Author Topic: Question on flywheel design  (Read 7573 times)

Offline Chuck in E. TN

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Question on flywheel design
« on: August 05, 2010, 11:16:21 AM »
I have been thinking about size and weight of flywheels for our little engines. I don't have a large selection of material on hand and want to make the best use of what I have.
I understand the basic flywheel effect, but was wondering about the relationship between size, both dia and width, mass and material.
Is a 1/2" thick 3" flywheel, made from aluminum as effective as a 2" dia x 3/8" thick flywheel made from brass? Why or why not?

Chuck in E. TN
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Offline kwackers

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Re: Question on flywheel design
« Reply #1 on: August 05, 2010, 11:55:21 AM »
you need to do the maths to answer that.

If you understand the flywheel effect then you'll know that it's all to do with the velocity and the mass - 1g at the edge is travelling faster than 1g in the middle and therefore has more inertia.
Your 3" disc has a larger circumference and therefore more inertia for a given mass than your 2". Different thickness and materials mean differing mass and therefore differing inertia.

In simple terms if your two discs weighed the same then the 3" disc would have more of it's mass travelling faster and thus would have more inertia.
Beyond that you need to do some maths to find the average energy in the disc for a given RPM and compare the two.

The relevant equations can be found here:
http://en.wikipedia.org/wiki/Flywheel
« Last Edit: August 05, 2010, 11:57:38 AM by kwackers »

Offline kwackers

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Re: Question on flywheel design
« Reply #2 on: August 05, 2010, 12:13:46 PM »
Further (whilst waiting for a build):

Volume of a cylinder = PI * R^2 * H
So your Ali = 3.142 * 3 * 3 * 0.5 = 14.14 cubic inches (0.000231 m³)
your brass = 3.142 * 2 * 2 * 3/8 = 4.712 (7.721584e-05 m³)
Mass of your ali disc is about 2700kg/M^3 = 0.624kg
Brass is about 8550kg/M^3 = 0.66kg

Moment of inertia = 1/2 * M*R^2
So for your Ali = 1/2 * 0.624 * 0.0762^2 (converted R to meters) = 0.00181
for the brass = 1/2 * 0.66 * 0.0508^2 = 0.00085

So your ali disc has about double the 'flywheel' effect of your brass disc.

(Assuming my calcs and assumptions are correct - haven't checked...)

Offline jim

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Re: Question on flywheel design
« Reply #3 on: August 05, 2010, 12:33:35 PM »
given my understanding of maths, i'm going to stick with my theory that if it looks ok, it'll be alright!! :D
if i'd thought it through, i'd have never tried it

Offline mklotz

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Re: Question on flywheel design
« Reply #4 on: August 05, 2010, 12:36:37 PM »
Kwackers,

Although it doesn't affect your conclusion, I believe you've used as your radii the diameters the OP gave.
Regards, Marv

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Offline kwackers

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Re: Question on flywheel design
« Reply #5 on: August 05, 2010, 01:13:56 PM »
Oops. :clap:

I was between builds and was typing furiously before the compiler came back (best excuse I can manage I'm afraid)...


You're right though - it doesn't affect the result. (I wasn't going to bother changing the units either since it makes no difference for a comparison but thought that might confuse folk!)

Offline Lew_Merrick_PE

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Re: Question on flywheel design
« Reply #6 on: August 05, 2010, 04:36:54 PM »
I have been thinking about size and weight of flywheels for our little engines. I don't have a large selection of material on hand and want to make the best use of what I have.
I understand the basic flywheel effect, but was wondering about the relationship between size, both dia and width, mass and material.

Chuck (and others), take a look at http://www.scribd.com/Lew%20Merrick for the paper Rotational Dynamic Design Formulae.  I use foot-pound-second values, but the information is there for rotational dynamics -- including (towards the end of the article) calculations for engine flywheels.

Offline Chuck in E. TN

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Re: Question on flywheel design
« Reply #7 on: August 05, 2010, 04:57:39 PM »
Way too complicated for a simple boy like me, lol. Anyone care to simplify? Just trying to compare making flywheels out of 3" aluminum vs 1 1/2" brass, vs 2" steel. all same width, say 1/2".

Chuck in E. TN
Chuck in E. TN
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Offline Miner

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Re: Question on flywheel design
« Reply #8 on: November 13, 2010, 11:52:48 AM »
Not to try and hijack Chuck's thread but for those far more mathematically advanced than myself. When it comes to model steam engines in my opinion they look far better when they can operate closer to the original full size versions. For example beam engines had a pretty slow rotational speed. I've seen a few videos of model beam engines where the builders tried to replicate this slow speed but the motion was always quite jerky and didn't have the smooth rotation they should. My thought was to increase the outer diameter flywheel weight by cutting a channel in the cast flywheel and then cast lead into that channel. Machine a accurate band of metal and then heat shrink that over the O.D. of the flywheel to hide the lead. It would be a lot more work but any thoughts about if this would be worth it?

Pete

Offline RichardShute

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Re: Question on flywheel design
« Reply #9 on: November 14, 2010, 10:32:31 AM »
The inertia is proportional to the mass. The density of lead is about 11.3 and iron about 7.8 so for each bit of iron you remove and replace with lead, the lead will be about 45% better than the iron you removed.

The end result will depend on the relative proportion of the iron you remove, but since if you made the entire flywheel of lead to the same design as the iron it would only be 45% 'better', frankly I doubt you would notice enough difference to make it worth the effort.

To get a 45% increase in flywheel effect you could far more easily increase the diameter of an iron one by 20% (flywheel effect is proportional to the square of radius). Obviously it would alter the look somewhat, it's just an alternative route.

Richard
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Offline cidrontmg

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Re: Question on flywheel design
« Reply #10 on: November 14, 2010, 06:36:26 PM »
Yes, Richard and Miner, that´s the way it is. Replacing iron with lead is not "cost effective" inertia-wise. It would be quite another thing if the original flywheel were of aluminium. Replacing aluminium (density=2.7) with even iron (7.8), brass (8.5) or bronze (8.7), let alone lead (11.3), would be a significant increase. Making even a thin(nish) rim to an ali flywheel of anything heavier is very worthwhile. And a brass/bronze rim around an aluminium wheel will (to most people) also enhance the visual aspect. Of course that´s not true, if you´re copying an old design, with exhibition/ museum quality ambitions...   :)
These are bronze. 300% more inertia compared to an all aluminium flywheel of same size.
 :wave:
« Last Edit: November 15, 2010, 05:59:59 PM by cidrontmg »
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Offline Miner

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Re: Question on flywheel design
« Reply #11 on: November 15, 2010, 11:23:21 AM »
Richard and cidrontmg,
Thanks for the information. I had a gut feeling that trying to cheat on the laws of physics wasn't going to work very well.

Pete