What Gear Shape Meshes With a Square?
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- čas přidán 20. 05. 2024
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How do you design the perfect gear to partner with a given shape? It's tempting to think the way to do it is to treat both gears as if they're rolling on each other without slipping, but it turns out most gears by their very nature must slip as they spin. Why is that?
Playlist of Weird Wheel videos: • The Wonderful World of...
=Chapters=
0:00 - Wheels are not gears!
2:03 - What's wrong with wheels?
5:32 - Ground News ad
7:21 - How to design actual gears
12:07 - Envelopes
18:50 - Parametrizing an orbiting gear
22:04 - Computing the envelope
25:22 - Example gear pairs
29:05 - Resolving road-wheel clipping
30:39 - Outro
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This video was generously supported in part by these patrons on Patreon:
Marshall Harrison, Michael OConnor, Mfriend, Carlos Herrado, James Spear
If you want to support the channel, you can become a patron at
/ morphocular
Thanks for your support!
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CREDITS
The music tracks used in this video are (in order of first appearance): Rubix Cube, Checkmate, Ascending, Orient, Falling Snow
The track "Rubix Cube" comes courtesy of Audionautix.com
The animation of the moving point of contact between two gears comes from Claudio Rocchini. Original source: commons.wikimedia.org/wiki/Fi...
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The animations in this video were mostly made with a homemade Python library called "Morpho". It's mostly a personal project, but if you want to play with it, you can find it here:
github.com/morpho-matters/mor...
Stay informed and get the full picture on every story by subscribing through the link ground.news/morphocular to get 40% off unlimited access with the Vantage subscription which is only $5/month.
Thanks. Even without knowing or having all the math skills, I still learned much.
very good video sir, but can you plz try to make video related to calculus and infinities , also matrix and why determinant as area moreover why cross product can be calculated as determinant, just what is linear algebra
Interesting video! I was wondering if you can create a gear pair for a fractal shape such as a Koch snowflake or the coastline of a country?
Make more vids for this
❤❤❤
As a mechanical engineer, I feel qualified enough to say this an amazing way to look at gear design. Definitely a different perspective than Ive seen, but I enjoy seeing it from someone with more of a math than engineering background
Obviously there are major things that this video doesn't take into account, but would this algorithm work at all for real-life gears, not caring about inefficiencies or wear?
@@nikkiofthevalley I will be printing gears tomorrow to find out lol
Just replying to stay updated
Interesting. Replying to stay updated, too.
This video is amazing, no qualifications needed.
It now makes a lot of sense why gearboxes are almost always lubricated - they need to slide past each other in order to work, even though they don't look like they're sliding!
That's also where a significant amount of driveline losses come from then! Lot'sa heat!
This is mostly wrong.
Gear shouldn't slide past each other. They would never last if that was the case.
@@electromummyfied1538did you watch the video lol
@@electromummyfied1538 mathematically wrong?
25:20 Great examples, but I kinda wish we saw them animated as actual gears too, in addition to the rolling versions
I was thinking the same ! Still a great video ! Thanks.
Ah, but that's easy to fix on your side: since the rolling has a constant angular velocity, you just need to stand in a hamster wheel rotating at the same angular velocity while watching the video, so it'll cancel out and you're just seeing the meshing gears as if they were rotating about fixed axles.
...what, you say I've spent too much time in a maths departement? No way...
Came to the comments to say that exact thing!
Despite gears being the posterchild of mechanical engineering and one of the first machines most kids are introduced to, they are absolutely one of the worst things to actually deal with in terms of designing (at least in terms of undergrad classes) There are an insane number of parameters you have to take into account and it quickly goes into a rabbit-hole of tables and equations. (At least if you want to design a set of gears that will last)
Yeah it absolutely sucks lol. My 3lb battlebot uses 3D printed gears in the drive train and they took forever to get running right. Making them herringbone was even harder.
@@DigitalJediyeah, gears kinda suck to make. I tried making herringbone gears for a small kinetic sculpture with a sla 3d printer, and I tried so many times before giving up because I was unable to make the gears work and have the right spacing to fit inside the gearbox I was using. I eventually gave up, and got rid of the nonfunctional 3d printed gears and the rest of the 3d printed parts. I probably still have the motor I was trying to use somewhere, but the rest of the stuff is gone
How have computers helped this situation?
@@rodschmidt8952matlab
Even when playing with physics based creative videogames, gears absolutely SUCK to design. Some of my most frustrating machines to get to behave properly in LittleBigPlanet were anything where two parts were interacting this way.
