Designing High-Performance Linear Actuators: Speed & Stiffness for my Juggling Robot | PDJ#19

[@harrisonlow]

Hey everyone! I hope you enjoyed this deep dive into the design process of the high-performance linear actuators for my juggling robot. I'd love to hear your thoughts on the design! What improvements would you suggest? Have you ever built or used linear actuators in your own projects? Share your experiences and tips in the comments below! 😃🤖🔧

[@IncendiaryMedia]

this is slick! have you considered routing the cables through the actuator arm itself?

[@paint4pain]

As others have mentioned, one or two PTFE or Rulon J linear sleeve bearings would be lighter and probably stiffer than 6 6001RS steel bearings. You could also swap to acetal plastic ball bearings instead of steel to lighten them up and reduce wear. Other than that you could replace the push and pull strings with a single string and have a tensionning spring at one end to remove backlash. You could also have the pulleys be tensioned against each other to acheve the same result but that sounds clunkier.

[@Alrion1704]

14:20 use a ceramic eye from a fishing rode for strung position 😉

[@MartinDxt]

u can use the bike/bicycle brake tubes those are quite tough instead of the ptfe

[@BlackHeartScyther]

I would put bearings on the 3 rods, back end of the extending arm, just for a bit more stabilization.

[@kurt7020]

For the string you might also look into braided uhmw, commonly used for high performance kites. It is not degraded by UV (if your robot has to perform outdoors) doesn't absorb water, and has far better abrasion resistance and has less friction than kevlar, also has nearly zero stretch for low backlash. Pretty cheap too. Excellent vid. Thanks for posting!

[@cgln8760]

I was thinking the same but with Braided monofilament as used in fishing, for the same reasons.

[@timotay22]

UHMWPE does have problems with creep, though. So you'd want to be sure that it's not stored under tension when not in use. Otherwise slack will creep in, and you'd need to re-tighten everything before you restart

[@genixia]

Use SK99 for the lowest stretch and creep.

[@James02876]

You could probably fit the ptfe tubes inside the brake cable housing for a bicycle. Alternatively, you could design an print a modular reinforcement part that surrounds the tube and can carry the compressive load. This is a great design, very cool project!

[@harrisonlow]

Thanks for the kind words! Re. PTFE ideas: a big consideration for these actuators in this project is that they need to be able to move around fairly freely, and I'm worried that bike brake sheaths would be too stiff, and I can't visualise how a printed support structure would be both flexible (in bending) and stiff (in compression). I would love to know if you have any suggestions on that front! FWIW, a few people have suggested trying brake cables and I'll be testing that (and a few other) ideas soon. I'm sure we can eventually find the perfect tube! (or completely get rid of them, as Eric McGraw suggested in another comment. So many options!)

[@NUeB_net]

@@harrisonlow Maybe Jagwire Elite Link can do the job.

[@James02876]

I was thinking about this while falling asleep last night, and I realized that simple beads-on-a-string will provide resistance against compression, but still allow flexibility while not under compression. Anything that allows the tube to freely pass through it, with a rounded edge to allow flexibility, would probably work. However, if it needs to remain flexible _while under compression_ it may be more of a challenge.

[@NUeB_net]

@@James02876 "beads-on-a-string" is basically the idea behind Elite Link and similar products. They allow smaller radius than conventional bowden tubes and at least one of such (of which I just can not remember the name) claims to be flexible under load… so further investigation may be worth it. I never used such myself because on all of my vehicles, classic bowden works just fine.

[@feiter]

Of you go down the bicycle cable option. Try with gear cable and gear cable housing rather than brake cable (thinner diameter cable and come with far more housing and connector options)

[@oscarhalse2026]

Wow this project presentation was EXCELENT! Besides the design being impressive, the presentation was beyond what you can expect from the average engineering video. It was very easy to follow, well paced, and most crucially it covered all* your design constraints which produced the end design. That piece of context is so incredibly important to make the listener understand. It demonstrated a deep understanding of the issue, it's multiple solutions, and that your design really *is* near the ideal design for your particular use case. I felt like I actually understood the design, I didn't just observe it. That level of maturity in design presentation just blew me away. Personally I think you outdid "Stuff Made Here", which in my eyes is the gold standard of engineering videos on youtube. Although the comparison is nto entirely fair to "Stuff Made Here", as he obviously targets a more casual viewer.

[@harrisonlow]

Wow! Thanks for the great feedback!

[@JCBeastie]

Great concept! An easy fix for the string getting pinched would be a small screw eyelet. Have you considered using Bowden cables? That would likely need you to change from spools to cams to drive it though.

[@weirdsciencetv4999]

This is absolutely brilliant work. I came up with a similar mechanism for my vacuum former. I needed to plunge the part piston rapidly to create a sort of surrounding bubble of soft plastic, before activating the vacuum. I'm surprised there aren't commercial products using this technique. Keep up the good work, I love your projects!

[@harrisonlow]

Cheers for the kind words! I agree that it's remarkable that this mechanism doesn't seem to exist on the internet anywhere else (at least not that I could find). Hopefully anyone else in the future who needs to build themselves a speedy actuator can now save themselves the ~1.5 yrs of development that I had to go through to get these working!

[@SouthSideChiTown]

This is so interesting! People like you are the future of technology, not big corporations. Thank you for sharing this! Subscribed!

[@harrisonlow]

Thanks for the support!

[@hylander2126]

For the string running between the bearing and the slide, I would suggest 'v-groove' bearings, which as the name implies, have a groove along the face of the bearing. Maybe someone already suggested it, but just my 2 cents (or pence) :)

[@victornovak1790]

Harrison, great job! Suggestion; instead of using the PTFE tubes for cable guides, why not incorporate the cables into the carbon fiber structure tubes?

[@Traqr]

You'd still want cable guides (shifter cable housing seems like the best bet) into the bottom block but yes, using the structural tubing as part of the string guide is a great idea.

[@harrisonlow]

Cheers! Routing the strings through the CF tubes would probably work well for many applications, but I'm using these actuators (there will be 6 of them) to power a Stewart Platform and they need to move around quite a bit. Having the strings routed through the PTFE tubes means that the strings don't get in the way of the actuator moving around

[@sandorkonya]

@@harrisonlow i had the same idea, why not routing the strings through the CF tubes. I don't exactly undestand why would the strings get in way of the actuator?

