how much rpms could a miniaturized mechanical crank get to. - Page 1
Forum section: electronics / beginners

author: Capernicus
author_rank: Frequent Contributor
timestamp: February 28, 2022, 01:58:46 pm
content: say you have a 1 millimetre conrod, connecting a 1 millimetre piston to a 1 millimetre crank shaft, how much rpms do you think are possible?(imagine a little mechanical computer.)

author: Cerebus
author_rank: Super Contributor
timestamp: February 28, 2022, 02:35:41 pm
content: More a question for a mechanical engineering forum, and you're missing about 10 bits of information necessary to determining the answer, materials, design life, forces to be transmitted, availability of exotic materials/manufacturing processes, cost and so on. Come back, preferably somewhere appropriate like a mechanical engineering forum, when you've thought it out a bit more, or in fact, thought it out at all (like how your 1mm con-rod isn't going to get stuck in your 1mm cylinder bore).

author: Capernicus
author_rank: Frequent Contributor
timestamp: February 28, 2022, 03:00:24 pm
content: Quote from: Cerebus on February 28, 2022, 02:35:41 pmCause I resin 3d printed it.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 28, 2022, 03:12:55 pm
content: I endorse Cerebrus' answer. His question wasn't about manufacturing. The problem he hints at is real and if you actually think for a few minutes you will understand the question.

author: Cerebus
author_rank: Super Contributor
timestamp: February 28, 2022, 03:25:16 pm
content: Quote from: Capernicus on February 28, 2022, 03:00:24 pmSo it broke. OK, that's not how we normally define "fits" unless your name is Bubba and your response to "It doesn't fit" is "It will" while walking up with a large hammer.Now we have some parameters - " I resin 3d printed it" we can now answer your question. The maximum shaft speed will be 0.5 revs per manufacturing time.

author: Capernicus
author_rank: Frequent Contributor
timestamp: February 28, 2022, 03:27:11 pm
content: Quote from: Cerebus on February 28, 2022, 03:25:16 pmOh it is a bit faster than that mate, have a look at this thing, and its macro sized. I was thinking reducing the inertia of the whole thing via miniaturization will increase stability alot at extremely fast rpms.about the con rod, what if its exterior poking out of the cylinder throughout the whole cycle- then it doesnt matter how thick it is.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 01, 2022, 01:22:58 am
content: OK. I will point you at some directions to think.1. In many piston crank systems the limit is strength of materials. The piston changes direction once per revolution and travels the stroke length. Simple physics tells you the acceleration, then the mass of the parts tells the force. Resin printed parts are pretty far down the strength curve, but the small size is a help here.2. Friction losses, particularly viscous friction become really important at small sizes. So much so that that for small insects like gnats flying is more like wading through mud than gliding. The low mass and hence low inertia that benefited you in topic one hurts here. But 3D printed parts tend to have relatively poor fits and horrible surface finish giving much higher friction. Where the pain is felt depends on your system. If torque is applied through an external system look at gear train losses and torque failures in the axles. If you are trying to make this an internal combustion motor look at thermal losses and ability to burn enough material in the small volume to cover losses. (Without even thinking about whether your plastic can stand the environment.)3. What I suspect Cerebus was hinting at is the geometry of your proposed system (1 by 1 by 1) is prone to dynamic issues. Rocking, rolling and all the other bad things that can happen. Which goes directly to items 1 and 2.There is no simple answer to the question of how fast it can go. But you should really think about the fact that your YouTube system has no reciprocating parts and the fastest widely available commercial rotors (dentist's drills) are turbine operated, not crank systems. The fastest crank and piston systems I am aware of are small model airplane engines which max out in the neighborhood of 30-40 thousand rpm running of fuels with a high nitromethane content.

author: BrokenYugo
author_rank: Super Contributor
timestamp: March 01, 2022, 01:49:22 am
content: If you're just spinning this hypothetical geometrically problematic thing in a frictionless vacuum, I'd bet whenever the rod yields, that's usually the practical weak point.

