Experimental Harmonic Drive Reducer - 3D Printed

5 Apr 2021
145 449 Aufrufe

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Over the last few weeks I've been investigating 3D printed Cycloid Drives. I thought it was time to take a look at a Strain Wave, or Harmonic Drive reducer. I'm using TPU for the flex-spline in this design, and the rest is PLA. I'm using the same brushless motor and the reducer is a 10:1 ratio to match the cycloidal drive so we can compare them.
CAD for this: github.com/XRobots/CycloidalDrive/tree/main/StrainWave
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Former toy designer, current DEhave maker and general robotics, electrical and mechanical engineer, I’m a fan of doing it yourself and innovation by trial and error. My channel is where I share some of my useful and not-so-useful inventions, designs and maker advice. Iron Man is my go-to cosplay, and 3D printing can solve most issues - broken bolts, missing parts, world hunger, you name it.
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  • Testing both Cycloidal and Harmonic Drive Reducers with an encoder and ODrive is coming up in next week's video, but Patrons and DEhave Channel Members have it already!

    James BrutonJames BrutonVor 14 Tage
    • The problem 3d harmonic drive is 3d printed material don’t survival very well on repeat turn and twist. The replacement rate is quite high.

      Raven kkRaven kkVor 7 Tage
    • dehave.info/down/mHx5XpXFf43bnYo/video remind me of that think you are using to flexible material. You only need a small flexing so it can get into contact not total deformation

      fabien herryfabien herryVor 10 Tage
    • Maybe printing the gears in transparent filament so we can see what's going On?

      diruvodest pdiruvodest pVor 10 Tage
    • maybe you could take some idea from this one : dehave.info/down/mHx5XpXFf43bnYo/video

      Patrick LepoutrePatrick LepoutreVor 11 Tage
    • Please, I implore you - go watch this video. I think this man's gearbox could be a potential win, with some modification. Compound carrier-driven planetary gearbox, with/without a sun gear on either side. I really would love to see this work. dehave.info/down/Z32Bks27sXbA3oY/video

      FuchsDaninFuchsDaninVor 11 Tage
  • Commercial Strain wave gears absolutely are backdrivable - even at 1:100 ratio. They are used extensively in collaborative robots for this very reason - otherwise you could not move the robot in teach mode. Internal friction in this prototype combined with the low stiffness of the flexspline is causing geometric binding. I think the ideal ratio for backdrivability is closer to 1:30 or 1:40. A lot of knowledge on this topic can be found in Harmonic Drive’s older (10+ years) catalogs.

    aftenstjernemixaftenstjernemixVor 4 Tage
  • I imagine the wave generator should not just be a narrow stick with two bearings at the end but rather an oval that conforms to the inside of the flex spline. This could reduce buckling of the spline and thus slippage. Also, consider using a chain rather than a flexible spline for more stiffness. That would, of course, make coupling with the output cup more difficult. Lastly, increasing the diameter of the bearings on the wave generator that push the spline outwards into the ring gear would increase meshing and thereby reduce slippage.

    surfcellosurfcelloVor 6 Tage
  • the harmonic drive flexring needs to be more rigid, and have much less flex when rotating, the industrial ones have so little flex you almost cant see it. the ones i used had less than a millimeter in flex.

    MrVeinMrVeinVor 6 Tage
  • is harmonic gear reversible or is there an equivalent mechanism that multiply gear ratio with similar compactness ? I saw a lot of method and mechanism for gear reduction, but not a lot on the gear multiplying department. For certain class of robotic actuator like shape memory alloy motor/magneto-restrictive motor/hydralic that have much higher specific work density compare to conventional electrical motor, but they have much low strain rate. So they are not really usable unless some gear multiplication is applied to magnify that low strain.

    Keen HeatKeen HeatVor 7 Tage
  • dehave.info/down/gmJ_k9WOcKi7uqQ/video thats a well simplified solid gears modell.