I love this approach. Not a lot of new work on gear shapes in the last century, but modern 3D printing makes it easier than ever to play around with fun and nonstandard gear shapes. If you’re researching this, “conjugate action” is the technical term for gears moving at constant angular velocities. Also if anyone wants to know why involute gears are the global standard, it’s because of one more requirement which is constant pressure angle which also reduces vibrations.
Also sliding action is often desirable for real world gears. The gears in your car transmission for usually kept in an oil bath and have hydrodynamic contact with each other so that the gear teeth never actually touch, they slide on a microscopic layer of oil. If you look closely, the spot on the teeth that typically sees the most wear is actually the one spot where the sliding velocity hits zero because that’s where they make metal on metal contact.
Didn't expect the envelope can be solved for a closed shape. That's so cool.
Jerkiness isn't always something you want to avoid. Look at Mathesian gears, for example, they convert a constant rotational speed into individual steps. It's useful in some cases.
I couldn’t find anything on Google for “Mathesian gears”, but what you’re describing sounds like an intermittent mechanism
@@Nicoder6884 Appearantly it's called a geneva drive in english, in German we call it Maltesergetriebe because one of the gears looks like a maltesian cross
Would clock hands be an example of this?
Isn't it this one that is used in film projectors?
@@buubaku Not really. Clocks rely on either a timing wheel or a pendulum to create the stepping action. In a timing wheel system, there's a specially weighted wheel that swings back and forth to keep the time, that's powered by the watch's main spring. There's a piece that looks kind of like a fork if the middle tines were missing, and that ticks between 2 positions every time either the pendulum reaches the apex (highest point) of each swing, or when the timing wheel changes direction.
Both of these forces are enough to make that little fork change what side it's leaning towards.
The long awaited sequel, I loved the road one.
18:50 - 25:20 Hmmmm I’m sensing a hidden connection to Fourier series and their epicycles when it comes to the construction of smooth gears. Seeing the formulas for the gears and then the algebraic construction of the gamma function parameterization with t in terms of s had had those ideas flowing through my head, Stellar work really sir.
You might be onto something… Epicycloids and hypocycloids can perfectly roll inside each other.
Cycloid gear already exist. They’re instrumental to clock making and are some of the few gears with zero sliding motion/friction. They must be spaced with extreme accuracy though, otherwise they go wonky. They most importantly can work without any lubrication, which is why watches and clocks can last so long.
The two parameter locus of motion (I.e. what you see in the thumbnail) for the generating gear is a field of epitrochoids for external spur gears and a field of hypotrochoids for internal spur gears.
So yes, the traced motion of each point on the generating gear is represented by the addition of two rotating vectors with some angular velocity ratio. You could call it a finite Fourier series if you wish.
More details are available in my larger comment on this video (a comment + larger one broken in two as replies to myself).
As a mechanical engineering student who has an interest in knowing how mechanics equations are derived from first principles, this is a satisfying and informative video. Very awesome.
One engineering solution to maintaining the same radial speed for meshed gears is to see the gear as 3-dimensional, and change the teeth from having their peak parallel to the gear's axis to being skewed, so when the gear is meshed with a similar (actually, mirror image) gear, the point of contact slides up or down in the direction of the gears' axes, but at a constant radius for both gears.
You're talking about helical gears, right? I kind of assumed that they were just normal gears twisted about the axis of rotation, and that if you untwisted them they'd work just like straight cut gears. I'm not sure if what you're saying means that assumption isn't true or not. Also, I thought the helical twist was mainly for noise and wear considerations.
Yeah, I'm in the same boat. I don't think helical gears suddenly are a whole different beast, but instead just twisted regular gears.
@@quinnobi42 While watching the video I thought of helical gears. It seems to me that those allow the point of contact to remain at the same radius from each axis.
Another important consideration for real-world gears is mass-manufacturability and interchangeability. This is the reason that the involute gear shape is so dominant: Unlike other shapes for the gear teeth, there the precise gear shape depends only on the pressure angle (the angle that the line of contact makes with a line perpendicular to the line connecting the centers of the two gears), the number of teeth, and the pitch/module (respectively, the number of teeth per unit diameter, and the diameter divided by the number of teeth), but **not** on the details of the meshing gear (though obviously pitch/module and pressure angle have to be equal between two meshing gears). This, and the fact that almost all gears use the same pressure angle (20 degrees) and manufacturing tolerances means that only a small set of standardised gear cutters are required to cut all gears of a given module, no matter how many teeth they have or which tooth number gears they will mesh with.
