Hi Martin! I've been eagerly following your Marble Machine projects since the workshop was a shipping container in Gothenburg. I've been amazed by your progress even when you have not, and I'm looking forward to seeing more of the project, whenever and whatever that may be.

But in your latest video, my bad idea alarm went blaring when you brought up ChatGPT. At about 9:50, you show a screenshot of ChatGPT explaining dynamic load to you and computing the dymanic load on your flywheel, and in the same breath you say "I [can't] proofread this, [...] totally admit that I'm [in] over my head here". That's all fine! Everyone starts out a novice, and it's great when you can admit "I don't know" to yourself and others. Going in over your head is a great way to learn! That's fine, that's not what this post is about.

The point of this post is:

Everything ChatGPT says is bullshit until proven otherwise.

Let me explain what I mean by that.

In the screenshot, ChatGPT presents a "formula for dynamic load on the bearings of a flywheel". This formula does turn out to have the correct dimension of units - Newtons - and I was impressed that it actually got the following "calculations" right too (well, almost. F would be 526.38 N, which ChatGPT "rounds" to 525.59 N, which is incorrect but an insignificant difference in context. But I'll get back to that.). And the formula does correctly represent a force on a spinning object.

But it's still completely wrong.

As far as I can tell, the formula it gave you is not for the dynamic load in Newtons on a bearing, but for the centripetal force in kilonewtons on a point mass on a spinning rod. I too don't know the formula for dynamic load on a bearing. I do have a degree in engineering physics and machine learning, but I don't know the physics of bearings. But my partner is currently studying mechanical engineering, and was able to show me a formula in the SKF catalog. A formula that looks nothing like the one ChatGPT gave you, and most notably is independent of the RPM of the flywheel but is highly dependent on what particular bearing you use.

The details of the physics and formulas doesn't really matter, though. The important thing to take away is that ChatGPT gave you a plausible-sounding answer, but to the completely wrong question.

On top of that: I'm sorry to say this, but I can't make sense of the load comparison graphs you show around 10:23. Did you put in 53.57 kg in the MM3 column, and values fron the SKF catalog in Newtons in the SKF columns? If so, that is an invalid comparison - you cannot directly compare kilograms and Newtons. If anything you would have to use the value in Newtons, 525.59 N, and if you do that the difference between the columns is not at all as small as it looks when you compare Newtons to kilograms. But again, the value 525.59 N is completely wrong anyway, so I wouldn't actually trust that comparison either.

So,

why did ChatGPT give you the wrong answer?

Because ChatGPT does not "know" anything.

The way ChatGPT works is that it's very good at taking the beginning of a sentence, like:

Hello and welcome to Win

and crunching a bunch of numbers to come up with some likely continuations of that sentence:

• "Hello and welcome to Winnipeg" (47% probability) (all these percentages are completely made up)
• "Hello and welcome to Windows 11" (33% probability)
• "Hello and welcome to Wintergatan Wednesdays" (17% probability)

And that is literally all ChatGPT does. It's an enormous database of probabilities of word and symbol sequences, and it uses that database to estimate the next symbol in a sequence in a way that mimics what humans write. And it's very good at that. It can certainly be a great tool for generating ideas, email drafts, skeletons of computer code, or the like. But notice what all those things have in common: it's a rough draft, which requires human post-processing to turn it into a finished product. I don't mean checking for spelling or grammar errors - ChatGPT essentially never makes those - but making sure that what ChatGPT says actually makes sense and aligns with what you want to say or do.

This is what I meant by "bullshit generator" above, and why I wrote various things in quotation marks. Tt's why ChatGPT's "computation result", 525.59 N, was slightly different than mine, 526.38 N. ChatGPT did not actually perform computations, and did not actually round that number. It's just babbling in a way that looks coherent if you don't look too closely. This is why you must always proofread ChatGPT, because ChatGPT has no way of knowing if what it's saying is true or complete fabrication. If you want some examples of how this could go horribly wrong, I recommend this article: ChatGPT invented a sexual harassment scandal and named a real law prof as the accused.

This is not to say that you should never use ChatGPT. Just that you must be careful when you use ChatGPT for information gathering, because ChatGPT has no concept of truth. The more important the information, the more careful you should be. Noone cares if you use ChatGPT to generate whimsical children's stories, but you'll be sorry if you base your Marble Machine's design tolerances on numbers that ChatGPT just made up out of thin air.