[@JH_Tech49]

Really nice. I wonder if it would be possible to route the Bowden string directly inside two of the carbon rod... This would make it looks like it's moving magically.

[@nemesis1588]

If two of the bearings that holds the shaft straight had a channel in it to guide the string this could be done easily.

[@Anonymous-ex3wq]

exactly what i was thinking

[@GnuReligion]

@@nemesis1588 Maybe turn the whole system inside out, where the actuator is a large tube on the outside, and the bearings are on the inside mounted on a smaller rod. Hide everything. Meh ... may make it too heavy.

[@ericcmcgraw]

Loving the progress! You can get rid of the bowden tubes and also stiffen your transmission coupling by guiding the cable around pulleys which rotate coaxially with your gimbal joints. The two cables need to wrap in opposite directions so that the total cable length remains constant. Normally this would slightly kinematically couple your linear motion with the rotation, but you can cancel this geometrically by offsetting the linear axis from the gimbal axis by the same radius as the pulleys. You can duplicate this concept rotated 90 degrees to get the other DOF of rotation. If you need help visualizing/designing the pulley layout, let me know, but I think it should be a feasible way of getting well-constrained cable motion to the actuator with almost zero flex/compliance or motion coupling. The pulleys could actually be a single, dual groove pulley since they will rotate in tandem, but you can also just use 2 parallel pulleys or one on each side of the joint itself.

[@harrisonlow]

Thanks! Hmm, I'm having difficulty visualising what you're describing. Do you know of any real-world systems that use something similar? I do quite like the sound of a tighter transmission 😁

[@ericcmcgraw]

@@harrisonlow Skyentific has shown a mechanism using pulleys co-axial with arm rotation, but for the purpose of actuating the arm itself. kzbin.info/www/bejne/q6Wnkpqlqt2Aa7c The idea I'm suggesting simply uses such pulleys to guide the cable onto and back off of the rotating body. I will try drawing the idea a little later and sending a picture if you would find that helpful.

[@harrisonlow]

​@@ericcmcgraw Very interesting! I just watched that video and I think I have a better idea of what you're describing. I have a few thoughts: 1) I'm quite averse to shifting the linear axis away from the centre of rotation as that makes the inverse kinematics for the stewart platform (what these actuators will be used for) significantly more difficult. I don't know exactly *how* much more difficult, but I started working through it a while ago and after a while decided to just make the joint axes coincident with the linear axis 😅 2) This design looks like it'd require quite a few parts, and at the scale I'm working at, they'd have to be fairly small. I'm trying to cut down on complexity as much as possible (especially where it concerns manufacturing). 3) As a slight (maybe) spoiler, I'm actually thinking I'll change the lower universal joint for a magnetic joint, in a similar fashion to the upper joints (this video if you haven't seen it already: kzbin.info/www/bejne/d3TMlZSqlKqEabM ). I'm not sure if this would affect the design you're describing, but it seems relevant to mention. Again, thanks for the idea! I'm interested to know what you think about the above points 😊

[@harrisonlow]

@@ericcmcgraw Grr, I think KZbin may have removed your last comment. I can see some of it in my notification, but nothing past a "small error" re. shifting the linear axis. Did you include any links? KZbin seems to dislike links to external sites 😡

[@ericcmcgraw]

@@harrisonlow Yep, I tried obfuscating the link by spelling it out, but still no luck. Here's the comment with the link removed: 1) I thought of that too, so I played around in Geogebra and found that you could still assume the linear axis rotates through a single point and you'd have relatively small error compared to reality. Assuming ±45 degree range of rotation, and 10mm diameter pulleys, the actual length error would only be ±0.1 mm throughout the range of motion. Here's the Geogebra file if you want to play around with it: (redacted) There do exist solutions which would cancel this error exactly, but the complexity is higher still. Let me know if you're interested in them. 2) I've found some design possibilities which add only a few additional pulleys to the part count you already have. Complexity is a little higher, yes, but the benefits are: no sliding friction in bowden, no risk of bowden buckling, higher tension and higher stiffness capability. It's up to you whether it's worth it. 3) Yes I saw those ball joints, it's a pretty elegant solution and I like how it makes the actuators detachable and modular. Of course the only drawback, as you mention, is the pulling force capability. As you start to push the limits of acceleration (how many balls can it juggle at a time, etc.) you may want to be able to apply high forces in both directions without worrying about the joints falling apart. Again, it's up to your own judgement (and level of ambition 😁)

[@mrday60]

Hello, try wrapping your ptfe tubes with a metal spring. will add some support about the length of the tubes in sections that demand a turn for fitment if you want to keep the cost low. otherwise you could consider a hybrid setup using metal cable bowden sections and then connecting the strings in the “interface” at the pushrod side and motor side. this should reduce stretch and keep the weight where it needs to be and where it shouldnt. keep in mind the material compatibility here, kevlar is strong and the ptfe will handle the abrasion for a little while, so periodic change outs may be required. nice job!

[@KaletheQuick]

Random ideas: Get a CF tube that the PTFE tube fits in, put it in there and now its stiff. Use as one kf the trio of stiffening buddies. take 3 of the stabilization bearings, and put them on the internal end of the big moving CF rod, instead of bearing in onto the central rod, bearing out onto the trio of kuter rods. Would increase the weight of the reciprocating mass, but perhaps will reduce wobble? IDK, thats all i got so far. Great work. I really hope to see this go somewhere awesome!

[@harrisonlow]

I like the idea of bearings on the main push-rod! I hadn't thought of that before! Might be tricky for a few reasons: 1) While the CF tubes are stiff, having the central tube pushing against the outer tubes (via the bearing) might cause the outer tubes to flex a little 2) I think you've hit the nail on the head with respect to excess weight: if the actuator's moving at > 2m/s, I want to keep that as light as possible. Cheers for the suggestions, and I hope to see some more of these ideas in future designs!