The only part of the math I understood was the comparison to the check for extrema in calculus, but it was still a nice video and I do now know what envelopes are and that complex numbers are good for calculating something with rotation. And it was very interesting to see the various partner gears that different gear shapes produced.
I'm so glad this video came! The variable angular velocity was something that I had noticed in the previous videos and was bothering me, so seeing more of an in-depth exploration of that and the difference between the wheel pairs and the gear pairs is very satisfying! I've loved this whole series!
This kind of content really scratches that curiosity based itch in my brain and I'm all for it
27:43 Incidentally, this internally meshed gear seems to be how the Wankel rotary engine is designed with a circular triangle inside forming an epitrochoid that the inside gear not only spin around, but also run the internal combustion cycle to run the engine.
The dot/cross product "trick" is because the complex numbers are the 2D Clifford algebra.
a•b + a∧b is the geometric product of vectors, but complex numbers are rotors not vectors
so this doesn't really explain it well
@@2fifty533 If you were to translate the common usage of complex numbers into geometric algebra terms, effectively what's going on is that all vectors are arbitrarily left-multiplied by e_x, which makes them into rotors.
e_x * v = e_x * (v_x * e_x + v_y * e_y) = v_x + v_y * e_xy = v_x + v_y * i
Complex conjugation corresponds to right-multiplication by e_x instead,
v_x - v_y * i = v_x - v_y * e_xy = v_x + v_y * e_yx = (v_x * e_x + v_y * e_y) * e_x = v * e_x
So his formula,
z^* * w
Effectively results in a geometric product,
= v1 * e_x * e_x * v2 = v1 * v2
It's just that that in common usage complex numbers are used to represent both rotors and vectors, the rotors are naturally identified with complex numbers, but the representation of vectors is a little bit strange when you translate it back into geometric algebra.
@@2fifty533 Yeah but (xe1+ye2)e1 = x + ye1e2, so they are very naturally isomorphic
"Babe, wake up, new Morphocular video just dropped"
Said no-one ever ;) Still I did find it very funny comment.
FreeSCAD library in 3...2...
Wow, such a great balance of show and science. Good graphics, just deep enough math, very good approach, humble person.
Nice to see another video on the series, i loved the series and am glad to see it return
I'm confused, how do you have a comment that is "2 hours ago" on this video that uploaded less than "2 hours ago?"
Weird, i uploaded it 40 minutes after the video went online
I saw that I wasn't subscribed, despite thoroughly enjoying your videos. I made sure to remedy that mistake as soon as I discovered it.
I'm a computer scientist/software engineer. These videos are like candy to me. Thank you so much for covering these fascinating topics in an accessible manner!
It was a doubt I had since long time ago and you solved it very nicely. Great video!
Exactly. Pressure angle is one of the two measures to know how much a gear should slip or "backlash" backwards.
My sheer happiness to see bezier curves on a video about gears
It's a beautiful day when both Sebastian Lague and Morphocular release videos relating to Beziers ❤
That's what I was thinking lol - I spoiled myself by watching the font rendering video first and being reminded of beziers being a lerp'd point on a lerp'd line segment
Oh wow, thank you! I've noticed the jerky motion in the last couple videos, and wondered what it would take to deal with it. And now you made a response to exactly that question, awesome!
Wow. I never thought about that but in retrospect it seems so obvious bc gears are either lubricated with lubricants, or made of inherently slick material like Teflon or nylon etc.
Wheels are generally maximally grippy
Thanks for making it clear that gears have to slide.
Especially around cycloidal gear teeth, there is a widespread misconception that the gear teeth are rolling against each other.
Yeah, learning there’s not just incidental/thermodynamically demanded energy loss from friction, but that sliding is literally necessary for smooth motion was an eye-opener.
As we are taught in undergrad mechanical engineering: theoretically, most gears have involute profile which perfectly roll over each other. But the speed ratio varies since the point of contact moves radially. I don't know why you did not mention this basic stuff. Clocks use cycloidal gears which often have constant speed ratios but have sliding and more strength which make more sound due to sliding.
I loved watching the series prior to this video. Cool to see a new vid on it!
I've been wishing for this video since Pt3, and never expected my wish to be granted!
