Compliant Harmonic Drive (3D Printed)

[@LeviJanssen]

Whoops! Every time I said "plastic deformation" I meant "elastic deformation". I guess that's what I get for not writing scripts. Also, I filmed this video with a new camera, so I was still figuring out things like focus and white balance. Oh well :)

[@averybrown8136]

I was just coming down to see if anyone had said anything about the deformation😂

[@PiefacePete46]

I guess the important word was "deformation". Didn't notice anything annoying with the camera... focus a little slow on close-ups, but better than most KZbin offerings.

[@wilheldp]

It was elastic deformation of plastic, so I’ll allow it.

[@KeithOlson]

Hmmm... What about making a dual-layer deforming piece, where one layer is conformal to provide springiness and the other incorporates a 'disc-and-socket' joint at either end of a 'bone' for resistance against torque without preventing deformation. Both layers would be part of a single model. There would just need to be a small gap between the disc and the socket to allow for rotation. (Three layers would be better as you can trap the rotating joint in between two conformal hinges, but that would require some thought in regards to support.) Does that make sense?

[@BricksanSticks575]

perhaps if u use metal like what actual springs are made of i could handle more pressure maybe?

[@qupbuqqoywogh1108]

Failure??? You were creative, you designed, you build, you tested, you learned and you shared. Hence, a big success! In one word: IMPRESSIVE!

[@Enkye_CA]

As a close friend once said to me: “We live and we learn. I am now wiser. This is not a complete loss.”

[@fowlerj111]

He was using the term failure in an engineers sense. He was measuring how much torque it took to break the mechanism. If the answer is higher than anything it will see in actual use, then it's strong enough. The safety factor is how much higher. If the answer is too low, he has information about how to adjust the design.

[@glow262]

Failure might just be as useful of a result as a success.

[@jawells1955]

As Thomas Edison said: “I have not failed. I've just found 10,000 ways that won't work.” “Many of life's failures are people who did not realize how close they were to success when they gave up.” I wonder how how two flex plates like you designed installed in opposite direction would hold up.

[@auralynn3862]

Same philosophy here. Despite the mechanism failing (i.e.: reaching a failure state such as slippage or breakage), the process of experimentation was a success as it yielded worthwhile information.

[@sermadreda399]

I think you can give the compliant spring more depth ( not width) it will keep its elasticity and give more support against torque

[@julstr6303]

Maybe also add some radius or chamfer to the attachment point from the spring to the internal and external gear so the plastic won't break at the sharp corner.

[@user-py9cy1sy9u]

@@julstr6303 ^ this

[@maibster]

@@julstr6303 very much dis^

[@frother]

And if you keep adding depth until it's as deep as a regular harmonic drive, you've basically created a stronger (better?) version of the regular harmonic drive.

[@maibster]

@@frother thats not how mafia works

[@combosloth1677]

you have no idea how much I appreciate you using metric instead of imperial

[@LeviJanssen]

Always have!

[@3DprintedLife]

I honestly don't know any engineers (I'm in the US) that use imperial. The only time we do is when we're forced to by dang archaic machine shops! Metric is love, metric is life

[@ALEXGIBSONCMG]

@@3DprintedLife imperial is less common in engineering, more of a general purpose measurement.

[@henmich]

Come on... how hard is it when the software does all the heavy lifting??

[@JohnMeacham]

Despite being in the united states I have fully metricized my shop, it makes collaborating online so much easier. Highly recommended. Although in my open source designs I try to use 3mm 8mm and 30cm dimensions as much as possible as there are very close imperial standard sizes so people can often use a design as-is with what they have on hand.

[@jamesridzon4707]

1:04 I think you meant elastically deformed. If that piece was being plastically deformed repeatedly, it would fail due to fatigue very quickly.

[@AleNXT97]

You mentioned that one torque direction could withstand more than the oher. So I thought that you could add the same pattern on the first (in the same part), like it is another 'layer', but mirrored, so it will compensate the inequality. Really loved this design, to me it's the most interesting I have seen that is 3D printed. I think you could expand the research from here to make a very good design, great work!

[@Peter-898]

Another option may be to include springs in alternating directions. That would probably come at the expense of radial space, but would still allow for a low vertical profile

[@LeviJanssen]

I was thinking the same thing, I wasn't sure how to implement it easily. I guess layering it wouldn't be too difficult.