    Levente LikhaneczLevente LikhaneczVor 7 Tage
  • Just spitballing but the flexspline may not be deforming enough to only have solid contact at just the two bearing points. The all metal one you showed doesn't need to flex much as the teeth height is extremely low.

    William AycockWilliam AycockVor 7 Tage
  • Why not replace the flex spline with a small chain that has triangular teeth?

    Helena Of DetroitHelena Of DetroitVor 8 Tage
  • When Reese’s goes mechanical

    The Z FamilyThe Z FamilyVor 8 Tage
  • Did you do any experiments with the parameters? The flex part looks to be much thicker than it needs to be and also much longer. Too much friction.

    SomunSomunVor 8 Tage
  • Wave generator shaped like and cross with a long section doing the driving and a short section keeping the tension throughout the flexi gear. I think this could help with the high speed issues

    Marco BurbiMarco BurbiVor 9 Tage
  • Might be handy for CNC projects, motors that should be stopped being driven by external force is a constant pain and commercial drives are super expensive. This has the speed and some torque to it, maybe the torque could be improved, plus a decent driver and the encoder and you're onto a winner.

    streakystreakyVor 10 Tage
  • The issue is the flex spline material. The plastic is too soft. To make a proper spline you gotta do some compliance calculations with pla or some other harder material. This is definitely not easy stuff

    Simon GormuzovSimon GormuzovVor 10 Tage
  • A bit new to these types of drives my self. I've noticed that the harmonic drives has less surface contact/engagement area amongst the sets of teeth. That would result in putting a lot more force between a small amount of teeth. Also, it's likely your flex teeth are also being heavily compressing between the wave bar bearings and the wall of outer teeth. Which could be allowing the flex teeth to compress enough for slippage to happen. If available, you could try another more solid, but flexible material that can handle the force between the low area contact with the teeth sets. Overall, it looks like a more finicky drive to figure out the tolerances in design. It's pretty cool that your trying it out and having fun with it! I've been enjoying the proxy ride along with your adventures into different types of reduction drives. As well as your robotic projects in general.

    David YoungDavid YoungVor 10 Tage
  • Not liking that reduction gear.

    Jeff SadowskiJeff SadowskiVor 10 Tage
  • Harmonic drives can be backdriven no problem. I would suggest instead of making the flexspline shorter to make it longer (more material to flex) and only use one set of bearings for the wave generator more to the open end of the flexspline.

    Tobias MortierTobias MortierVor 10 Tage
  • Hypothetically, could the flex spline be replaced a roller chain like a bicycle uses running on some idler sprockets where the bearings on the wave generator are and an elliptical track? Output would be derived by having longer pins on the chain joints operating in a series of holes on the end cap, ideally via cams on bearings to reduce friction, similar to how the cycloid gears are engaged. Such a system would probably need to be completely packed in grease and the chain track would require a bit of metal inside so the chain wouldnt just chew through the plastic. Additionally the teeth on the idlers and the ring gear (ring sprocket?) Would have to be carefully designed so they dont smash into each other as they engage the chain (doesnt matter if the idler skips so probably doesnt need full depth teeth if it needs teeth at all, just whatever it takes to keep the chain aligned)

