios are reversed, leading to high output speed with less torque.

The demonstration works somewhat differently than modern automotive models, but the basic concept is the same. It’s also limited in its torque transfer ability by the coefficient of friction of the plastic Lego parts. Despite this, it’s a quick way to illustrate the mechanisms at play, and where some of the common losses are in such a system. If you prefer your gearboxes of a more classic sequential design, we’ve seen those too, of course. Video after the break.

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