[@gordonpromish9218]

ah, you had the same thought

[@red13emerald]

@@LeviJanssen I mean you could always just print two parts and assemble them together in some way. You could also print supports between the two layers of springs, but those would be hard to remove, I reckon.

[@brianfoster7064]

@@LeviJanssen you can do that as a single print leaving a small gap between the two spring layers keeping a single hub. Also, increase the depth of the springs and gear without modifying the thickness. That should provide more strength without compromising the elasticity. Also, consider using TPE. Though, a TPU spring with PLA center would be even better. That, however, would require a printer able to print with two filaments, or a lot of fun doing it manually.

[@BrassLock]

I first saw a 3D printer in Chiang Mai a few years ago and realised how the future would unfold rapidly in the hands of young inventors. You are a great example of how mechanisms can be rapidly prototyped and improved, in ways that could never have been done last century. It's exciting to hear your thought processes, and see your working models.

[@2danshepherd]

Compliant mechanisms and strain wave drives are both cool. So combining them is even cooler! I'd love to see more development of this project.

[@Niro_sounds]

It's cool that people just put information like this out there. There's an entire different side of the spectrum which would try to find ways to patent and close off this knowledge you're willing to share. We really are coming into a new age. It makes me happy.

[@nikolatotev]

Something that comes to mind, you could alternate the spring "direction" so that for every "left" spring there is a "right" spring. That way the mechanism has support in both directions. Another improvement would be to make the springs thicker vertically.

[@davidaustin6962]

There's a youtube channel where all he does is test people's fan blade design. You should do the same, but with a torque transfer plate (or whatever you call this). I have an idea that I think would work.

[@Barak43]

that's a good idea ^^

[@stephensomersify]

Not sure what i mean - not sketched - 'radial slots'

[@xaytana]

At the very least give the name of the channel, Major Hardware, and you should also better describe the channel as he does far more than just the Fan Showdown series.

[@dapperbrick7516]

@@xaytana I came here to say this. He's a smart guy over at Major Hardware, moreso than he lets on in his videos

[@nathanmaher2846]

'Elastically' deformed, not plastically. Also, this project was AWESOME! Please keep developing. Torque is usually expensive - either in size or cost, so cheap torque will always be in demand.

[@LeviJanssen]

Yeah, I had to correct myself at least once, I probably missed other occasions.

[@RinksRides]

...Like Ford Mustangs.

[@bishopp14]

I agree. Keep up the good work! What you're doing is awesome. And interesting to boot!

[@colbylippincott7173]

What if you replaced the longest section of each of those flexures with an off the shelf metal strip or wire or something? You could beef up the plastic sections and have all of the bending be done by the metal.

[@anoirbentanfous]

the solution should be from the design first before looking for the material science...

[@antno9158]

Surely design is interactive with material dcience? Different materials have different properties to design with. PLA isn't a great choice for a flex element.

[@revimfadli4666]

@@anoirbentanfous shouldn't you keep material properties in mind when designing? And designing for one material might not work for another

[@colbylippincott7173]

As long as you designed the mechanism to stay below the fatigue limit of the material, any material is a good material for the application. It all depends on what you are shooting for. If you are looking to minimize cost, but don't care about size, you could use a cheaper material and give it more room to flex without stressing the material much. If you care about size, but not cost, you could make it from a more expensive material that is better for higher stresses. And if you are really creative, you can come up with a design that optimizes cost, size, and performance using a mix of materials. Engineering should never be constrained to "this is the material for this application". That will severely limit creativity. Creativity, plus a clear, unrestrictive definition of the problem, the willingness to fail, and trusting but testing other's advice equals finding the best solution possible.

[@OpenSourceLowTech]

Swap in some spring steel strips with the PLA as just housing? Junior hacksaw blades are a good source of thin spring steel which can (usually) be bent 90 degrees without breaking...

[@NistenTahiraj]

I think this is now my fav new channel for 3d printing. Original ideas, competitive, first principles thinking, proper testing; keep it up dude.

[@RecreationalOverEngineering]

Very interesting! I like your style of going through the design history, so we understand the reasoning behind it. Keep up the good work!

[@liborferencak8297]

Please continue with this project. Its really interesting.

[@stefanp7603]

Great engineering video!