[@charliewynn3210]

What if the spacers were eccentric so you could rotate them for a perfect fit? Might not work with a 3d printed part. Seems like your solution with a few sizes works great already though 🙂

[@harrisonlow]

That's a very interesting idea! One potential issue I can see with that is that it'd make straightening the extending rod a little tricky. I didn't explicitly mention this in the video but I've been keeping each "set" of spacers (ie. top three or bottom three) the same size so that the central rod is kept centred. Having the spacers be eccentric might make this a little difficult. Very cool idea though and I'll keep that in mind if I need to do anything similar in the future. Cheers!

[@charliewynn3210]

@@harrisonlow ooh, very good point about wanting to put the same spacers around so it stays centered!

[@DRKSTRN]

Wanted to give a Hard Congratulations for hitting this milestone through your Project. 👏👏👏 Looking forward to see where it goes

[@harrisonlow]

Cheers!

[@kingofcastlechaos]

Great build. Just found your channel so excited to see what is next. In addition to the hoover method for string, I have used a can of compressed air to blow the string through. This works well on small strings in tubes too small to use the suction method. (Saved our butts at a robotics competition when rubber tubing began to stretch- we blew string through and tied it tight and it got us through.)

[@harrisonlow]

Great idea! Never heard of that and I've got a can of compressed air on hand. Cheers!

[@laundmo]

tldr: both ptfe tubes can probably terminate at the bottom part. thin brass/copper pipe for redirection. down-stroke the down-stroke string can pull the tube towards the bottom directly, instead of pulling on the topmost part. This allows more weight to be shifted to the less moving part and reduces ptfe tube length. It would require a redirection bushing or similar in the bottom part, but as that moves less i think it might be advantageous. up-stroke the up-stroke string can run alongside one of the structural carbon fiber tubes to the top part, where it gets redirected back down. this reduces ptfe tubing and probably tube compression issues. redirection solutions for redirecting the string, i would evaluate whether a small bent brass tube which you feed the string through wouldn't be enough. all this would require is a section on the printed part to glue this into (so that it doesnt fall out while assembling/maintaining, while in use the string tensions should keep it in place on its own) which might be easier for assembly and even lighter. another crazy idea: it might be possible to reduce this design to a pair of telescoping pipes, by using a ha:f open linear bearing (like the LM12UU OP) at the top and running the string between the 2 carbon fiber pipes. though this would likely require a larger pipe for the outer part. This would probably be overengineering for this usecase tho.

[@leelipinski4667]

Ive just just read every suggestion and thought of the most mentioned one's. String in support legs, use of bike cable sheathing, different shaped bearings. But didn't see any suggestion of just running a rubber pully wheel at the top of the 3 CF legs that snugs up against the CF tube with a tension adjuster. So your motor would spin the rubber wheel instead Would save string going to top and bottom of main CF leg. Obviously there could be slippage, just a thought. Great idea thou. Im subscribing............. Now. Thanks for sharing 👍

[@Rob_65]

This looks very nice and I can't wait to see this running. @11:57 - is exactly why kiting shops don't sell these kevlar lines over here any more. These lines cut fingers too so we are now allowed to only used kevlar with a braided cover. @James Petersen suggested the use of bicycle brake cables but this is the wrong type to use. Outer brake cables can be compressed. You could try to use the outer gear cables instead, a lot of them even already have a PTFE liner inside to reduce friction. But do go out to a bike shop, tell them why you need these and check yourself if these are flexible enough for your use. Original Shimano cables are most likely way too stiff.

[@harrisonlow]

Cheers for the suggestions! That's interesting re. kevlar strings. Thanks for sharing that! Thankfully I'm pretty sure I have both brake and shifter sheathes lying around that came with cables for my bike. I'm planning on making a testing rig to test a bunch of the ideas that everyone has suggested, and I'll absolutely be trying shifter sheathes. Cheers!

[@AJMansfield1]

For the cables, you can reduce buckling if you pair the string with a tube whose inner diameter more closely matches the string diameter. That could mean a thinner tube, but it could also just mean thicker string: for instance, type 95 paracord has an outer diameter of 1.75mm, which would be a pretty good match for the tube you're using.

[@harrisonlow]

Yup, I 100% agree. I actually have some very thin PTFE tubes on hand but I don't have any way to fix them to the printed parts on each end, so I just opted for the larger ones. Now that I think about it, I might be able to put the smaller tube INSIDE the bigger tube 🤔 Thicker string is a very interesting idea that hadn't crossed my mind! I'll pick some up and give it a test! Cheers for the suggestions!

[@Daniel-iq5oo]

You could use car break lines. Basically the hydraulic equivalent to those pneumatic tubes and pass throughs. They can be bent easily and you should be able to find threaded connectors for the plastic parts

[@marktahu2932]

Another suggestion for your string issue - try having a chat with Otis Elevators, they use a flat rubberised material in their elevator shafts instead of cables, they may have a few ideas that you can experiment with.

[@markjmaxwell9819]

Hydraulics have uses, but as more all electric fast acting and accurate systems are built going forward. Iinear actuators and servos will become even more prominent in new designs. Nice job 👍

[@randallblake1213]

Impressive. I need linear actuators that are both fast and strong. That means very expensive. 😢 I think I’ll try yours! 😊

[@inherentlybadnoodles]

thats a neat actuator! one problem i can think of is the off axis load created by the cables pulling on one side of the rod. this might not be an issue with light loads, but with higher forces along its extending & retracting axis it would require increasingly high lateral stiffness between the bearing assembly and the rod. as for the ptfe lines. maybe running them through the existing carbon fiber rods could help. it shouldnt interfere too much with the movement of the base hinge as long as you run it directly through the pivot axis of each hinge. if that is too complicated / would require too much precision, you could still shorten the length of the ptfe tubes significantly by only running them from the base of the carbon rods to the motors.

[@inherentlybadnoodles]

btw if you really want to lighten all the moving parts as much as possible, theres always the option of a cable driven parallel mechanism!

[@sqwearl1392]

Silicone socks for the OD of the bearing, you can put a radius on it increasing the contact area, and you can put a recess for the string

[@michaelguzzi1]

Man what a great design! gonna print one to test some ideas I have

[@yeroca]

Wow, that's an amazing mechanism! It's a bit like a hydraulic actuator, in some sense, because they typically (?) apply force through high pressure to one side of the piston at a time, rather than using pressure and vacuum on one side.