I was able to get Desmos's graphing calculator to make the envelopes and I think your analysis of what goes wrong with the ellipse is correct. As the distance between the axles decreases eventually the inner envelope starts to self intersect, which in this case indicates that there are moments where the source gear is no longer in contact with the partner gear assuming you shave off the areas created by the self-intersection. Interestingly, as you continue decreasing the distance the inner and outer envelopes meet and then each become discontinuous, forming two new curves - I think this is when the output is an error for you.
I plan on doing the same for a rack and pinion using a given pinion (and maybe vice versa, though that might be harder).
Sweeping out negative space is essentially the way that some types of gear cutting machines work. You have a tool which is shaped like a gear with cutting teeth, and you rotate it together with a gear blank in the same way that two meshing gears would move. All the parts of the blank that aren't part of the matching gear shape get cut away.
The one issue is that there are a whole category of gears with minimal to 0 sliding motion that do exist all around us. Cycloid all gears have for a long time been part of clock and watchmaking. Their contact allows them to have zero sliding friction as the gears themselves must have minimal friction and be never lubricated in order to prevent dirt build up. Other forms of cycloidal gears can be found in roots blowers and such. Having played which watch parts as a child and assembling a couple watches from parts, almost any sliding friction in watch wheels (gears) causes the rapid wearing out of gears that should never wear. This causes friction to increase rather exponentially until the watch spring can’t power the watch anymore, and in that case, every gear would need to be recut and at best, the plate that holds the jewel bearings be drilled again or tossed out.
But I believe those gears don't maintain a constant angular velocity.
so brilliant!! I'm astonished!
That was amazingly put. Congrats, I really learned from it.
We've all been waiting for the next episode, very fun to learn that way :)
20:00 - after seeing triangles and hexagons I believe it's the constant rate of change of R along the edges.
Since they're straight it helps; also in the limit with infinite sides it becomes a circle so more sides should make them more alike;
Your intuition about some kind of "self-intersection" of the envelope is on the point for the artifacts. Just like how zero derivative is necessary but not sufficient for a maxima or minima, the envelope condition used is necessary but not sufficient for the type of envelope wanted here. If the curve traces out some kind of interior envelope, that will be caught too, and mess up the result. Additionally, the full failure is probably since not all positions of the gear necessarily have to correspond to being part of the envelope. That is, the gear at the positions for which the formula fails is entirely inside of the envelope, not touching it. I'm also not sure it would handle correctly the cases where multiple points or sections of the gear shape at a position are part of the envelope.
In any of those cases however, the real-world implications is that the parameters set up are impossible to construct a normal gear for. Either the force transfer is not in the correct direction to couple the motions, and/or the gears would physically separate and not transfer motion. It may still be useful for things like cam systems, where the motion wanted is to pause (while the gears are not in contact), like in watch escapements or film projector reels, or if the intent for the gearing is to synchronize motion rather than transfer forces.
Brilliant as usual
you've nailed teaching
great work/presentation! you got a new fan ;)
Yessssssss
Finally a new morphocular vid
Envelopes are like, my favorite thing, I particularly like the envelope I discovered independently of a line segment of constant length, with the endpoints bound to the x and y axes: the astroid, with equation x^(2/3) + y^(2/3) = 1, and somehow a length of exactly 6.
Finally, my favorite wheel math content creator uploaded!
I love math but something about the music in these videos and your voice is soothing and makes me so sleepy sometimes. I’ll doze off until halfway through the video and then I have to go back several chapters 😅
Excellent video. Thank you very much.
one of the best videos I've seen for some time
I love how some of them ends up as geneva mechanisms.
Also great video. Gonna experiment myself with a custom slicer based on the knowledge you provided me and attempt to 3D print them :)
Subscribed for the great animations and the humor included in the educational videos!
26:17 this reminded me of the mathologer video about modulo times tables.
I bet that a gear that is just a line would pair with a cardioid gear.
I remember suggesting the clipping thing! not sure if you came up with it on your own before me, or my email was what gave u the idea, but either way im happy to see it
awesome video bossman!
This is amazing.
I saw the painted gear part and had to thumbs up and give a comment. That is so cool!
I really enjoyed this very mathematical take on the concept of gear engineering, very interesting, informative and fun. Also damn, that offset axle oval gear looks so interesting! I wonder if making it that much larger would produce more of the indents that it produced on a smaller scale, as currently it only has two.
