Cheap Big Servo For Robot Arm

November 19, 2021 by Lewin Day 10 Comments

[Skyentific] is looking to push the hobbyist robotics state of the art. Motors and their gears, the actuators, are typically the most expensive part. For his build, he realised he needed big servos capable of delivering plenty of torque. Thus, he set about creating a 3D-printed design to get the job done on a budget. (Video, embedded below.)

Stepper motors are the order of the day here, chosen for their low cost compared to brushless solutions, particularly when taking control hardware into account. In this design, the stepper motor drives a sun gear as part of a bigger planetary gearbox with a high gear ratio. Cross-roller bearings are used to allow the servo to effectively handle both radial and axial loads. The servo as a whole is designed to fit neatly into the joints of the robot arm itself, and has external mounting points provisioned as such.

It’s a neat servo that somewhat apes those used on full-sized industrial designs, at least in the sense of being an integrated part of the joints of a robot arm. It also comes in at a relatively-cheap $32 based on the materials used by [Skyentific].

We’ve seen some related work from [Skyentific] before, too – like this interesting cable-driven joint. Video after the break.

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A Simple LEGO Automatic Transmission

July 22, 2021 by Lewin Day 9 Comments

The automatic transmission in your average automobile can be a complicated, hydraulic-y thing full of spooky fluids and many spinning parts. However, simpler designs for “automatic” gearboxes exist, like this Lego design from [FUNTastyX].

The build is based around a simple open differential but configured in a unique way. A motor drives what would typically be one of the output shafts as an input. The same motor is also geared what would normally be the main differential input shaft as well. In these conditions, this double-drive arrangement would sum the speed input and lead to a faster rotational speed at the other shaft, which becomes the output.

However, the trick in this build is that the drive going to what would be the usual differential input is done through a Lego slipper clutch. This part, as explained by [TechnicBricks], allows the outer teeth of the gear to slip relative to the shaft once torque demand is exceeded. What this functionally does is that when the output of the “automatic gearbox” is loaded down, the extra torque demand causes the clutch to slip. This then leads to only one input to the differential doing any work, changing the gear ratio automatically.

It’s likely not a particularly efficient gearbox, as there are significant losses through the very simple clutch, we suspect. However, it does technically work, and we’d love to see its performance rated directly against other simple Lego gearbox designs.

It’s a little confusing to explain in text, but the video from [FUNTastyX] does a great job at explaining the principle in just a few minutes. We’ve seen plenty of crazy Lego gearboxes over the years, and we doubt this will be the last. Video after the break.

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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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A Stackable Planetary Gearbox You Can Print At Home

May 14, 2021 by Lewin Day No comments

In one little corner of YouTube is a small but vibrant community sharing videos about gearboxes of their own design, particularly those with very high ratios or other quirky features. Adherents of the subculture are known as gearheads, and [Let’s Print] is among them. His latest creation is a 3D printed planetary gearbox design with a focus on easy assembly and versatile ratio choice. (Video, embedded below.)

The gearbox came about as [Let’s Print] grew weary of designing bespoke geartrains for each of their individual projects. The planetary design they landed on has the benefit of being stackable, with each reduction block fitted adding a 1:3 stepdown to the train.

For testing purposes, four stages were ganged up for a total reduction ratio of 1:81. The resulting gearbox was able to lift 40 kg before its output coupler failed, no mean feat for some plastic squirted out of a hot nozzle. It’s a common problem with huge ratio gearboxes made out of plastic – often, the very components of the gearbox can’t hold up to the huge loads generated.

Regardless of the limitations of the material, we’re sure the gearbox will prove useful in future projects from [Let’s Print]. We’ve seen other tough 3D printed gearbox builds before too, such as this anvil-lifter from the aptly-named [Gear Down For What]. Dive into the online gearbox subculture yourself.

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Testing 3D Printed Worm Gears

May 12, 2021 by Lewin Day 24 Comments

Worm gears are great if you have a low-speed, high-torque application in which you don’t need to backdrive. [Let’s Print] decided to see if they could print their own worm gear drives that would actually be usable in practice. The testing is enlightening for anyone looking to use 3D printed gearsets. (Video, embedded below.)

The testing involved printing worm gears on an FDM machine, in a variety of positions on the print bed in order to determine the impact of layer orientations on performance. Materials used were ABS, PLA and PETG. Testing conditions involved running a paired worm gear and worm wheel at various rotational speeds to determine if the plastic parts would heat up or otherwise fail when running.

The major upshot of the testing was that, unlubricated, gears in each material failed in under two minutes at 8,000 RPM. However, with adequate lubrication from a plastic-safe grease, each gearset was able to run for over ten minutes at 12,000 RPM. This makes sense, given the high friction typical in worm gear designs. However, it does bear noting that there was little to no load placed on the gear train. We’d love to see the testing done again with the drive doing some real work.