[@JordanMalone2014]

Had a similar problem at work, use 1/4 in copper tube and a pipe flare tube. Decently slippery and much much more robust

[@geckocave]

This is awesome! I recommend bike shifting cable lubricated with silicone spray, its thinner and has a lot less resistance than bike brake cable. The housing is reenforced to prevent buckling and you can easily refill it with silicone if it starts to stick

[@conorstewart2214]

How would it hold up long term? When used in bikes those cables don’t need to move an awful lot compared to what they would in this application.

[@degreeless_engineering]

I wonder if you could replace the PTFE tubes with linear bicycle brake cable housing, but replace the steel cable with the Kevlar string. Have the string and cable housing extra long so you could add a loop, that should give you your flexibility (housing wise). The housing is also lined with PTFE, so wear should not be an issue. The brake cable housing should eliminate the buckling issue and you could even incorporate threaded linear adjusters (like the ones on bicycle brake levers) to tension the Kevlar string if need be. I like your design. I have been thinking about doing something similar, but also very different. I like the use of the Kevlar string. Great idea!

[@IdRatherBeMaking]

Outstanding video, your approach is very clever. Questions. 1) Why are you using such large bearings? You could save considerate weight by using smaller bearings. I use 14mm bearings in my home made linear systems and they have a 400N static load. 2) If the cords are always under tension, why do you need the tube? Couldn't you just run them uncovered between a couple of needle bearings or a roller?

[@harrisonlow]

Thanks for the kind words! 1) Mostly just because these are the ones I had on hand 😅I need 6 of these actuators and I don't have enough of any other bearing size. If weight becomes an issue then I will absolutely change to smaller bearings 2) Good question! For many applications, you probably wouldn't need the tube, though I'm using 6 of these actuators to power a Stewart Platform and they need to move around over a fairly large range of motion (+/- ~45 deg about x/y axes) so I figured it would be easier to route the cable through a sheath rather than having a mechanism that guides the string perfectly throughout the entire ROM. Another commented (Eric McGraw) has gone a long way towards convincing me that removing the tubes is a good way to go (and somewhat do-able) so that may be how the next version of the actuator works.

[@conorstewart2214]

If you could mount the motor at the base of the actuator you may be able to use a belt system similar to what bed slinger 3D printers already do. Another option would be to use a rack and pinion system. Hydraulics or pneumatics are extensive but they would be ideal for this.

[@ferdyg3520]

this is some seriously high quality stuff

[@JeffBradway]

You could probably save weight by swapping the bearings for some linear bushings like IGUS drylin ones. They do have carbon fiber shafting as well. Be sure to get a decent spacing between your bushings.

[@triffid0hunter]

I'd use bicycle brake lines for the tubes, and kite string which is specifically designed to have very low stretch under tension

[@Killernova86]

You could use PA(nylon) tubing instead of a ptfe, they're way more ridgid and are meant to withstand +100bar of hydraulic pressure (they're used for grease in industrial machines). You can keep the fittings as well since they're available in 4&6mm outer diameter

[@yeinierferras2721]

Hi!!! You can use a V profile bearing, perhaps using TPU as a bearing cover. I think that will provide more stability (at least two contact points per bearing). This V profile can allow you to run the thread close to the apex of the V.

[@dittilio]

Easy upgrade for your string tubing, insert a spring inside it of the correct diameter, then pull. You should be able to get one spring turn per 20mm or so, which yields both an internal structural advantage (no collapsing tubes), and reduces the string contact area to 1/spacing, the kevlar will ride on spring instead of the entire length of the tube. Much less friction.

[@alekseikhudoshin4481]

Hey, I'm actually developing similar actuator but much more miniature and with inside-out design similar to yours. Just a couple of ideas that may suit your application: 1. Design pulleys in a way that string is always wind up row to row in one direction every time. 2. If you can put consistant helical path for strings to fall into. For both accurate angular to linear meaurment and to avoide slippage. String and fdm resolution may be too low for this though. 3. Align the strings parallel to linear axis as close as possible. On improving bowden cables: 1. Best kind of sleeve is coiled metal sleeve - can be bent and partly fixed. Finding them cheap is not an easy task, since most of them are medical use or quote to order. Best kind of cable on your scale is literal cable (not just one wire) 2. Second best option is indistrial bowden cables. At least they are tested and can be trusted in pull operations. 3. If you are going to go more diy and cheaper using PTFE: Then best fit you can probably get easily is exactly 16S PTFE tubing (1.34mm inside diameter) and nice 1.2mm cable. Alternatively you can use other 10-20S PTFE tubing with cable 0.08-0.175mm smaller than tube ID.

[@harrisonlow]

Great comments; thanks for the suggestions! What are you using your actuator for?

[@TheRobojay]

You could make one of the bearings be in a tab that flexes a bit. You flex it out to fit the rod then the bearing will have tension against the rod. It's like making the bearing be pushed by a spring. I would recommend PC filament for this kind of thing as it retains its shape well when tensioned and is strong

[@lawmate]

Awesome work, love the progression! With the bearings, you're running the hardened steel outers against (relatively) soft carbon fibre tube, which will have a bit of diameter variation along its length. The contact area between the bearing and tube is pretty small, so will exert a very high pressure. You'd be better to increase this contact area by using smaller bearings and adding a plastic sleeve that fits the tube diameter. You can buy these as track rollers, but you might not be able to find one for the exact diameter of tube you have. The sleeves could be 3D printed then turned true on a lathe. If you haven't got access to a lathe, you could use a drill with a mandrel and a piece of sand paper wrapped around a rod. The plastic sleeve will give a bit of compliance too so you can preload the bearings without risking damaging the CF tubes. Look forward to seeing the progress!

[@harrisonlow]

Big thanks! The "turning true" with a drill and sandpaper is a great idea!