    David ThomasDavid ThomasVor 10 Tage
  • From what I can tell, harmonic drive designs seem to favor a tooth difference of 2, and they seem to favor high reduction (high tooth counts), which makes a low reduction harmonic fairly rare. Most of the issues more than likely come down to tooth profile, just as involute teeth on gears are the optimal profile, there's more than likely an optimal profile for harmonic drive teeth that's dependent on tooth offset, as a different number in offset requires a different movement of the strain wave, where a design meant for an offset of 2 may not necessarily scale to an offset of 4. It's just not as simple as having a triangular or trapezoidal profile, as the flex spline itself is moving in and out while also rotating; again, involute gear teeth are optimal for their movement, a flex spline will also have an optimal tooth profile. There's also loads of different designs. One uses a compliant mechanism for the flexing rather than transitioning from a flex spline to a hub via a solid piece. Another design uses multiple sets of bearings to better mimic the desired shape of the flex spline, rather than relying on just two bearings to generalize the flex spline shape. Material choice is also important. Tolerances are also important. Etc. Deformation is an issue that plagues printed variations of harmonic drives, there's a reason why actual ones are all metal, either to the teeth and/or the wave generator bearings not being square. The downfalls seen here could easily just come down to geometric issues that need solving. It could just as easily be a tolerance issue. Like all developmental projects, this just needs refining and finessing to get it to work. A cycloidal drive is probably the better choice for the reduction you're wanting, there's a reason why low reduction harmonics are rare, if not entirely unheard of, and I don't think any hobbyist wants to reinvent the wheel just for a singular project when other wheels are more readily available. Maybe save looking into harmonics when you actually need a high reduction drive, where they're most commonly used.

    Ξανδρος PeachesΞανδρος PeachesVor 10 Tage
  • Your output sounds quite painful....

    Dotsch GoubbraDotsch GoubbraVor 10 Tage
  • Seems like this type of drive would be inefficient.

    EnlightenedSavageEnlightenedSavageVor 10 Tage
  • "Very slightly... only about 0.4 mm ..." Seems like 0.2 is worth a shot?

    toxaqtoxaqVor 11 Tage
  • the flex drive has baring that runs along the inside of the flexible belt. Because of this, I believe, that when you stop it that lets the barrings spin freely. There is nothing stopping the inside barings from spinning feeling along the inside of the flexible belt.

    Chad LeeChad LeeVor 11 Tage
  • Can we see it spinning at 20k rpm?

    AaronAaronVor 11 Tage
  • You should do a Rev robotics Neo motor, it's a brushless motor with build in encoder

    Tyler VanProoyenTyler VanProoyenVor 11 Tage
  • I'm fairly new to 3D printing, so I don't know if this makes sense, but would it make sense to use 2 materials for your flex spline, where the teeth are hard at the tips, but flexible where they connect to other teeth. Would this allow the spline to be flexible enough without the risk of the teeth deforming and skipping when you try to stop it by force?

    Andras BakAndras BakVor 11 Tage
  • Let me guess, you cannot get another motor because of brexit.

    Chris LambeChris LambeVor 11 Tage
  • On the left, below Bender's head. Are those Wolverine's claws? What, exactly, are you planning for this robot dog to do?

    AlansshowsAlansshowsVor 11 Tage
  • Just out of curiosity what filament do you use most PLA/PETG/ABS when you print your parts

    - prime -- prime -Vor 11 Tage
  • Great job! I work at a robotics company and have designed and built robots that use both types of gearboxes on mobile platforms and fixed robot arms. Your videos perfectly show the operational and engineering pros and cons of both reduction types. Harmonic drives are the preferred gearbox for applications that require no backlash and high efficiency while cycloidal drives are actually notorious for being inefficient given all of the surfaces they have rubbing against one another, but they are typically much cheaper than harmonic drives so there is a design tradeoff. I will say though that I have been able to back drive harmonic drives with ratios up to 40:1 without issue, so I think your back driving problem with your own harmonic drive lies somewhere in your design (to be expected though given how temperamental they are). There are companies with large teams of engineers designing these things, and the fact that you were able to make one that works this well on your first try is nothing short of amazing! Looking forward to more videos on your projects!

    Chris LaBordeChris LaBordeVor 11 Tage
  • the flex spline material is too soft. try a little harder material at least for the teeth.

    TheMusicGeekTheMusicGeekVor 11 Tage
  • AMAZING!!!