[@ahaveland]

Great work Levi! I have a couple of suggestions - use fillets at each end of the compliant spring to distribute more force and reduce stress risers. You could have more of them per revolution, and you aren't limited to having them in just one plane, so you could layer them up for even greater torque transmission.

[@Magnum3144]

I'm just impressed all around. Well done and nice display of your knowledge.

[@safetyinstructor]

Appreciate the use of engineering units ;)

[@gregarmstrong4653]

well done, you are really close to a solution, both the vibration and the strength. All you have to do is print the compliant mechanism seperate from the spline, print two, put one of them up side down and glue them both into the spline. Now the two compliment each other, and the mechanism should be much stronger and there will be less vibrations

[@3DprintedLife]

Awesome video! I'm working on my own harmonic drive myself aiming to be super compact as well, but using an inside-out HTD belt instead of 3D printing a flex spline. Initial results are promising but I still need to get a high power motor driver and do some torque tests!

[@Roadhouseee]

Super cool video dude! First I’ve heard of this particular mechanism

[@GabeIsko]

Cool Project! That's the tradeoff with Harmonic drives - no backlash, but your stiffness is compromised. Even happens on the indutrial drives. They are a lot stiffer than the plastic ones because they are metal, but not nearly as the stiffness you get out of a normal gearbox.

[@julienboucaron9780]

Pretty elegant design. Curious to see what geometry, layering and other stuff you will create to put more springs and material to handle the torque.

[@DiekiKondrael]

I think the best bet is to use a more flexible material. With something like ABS or PETG, you could get the same flexibility from thicker (and thus stronger) parts.

[@KageBlink]

Thats amazing, I can see PETG or ABS making a huge difference in strength. I would go at it again, good video! Thanks for sharing

[@SianaGearz]

ABS/ASA are finicky but they could work indeed. PETG only works up to a point, under repeat stress and certain applications of force it paradoxically crystallises and becomes brittle! But there are printable TPU/TPE varieties, and not all of them are that low Durometer, they could be of use as well, they're basically made to handle repeat stress. Polyamides (Nylons) as well after one-time conditioning.

[@keithduffield5239]

Thanks ever so much. Especially for your explanation and show and tell on how a harmonic drive works. I have used many HD's in steerable antenna, but never understood exactly how they work, despite trying.

[@CoolAsFreya]

Can't believe this is the first time I'm finding your channel! This video alone has earned a sub! This design with a less brittle material for the compliant gear could be very good.

[@cyborgmike9210]

Addressing favoring direction: move the inner support to the middle and double the outer to both sides of the "spring". Angle the inner as "v" hitting the core allowing load to be transferred via compression and extension. You can do the same with the outer.

[@aditya95sriram]

The KZbin algorithm brought me here and I'm not complaining :) Good work!

[@svampebob007]

Hey if you want less "wobble" without over stressing the threads I would suggest adding some springs to the bolts. It does add "more load" to the threads at the same torque, but that also allows you to convert that load into frictional between the 3d printed part and the metal beam. This is kinda how we use springs on watercoolers for CPU's/GPU's, you don't want to crack the chip so you add spring loaded mounting brackets that distributes the load more evenly ensuring a "tighter" gap between the cooling surface and die... so in turns the force is applies more evenly. If you ruffen up the metal beam that would add more friction between the metal and plastic, kinda the opposite that we do with coolers. it's probably not going to make it "stiff" but it should prevent a couple of wibbly wobbly, timey wimey stuff to your presentation :)

[@SamAlderDesign]

Very clever, and clear explanation of the results. Appreciated.

[@Bigman74066]

I think that the curved part of the complient mechanism could be made thicker without compromising springiness. It may also be beneficial to use alternating left and right turning springs. Just a thought... Nice video! Thanks!

[@solosailorsv8065]

Rather DeVinci of you , Well Done ! Made of traditional spring steel, your flexure design is unique & novel

[@dekutree64]

Great idea! The most obvious way to increase the capacity is to simply scale up the springy part in the Z direction. Why do you (and so many other makers) use such small bearings on the bearing bar? Seems to me that you'd want to use the largest you can fit, to produce a smoother bend in the flex spline, reducing fatigue and getting more teeth engaged and supported. Either that or use a bunch of small bearings placed along an ellipse to simulate the proper elliptical bearing of a harmonic drive, counting on the stiffness of the flex spline to produce a somewhat smooth curve across the spaces between each individual small bearing.