It also bears noting that worm drives typically don’t run at 12,000 RPM, but hey – it’s actually quite fun to watch. We’ve featured some 3D printed gearboxes before too, pulling off some impressive feats. Video after the break.

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7000 RPM On A 3D-Printed Gearbox

April 25, 2021 by Chris Lott 17 Comments

[Steven] at the 3D Printer Academy has been working on a variety of different gear designs. He recently embarked on a series of experiments to see how fast he can spin a 3D-printed gearbox.

After testing several kinds of gear teeth, gear diameters, and gear spacing, he finally struck upon an 81:1 ratio gearbox. It has six gears: five stepped gears and one drive gear on the input shaft. First tests are accomplished with a 3D-printed handle, similar to a hand crank used to start really old cars. But unlike those cranks, [Steven]’s doesn’t have any release provision. While the handle can be removed, it can’t be removed while spinning.

We think it would be helpful to revise the drive shaft coupling method, allowing the handle or drill to be easily removed from the gearbox once it’s attained speed. This would be more convenient, and it seems prudent from the workbench safety point of view as well.

Example of a crank quick release mechanism

[Steven] manages to get the final gear spinning at 7000 RPM in video #2 of the series by hand cranking it “as fast as he can”, a speed measured by using the metronome app on his smartphone. He begins driving the gearbox with an electric drill in video #3, with some mixed but promising results. We think he will ultimately succeed in his goal of a high-speed, electric-drill-driven gearbox after a few more tests. If you want to have a go at this yourself, the design files are posted online.

How fast do you think he can eventually get this gearbox spinning? Are there any physical limitations of the assembly or due to the 3D printing materials/process? We certainly know that high torque can tear 3D-printed gearboxes apart, but how does the speed affect things? Let us know in the comments below.

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Hackaday Links: March 21, 2021

March 21, 2021 by Dan Maloney 31 Comments

If you think you’re having a bad day at work, pity the poor sysadmin at Victoria University of Wellington in ~~Australia~~ New Zealand, who accidentally nuked the desktops of pretty much everyone at the university. This apparently happened last week and impacted everyone connected to the university network with a Windows machine, which had any files stored on their desktops deleted and also appears to have reset user profiles to the default state. This caused no end of consternation, especially among those who use their desktop folder to organize work in progress; we’d imagine more than one student at VUW is hating life right now for not storing work on a backed-up network drive. The problem seems to have started with an attempt to clean up files and profiles left behind by former students; how that escalated to nuking files on the desktop will require some ‘splaining.

Speaking of mea culpas, there was quite a dustup this week in the Cricut community. It started when the maker of CNC cutting machines announced its intention to limit uploads to their online design software unless the user signs up for a $10 a month account. After getting an earful from the users, the CEO of the company announced that these changes would be delayed until the end of 2021. That decision still didn’t sit well with the community, which includes a fair number of users designing PCBs, and two days later, the CEO announced that they were throwing in the towel on the whole plan, and that everything was going back to status quo ante. Story over? We’ll see — it seems like Cricut has tipped its hand here that they’re looking to extract more money from the users, and the need for that likely hasn’t gone away just because they relented. As Elliot Williams pointed out when we discussed the whole debacle, it’s easy to see how Cricut could start adding new features to the paid version of their software, basically abandoning the free user base. We’ll have to see how the obviously vociferous community responds to something like that.

Much interesting news from Mars this week, where the Perseverance rover is getting used to its new home and getting itself ready to roll. Late last week, Perseverance successfully dropped the “belly pan” that was covering the sensitive instruments under the rover, including the Adaptive Sample Caching system that will seal up Martian core samples and drop them out onto the surface for later pickup. This seemingly simple task was a critical one; had the pan not cleanly separated, the mission could have been severely impacted. Perseverance also did a little test drive this week, and recorded what it sounds like to drive on Mars. The audio clip is 16 minutes long, and the noises coming from the billion-dollar rover are just awful at times. We hear clunks and clanks and squeals galore, and while we’re sure they all have a good explanation and will provide valuable engineering data, they sound somewhat alarming to us.

But not so alarming as the sounds that must have come from a Jeep that suffered a bad tow job recently. The cringe-making story starts with a brand-new Jeep being towed on its wheels behind a motorhome, which allows the RV owners to park their rig and still have something to drive around in while they camp. The towed vehicle, or “pusher”, is normally equipped with a manual transmission, as towing with the wheels on the ground for extended distances is easier with them. Unfortunately, the Jeep’s owner set up the shift levers wrong and left the transmission in first gear, with the transfer case in low range. The linked article estimates the gearing ratios meant that the poor Jeep’s engine was being spun at something like 54,000 RPM; chances are good the engine exploded long before that point. The damage shown in the video accompanying the article is just brutal — the oil pan and bell housing are gone, the bottom of the crankcase is blown out, and at least two pistons and their share of the crankshaft are missing in action. We feel sorry for the owner, but really wish the Jeep had had a belly cam like the one on Perseverance.