Differential Drive Doesn’t Quite Work As Expected

June 6, 2021 by Bryan Cockfield 10 Comments

Placing two motors together in a shared drive is a simple enough task. By using something like a chain or a belt to couple them, or even placing them on the same shaft, the torque can be effectively doubled without too much hassle. But finding a way to keep the torque the same while adding the speeds of the motors, rather than the torques, is a little bit more complicated. [Levi Janssen] takes us through his prototype gearbox that attempts to do just that, although not everything works exactly as he predicts.

The prototype is based on the same principles as a differential, but reverses the direction of power flow. In something like a car, a single input from a driveshaft is sent to two output shafts that can vary in speed. In this differential drive, two input shafts at varying speeds drive a single output shaft that has a speed that is the sum of the two input speeds. Not only would this allow for higher output speeds than either of the two motors but in theory it could allow for arbitrarily fine speed control by spinning the two motors in opposite directions.

The first design uses two BLDC motors coupled to their own cycloidal drives. Each motor is placed in a housing which can rotate, and the housings are coupled to each other with a belt. This allows the secondary motor to spin the housing of the primary motor without impacting the actual speed that the primary motor is spinning. It’s all a lot to take in, but watching the video once (or twice) definitely helps to wrap one’s mind around it.

The tests of the drive didn’t go quite as planned when [Levi] got around to measuring the stall torque. It turns out that torque can’t be summed in the way he was expecting, although the drive is still able to increase the speed higher than either of the two motors. It still has some limited uses though as he notes in the video, but didn’t meet all of his expectations. It’s still an interesting build and great proof-of-concept otherwise though, and if you’re not clear on some of the design choices he made there are some other builds out there that take deep dives into cycloidal gearing or even a teardown of a standard automotive differential.

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Tiny Gasoline Engine Fitted With A Custom Billet Waterpump

May 31, 2021 by Lewin Day 31 Comments

We don’t typically use gasoline engines smaller than 50 cc or so on a regular basis. Below that size, electric motors are typically less messy and more capable of doing the job. That doesn’t mean they aren’t cute, however. [JohnnyQ90] is a fan of tiny internal combustion engines, and decided to whip up a little water pump for one of his so it could do something useful besides make noise.

The pump is built out of billet aluminium, showing off [JohnnyQ90]’s machining skills. The two pieces that make up the main body and cover plate of the pump are impressive enough, but the real party piece is the tiny delicate impeller which actually does the majority of the work. The delicate curves of the pump blades are carefully carved out and look exquisite when finished.

The pump’s performance is adequate, and the noise of the tiny gasoline engine makes quite a racket, but it’s a great display of machining skill. If so desired, the pump could also do a great job for a small liquid delivery system if hooked up to a quiet electric motor, too. The aluminium design has the benefit of being relatively leak free when assembled properly, something a lot of 3D printed designs struggle to accomplish.

We’ve seen [JohnnyQ90]’s micro engine experiments before, too — like this small generator build. Video after the break.

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Spinning Up A Water Cooled 3D Printed Stirling Engine

March 31, 2021 by Tom Nardi 7 Comments

The Stirling external combustion engine has fascinated gear heads since its inception, and while the technology has never enjoyed widespread commercialization, there’s a vibrant community of tinkerers who build and test their own takes on the idea. [Leo Fernekes] has been working on a small Stirling engine made from 3D printed parts and common hardware components, and in his latest video he walks viewers through the design and testing process.

We’ve seen Stirling engines with 3D printed parts before, but in most cases, they are just structural components. This time, [Leo] really wanted to push what could be done with plastic parts, so everything from the water jacket for the cold side of the cylinder to the gears and connecting rods of the rhombic drive has been printed. Beyond the bearings and rods, the most notable non-printed component is the stainless steel spice shaker that’s being used as the cylinder.

The piston is made of constrained steel wool.

Mating the hot metal cylinder to the 3D printed parts naturally introduced some problems. The solution [Leo] came up with was to design a toothed collar to hold the cylinder, which reduces the surface area that’s in direct contact. He then used a piece of empty SMD component feed tape as a insulator between the two components, and covered the whole joint in high-temperature silicone.

Like many homebrew Stirling engines, this one isn’t perfect. It vibrates too much, some of the internal components have a tendency to melt during extended runs, and in general, it needs some fine tuning. But it runs, and in the end, that’s really the most important thing with a project like this. Improvements will come with time, especially once [Leo] finishes building the dynamometer he hopes will give him some solid data on how the engine’s overall performance is impacted as he makes changes.

If you’ve got a glass test tube laying around, putting together a basic Stirling engine demonstration is probably a lot easier than you might think. Commercial kits are also available if you’re looking for something more substantial, but even those can benefit from some aftermarket modifications. With a little effort, you’ll have a power plant ready for the surface of Mars in no time.