[@maxk4324]

Mostly cost I bet. Smaller bearings are cheaper both in material cost and in shipping cost.

[@jurajvariny6034]

What if the whole bearing bar would be smaller, say only 2/3 of the diameter of the flex spline? Now you have whole outer 1/3 to try various compliant mechanisms to optimize both flexing and torque. And leading to space saving overall.

[@swannschilling474]

I designed a backdrivable 40:1 Cycloidal Drive which I am super happy with...all my attempts to get a harmonic drive going failed terribly!! This is super interesting!! Keep it up!! 🤩

[@DailyFrankPeter]

Very educative thank you. Good work with your design, too! It seems like a natural fit (harmonic drives being almost compliant mechanisms themselves).

[@mikebennett2015]

I used a harmonic drive back in the eighties, but it was different to the one you have. It had two outer gears, both same pitch diameter but one had one less tooth. The flex spline engaged in both the output would rotate one tooth for each rotation of the flex spline.

[@mosesturner4018]

This is freakin awesome!! I'd been thinking about harmonic drives for like the last year and a half; this never even occurred to me!

[@MaxMakerChannel]

Very specific, very cool. Loved the style!

[@deschoee]

Awesome! More compact, less parts and no imbalance.

[@wobblyjellyman69]

Once again an original idea and a great quality video. Keep it up mate! Great stuff

[@RinksRides]

Brilliant start of something bigger. I'm calling it.

[@KingdomOfHeavenPictures]

Wow, your videos have become much better since 2019. Great work!

[@WungoBungo]

this is absolutely genius design. very well done

[@christianmose9450]

Impressive! Harmonic Drives do fill a gap where hole bunch of alternative solutions should be possible..good approach!

[@damianbrisendine5154]

Super cool would love to see it scaled up and tested again

[@uint16_t]

Amazing work! Well done.

[@artantme]

Man, congratulations! That's impressive. It broke on load, but it's just a trial and error process. Please keep on!

[@nikitanugent7165]

Very well done! I'm keeping my eye on this. I'm hoping to start working on a small robot arm soon, and this is definitely has potential.

[@KW-jj9uy]

you can introduce some sliding slots to transmit the rotation force, and then use smaller deeper springs for radius changes.

[@that_penguin1605]

you could make half of the springs one direction and half in the other direction so it is still springy in opposite directions and a higher strength for both directions

[@AllanKobelansky]

Very educational. Well done Levi.

[@TheMadManPlace]

Not a "simple" concept but a very intelligent and eloquent solution to the problem - inspired work - congratulations. May I suggest movement limitation stops in the flexing wheel that will prevent the spring element from being overtaxed? This could cause lockup if the spring element is over-stressed but that is good - don't drive the mechanism beyond its mechanical limit. I wish that I could share a doodle here to visually explain what I'm talking about...

[@dythe7858]

So cool, hope I can learn to use what I learn at uni like you do. Best of luck making a reliable 3d printed harmonic drive. It would be fun to experiment and take your compliant mechnisum and extrude it on the y axis to add more torque but keep it compliant on the x and z axis. (Extrude the y axis until it's big enough to fill the empty space from the regular design)

[@nsshurtz]

If the spring is indeed stronger one way vs the other way, you could possibly add in a mirror image of the spring system and that would definitively help, that way it can be under tension both ways instead of just one.

[@-powerband-gamer-6117]

You have a bright future ahead of you my friend.

[@eggspresso]

The super low parts cost for 3D printed harmonic drives makes them really favorable to cycloidal drives that rely heavily on bearings. To the point of the compliance being variable in rotation direction, you can mirror one of the flex splines and run 2 inside the same rigid spline connected to the same output. That should give you more symmetric torque, and also double the amount of material you've got in contact.

[@traviscecil3903]

Would love to see you try this again. But this time, turn your spring. An inner core, an outer gear set, and two sidewalls with outward flex, but solid and without flex in the direction of motion. Think Goodyear tire. For bonus points, look up some of the solid tire concepts from Goodyear for the military.

[@Molb0rg]

that is super cool, this is indeed how they have to be made. Very cool, thanks!

[@asid61]

This is an excellent design! Those springs solves a big problem with printed harmonic drives.

[@ShivaprasadDabholkar]

Wow! Really enjoyed the video.