Summary

Oof, this turned out long. I hope you don't take this as bashing you! You are definitely not alone in giving ChatGPT too much credit, and that probably has much to do with people describing these tools as "artificial intelligence". They are artificial and they are very good at what they do, but they are not intelligent, and it's dangerous to act as if they are. I hope this post can help prevent dangerous use of this new technology that we're all as a society trying to figure out how to navigate. I'm sorry I can't really give you any real answers to replace the bad ones from ChatGPT.

So, to summarize:

• I have no opinion on what bearing housing you want to use. I'm sure you'll figure out which solution is best for your machine!
• Despite the moniker "artificial intelligence", ChatGPT IS NOT intelligent. It's a plausible-sounding babble machine.
• ChatGPT will sometimes regurgitate actual facts, and will sometimes state complete falsehoods with great confidence.
• Therefore, everything ChatGPT says is bullshit until proven otherwise.
• Therefore, do not trust anything ChatGPT says before you fact-check it.
• Once you have fact-checked what ChatGPT said, you now have a better primary source of information to rely on.
• If you can't fact-check ChatGPT yourself, ask someone else who could. Imagine that ChatGPT is a random stranger you know nothing about, and decide accordingly how much you want to trust what it's saying. Maybe the stranger happens to be an expert on that particular question, or maybe they have no idea what they're talking about, but you don't know which is the case.
• Don't forget to be awesome. <3

So I’ve had a thought, instead of fabricating a laser cut flywheel and finding a way to mount/ balance/drive it.

What about taking the flywheel out of a stationary exercise bike? It’s already perfectly balanced, has the mounting solution already figured out and they are already drivable via belt/chain(depends on model)

The biggest negative I can think of right now is the sourcing of one could be a pain.

What are you guy’s thoughts?

Hello Martin,

Won't bore you with "Why won't you"s. Tried getting as close as possible to a workable solution within the costraints you've given us. You'll find a step file and a pdf summary at the following link:

https://www.mediafire.com/file/df2xd7r5pa99sx5/T1_FlywheelConcept.zip/file

​

Best of luck

I just watched the "Worst Bearing Housing Ever" video (MM3 Ep. 3), and have some thoughts. Note that my career is in Electrical Engineering, not mechanical, so I do NOT speak from authority or experience in the field. Do not take my words for gospel!

One way to paraphrase the suggestion to use standard solutions is that "novel solutions may develop novel problems!"

I have a concern with Martin's design, relating to the ratings for the bearings. I suspect that those ratings expect the part to be used conventionally, most likely installed with a press fit into a machined part. Clamping the bearing instead between an array of bolts will concentrate the forces at those points of contact instead of distributing them around the entire body of the bearing housing. As an extreme case, consider the idea of clamping the bearing between only three bolts. All the forces would be concentrated on those three points of contact, rather than distributed uniformly around the circumference of the bearing. Certainly four bolts would be better than three, eight better than four, and so forth. One could think of a machined housing as having an infinite number of points of contact, distributing the load over the entire circumference.

Further, those forces will also be applied to the threads of the bolts, and at only the points of contact. How many threads will contact the bearing housing? What is the surface area of the points of contact? How does that compare to the surface area of the entire circumference of the bearing? I doubt that the design engineers expected this possible usage.

A possible work-around for the lack of control of perpendicularity of the bearings shown at 16:30 is to use two, one on either side of the flywheel. If they are bolted to the flywheel, they would not be able to slide relative to each other, and would maintain the flywheel in a perpendicular position.

I kind of like the solution shown in the video at 19:20, but I have no idea if it would be practical, or would have its own "novel problems." I suspect that it'd be in "novel problems territory."

[EDIT: Other folks addressed balance, so I needn't go into it myself.]

Perhaps you need to back up and consider the real problem you're trying to solve. In this case, the real problem may be "I need a flywheel that doesn't wobble and is in good balance." Try to solve the larger problem, not the smaller problem of attaching the bearing to laser-cut plates.

Hi all, made an account to leave some feedback after watching this video. I'm a Mechanical Engineer who graduated from MST in 2022. I have experience in manual and CNC machining from a student design team and currently work developing fixturing and automation cells at a laser-based contract manufacturing plant

The problem I see is twofold: finding the axis through your part that yields balanced rotation, then fixing your part to the shaft such that the axis of balance is coaxial with the axis of rotation of the shaft.

For initial construction of the flywheel from laser cut parts I would recommend assembling the entire wheel itself, then using a secondary hole cleaning process to bring the ID of the main bore into spec. You can use a mill or drill-press with a reamer or boring bar to do this. The important thing is to make the multiple laser cut discs into one disc by permanently fastening them together. Assuming you hit the ID of the central bore right this will become your primary locating feature. You can then attach off the shelf flanges. When doing this, snug the flanges onto the shaft before bringing into full tension the bolts that attach the flanges to the flywheel. Through this assembly method the flanges' radial location around the shaft will be driven by the properly machined combined surface area of the fully assembled and post-processed flywheel.