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Prototyping A Turbojet Engine In The Home Shop

March 16, 2021 by Lewin Day 21 Comments

The development of the turbojet engine was a gamechanger in aviation, as no longer would aircraft designers have to struggle with ever larger and more complex piston engines, nor would propellers keep planes stuck below the speed of sound. However, the turbojet is an exacting device, demanding the utmost of materials in order to work successfully. [Integza] discovered just this in his quest to build one at home.

Unlike most home jet engine builds, this one doesn’t use a turbocharger or go with a simpler pulse jet design – though [Integza] has built those, too. This is a proper radial-flow turbojet design. The build uses a 3D-printed compressor, which is possible as it doesn’t have to deal with much heat. However, for the turbine, [Integza] realised that plastic wouldn’t cut it. After experiments with ceramic resins failed too, a 3D printed jig was instead built to allow sheet metal to easily be crafted into a workable turbine. Other internal components were made out of concrete for heat resistance, and a combustion chamber welded up out of steel.

The engine did run after several attempts, albeit for just ten seconds before components started to melt. While the engine is a long way off being flight ready, it goes to show just how hard it is to build even a bench-running turbojet. Even major world powers have struggled with this problem over the years. Video after the break.

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The Rotary-X Engine Is A Revolution In Thermodynamics

February 15, 2021 by Kristina Panos 119 Comments

If you’re running an army, chances are good that you need a lot of portable power for everything from communications to weapons control systems. When it comes to your generators, every ounce counts. The smaller and lighter you can get them, the better.

Connecticut-based company LiquidPiston is developing a high-powered generator for the US Army that uses the company’s own rotary x-engine — a small, light, and powerful beast that sounds like a dream come true. It can run on gasoline, diesel, natural gas, kerosene, or jet fuel, and is scalable from 1 to 1,000 horsepower (PDF).

Co-founder and CEO Alex Schkolnik describes the design as a combination of the best parts of the Otto and Atkinson cycle engines, the Diesel, and the Wankel rotary while solving the big problems of the latter two. That sounds impressive, but it doesn’t mean much unless you understand how each of these engines work and what their various advantages and disadvantages are. So let’s take a look under the hood, shall we?

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Steam Engine Replica From LEGO

January 25, 2021 by Bryan Cockfield 25 Comments

If engineering choices a hundred years ago had been only slightly different, we could have ended up in a world full of steam engines rather than internal combustion engines. For now, though, steam engines are limited to a few niche applications and, of course, models built by enthusiasts. This one for example is built entirely in LEGO as a scale replica of a steam engine originally produced in 1907.

The model is based on a 2500 horsepower triple-expansion four-cylinder engine that was actually in use during the first half of the 20th century. Since the model is built using nothing but LEGO (and a few rubber bands) it operates using a vacuum rather than with working steam, but the principle is essentially the same. It also includes Corliss valves, a technology from c.1850 that used rotating valves and improved steam engine efficiency dramatically for the time.

This build is an impressive recreation of the original machine, and can even run at extremely slow speeds thanks to a working valve on the top, allowing its operation to be viewed in detail. Maximum speed is about 80 rpm, very close to the original machine’s 68 rpm operational speed. If you’d prefer your steam engines to have real-world applications, though, make sure to check out this steam-powered lawnmower.

Thanks to [Hari] for the tip!

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A 136 Euro Pulse Jet For Some No-Firework Lockdown Fun

January 15, 2021 by Jenny List 21 Comments

With the aim of reducing virus transmission due to gatherings during the pandemic, the Dutch government have banned fireworks. The people of the Netherlands like their noisy things so we’re told that the ban has been widely flouted, but [Build Comics] are a law-abiding group of workshop tool heroes. For their lockdown noise, they created an entirely-legal pulsejet. The interesting part is that it was made entirely using fairly basic tools on a minimalist budget, with TIG and MIG eschewed in favour of a mundane stick welder.

The form of the pulse jet will probably be familiar as it has been taken from other published designs. A long tube is bent back upon itself with a combustion chamber placed in one of its arms such that the jet forms a resonant chamber that produces continuous pulses of exhaust gas. This one is made from stainless steel tube, and the exhaustive documentation should be worth a look for anyone tempted to make their own. Welding thin sheet with a stick welder requires quite a bit of skill, and in a few places they manage to burn a hole or two. One requires a patch, but the time-honoured technique of running a bead around the edge manages to successfully close another.

Their first attempt to fire it up using a leaf blower with a 3D-printed adapter fails, but following the construction of a more resilient part and a more efficient gas injector the engine starts. It’s then taken out on a farm for some serious noise without too many angry neighbours, as you can see in the video below the break.

The hero tools of Build Comics have appeared here before, most recently with an analogue meter clock.

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