[@MongrelShark]

Really good to see an Aussie doing great content. A few tips from a more experienced self taught engineer. Your interference fits are probably weakening the parts. Maybe not enough to worry about. Epoxy or similar would be another way to get the perfect fit you want without forcing stuff. Just make the socket about 0.1 - 0.2 bigger and fill the space with resin. If you do stick with interference fit. I'd suggest using a vice jig or car jack jig to press the parts rather than a hammer. Lube is your friend here. Same for your bearings. I suspect they might be a bit too firm. Rather than 0.1mm spacers (0.1mm is quite a lot in tollerance fitting) perhaps an adjustable system, like a cam and set screw might be better. I've done interferance fit in PVC injection moulded parts, 0.02 increments where big changes. In 3x 0.02mm steps I could go from falling out, to pretty good to dosen't fit without a 2 ton press. The ptfe cables are a great idea. Reminds me of the richard 9000 robot another channel made a while back. I do wonder how long before the string cuts into the tube? I guess 2k chooches is pretty good though. I wonder if a good glue lined heat shrink around the ptfe would add the stiffness you need? A bigger size would be stiffer, due to lots of shrinkage thickening the heatshrink wall, while a closer fitting size would be less stiff due to less shrinkage. I've used heatshrink to aleviate cable flex on electrical wires many times. Works wonders on guitar-amp leads to stop them bending at the plugs and breaking. I suggest glue lined (double wall) because it shrinks a lot more and can get much stiffer. re the current sense end stops. I cringed a little. Hope your detection time is very short. Its being pretty hard on a few things, heat in the motor driver and mechanical force on parts being the 2 main concerns. As well as pwer consumption. Perhaps a short stiff spring, or a fit of rubber foam, at ends could allow sensor to work while reducing extra wear and tear? on the plus side, if you hjave something fowling your bearings, or needs lube, etc etc, the sensor will let you know early. Last tip. When you said at the start, that it needded to be perfectly acurate. I felt much pain in your future. As you are learning. there is no perfect movement. Only less play. As your go down from mm of play to microns, your parts need to become exponetially stronger due to all the forces. Suggest aiming for a sensible amount of play and backlash. say +/- 1 or 2mm? should be fine for juggling robot. Also perfection is the enginners enemy of finnished projects. got a sub from me. look forward to seeing how you go with this and hopefully many more projects.

[@MongrelShark]

Oh, one last thing. Rubber tyres for the bearings. Probably print, maybe cut 700c bike tube (seam might be a problem)? They want to be thin to get your no play, but a bit of softness and grip could help performance a lot while reducing tube crush.

[@harrisonlow]

Cheers for the suggestions! Yeah I'm not a huge fan of the hammering needed for the interference fits... A few of the other actuators that I made cracked 😬. Nothing too bad, but not great either. I like the idea of lube here! Others have suggested heating the CF tube before inserting, and I think I'll give that a go as well. Not too keen on epoxy/glue, though; It's quite likely that I'll be cannibalising these actuators for parts later and those tubes are easily the most expensive parts on them, which I'd like to reuse if possible. I'll probably be cutting the printed parts to get them out right now, but at least that should be possible. Right now I'm not too worried about the PTFE tubes; I spent most of yesterday redesigning the actuator so that the motor is in the actuator itself, removing any need for tubing as the strings only travel straight (or around pulleys). I think I'll live with the PTFE tubes as they are right now while I test the new design, and if any other PTFE-related issues come up, I'll just swap to the new design. Interesting idea with the heat-shrink, though! Nobody else has suggested that (from what I've seen) and on first thought, it seems like it might work pretty well! Haha yeah the current-spike-end-stop is a little rough. I really like the idea of foam/rubber pads. I think I have the perfect thing lying around so I'll give them a go when I have time. Cheers for the idea! Yep, I am painfully aware that "perfectly accurate" doesn't exist; I think I just spoke _a little_ hyperbolically in the recording for this video. ~1-2 mm inaccuracy should be tolerable for the Stewart Platform on the whole, though I reckon these actuators should be able to pull off much tighter control than that! Re. rubber tyres: I really like the idea that a few others have mentioned of printing TPU "sleeves" to fit over the bearings. They can be printed so as to perfectly match the diameter of the inner CF tube and should help a lot with getting a tight-but-not-too-tight fit. Cheers!

[@MongrelShark]

After commenting, I thought heatshrink on bearings might work too. Again I'd be going for the biggest glue lined size that will shrink on tight for thicker wall. Its too cheap and easy to not try it. Re interference fit, have you considered a clamp design similar to push bike seatpost? The press fit requires tolerance matching hard to reproduce consistently on rdm printer. Its likely that a strong fit is putting micro fractures in your parts, limiting re-use anyway. You wont know about the micro fractures untill stuff starts suddenly failing some time later... Maybe print in abs and lube it withe acetone? For a melt fit. I like the idea of heated parts! If you heat the printed parts it might go well too. A few videos on annealing pla on KZbin. It'll get a bit soft at say 80c - 90c, then shrink smaller than original when it cools. A common press fit with metal parts involves heating outer to make it bigger, and also cooling inner in freezer or cryo, to make it smaller. When cold shaft goes in hot cavity the parts create a fit thats almost as good as a weld. Similar tech in glass to create very strong laminated glass.

[@harrisonlow]

Hmm that's an interesting idea! I'll have a look around for some really big glue-lined heatshrink! I've tried a few clamp-like mechanisms on for size but couldn't find one that worked universally well. Always either too big or too many parts, making construction difficult. Re. printing in ABS, my printed is

[@FutureEon]

💡🌬 Have you considered using braided UHMW for the string? It's commonly used in high-performance kites and offers several advantages. It's UV-resistant, water-repellent, and has excellent abrasion resistance. Plus, it has lower friction and almost zero stretch, resulting in minimal backlash. Another bonus is its affordability. Great suggestion! This video is full of excellent insights. Thanks for sharing! 👍🤖🎪

[@SDesigns2023]

Awesome!! So many ideas!! I had an idea regarding the string under the bearing, but then the person's comment about the Gothic Groove Bearing seemed better. I was going to suggest doing something where it uses two strings instead of just the one, a string on each side of the bearing somehow. Maybe something like that with the grooved bearings as well could work? Great stuff!!

[@TheThunder005]

Very nicely structured and well explained video! Keep up the good work! Edit: i really like the simplicity of the design with the range of motion it offers. Thinking outside the box work here!

[@harrisonlow]

Cheers!