[@jakewagner7416]

About a month ago I designed and 3d printed a flex-splineless harmonic drive. I was working out shrinkage dimensions on new filament at the same time, so mine is quite clunky, but it basically negates the need for the spline altogether and worked well enough as a proof of concept in my view. Maybe it's not of interest if you're specifically looking to explore compliant mechanisms, seeing as I took the flex out of flex-spline lol.

[@Lakitu1446]

I really liked this video, the way you explain everything and why you do it. That's the scientific approach i like to see with makers i like and subscribe for more, continue like that ;)

[@0LoneTech]

The broken springs are mostly broken past the bend. I don't think they need to even have a bend like that, and it doesn't need to be where they flex the most either. I'm thinking a design with concentric circles connected by short thick spokes, out of phase such that each outer spoke sits in the middle between two inner spokes and vice versa, may be worth trying out. The test showed where the weakest point for this particular design was for the rotary force, not that it had to be that weak there. We want rotational stiffness and radial flexibility for this task. We could also make approximate evaluations with FEM (built into e.g. FreeCAD). A part with these same goals is typically mounted in 3D printers: the shaft coupler. Compliant versions usually have a spiral cut in their midsection, allowing some flexing. There its main purpose is to adapt to slight differences in axis alignment. Unfortunately, many makers then skip the part about stabilizing the output shaft against linear axial force.

[@txikitofandango]

I don't know much about this stuff, but I found your video really easy to follow

[@GeovaniNogueira]

You can maybe achieve an bidirectional rupture force if you mirror the design and kind of glue them together... Furthermore, you may be able to increase the number of Springs of this design if you give some angle to then and pile one above (alongside) the other...

[@MecheMike]

Great job! Keep developing.

[@chrisBruner]

I've done similar things, with similar results. Hoping you come up with something even better. Good job.

[@i-make-robots]

nice! thorough! good work.

[@CatNolara]

I was building something similar, only a little bit more fancy. My thought: it's not ideal to make the compliant struts curved, since they would bend under tension -> reduced accuracy. I would make those struts straight, maybe even with thinned out joints at the ends. Also something I was trying out as well: there's a type two harmonic drive, where you have one flex spline that is basically just a toothed ring on an eliptic bearing, and two rings with inside teeth. One of those rings stays stationary while the other is connected to the output and they both have a tooth count difference of 2. This also makes a very compact drive, but it has high demands on the accuracy of the inside gears since any deviation of the tooth shape would result in backlash or binding.

[@ModelLights]

I'm looking at similar aspects, but somewhat different conclusions.. First, at the end with it broken, notice how they seem to have 'curled up' at the ends. The opposing curves at the ends of the elements makes the longer element want deflect on a diagonal when under the 'compression' direction force. Instead of a curve and flexing at each end, they need a trapezoidal pillar as an anchor for each base connection. Then the element joined flat to the top of the trapezoids, make the joint much stronger so it can't bend out of line or curl. IOW strengthen the ends, and make all the flex of these elements happen in the longer middle area. Stronger end corners will keep the entire element from shifting out of line and make the force spread out into most of the longer element between the base pillars..

[@leosbagoftricks3732]

Very interesting project- enjoyed and subscribed.

[@ArunUdai]

Awesome design and your research. Great demonstration 👍

[@TrueSighted]

Try T with a small bump on the ends sort of like a Y shape, but more ability to bend inwards in your mechanism. Yet still connect to outer ring shape. Should bend but still be rigid in both directions. Allowing your reverse drive. just a thought.

[@foureyedchick]

The red plastic inner gear reminds me of the adapters used to play 45 RPM records on 33 RPM turntables.

[@davidaustin6962]

Get rid of all curves in the torque-transferring spokes. For example the outside (teeth) and inside (hub) should be 2 separate pieces with STRAIGHT spokes connecting them, hinged at the connection points (from the inside hub to the flexible outside gear). There will need to be some axial-movement play in the hinges.

[@sage5296]

I think you should alternate the direction of the springs, so that way half are always in tension. Should make the failure load a lot higher for the device

[@alexkuhn5078]

Behold, a whole compliant CVT! I can't wait to put it in my Compliant Robin

[@franciscoanconia2334]

That's a VERY nice design mate... The compliant cage makes it actually quite feasable to do in steel and just water-jet cut the middle. Maybe with a water-laser to keep as much material as possible.