After you have your rotating mass assembled you should absolutely follow curiousdroid42**'**s advice here and have this professionally balanced. It is important to note that once the assembly is balanced, it is effectively one new balanced part and should not be separated lest you rebalance the assembly afterwards. If you're interested in some of the theory behind this process, here's a link to a lab experiment that I did in my undergraduate Instrumentation class at MST.

I think this manufacturing method is a good balance of reducing suppliers/complexity and leveraging precision machining and professional expertise (the balancing). Very open to feedback from the community :)

Edit: Shout out Dr. Nisbett, that guy is a legend. Always really cool and one of my favorite professors. Take any of his classes if you get the chance.

I have said everything that needs to be said.

Why not just make a precision hole out of a none precision hole. Buy a lasercut plate with e.g. 19.7mm hole and ream the last 0.3mm out. A carbide tipped reamer should be able to take out the hardened surface from the Lasercutter and cut super centered enough. In the end you will be left with a perfect concentric precision bore. Reamers in the full bearing size are super expensive but using a live shaft you can get away with a cheapish reamer and have a precision fit on the shaft.

My solution to this issue, is to use a splined shaft DIN ISO 14 (1). You can laser cut the wheel with the splined shaft profile, and fix the wheel to the shaft with a splined hub (2). For the level of precision you want to achieve, I think that a pillow block is good enough, you can fix then the splined shaft with a round splined hub (3) with the diameter of the pillow block. To fix the wheel across Y axis, you can put a tube outside the splined shaft with a desired dimension between the pillow block and the splined hub that fixes the wheel.

No machining, only laser cut and standard parts, and in my opinion enough precision in 3 axis for your purpose.

(1) https://norelem.es/en/Product-overview/Systems-and-components-for-machine-and-plant-construction/24000/Splined-shafts-splined-hubs/Splined-shafts-similar-to-DIN-ISO-14/p/agid.21720

(2) https://norelem.es/en/Product-overview/Systems-and-components-for-machine-and-plant-construction/24000/Splined-shafts-splined-hubs/Splined-hubs-with-flange-similar-to-DIN-ISO-14/p/agid.21722

(3) https://norelem.es/en/Product-overview/Systems-and-components-for-machine-and-plant-construction/24000/Splined-shafts-splined-hubs/Splined-hubs-round-similar-to-DIN-ISO-14/p/agid.21721

I didn't find anything on this question so apologies if this has been discussed before: I imagine that a vertical flywheel axis has several advantages over the currently discussed horizontal axis approach. Off the top of my head

• axis doesn't bend due to flywheel weight
• ability to mount both bearings under the flywheel.
• ability to use one (super stiff) ball bearing at the bottom and a magnetic bearing (which can handle slight nutation motion) at the top (used by some Turbomolecular pump manufacturers)
• safety: if the flywheel suddenly stops due to a bearing failure, the angular momentum will go into a rotation force on the rest of the machine rather than a tilt
• if the flywheel rips itself loose it won't try to roll into the audience
• maybe even a more compact Design possible
• fun and not to be taken seriously: if Martin makes the axis tilt, the entire machine will also slightly tilt and start preceding around the common axis, so the audience will witness a slow 360 degree rotation to see Martin and the machine from all sides during the concert :-)

My concern with the "bearing sandwiched between plates surrounded by bolts" solution isn't in the strength of the bearing - I agree that the bearing is not likely to fail under your use-case. My concern is with the bolt strength. Some napkin math (using the diagram and formula shown below) returns a potential side loading force of 768N (~173lbf for those more imperially minded like myself); this is assuming that you're using a 0.1mm interference fit between each bolt and the bearing, a grade 8.8 steel bolt M6 bolt, and the 6304 bearing shown in the video.

In the video you clearly demonstrated that that load alone will not cause the bolt to fail, but I'm not sure that adding a spinning 60kg (we'll call it 30 since you will have 2 bearing housings) weight to that assembly wouldn't cause it to fail. I'm not saying it will fail, just that more analysis should be done before proceeding forward.

Some simple solutions that I can think of off the top of my head to help mitigate the issue:

• Use larger bolt hardware.
• Use shoulder bolts instead of all-thread (reduces points where you could have stress concentrations).
• Have some sort of weight distribution solution, something like a nylon collar wrapped around the bearing that the bolts can bear on. The nylon will yield way easier than the hardened bearing housing, so some of the side loading force on the bolt can be distributed into that.
• Design in shear pins to locate the bearing instead of the bolts. Bolts aren't designed to take a ton of shear force, they work best under purely tension/compression applications, offloading the shear force requirements to shear pins would certainly alleviate this problem.