[@JustPlainRob]

For a cable system, just look for something like a bicycle brake cable or derailleur shift cable. They're usually braided steel cables with a solid shield tube and some kind of plastic (probably PTFE) lining. It's easy to cut with hand tools, and is intended for durability and longevity in some nasty environments. Have you tried using one of those constant-force springs inside the cable spool to take up backlash?

[@halted_code]

the issue with the ptfe buckling could be solved by using an actual tensioning system and a string path under tension, it will require a path under tension all the way back to the motor, but if you do it right that wont be an issue say building the motors into the base of the robot so you dont have to run a long run of tensioned cables, also for ease of use you could have all of your cables terminate before the motors in a loop (possibly reinforced with some sort of loop eyelet) and have the motors connect using a sort of like fishing line swivel (or you could use 2 swivels) this way you can keep things modular.

[@abdullahshuaib8903]

If there is no requirement for the ptfe to move/flex the you could possibly use 2mm copper/aluminum tube and run 1.2mm or 1.5mm PVC coated steel cable through it.

[@glorhi]

You may remove tubes completly, just move the motor to one of the end of actuator. Also , for brushless motor, you can use the body of motor as the base for spool - that way you have ability to center the motor relative to the plane along the actuator. Event more, you can use closed loop string, or even chain.

[@Hyperian]

these adult toys are gonna be super quiet and reliable!

[@jwinfieldfrank]

I like the coil spring idea. You can integrate it into the bearing assembly.

[@petersvideofile]

This is super cool! Thanks so much for sharing this!

[@1238a8]

"so, there mustn't be any slack in system" I can't imagine how to achieve that condition in that construction. I would like to see additional video with clarification.

[@harrisonlow]

I've actually been quite surprised by how well this has worked. My methodology is: 1) Build the actuator as shown in the video 2) Connect the motor pulley to the strings by feeding the string through a hole in the pulley and tying a knot in the end 3) Loosen the bolts that fix the string to either end of the central CF tube 4) Tighten the strings by hand and re-wrap them around the bolts 5) If the string is wrapped the correct way, tightening the bolts will further tighten the string By doing that, I've gotten the strings so tight that I've actually loosened them again for fear of snapping them. They sang a very clear, high-pitched note when plucked

[@ray-charc3131]

Bearing should runs on flat surface, when it run on round tube, there is only a line contact. Just using metal tube and brass brushing is enough. For frictionless, using the brushing with small rolling balls inside

[@davegage1249]

I've had a similar problem with guiding tension lines. I ended up using endoscope grade coil pipes and I was able to get them fairly cheaply.

[@steve_jabz]

Would be really interested to see a torque test. Also, can't help but feel a belt drive would work better than cable transmission. Haven't really seen it done well in cheap DIY systems. They all kinda have the same issues

[@PuFF1kPuF]

I already see where this fast linear actuators can be used)

[@jaydekaytv]

Check out bike cables to house your strings, they’re durable and would be friendly to your setup.

[@krisnadiimam4556]

to solve the ptfe tube is to use steel hoses on the outside, like the one used on bike brake wire

[@Jakefrc]

If you don’t want the ptfe to buckle use the Capricorn stuff that has a thicker wall.

[@planckstudios]

The CF tubes look like they're leaving some marks on your bearings. Worried that over time you'll experience drift due to uneven wear. I've printed springy collars for bearings in FilaFlexible40 - itd allow you to compress the contact area (more than just a sliver per bearing) and get better grip and less wear. when sanded it bonds to the metal bearing with super glue. If you get probs with wear and tear, give that filaflex40 a try. It's tough as a honey badger

[@harrisonlow]

You've got a keen eye! Yeah, some of the bearings are wearing at the contact point. I love the idea of a rubbery sleeve. I'll see if I can find any of the FilaFlexible40 in Aus! If not, I'll at least try the TPU I've got lying around. Cheers for the suggestion!

[@planckstudios]

@@harrisonlow happy to help! the speeds you're getting are awesome. I order mine from Prusa - it prints super well, but ya gotta glue up the bed to protect a smooth sheet from damage when removing the print. still thinking on how you could replace the ptfe tubes...

[@capitalinventor4823]

Thank you for sharing your wonderful design with everyone. Near the beginning you were trying to design something that would allow you to remove the carbon fibre rods without breaking everything. Have you seen a the router bits from Freud that come in a two part plastic case, mostly clear with a black bottom section? There’s actually a third part to the case in the middle of the black bottom section that holds the router bit. The third part is made of a slightly flexible plastic which grips the base of the router bit. Sometimes it’s very difficult to get the router bit out of the case and it would be possible to create a model, if one does not already exist, and print parts out of TPU. Another design to draw inspiration from is the RackBit system from Woodpecker. That system has plastic bases that store router bits, with either a 1/4” or 1/2” shaft (sorry for the Imperial numbers but being in Canada the woodworking and construction industries are heavily influenced by the US). I know that you have decided to go ahead and just make the connection permanent but perhaps you will find the information useful in a future project.

[@harrisonlow]

Interesting, this is good to be aware of! I actually haven't seen that design before and I'm curious now: is the case hinged, so that the rubbery section is clamped on to the bit to hold it in place? Or is it basically an interference fit with rubber so as to be more grippy? Cheers for the suggestion!

[@capitalinventor4823]

@@harrisonlow I had to look at the case. The black hard plastic (could be done with PLA, PETG, or any filament like that if you were to print the equivalent) has a well for the rubbery, or more flexible, part to fit in. If one defines that the router bits fit into the top of the part then at the bottom there are four parts that fit through slots in the bottom of the well. To remove the flexible part one must move the flaps towards the centre slightly such that an extended part of the flap, which is angled to slide in when the part is inserted but a 90 degree surface to prevent it from sliding out, to pass through the slots. A basic snap fit part. It's a nice piece of industrial design as it lets the company use the same rigid base for either 1/4" or 1/2" shaft router bits by just changing what bit holder is placed in the base. If you would like some photos I could contact you on Printables.

[@harrisonlow]

@@capitalinventor4823 interesting. I'd appreciate photos if it's not too much effort! I'm not sure I understand what you're describing. Cheers!

