Behold A Geared, Continuously Variable Transmission

December 14, 2024 by Donald Papp 72 Comments

When it comes to transmissions, a geared continuously-variable transmission (CVT) is a bit of a holy grail. CVTs allow smooth on-the-fly adjustment of gear ratios to maintain a target speed or power requirement, but sacrifice transmission efficiency in the process. Geared transmissions are more efficient, but shift gear ratios only in discrete steps. A geared CVT would hit all the bases, but most CVTs are belt drives. What would a geared one even look like? No need to wonder, you can see one for yourself. Don’t miss the two videos embedded below the page break.

The outer ring is the input, the inner ring is the output, and the three little gears with dots take turns transferring power.

The design is called the RatioZero and it’s reminiscent of a planetary gearbox, but with some changes. Here’s how the most visible part works: the outer ring is the input and the inner ring is the output. The three small gears inside the inner ring work a bit like relay runners in that each one takes a turn transferring power before “handing off” to the next. The end result is a smooth, stepless adjustment of gear ratios with the best of both worlds. Toothed gears maximize transmission efficiency while the continuously-variable gear ratio allows maximizing engine efficiency.

There are plenty of animations of how the system works but we think the clearest demonstration comes from [driving 4 answers] with a video of a prototype, which is embedded below. It’s a great video, and the demo begins at 8:54 if you want to skip straight to that part.

One may think of motors and gearboxes are a solved problem since they have been around for so long, but the opportunities to improve are ongoing and numerous. Even EV motors have a lot of room for improvement, chief among them being breaking up with rare earth elements while maintaining performance and efficiency.

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Lathe Outfitted With Electronic Gearbox

September 19, 2024 by Bryan Cockfield 45 Comments

Running a metal lathe is not for the faint of heart. Without proper knowledge and preparation, these machines can quickly cause injury or destroy expensive stock, tools, or parts. The other major problem even for those with knowledge and preparedness is that some of their more niche capabilities, like cutting threads with a lead screw, can be tedious and complicated thanks to the change gear system found on some lathes. While these are useful tools for getting things done, [Not An Engineer] decided that there was a better way and got to work building an electronic gearbox to automate the task of the traditional mechanical change gear setup in this video.

What makes change gears so tricky is that they usually come as a set of many gears of different ratios, forcing the lathe operator to figure out the exact combination of gears needed to couple the spindle of the lathe to the feed screw at the precise ratio needed for cutting a specific thread pattern. It is possible to do this task but can be quite a headache. [Not An Engineer] first turned to an Arduino Nano to receive input from a rotary encoder connected to the shaft of the lathe and then instruct a motor to turn the feed screw at a set ratio.

The first major problem was that the Arduino was not nearly fast enough to catch every signal from the encoder, leading to a considerable amount of drift in the output of the motor. That was solved by upgrading to a Teensy 4.1 with a 600 MHz clock speed. There was still one other major hurdle to cross; the problem of controlling the motor smoothly when an odd ratio is selected. [Not An Engineer] used this algorithm to inspire some code, and with that and some custom hardware to attach everything to the lathe he has a working set of electronic change gears that never need to be changed again. And, if you don’t have a lathe at all but are looking to get started with one, you can always build your own from easily-sourced parts.

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Magnetic Gearbox, Part 2: Axial Flux Improves Performance

July 27, 2023 by Dan Maloney 13 Comments

The number of interesting and innovative mechanisms that 3D printing has enabled always fascinates us, and it’s always a treat when one of them shows up in our feeds. This axial flux magnetic gearbox is a great example of such a mechanism, and one that really makes you think about possible applications.

The principles of [Retsetman]’s gearbox are simple for anyone who has ever played with a couple of magnets to understand, since it relies on that powerful attractive and repulsive force you feel when magnets get close to each other. Unlike his previous radial flux gearbox, which used a pair of magnet-studded cylindrical rotors nested one inside the other, this design has a pair of disc-shaped printed rotors that face each other on aligned shafts. Each rotor has slots for sixteen neodymium magnets, which are glued into the slots in specific arrangements of polarity — every other magnet for the low-speed rotor, and groups of four on the high-speed rotor. Between the two rotors is a fixed flux modulator, a stator with ten ferromagnetic inserts screwed into it.

In operation, which the video below demonstrates nicely, the magnetic flux is coupled between the rotors by the steel inserts in the stator so that when one rotor moves, the other moves at a 4:1 (or 1:4) ratio in the opposite direction. [Retsetman] got the gearbox cranked up to about 8,500 RPM briefly, but found that extended operation at as little as 4,000 RPM invited disaster not due to eddy current heating of the inserts or magnets as one might expect, but from simple frictional heating of the rotor bearings.

Torque tests of the original gearbox were unimpressive, but [Retsetman]’s experiments with both laminated stator inserts and more powerful magnets really boosted the output — up to a 250% improvement! We’d also like to see what effect a Halbach array would have on performance, although we suspect that the proper ratios between the two rotors might be difficult to achieve.

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A line art schematic of a bicycle CVT drive. Two large green circles at the bottom have the text "1. Increases speed" where the crank arm would enter the system. A series of cam arms highlighted in red say "2. Converts from rotary to reciprocating motion." Finally, a blue highlighted bearing says "3. Converts from reciprocating back to rotary motion."