  • Idk why but this makes me dizzy

    KhalifKhalifVor 11 Tage
  • two Ideas: 1. Make the flexdrive with dualextruding, with ridged teath. 2. The harmonic drives I have seen before don't mount the output to directly to the flexdrive, but have a second ridged tothring, that gets alined with the stator ring by the flexdrive.

    MusikCassetteMusikCassetteVor 11 Tage
  • compared to some commercial variations your wave generator is not engaging very many teeth at a time and the teeth are already seem somewhat large for a material that is that flexible, maybe try finer teeth with more solid engagement?

    direthingdirethingVor 11 Tage
  • Hello, please consider testing Constantinesco torque converter

    Orly K.Orly K.Vor 11 Tage
  • @12:56 _"Yooo tell me what you want what you really really want"_

    DurahlDurahlVor 11 Tage
  • The circular disc drive system, uses a set up, that looks like a spirograph. This new set up, has a similar feel. Even then, it looks, awesome.

    James HamakerJames HamakerVor 11 Tage
  • Just use a brushless drill. All the parts plus rechargeable batteries. Stick it in the drill chuck.

    AssassinlexxAssassinlexxVor 11 Tage
  • im not feeling worthy to watch this, you know so much

    the madtraxthe madtraxVor 11 Tage
  • I think you made the flexy part too soft, if you look at the video you linked, that part in the industrial unit was made out of metal. It needs to flex, but not much, plastics should work great

    fiskfisk33fiskfisk33Vor 11 Tage
  • VESCs can in fact use encoder feedback from a number of different sources such as ABI encoders and AS5047 magnetic encoders - even the older 4.8 hardware shown in this video

    Josh GeatingJosh GeatingVor 11 Tage
  • The teeth are climbing each other as rotation is inhibited, squishing them at point to point because the leverage is significantly against them. You probably have minute warping as the inner PLA shell is warping also to allow. Maybe if the material was a denser, non flexible type and lubricated so the teeth are forced to slide into being seated into the outer teeth.

    Sean DaughertySean DaughertyVor 11 Tage
  • Cool stuff!. Actual harmonic drives are back-drivable with little losses to friction. They key is to have the wave generator fully support the path of the elliptical deforming gear. Otherwise it will just try to flex into itself and cause lots of friction. I am testing a 3d printed version with large offset bearings similar to the center shaft of your cycloidal, but have both of those support the elliptical gear. Also, since you have very large teeth thus large deflections, consider the pancake version of the harmonic drive. You won't lose as much energy where the flexspline meets the output as the whole flexspline stays deformed.

    Jeffrey ZhuJeffrey ZhuVor 11 Tage
  • Put a thin walled cup of PLA or ABS inside the flex gear that the wave generator has to deform first, I think the reason you’re able to stall the drive but not the motor is because the TPU is just squishing and letting the wave generator spin.

    Phoenix540Phoenix540Vor 11 Tage
  • I really like watching you working through these designs! Keep it going!

    Will HuffWill HuffVor 11 Tage
  • Sorry I'm stupid, Why make this? Is it like a sholder?

    ThetreetrollThetreetrollVor 12 Tage
  • Video title prediction for next month: "Experimental Compound Planetary Reducer - 3D Printed"

    Karl PfalznerKarl PfalznerVor 12 Tage
  • Gah. The harmonic drive is DRIVEN by wedge force of the teeth. Pure friction force. With no torque the wasted energy JUST IN DRIVING is going to be the cosine of the gear face angle. IOW if you have 20degree tooth angle only 7% of your energy (and force) is going towards moving the output round. Sure you're getting a lot of force ... again that wedge angle, but it's horribly inefficient. It's worse against substantial torque because the central belt i s now deformed against the downward force...,, more force and energy percentage would go to move the drive forward .... but now the resistance is increased horribly further increasing the deformation. It's a lose-lose vicious circle (well I didn't intend the pun before I made it, but I intend it now). I'd also point out that there's NOTHING preventing the belt from backsliding against it;s forward motion EXCEPT it's own deformation force. Given that there's only a few millimeters of deformation doing that forward drive and what, 70 to 80 mm between the cam bearings there's plenty of room for it to slip backwards if there is ANY play where the teeth meet. It's cool that it CAN do that kind of thing but it's deficiency of efficiency and compromised structure by design make it very dubious for larger loads. Possibly a metal flex belt would work in some odd particular circumstance. but it's poor choice for most cases (IMO, I'm a "software engineer" who once studied some physics, not any kind of authority here) EVEN IF you can get the tolerances down to where you can make it work with some reliability and at least moderate force, the inefficiency means HEAT. PLA and flex plastics?? If you run the thing about five minutes it's gonna be "interesting". But not at all purty.