Hey morpho! I think it would be easier to say that, if velocity vectors of changing s and t are parallel [17:11], then del gamma/del s = (lambda) * del gamma/del t
I solved the example envelopes as well as the general equation using the lambda parameter and it doesn’t involve the “unusual” albeit beautiful step of pulling out f’(s) from the Re{.} part (which you did in the complete derivation).
Both the conditions are essentially the same but i thought i would share this. Great video btw!
23:39 i think some of the expressions might become simpler or at least more intuitive if you go back to vector representation somewhere here. in particular, Re[f’(s)/|f’(s)|*f(s)] is just the projection of f(s) onto f’(s).
-in other words, it’s the radial component of the derivative-
you might also be able to eliminate the cos-1, since we immediately take the cosine of it afterwards, but maybe not, since we’re multiplying it with things in the meantime
Submit this to Summer of Math Exposition!
Fantastic video
Turns out I solved the envelope problem to draw very accurate involute gears for my own need recently. Being the caveman I am, I did it much less elegantly, brute-forcing it with algebra and questionable calculus. Your approach is so much more elegant
Camus' theorem would give good insight.
27:00
The error can be interpreted as being caused by the gear gets inside-out in some point. It is interesting problem that how much the gear's projection can gouge out its pair-gear without causing errors or slipping through.
I love how you played the algorithm and annoyingly me while im studying for my topology and fluid mechanics exams this week.
Best animations I've seen, if some4 will come out, you can easily win
Great video!
With the internally meshing gears, is it possible to stack multiple gears to create a sort of rotary engine? My understanding is that you could give the shape of a single rotor and create the housing and then another internal gear inside the rotor for the crankshaft. Rotary engines commonly use a gear ratio of 2:3 between the spinning rotor and the crankshaft, but i wonder if there are any other ratios that would work
Thanks doc
Your explanation of the envelope is fascinatingly similar to the math behind (I think) splines (or was it bezier curves?). Very interesting!
Awesome ⚙️s video. Thanks. Advertized it on X-platform.
My approach to this sort of gear question when playing around with it in the past was to define a shape as a function of radius over angle (0 to 2π). Then using that function to approximately generate a polygon. Interesting.
15:44 This part is really clever!
YEEES A NEW EPISODE OF WEIRD WHEELS SERIES
Great video! I think it would be cool to see the last animations with both gears at fixed points to see what they would look like in real life.
Thanks, you gave me an idea of a breakthrough in one of my math-heavy projects, I will spend countless hours researching and it will all be your fault. Sincere thanks.
Interesting domain for that solution! In the parens we have dot(normalized tangent, radius vector), so all in all this means "gear radius projected on tangent to contact point is no greater part of R than w'/(w+w')". It's sort of a lever rule, but for angular speeds, and reflects the common design that the gears' average radii are in ratio with their number of teeth (in that case you can make all the teeth the same).
Great work and great content
Thank you!
I watched the whole ad to support you
A true popularization masterclass! Thank you
The serie remind me slightly of the news of a team that invented an algorithm to create a 3D shape that would follows any predetermined path (trajectoïd)! Maybe an idea for a futur video? :)
This incredible! I'm curious if there's a way to solve for f(s) such that, we could find a function whose gear partner envelope is the original function, probably with some angular offset. I know a circle is a trivial solution to this, but, I wonder if there's a whole family of functions.
Do you know what's most impressive to me? When someone shows me how to use basic tools and put them to real life use, in a very out of the box. I try to do this often, but it's very hard to come accross things like these!! How do you come across such things, and then also put it so beautifully in a video??
I am yet to be a mechanical engineer, and ai find this very cool, I think this can be used in improving rotary engine design if they didn't already use such a technique for doing so.
I'm imagining an iterative process where we start with a gear and assign it a number of "teeth", select some other number of teeth to construct the partner gear with an appropriate ratio of angular velocities (there seems to be some flexibility in selecting R, which might yield an interesting constraint to explore), construct the partner gear, and then repeat with another number of teeth (again there's flexibility here, making for another interesting tweakable attribute on the iterative process), etc.
It seems obvious that for suitably chosen R, the collection of circles would make something of a fixed point for a dynamical system constructed around such an iterative process; is it attractive? What's its basin of attraction? Are there other attractive fixed points? Do any of them closely resemble gear profiles currently in widespread use? What about repulsive fixed points?
The interlocking wheel are gears - a gear is a spinning device using mechanical interlocks to transmit power. There is no requirement on slipping or continuity and there are gears that are specifically designed to give non-uniform rotation even to the point of not rotating at all for large parts (geneva drive).