A Look Inside Bicycle Gearboxes

March 2, 2023 by Navarre Bartz 23 Comments

While bicycle gearboxes date back to at least the 1920s, they’re relatively unseen in bike racing. One exception is Honda’s race-winning mid-drive gearboxes, and [Alee Denham] gives us a look at what makes these unique drives tick.

Honda has developed three generations of bicycle gearbox as part of their company’s R&D efforts, but none have ever been released as a commercial product. Designed as a way for their engineers to stretch their mental muscles, the gearboxes were only used in bike races and seen at a few trade shows. In 2004, the third gen “derailleur in a box” led to the first gearbox victory in the Downhill World Cup Circuit.

The third gen gearbox differs significantly from the CVT drivetrains in the first and second generation gearboxes, but it is unclear why Honda abandoned the CVT. [Denham] has a nice animation detailing the inner workings of these CVTs based on information from the original patents for these rarely seen gearboxes.

Derailleurs remain the primary drivetrain in racing due to their lighter weight and higher overall efficiency. While still expensive, the decreased maintenance of gearbox drivetrains make a lot of sense for more mundane cycling tasks like commuting or hauling cargo, but only time will tell if the derailleur can be supplanted on the track and trail.

For more on bicycle drivetrains, check out this chainless digital drivetrain or the pros and cons of e-bike conversions.

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Cut Your Own Gears With This DIY Machine

January 27, 2023 by Lewin Day 29 Comments

You can buy gears off the shelf, of course, and get accurately machined parts exactly to your chosen specification. However, there’s something rugged and individualist about producing your own rotating components. [Maciej Nowak] demonstrates just how to produce your own gears with a homemade cutting tool.

The cutting tool for the job is an M16 machine tap, chosen for the smaller flutes compared to a hand tap. This makes it more suitable for cutting gears. It’s turned by a belt driven pulley, run by a small motor. The workpiece to be cut into a gear is then fed into the cutting tool by sliding on a linear bearing, with its position controlled by a threaded rod. The rod can be slowly turned by hand to adjust the workpiece position, to allow the gear teeth to be cut to an appropriate depth.

The method of action is simple. As the tap turns it not only cuts into the workpiece, but rotates it on a bearing as well. By this method, it cuts regular teeth into the full circumference, creating a gear. Obviously, this method doesn’t create highly-complex tooth shapes for ultimate performance, but it’s more than capable of creating usable brass and steel gears for various purposes. The same tool can be used to cut many different sizes of gear to produce a whole geartrain. As a bonus, the resulting gears can be used with M16 threads serving as worm gears, thanks to the pitch of the tap.

If you find yourself needing to produce tough metal gears on the regular, you might find such a tool very useful. Alternatively, we’ve explored methods of producing your own sprockets too, both in a tidy manner, and in a more haphazard fashion. Video after the break.

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Magnetic Gearbox Design Improvements Are Toothless But Still Cool

October 8, 2022 by Dan Maloney 9 Comments

Any project that contains something called a “flux modulator” instantly commands our attention. And while we’re pretty sure that [Retsetman] didn’t invent it after hitting his head on the toilet, this magnetic gearbox is still really cool.

Where most gearboxes have, you know, gears, a magnetic gearbox works by coupling input and output shafts not with meshing teeth but via magnetic attraction. [Retsetman]’s version has three circular elements nested together on a common axis, and while not exactly a planetary gear in the traditional sense, he still uses planetary terminology to explain how it works. The inner sun gear is a rotor with four pairs of bar magnets on its outer circumference. An outer ring gear has ten pairs of magnets, making the ratio of “teeth” between the two gears 10:2. Between these two elements is the aforementioned flux modulator, roughly equivalent to the planet gears of a traditional gearbox, with twelve grub screws around its circumference. The screws serve to conduct magnetic flux between the magnets, dragging the rotating elements along for the ride.

This gearbox appears to be a refinement on [Retsetman]’s earlier design, and while he provides no build files that we can find, it shouldn’t be too hard to roll your own designs for the printed parts.

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3D Printed Strain-Wave Gearbox Turns Up The Torque

September 17, 2022 by Dan Maloney 6 Comments

3D printers are good for a lot of things, but making parts for power transmission doesn’t seem to be one of them. Oh sure, some light-duty gears and timing belt sprockets will work just fine when printed, but oftentimes squooshed plastic parts are just too compliant for serious power transmission use.

But that’s not a hard and fast rule. In fact, this 3D-printed strain-wave transmission relies on the flexibility of printed parts to work its torque amplification magic. In case you haven’t been briefed, strain-wave gearing uses a flexible externally toothed spline nested inside an internally toothed stationary gear. Inside the flexible spline is a wave generator, which is just a symmetrical cam that deforms the spline so that it engages with the outside gear. The result is a high ratio gear train that really beefs up the torque applied to the wave generator.

It took a couple of prototypes for [Brian Bocken] to dial in his version of the strain-wave drive. The PLA he used for the flexible spline worked, but wasn’t going to be good for the long haul. A second version using TPU proved better, but improvements to the motor mount were needed. The final version proved to pack a punch in the torque department, enough to move a car. Check it out in the video below.

Strain-wave gears have a lot of applications, especially in robotic arms and legs — very compact versions with the motor built right in would be great here. If you’re having trouble visualizing how they work, maybe a Lego version will clear things up.

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