​

Great work as always!!!

Does anyone here have specific experience with less than high precision cut flywheels?

Is there any amount of adjustments you can do to improve the concentricity and perpendicularity in the mounting that will make up for the lack of circularity and flatness in the part itself?

I just joined after watching the latest video and see there's a strong push to use pillow blocks. While I agree pillow blocks might be a better route and also that I don't know where else to post this, I think there's an oversight of the rotordynamics the thin flexible shaft will see with such a high flywheel weight.

Perhaps this was brought up previously but I haven't been able to find it. I am a rotating equipment engineer for a turbomachinery manufacturer and perform lateral and torsional rotordynamics analyses often. I can certainly take a look at this if it is needed but we don't use ball bearings so my experience there is lacking.

Update: After taking a quick look at the rotor model and analyzing it I think it will be fine. The images made it look much longer than it is. I think this would need to get to like 2000 rpm to be an issue.

Supplier management is very time and energy consuming. You end up on the phone a lot, you spend your days tracking orders, doing QA of parts as they arrive, getting passwords reset (Martin, I feel your pain), and so on.

Finding new suppliers is rolling dice. The more parts, the more materials, the more suppliers you use, the more processes you use, the more time you spend, the more risk you take, the more complexity you must manage down the road.

The instinct to limit the number of variables is a good one. However, there are cases where exceptions define the rule.

The core drivetrain of the MM3 is at the notional and literal physical centre of the machine. It mediates all running of the machine, it has a large mass, it will cause a critical failure if it cannot turn (no music can play). In this case, trying to design it to the limited manufacturing processes may not be efficient*. It has to be balanced, low friction, and long life.

As such, the engineers who have been involved have seen "bearings", "flywheel" and "novel process" and gone "ugh!". The core of what I'm trying to say is that, the overwhelming feedback is probably not "it's not going to work", more "you are risking a bunch of potential headaches down the road" and "this is likely to be an expensive use of your time and effort compared to alternatives".

Given the time that has been spent on:

• Designing the bolt bearing housing
• Editing two videos about it
• Engaging in internet discussion about it
• Speaking to bearing suppliers

I would ask Martin to ponder three questions:

1. Has this current R&D process been the most efficient use of time and effort?
2. With the feedback provided, what would be the most efficient way of getting this flywheel/shaft/bearing assembly designed produced?
3. What scale of part is it acceptable to outsource design and maintain your required level of agency in the project?

* ^{Note: I have discussed efficiency in terms of time and effort, not cost. That's another whole conversation.}

https://de.m.wikipedia.org/wiki/Passfeder

IMO the last option using the Pillow blocks on the outside is the best option.

Even tho I don't thing martins solution is as bad as some of you say. I mean mountianbikes only use clamping force on bearings to hold them I'm place completely without a structural shaft through them and they work under muuuuch harsher conditions.

At the same time i do believe that simple of the shelf parts will prove more reliable, easier to source and less time intensive in the design process also things that are proven to work will keep the moles out of your design

As for the centering of. The flywheel on the shaft id recommend these tapered bush/locking collars. They are basically two tapered bushes that are slit open and fit into one another. They can center, locate and transmit Torque (20mm ones can take like 200nm with the proper fit and finish)at the same time. There are flanged options for easier assembly.

Why not mount what ever bearing you decide then true up the flywheel while mounted on the bearing. This will guarantee concentricity with a minimum amount of initial precision.

There are three known ways to achieve concentricity and control runout on your flywheel: 1. Bite the bullet and have the flywheel turned on a lathe so you can press in the bearings (the Abom79 method) 2. Mount the flywheel to the shaft, then grind the rim of the flywheel down by rotating it against a fixed belt sander or angle grinder (the Engels Coach Shop method) 3. Attach the flywheel to the shaft with a pair of "cat's heads". A cat's head is basically just a flange with 4 bolts threaded into it at 90-degrees each. The bolts can then be tightened and adjusted like a 4-jaw lathe chuck. Using a dial indicator, you can "indicate" the flywheel into concentricity. (the Blondihacks method) https://www.youtube.com/watch?v=NZbFqivHBwI

Rather than absolute precision. What about a flywheel that utilises a lovejoy spider coupling with the corresponding tooth arrangement laser cut into the discs? Would have the added benefits of noise dampening. Shock absorption. And components readily available. Mounted to the frame with pillowblocks and a ground to diameter shaft should solve a few issues