author: CaptDon
author_rank: Super Contributor
timestamp: March 01, 2022, 01:56:42 am
content: I believe that with any obtainable materials in this current time the mentioned 30 to 40 thousand RPM's seems to be the practical limit. One limiting factor is ignition rate or flame travel. And a 2 cycle producing power on every downstroke without the drag of mechanical valves will be able to attain a higher overall RPM. On the mechanical side of things the piston travel obeys the speed laws related to a sinewave slowing as it reaches T.D.C. or B.D.C. and then accelerating to the fastest speed at mid-stroke. But even so the acceleration and deceleration forces are either going to break the connecting rod or break the crank pin as RPM's reach the attainable limit. Reciprocating engines have suffered this pitfall ever since their design. Even the wobble-plate 'engines' still encounter a similar 'piston speed limited' maximum RPM. The Wankel tried to get around this piston speed limit but even they hit a limit where the rotor will beat the seals to death. Bottom line, if you can get your fuel cheap nothing beats a turbine and if you can operate the fuel fed turbine in conjunction with a combined cycle steam turbine including heat recovery (incoming air preheat) and condensate recovery you can hit total plant efficiencies that reciprocal power will never match. Now a newer question, what would be the practical RPM limit for a miniature turbine?? I know some have already hit around 100,000 RPM. Cheers Mates!!!

author: Circlotron
author_rank: Super Contributor
timestamp: March 01, 2022, 02:24:46 am
content: Quote from: CaptDon on March 01, 2022, 01:56:42 amAccording to Garrett who manufacture turbochargers, some get to 350,000 rpm.Must be very small ones. I had heard figures of half that many years ago, so yeah, I was more than a bit surprised.https://www.garrettmotion.com/turbocharger-technology/how-a-turbo-works/

author: CaptDon
author_rank: Super Contributor
timestamp: March 01, 2022, 03:30:11 am
content: We had some Czechoslovakian built turbochargers on our early EVO locomotives (I always think top quality well balanced items when I think Czech..hehehe) and the rotating mass was easily 80 pounds running around 18,000 RPM. They had a tendency to fatigue crack and split and when they did they came flying right through the engine cab doors!!!! Several class 1 railroads forbid employees standing on the catwalks above notch 4!! I know they found chunks of turbo wheels over 100 feet from the tracks!! Usually the oil seals self destruct and the engine goes into hyper-speed using its own lube oil for fuel until either the pieces go flying into low orbit or it finally seizes due to no more lube oil. Either way, its like the locomotive has its own built-in 'cash for clunkers' program. The damage is spectacular!!

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 01, 2022, 07:11:02 am
content: You can make gear ratios any amount, even 1:1,000,000. The effective mass is counteractable by making the gear chain smaller and smaller as it goes. So that fact has me hooked on this idea, It should actually be liftable even with a small motor... but I wonder what's going to go wrong, I figure friction will cause it to heat up.So I'll put my micro transmission together and see how it goes.

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 01, 2022, 07:31:29 am
content: cool video.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 01, 2022, 02:18:27 pm
content: Just remember that gear ratios work both ways. That speed ratio of one million implies a similar factor on torque. A tiny amount of friction on the output gets a million times larger at the input even in the ideal case. The reality is worse.

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 01, 2022, 03:31:19 pm
content: Quote from: CatalinaWOW on March 01, 2022, 02:18:27 pmThanks for the warning. But friction is based on surface area right?If I reduce the size of the cog, as it gets faster and faster, this will reduce the weight and the surface area. but I spose the two are different ratios, as I reduce the size.So It will come to less friction as well as inertia, for a 1mm crank - to counterbalance the speed/hz.Ill be happy with quarter of a megahert (going for a fair bit heheh), I'll see if I make it!

author: Cerebus
author_rank: Super Contributor
timestamp: March 01, 2022, 03:57:55 pm
content: Quote from: Capernicus on March 01, 2022, 03:31:19 pmNope. The force(s) due tofrictionare proportional to applied force(s), the proportionality factor is known as the coefficient of friction, area is not involved.Have you considered taking, understanding, and passing, a high school Physics course before setting out to design novel, never done before, mechanical devices? There's a saying "Don't try to run before you can walk.", in this case I'd amend that to "Don't try to run before you can crawl.".

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 01, 2022, 04:43:55 pm
content: The advice to take some courses is excellent. But I am also a strongbeliever in practical experience also. Since you are 3D printing your device you should be able to do some trial runs to gain experience in a day or two. Start with a 100 or 1000 to one in a size that doesn't challenge the accuracy of your machine. You are likely to learn a lot at that step. Then either downsize or up ratio and see if the results match your expectations. The coursework can help you understand what happened.

author: Capernicus
author_rank: Frequent Contributor
timestamp: March 01, 2022, 05:45:29 pm
content: Quote from: CatalinaWOW on March 01, 2022, 04:43:55 pmWhen I get it done I'll post here and show off the print, I want ppl to see the results, I'm not keeping this one a secret, if it works it would make me happy if someone else gave it a go.