    Terrence ZellersTerrence ZellersVor 12 Tage
  • Loving this stuff you're doing, but could you please balance your voice audio? Hearing it more in one ear than the other is really distracting

    LT CuddlesLT CuddlesVor 12 Tage
  • Spline is my favorite word

    L Train45L Train45Vor 12 Tage
  • How not support in middle of wave generator?

    MopedMikeMopedMikeVor 12 Tage
  • The issue looks to be a combination of pressure angle and over flexibility. A suggested improvement would be steeper tooth angle. "AAAAAA" instead of "".

    Jason Burton LatheBuilder LabJason Burton LatheBuilder LabVor 12 Tage
  • Sounds like Wall-e

    Chaotic GoodChaotic GoodVor 12 Tage
  • I would think that you'd want to gear the wave generator so the flex has gears on both inside and outside and then clock the gears in the wave generator together so you can't slip at all. I can only guess that you're slipping the wave generator around the inside of the flex drive.

    cfabercfaberVor 12 Tage
  • Hi James, how about changing the teeth geometry? Triangle to half circle or square tapered. Cheers m8!

    Julien BietlotJulien BietlotVor 12 Tage
  • Try 2 narrower flexible spline. One on each side of the mount plate with single stack of bearings on each side of the mount plate. You could even position the bearing holders perpendicular to each other.

    Jason HornJason HornVor 12 Tage
    • Oh geez simplest solution triangle single bearing layer wave generator. Simple and quick test.

      Jason HornJason HornVor 12 Tage
    • Hmm actually any flex friction savings would probably be lost in added complexity and weight. Alt material like nylon or something self lubing, triangle single layer bearing stack, narrower flex spline as close to the drive plate.

      Jason HornJason HornVor 12 Tage
  • I have been thinking of a drive with 3 lobes on the wave generator instead of 2. I think the geometry of this could do something useful for taking the power off the flexible element.

    Ken SmithKen SmithVor 12 Tage
  • I can see how you came to The conclusion that harmonic drives are made this Way. But the belt in the center of a harmonic drive is normally not a cup and that's also why you have a lot of extra friction. You should have just made it as a normal belt with 2 parts interacting with the belt, one acting as the stationary One, and one as the output it should reduce the friction a lot. I assume at least. Don't now about the teething though since I'm not an expert on this. Probably won't be back drivable eatherway but it's still beter if you're only trying to make a harmonic drive

    bjornbjornVor 12 Tage
  • dehave.info/down/aYaZlbnHcKSnvaQ/video shows that the flex part wedges two different gears together. What you have done is take just one gear and put all strain on the flex gear, while the flex gear is meant to wedge between two real gears, as a wedge. What if you: 1) free the flex gear, but make sure it doesn't run off. 2) put a real gear with a 2 teeth difference ratio next to the other. 3) if possible: put that same gear on the other side too... This way all strain should be located near the teeth, and not the whole flex gear body. I think they use steal flex gears in the industry so they don't need the extra gear to wedge between.

    bloepjebloepjeVor 12 Tage
  • What would happen if you used toothed gears, instead of the bearings, against the flex spline?