[@perrinsilveira6759]

As someone who built something similar for a robotics club, and did a decent amount of testing, my 2 cents would be to ditch the ptfe altogether and route both strings through the bottom, the one currently connected to the top can connect to the base via throughhole, and the one going up can come straight back down. Also, just a note on names for search visibility, "cable sheave" is the name a decent amount of websites including mcmaster-carr use for pullys made for "wire rope". Took me way too long to find the right names for search results to come up. I think the kevlar is probably all you will need as long as you actually have current sensing. I think if you wanted to really make them modular without the ptfe and able to redirect to the motors as needed you could use a caster wheel the string goes down through the main bearing and gets redirected by an offset wheel.

[@harrisonlow]

Cheers for the heads-up re. naming! Something that I didn't say explicitly in this video is that these actuators (there will be 6 of them) need to move around quite a bit (relative to the motors, which are fixed on "ground"). This makes removing the PTFE tubes a little trickier as I can't rely on the routing being in the same direction at all times. Eric McGraw had a very interesting idea to get around this (comment thread is under this video) and I may move to a routing system like that for the next version of the actuators. Out of curiosity, what was it that you made with the robotics club?

[@perrinsilveira6759]

@@harrisonlow Good luck with the kinematics then^. I still think as long as you are in the same hemisphere [the cable going down and out from the base of the mechanism] the caster cable diverter would work although I was working with much larger loads and friction was a much larger factor. It was a cable driven double jointed arm and a springsteel strip bent into a loop as a grasping mechanism that got pulled closed in the same way your actuator is set up.

[@neutron7]

Perhaps you could use bicycle brake cable housings, good ones even have ptfe tubes inside.

[@brianii5809]

PEX tubes can be found in the same diameter and are much stiffer and stronger while still flexible enough.

[@fordgeher]

Pretty interesting design. Besides the speed endurance test with 2,6 m/s (which is quite fast), have you done a load test? Which load can actually be lifted/pushed until it will collapse and also in endurance run? Would be pretty interesting. Those Kevlar strings should be able to withstand a lot of force I guess.

[@harrisonlow]

Cheers! I actually haven't run a force/power test, for two reasons: 1) I don't need a crazy amount of force for jugglebot, and 2) I couldn't think of an easy way to mount a load to the CF tube I reckon it should be pretty strong though! The strings are definitely the weakest link, and they've got a breaking strength of ~35kg. I'm also likely going to replace them with braided cable so it should be super strong then!

[@fordgeher]

@@harrisonlow dyneema cables/ropes with a small diameter could maybe be a good option? Very little elongation under load, good for precise control and positioning.

[@Z0DI4C]

Either a bushing instead of bearings or a bearing with a curve, and also, proper steel Bowden cable

[@ZchDude1]

Have you considered running the string through the shafts? Would look much cleaner. Great project!

[@lam_xyz]

Cool project! What is the max force of the actuators? Funny, when I first heard you talk about threads I thought about when I used to build kites 30 years ago and the kevlar threads I used at that time because of their strength and stiffness.

[@TroyWuelfing]

What would stop you attaching the retraction tendon to the opposite end of the tube from where you have it now? Like don’t pull from the far away end pull from the close end of the tube. Fun project keep it up!

[@ndrewlee]

Better patent this if you can. I see a big potential for a quick extending lightsaber that out matches Disney's and is robust even for lightsaber duels since it is made from carbon fibre. Happy May the 4th.

[@chipcode5538]

Nice work this design could also work as the muscle in a walking robot. 👍

[@cubiee-sci6190]

I realy like the design this could come handy in so many way 👍🏻 i have two ideas for it that could help. It could help to skip the roller bearings completely and go straight to linear bushingsor bearings. 3d prints like you found out are never really concentric and fitting six bearings precisely inside the print, to get repeatable results can be next to impossible. Linear bushings or bearings on the other hand constrain the movement of the rod completely into one dimension and are manufactured directly into the Right tolerances and can be fittet really easy into your design. Second if you want to tension your strings you could run both strings before going onto the motor pulley over two pulley’s sitting on one movable axle. This way the forces should cancel out while tightening and you have no problems of one side being pulled harder than the other.

[@harrisonlow]

Cheers! I hope it helps with your designs! Thoughts on your suggestions: 1) I previously tried a linear bearing instead of the 6 radial bearings, but the carbon fiber tubes that I'm using aren't specced for that level of tolerance; one manufacturers tubes had a loose fit, another didn't fit at all. Linear bearings would be awesome (smaller, lighter, more elegant) but I don't have the cash to get high tolerance, light tubes... yet 😉 2) It's actually not so bad to tension the strings; I tie off the motor pulley end first and then adjust the length with the bolt clamp thing at each end. If I wrap the string around the bolt the correct way then as I tighten the bolt, it pulls the string even tighter, so it's possible to get the string extremely tight. Your idea would certainly be faster for small "maintenance" adjustments so I'll keep it in mind for future versions! Space is at a premium though and I'm not sure if there's room for that sort of mechanism in the current design. Cheers for the ideas!

[@StreetSurfersAlex]

Cable Transmission System idea - use the tubes from a bike brake cable

[@MatSmithLondon]

KZbin magic (stalky / scary) algorithm suggested this video to me, as I embark upon my design of a projector lift (high spec, as in - projector comes out of ceiling perfectly square, minimal maintenance, strong and long lasting design) using linear actuators. I think a lead screw / stepper motor design is all I'll need for this application (similar to a 3D printer Z axis), although the cheapo ones on amazon aren't hitting the spot just yet...

[@harrisonlow]

FWIW, I made a video a while ago on my last design, which used a ballscrew and stepper motor. It was fairly cheap and easy to hack together. I've only bought proper linear actuators once and they were pretty garbage. Whacking a ballscrew (IIRC ~AUD$30-80, depending on length) on a stepper motor works quite well if you're after high force and precision, and don't care too much about speed. Here's a link to that video if you're curious: kzbin.info/www/bejne/Z3-3hZ9slL6CY9U

[@jesperkock5969]

You might want to use groove ballbearings. It gives you space for the string and supports the rod in to places

[@gomdolri]

플라스틱재질의 아세탈 베어링을 사용하세요 이전 액추에이터는 1회전당 이동거리가 먼 나사를 쓰면 많이 좋아집니다.

