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.
As we’ve learned from past experience, videos from [HowToLou] tend to be a bit controversial. His unique style of expedient engineering isn’t everyone’s cup of tea, especially when it’s combined with a devil-may-care attitude towards safety. On the other hand, there’s no arguing that his methods get results. His video on converting an 18 HP riding mower into something akin to a go-kart is a perfect example.
The first phase of the project involves removing all the hardware related to mowing, as obviously you won’t be cutting any grass while pushing speeds of 48 kph (30 mph). This both saves weight, and removes a lot of mechanical complication that would be in the way of further modification. That said, it also leaves the mower immobile, as there’s no longer be any connection between the engine and transaxle.
In its place, [HowToLou] installs an off-the-shelf torque converter kit that uses a continuously variable transmission (CVT) clutch. As he quickly demos, the CVT technology allows the gear ratio to automatically adapt to the engine RPM thanks to pulleys that change their size depending on how fast they’re spinning. It’s a big improvement over the system he originally yanked out, though as you might expect, fitting it into the mower required some custom work. The final step was to pull the old pulley off of the transaxle and replace it with one that’s less than half the original size.
Wearing his protective flip-flops, [HowToLou] hops on the souped-up mower and is nearly thrown off the back of it as soon as he steps on the gas. Clearly the modifications were a success, and the video ends with some open road testing — presumably he’s riding off to the store to go buy a helmet.
We actually missed this video when it first made the rounds, but it has since picked up steam and is pulling in some impressive numbers. [HowToLou] tells us he thinks it’s due to the fact that a lot of people are upgrading to more modern zero-turn mowers, meaning there’s a surplus of these second-hand mini tractors on the market. Whatever the reason, we’re happy to see this backyard engineer get some mainstream success; his methods might not always be by the book, but they’re always entertaining.
[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.
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.
The electric motors generally used in robotics can be extremely efficient, often topping 90% efficiency at high speed and low torque. Slap on a traditional fixed-ratio gearbox, or change the input speed, and efficiency is lost. An infinitely variable transmission, like [Alexander Kernbaum]’s cleverly named Inception Drive, allows the motor to stay at peak efficiency while smoothly changing the gear ratio through a wide range.
The mechanism takes a bit of thought to fully grok, but it basically uses a pair of split pulleys with variable spacing. The input shaft rotates the inner pulley eccentrically, which effectively “walks” a wide V-belt around a fixed outer pulley. This drives the inner pulley at a ratio depending on the spacing of the pulley halves; the transmission can shift smoothly from forward to reverse and even keep itself in neutral. The video below will help you get your head around it.
We’ve seen a couple of innovative transmissions around here lately; some, like this strain-wave gear and this planetary gearbox, are amenable to 3D printing. Looks like the Inception Drive could be printed too. Hackers, start your printers and see what this drive can do.