    Aaron CunninghamAaron CunninghamVor 12 Tage
  • Great job. I would recommend you to try this using an involute eccentric gear with two ring gears. The two ring gears would have different number of teeth and thus achieve the gear ratio (you can also try a Wolform gear set in a similar fashion). In this case, you would not have eccentric bearings, wich can be a big issue. All the "work" would be done in the involute gear meshings. You have to consider only the backlash. Cheers!

    Dr_RadDr_RadVor 12 Tage
  • What about a planetary gear?

    Ein Herz für LandwirteEin Herz für LandwirteVor 12 Tage
  • you need to use a rounded oval to push the belt to get more contact and engagement on each of the teeth also the flexible part is to soft

    *iTs STill ME* V-i-P*iTs STill ME* V-i-PVor 12 Tage
  • Make something Clever like square wheals or something

    Peter MarshallPeter MarshallVor 12 Tage
  • ... may by print this harmonic wave gear by PA1010 ? This PA1010 (nylon) more flex then PA6, but it more stronger on shift tension then TPU

    Артем ГорловАртем ГорловVor 12 Tage
  • what i learned today: these gearboxes are garbage. They are hard to make. They are overcomplicated. They cant provide much torque. They have major friction losses. They easily skip gears. They rely on constant material flex. I don't see any advantage to this.

    T_ CT_ CVor 12 Tage
  • I've never used flexible filaments, but is there an endurance factor at play or do they generally not wear out?

    Lawrence ManningLawrence ManningVor 12 Tage
  • Idea for future video: Make a gear that expands while rotating and only locks into place when it reaches a certain rpm. Probably not useful for this project, but maybe for something else :)

    Denis ŠaškoDenis ŠaškoVor 12 Tage
  • My suggestion - make the wall of the flexible insert thinner to remove the tolerance capacity. I believe, what is happening is that you simply squeeze the wall of the flexible insert so much that the bearings are able to pass around and will simply turn. So thinner wall should address this - give much less capability for the wall to flex thinner and allow the bearing to run around. But it's just a guess...

    Robert KlaucoRobert KlaucoVor 12 Tage
  • I really want him to build a copy of spot but he has to make it as small as spot and that he has to 3D print the entire things But he can ask Boston dynamics if he could borrow the 3D model and have it exact or even ask Boston to 3d print the parts for him And that it would be cheaper then!

    ii12x_xii12x_xVor 12 Tage
  • why not use a planetary gearbox?

    Siddh NarhariSiddh NarhariVor 12 Tage
  • Would 3 points of contact work with this design? Maybe reduce the skipping?

    Pete KastnerPete KastnerVor 12 Tage
  • How about using magnetic gears?

    Edward LymerEdward LymerVor 12 Tage
  • Are resin UV 3D printer working for printing this stuff in the video? (I'm a completely nooby about this stuff 😅)

    Maz LionMaz LionVor 12 Tage
  • You can also run this with a triangular wave generator for more teeth meshed.

    Kenneth FeaginsKenneth FeaginsVor 12 Tage
  • Hi James, you might already have looked into this but I'll add this anyway just to be sure. Did you try updating the VESC to the latest firmware? You should try the latest version of the VESC tool and do that and then have a look at running the motor in FOC-mod after calibrating it. You can also connect an encoder to the VESC, I myself use three externally mounted hall-effect sensors to sense the rotation of the rotor. The VESC has automated calibration sequences to synchronize these. When you are in FOC-mode you control the current through the motor which is then proportional to the amount of torque you have at the shaft. Standard voltage control will only set the speed of the motor disregarding torque. IMHO it's better to run it in FOC mode and then adding or reducing current depending on potmeter readings you have in your Arduino. Latest project website is here: vesc-project.com/

    The Ohmega ProjectThe Ohmega ProjectVor 12 Tage
  • Metal and high strength plastic (nylon has been used commercially for the gears near the motor side) planetary gear systems is where you are going to end up. It has everything you are looking for, it just involves using at leas mild steel or sintered metal gears. Maybe experiment with 3d printing and then casting the gears? The router you have is not going to do the small size the planets need to be made out of. Besides, they couldn't be aluminum alloy that it could cut anyway.