[@danon-theautisticmaker8112]

1st - very cool! 2nd - subd & rung, first time seeing ur vids. 3rd - You could use more carbon fiber tubes in place of the PTFE tubing. You did the other testing & showed the results, but didn't test or show what kind of torque it can handle. O' & you could route that one problem string threw the plastic part a bit more, like right out the top of it. But also line that path with PTFE tubing, or a material that wont so easily succumb to the strings movement. 4th - Using this design in a robot arm, leg etc would be seriously interesting & Very cool!! Have you considered collaborating / working with other KZbinrs? Ie: James Bruton. this looks like just the kind of design that he'd really like & might be interested in incorporating into one of his designs.

[@CameronTacklind]

You should consider PEEK or PEK rollers instead of bearings. Much lighter. It's also possible to make a small "maze" for the cables to go down the legs directly. Having a stiff reference for them will be critical for fast movement.

[@harrisonlow]

Good to know! Are they easy to source? Something that makes routing the strings directly down the legs tricky is that for Jugglebot, the actuators will move around quite a lot with respect to the motor, so the string will be getting pulled in any direction. I believe it might be possible to route the strings directly, without any sheath (and no-or just known?- kinematic coupling), and I do think there are pretty good advantages to a routing path that could do that. If you can think of a way to do it, I'd love to know!

[@CameronTacklind]

@@harrisonlow It takes more pulleys for the cables but you can move the motors offboard completely.

[@assassinlexx1993]

I suggest you a photo eye and disk attached to one bearing with black bars. So travel can be counted.

[@lusher00]

This is clearly for your fleshlight. Brilliant!

[@arnoldbailey7550]

Have you considered using mechanical or light stops switches at the end of the rods? Much like 3D printers use in their limits along with known rod length values. This would also allow for precise and fast calibration if anything did loosen or need adjustment. Just a thought.

[@harrisonlow]

Yeah I was using those in a previous version of these actuators, but now that I'm using ODrives to control the motors, I can much more reliably monitor the current passing through the motors to "home" the actuators on startup without needing extra parts

[@ibrahimkocaalioglu]

Also you may try to use a linear rail and wires. You move rail car via wire.

[@Tora1337]

There is a type of rope, which is high tensile strength and very low wear, which also is very low stretch, I triducing little to no backlash over time. Dacron. You could also look into Kevlar rope sheathed in polyester for weather resistance but I think that might be over expensive. Dacron is Amazon obtainable for not much for a lot of rope. $50.50 U.S. for 1000 ft. spool.

[@harrisonlow]

Interesting! I haven't heard of dacron before. I'll pick up a spool and give it a test! Cheers for the suggestion 😊

[@MK-lk7nc]

What a fantastic design you've made here. Really neat. It reminds me a bit of a project I saw on Thingiverse you might like to have a look at, if you haven't allready.... the "Mostly Printed CNC", which makes extensive use of electrical conduit and a roller system to traverse the conduit. Speaking of conduit, in short lengths it's actually quite stiff and might make a suitable replacement for your three outer carbon fiber tubes if you'd like the driving piston to be completely internalized inside a housing or whatnot. Heavier tho, a bit.

[@sermadreda399]

Genious man , thank you for sharing

[@justinsilewski]

Really impressive, I love the pneumatic tubing to protect the string. Did you consider using linear ball bearings for the slides as opposed to the radial ones? It makes packaging all the bearings so simple, we have used them in the past for linear motion in FRC which has similar design constraints as this.

[@kurtnelle]

Is your tripod on top of the table with the camera pointing down? If so, I admire your bravery.

[@harrisonlow]

Haha yep, it was for this video. Now I've got a microphone boom arm to hold it so it doesn't take up space on the desk and is far more stable

[@Mythobeast]

The PTFE tube buckling makes me wonder if copper tubing would work. Definitely try suction for routing the strings. Amazing results! Have you measured the power?

[@nicolasleroy3796]

for your tubes you can try spring at every edge

[@brodiepasker9918]

Maybe try to get some Capricorn tubing instead of PTFE, idk but maybe it'll help

[@exhibit_fx]

I think a tightening screw with spring can stop the wriggling in carbon rod at the ball bearings.

[@ibrahimkocaalioglu]

You may use bicycle brake cable tubes. They ate made of metal. Or cars hand break mechanism.

[@enriquecamou6913]

Just ran into your channel. Looks very interesting. Kudos! Just wondering what you’d think about running the two kite lines through inside two of the three support struts and thus running the Bowden tubes into the bottom of the actuator instead of running them outside all the way to the top…? This might reduce the total mass of the arm around the top end. 🤷🏻‍♂️

[@charlestaylor3195]

Your face is lagging behind yoice. Really intresting video, this is awesome. That is going to be neat seeing mechanical juglging. Timing is everything.

[@harrisonlow]

Yeah my old editing setup made me completely unaware of the audio/video sync issue. Bought proper editing headphones and now the issue is clear as day! Hopefully won't happen again in future videos 🤞 I agree! It's going to be awesome 😁

[@Matze0480]

Just a proposal for a more rigid structure. What if you use a sqare tube inside a square tube. As bearing a kind of FF2515 INA. So you are able to cover the tourqe in the System. Motor bearing in the middle of the outer tube perpendicular to the tube.. Cable system outside

[@matthewmcgraw8901]

Is there a reason you didn't use linear bearings? Super curious.

[@harrisonlow]

Yep! I tried them but the carbon fiber tubes aren't manufactured to the necessary tolerance for that to work; some CF tubes from one manufacturer fit in the linear bearing with some wobble, other CF tubes from another manufacturer didn't even fit at all!

[@rylangrayston]

Hey how about a grooved bearing, its groove could stratle the string along the tube. Thanks I really enjoyed this!

[@Roobotics]

I'm actually wondering if you could do away with the 12 PTFE tubes and couplers almost entirely.. or at least feed them all into the bottom area of the hinges for nearly no inertial mass penalty, both strings up into the bottom area then one of them looped up and around a pulley at the top for the primary 'up' stroke that gives the needed power to toss the ball. Also fair warning that PTFE tubes are subject to wear-through, that kevlar line will slowly bite channels into the areas that are bent against it.

[@tobyhinton739]

love this video, couldn't you run the kevlar string through the carbon tubes?