27:20 i have not tried to prove it but from how the shape looks like i'd assume that if you took the negative space and tried to roll the shape around it with the given parameters you'd get points in time where you actually have no contact points for the shape because the backswing moved into those contact points erasing them from the final negative space
the gear itself does not slide, but the teeth of the gear uses slide properties to make it work smooth and maintain constant rotation. So to say it slide or not is more about the perspective in what are you looking at. If one gear is rotating clock wise the other has to rotate in the opposite direction, in this sense they don't ever slide. How the gear really works in a micro perspective they do use sliding properties on the teeth to make the rotations constant. It is all about perspective. It is weird as to make a gear not slide it must slide? I guess so, both things are right, depending on what are you referring to.
I usally skip sponsors but bc u were so polite i didnt :)
The well anticipated sequel finally comes.
I was thinking of a weird way to do this. Create a line, where the height of any point on the line matches the length of a line from the center of the gear to the edge of the gear, rotate until you return to the starting position.
Once you do this you have a line the length of the border of the original gear.
Then you start a new object. Start drawing a line that is as far from the center as the height of this first line. Rotate and keep drawing at that height value. Once you return to the start you will have drawn the second gear. Just keep restarting at the beginning of your height line any time you reach the end.
I wonder if i deacribed that well enough...
Oh I guess you are doing this with smart pants math stuff.
You seemed to me to imply that you can arbitrarily define both rotational velocities as well as the axle distance separately while maintaining a fixed shape & scale for the arbitrary gear. To my understanding the shape & scale with a given rotation ratio should require a specific axle separation and the shape & scale with a given axle separation should require a specific rotation ratio. Something something conservation of torque & energy
To clarify I'm more curious than anything
i thought so too, it didn't seem like you could specify the angular velocity of the partner gear as a free parameter, it seemed dependent on your axle separation and source gear shape and velocity
30:00 would it also work to mirror the overlapping envelop at the road and cut that mirrored version out of the triangle wheel?
i'll admit it, i wasn't expecting the parametric equations, the partial derivatives and specially the complex numbers
i didn't quite get it why this solves the switch problem, you are using another type of switch? also kudos for the project, nicely done!
If we flatten gear shape (same way as from circular coordinate system) and calculate R-"shapefunction" will it give us flatten form of shape we need? Or there is a problems with neighboring collisions or revolutions speed??
@24:00 I'm astonished that the solution stays closed form when I imagine all of the different types of gears in my head in particular, square and triangular teeth. To no surprise, as you developed your solution your mathematics are starting to look more like the equations used for cams and lobes. At the end of the day, all mechanisms are going to be an inclined plane, lever arm, wedge, pulley or some combination.
is there a way too have the shape of the source as a variable and equal to the partner?
yay another video
I think it's worth noting that the gear ratio has to be rational. This becomes very clear when using the 'carving out' visualization, since the source gear needs to make exactly a whole number of rotations for each orbit (rotations of the partner gear). The gear "ratio" is then a rational number: GR = (N_rot_source / 1).
Also, as was touched upon, there are many restrictions on the initial parameters for this to work; such as the rotational speed of the source gear. Imagine a shape, size and distance such as what was portrayed in the video, if the source gear spins super-fast, then the envelope will approach a circle.
Furthermore, if the shape of the source gear is weird, then the envelope may no longer represent the best partner gear, even if there exists a perfectly viable option. (I'm pretty sure)
18:50 just an aside: would that graph be described as a45 degree rotated parabola? My mind went in the direction of a graph for an equation similar to y=1/x where x is greater than 0. I don’t remember the specifics or how to test in this instance, but would that actually satisfy the requirements for a parabola?
One other condition you completely failed to discuss is that the relative angular moments of the two component gears need to be rationally related. In other words, when the "bigger gear" rotates through a whole circle, the smaller gear needs to rotate through an integer number of revolutions. You could break this requirement somewhat if you are able to take advantage of internal symetry of one (or more) of the gears such that, for example when the larger gear spins once, the smaller gear spins 5 1/2 times. As the smaller gear makes the second circuit, the contact point between the gears will be exactly halfway around the smaller gear than it was during the first revolution. The allowed fractional components are determined based on the symetry present in the given shape.
Maybe you discussed this issue in one of your "rolling" videos?
If this isn't already known in the literature, I feel like this might be publishable. Some engineers would find this useful. You might consider emailing some engineering professor who would know and offering to coauthor the paper with them.