    KnightsWithoutATableKnightsWithoutATableVor 12 Tage
  • Maybe add bearings perpendicular to the flexing bearings to prevent it flexing further then it should

    Niek ZwartNiek ZwartVor 12 Tage
  • I'm very familiar with harmonic drives.. I have worked with them for years in Fanuc robots... They can be back driven but only under high torque... They are usually used in much higher gear ratios and with much tighter tolerances than a 3d printer can provide.. Long story short... You probably need to shoot for a higher gear ratio with more tooth contact.

    Rusty ShacklfordRusty ShacklfordVor 12 Tage
  • You should hook up a laptop to the VESC! Get some telemetry data and monitor how much current it takes to spin the motor. Gives you a good idea of how efficient it is :)

    Tower CrisisTower CrisisVor 12 Tage
  • A harmonic drive's spline is more efficient the better it is at retaining its circumference without losing engagement with the exterior teeth. This suggests several optimizations: * Print out of a rigid material and keep the deformation juuust this side of plastic deformation. This allows you to make the spline much thinner without compromising on resistance to tension. * Print out of a rigid material. You're likely losing a ton of energy into compressing the *teeth*, which costs energy directly and allows the teeth to engage badly. * Introduce little metal belts. These will resist tension without introducing resistance to bending. * Don't extend the teeth all the way down the outside of the spline. Corrugations are *fantastic* for introducing resistance to bending, which is what you don't want.

    Saul Reynolds-HaertleSaul Reynolds-HaertleVor 12 Tage
  • The fact that the flex spline can compress (not just flex) is probably causing problems. Try designing a solid flex spline instead.

    Jens the BenzJens the BenzVor 12 Tage
  • It's possible your flexible material is _too_ flexible; perhaps that's why the commercial version is made of metal. I suggest at least exploring the possibility of using a planetary gear set, it can be back-driven (at least in a single stage), which you mentioned was a requirement. Helical teeth would reduce friction.

    Jeremiah BullfrogJeremiah BullfrogVor 12 Tage
  • Hey James please revisit the iron man project

    Billy PitcherBilly PitcherVor 12 Tage
  • I think your flex spline is too flexible

    matt cmatt cVor 12 Tage
  • In the flex spine you are looking for a flex not a deformation. The long and short chords of the wave generator need to be calculated from the tooth profile. That is, at the long chord, the teeth of the circular spline and flex spline are fully engaged, but along the short chord, the teeth are just barely separated. The fact that you could rotate the flex spline when the wave generator was not inserted is probably not good. Diagrams often exaggerate the flex to show the concept, but the implementation is rarely so eccentric. With the minimum clearance, you can get more teeth at each major axis to share the load. More teeth with smaller depth maximizes the efficiency so you can get up to %60 total teeth sharing the load. Because you are flexing the surface, I think you can use a much more aggressive pressure angle than normal gearing. The reason that metal flexsplines are deeper cups are to allow the needed flex, but that is balanced by the twist of the cup which can distort the tooth geometry. I think that is why you were able to stop the rotation with your hands. If you are going to continue to use a TPE, you should use the hardest (PCTPE/TPA), switch to involute teeth, with an aggressive pressure angle, and calculate your wave generator ellipse dimensions based on your tooth working depth. Creating a wave generator with bearings all around the ellipse (at least at the major and minor axis) should also help keep your flexspline from deforming rather than flexing. If you go for a more rigid material for your flexspline, balance the depth of the cup to allow for flex, but minimize twist.