[@pauljs75]

Add some spring layers in opposing directions. Slightly thicker, but should work. Other than that, a better material choice than PLA would help.

[@saxd0ct0r]

So I just discovered this video… fascinating for a non-engineer like me. My initial thought was that the compliant spine could be duplicated with the reverse orientation and combined. Therefore, when one is being stretched by the torque, the other is being compressed, and neither direction is favored.

[@fernandoi8958]

This is great design and video, congrats! Would love to see a metal version of this...

[@sephjfox]

looking forward to seeing another iteration of the design

[@Decco6306]

To reduce wear on it, put petroleum jelly in it as a grease.

[@daniellclary]

Very interesting way to get a gear ratio like that. Although you can increase torque in these methods, You still will only get the torque that the materials can handle. And thin plastic parts don't handle torque very well. But this does show potential if you are able to get your hands on more appropriate materials. Be cool to see that done. Also the only experiment that is a failure is one where you don't learn anything. I think we learned alot from this.

[@SimpleElectronics]

Hey Levi, I love your content! Subscribed, keep it up!

[@Untaramar]

great video! One comment, for those of us who are not in the know of what these sort of devices are used for, a intro of common uses or what you might be using them for would be spectacular. 👍

[@Gallardo6669]

Excellent video, thanks!

[@cirnet]

What if you don't drive the output off of the flexspline's hub, but instead the flexspline meshes with both the input and output gears, which are concentric but out of plane with each other? I feel like you could make the mechanism even more compact, and your torque would only be limited by the strength of the teeth themselves.

[@mineterragaming1796]

this made with titanium and more springs would work great. the titanium would be flexible enough to work as a spring and be strong enough to not snap due to torque. good idea!

[@TheRainHarvester]

Heck yeah there are more optimizations to come! Make that spring twice as thick. Use 2 springs, one flipped for symmetrical load. Make more springs instead of only 6 around the circle. Great design! I think you've started something great!

[@TheRainHarvester]

4:36 in the top left...What went wrong with that one? It looks the same as the final spring, but had 8 springs around the circle. Was it too stiff?

[@LeviJanssen]

It had eight springs and was significantly stiffer, yes.

[@Mr30friends]

I wonder how hard it would be to make a design that replaces the springs with metal spring blades and the outer circle that deforms with a timing belt. The only challenge with that would be to somehow attach the metal spring blades to the timing belt. Some kind of small riveting or stapling might be able to work with that if the timing belt is wide enough to allow enough teeth area to remain intact despite having the rivets/staples there. If it all came together you could get some really solid performance out of that thing. At that point it wouldn't matter if going one way is stronger than the other way, cause event the weaker one would be insanely strong for a mostly 3d printed design.

[@alinoori3350]

This was awesome I love to see an optimized version of it👌

[@mdchaney]

I think you can add a lot of overlapping springs, seems like a great idea and great base for further research.

[@Daniel-rj2ci]

Great video! Could you maybe use nitinol to transfer heat to rotational energy?

[@jasonorozco1587]

"It's not actually this bad, it's actually really good." Best disclaimer ever. Will keep that one in my back pocket. 😂

[@xaytana]

The direction bias could probably be fixed with mirroring the springs backwards, like a modified Y shape. Maybe have to sets of rings s othere's no bias between the hub or ring having more connections to the Ys, as in invert every other one so that the connections are -1-2-1-2- on the ring and -2-1-2-1- on the hub, where the horizontal of the Ys are offset to form two rings of the concentric portion; this should also add some strength overall. Better materials would also help with the strength quality of a compliant harmonic, just as there's a reason industrial-grade harmonics always use metals and not plastic. This could easily evolve into a viable solution, and with it being open sourced it'd make industrial-grade robotics more affordable.

[@typha]

why not stack two of those flat axial compliant springs on top of each other in opposing directions. Also you could try thickening the material in the places it broke before, it looks like that was mainly right where the flex rod things connected to the inner wheel.

[@douglarue8485]

Add some meat/plastic on the gear ring between each spring and design a slot through with a metal pin could be slipped in during 3D printing. The torque could then be transferred more directly from the outer ring to the center via the pins as opposed to the being transferred via the springs. with a hole in the center of the long straight section of the spring tuning the spring dimension might be required. Very nice though.