    Mark HorstmeierMark HorstmeierVor 12 Tage
  • "wave generator... strain weave... wave generator" Man that grinds my gears.

    batlinbatlinVor 12 Tage
  • Try making the circular spline in a conical shape and not a cilindrical shape (deform manualy the flexpline and predict the angle it will deform so you have full mesh between the flexpline and the circular spline), also the bearings should be "tilted" so the surface they contact in with the deformed flexpline remains constant and helps to make the meshing better.

    Sebastian BasaureSebastian BasaureVor 12 Tage
  • start this video: wow thats a cool gear Next screen: fully 3d printed boston dynamic clone: "OH SH-"

    punkrock4401punkrock4401Vor 12 Tage
  • Hey I'm not certain but you show that the drive is jumping 2 teeth per rotation early in the video. Should it be a single tooth jump? Could this be deforming too much, and not acting predictably?

    Eric WhewellEric WhewellVor 12 Tage
  • Very nice. Why not INVOLUTE profile ? Basically gear profiles ? That would (I think) properly handle force vector to a constant, much shallower pressure angle (i.e. 20 deg). I suspect your basic triangle profile just put a ton of useless radial load, causing it to skip and adding ton of extra friction. Also, I think using a big rigid cam mounted on an extenctric bearing would be way more efficient compared to those smaller bearings. It would support the back of the strain gear so it doesnt collapse and reduce local deformation (less hysterisis loss). These two factor could actually make it back drivable and more reliable. Please try these I'm really curious to know ! Cheers !

    Antoine TalbotAntoine TalbotVor 12 Tage
  • Hi, where do you get your bearings?

    Ivan RamosIvan RamosVor 12 Tage
  • You pretty much nailed a lot of the drawbacks of the design. So why do industrial robotics companies use them so often? Because they have the lowest backlash and are lower maintenance than cycloidal drives.

    Scott JacksonScott JacksonVor 12 Tage
  • Very cool thoughts and design iterations on both drives. I suspect the triangular teeth are not a great choice for friction or smooth radial velocity. E.g they must be generating a big radial force vector, which must also be doing some work. Consider trying ring and pinion gear design formulae. This looks like a nice discussion: gearsolutions.com/departments/tooth-tips/internal-ring-gears-design-and-considerations/#:~:text=These%20gears%20are%20composed%20of,is%20opposite%20of%20internal%20gears.

    Gene ResslerGene ResslerVor 12 Tage
  • Over long periods of operation wouldn't the flex spline get ground into bits eventually?

    Pedro Paramo del vallePedro Paramo del valleVor 12 Tage
  • Been loving these videos on reducers. I was trying to make a 3d printed harmonic drive myself a while back. I think one thing you could try is to add additional bearings onto the wave generator so the bearings form a sort of ellipse shape to keep the flex spline from flexing in an undesirable way. I think this may solve the issue you were having when the motor would rotate but the output was able to be held in place.

  • To echo other commenters, try again with the flex spline printed in a rigid plastic. To regain flexibility, add a thin cylindrical section under the teeth instead of continuing them all the way to the output. (just like how metal harmonic drives are built) It might also help to buy some Delrin bearing balls (at least 8mm) and 3D print your own elliptical bearing wave generator.

    Marty lawsonMarty lawsonVor 13 Tage
  • Harmonic drives are not vack drivable, to back drive them would be to shear off the teeth

    Shirdel YanShirdel YanVor 13 Tage
  • I know why the harmonic drive isn't working. Because it is an abomination to God, and all that is holy. (Sarcasm) There is just something so wrong to me in the combination of gear and compliment mechanisms. I think it's because anything that bends will be the first to break in a mechanical system.

    Belias PhyreBelias PhyreVor 13 Tage
  • I've been watching a lot of harmonic/cycloidal drive videos lately. Somebody printed in straight PLA and it "flexed" enough to work well. Finding that goldilocks zone of stiffness and rubberyness is where I imagine a ton of research has gone into for industrial drives, and the most frustrating to dial in as a DIY.

    Saccharine SasquatchSaccharine SasquatchVor 13 Tage