# flywheel size



## itowbig (Apr 13, 2010)

its prob been covered some where on here but here it goes *club* . how do u determine what size flywheel to use. (im quessing u would also need to know what type of engine too) but i dunno. is there a simple way of determining what diameter, weight ect to use.
thanks *discussion*


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## Brian Rupnow (Apr 13, 2010)

Itowbig---There are a few really complex calculations that will tell you.--Or --Do like I do. Find an engine close to the same bore and stroke that someone has ran succesfully here on the forum and make your flywheel close to the same size and same material (weight does matter).---Brian


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## Cheshire Steve (Apr 13, 2010)

Well now, that all depends ... It depends on what sort of engine, and it depends how little variation in crank speed you want. So, for example, a single cylinder stationary gas engine used for generating electricity might have two gurt big flywheels to keep the AC frequency steady even during the compression stroke. A multi-cylinder double acting steam engine might not need much flywheel at all.

I did dig out some equations from old books on this topic as I wondered how flywheels scaled, and as far as I can see they don't scale well, not being so effective at small scale, especially if you want to run at slow speed so the gubbins can be viewed working. However, for a scale model, people aren't generally so fussy about the speed varying a little through each revolution (provided it doesn't stop). 

If you would like I can root around and dig the equations out, but it might be useful to tell us a little more about your application. I suspect what I have is for single cylinder double-acting steam engines, but memory is a bit vague. 

Steve


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## Maryak (Apr 13, 2010)

Sid,

This is as rough as it gets but here goes. :-\

First up it's only the rim of a flywheel that is counted as providing the energy, ( forget about the hub and the spokes), so the bigger the diameter the less weigh needs to flanging around to provide that energy, i.e. bigger diameter narrower x section, smaller diameter wider x section.

For a very very very simplistic starting point you need around 10 - 15 lbs of weight per HP to maintain the speed within a 4% to 2% of the desired speed.

Lets say we have a simple wobbler of 3/8" bore and 1" stroke supplied with 50 psi steam and we want it to run at 900 rpm. The HP of such an engine is around 0.012 so our flywheel needs to be around 3.0 ozs.

1" steel plate is also known as 40 lb plate i.e. 1 sq ft weighs 40 lb so 1 cu ins weighs around 4 ozs. Lets say our flywheel is 2" diameter then our rim needs to have 3/8" x 3/8" cross section to be somewhere around the mark.

It's not an exact science and we have not touched on many factors such as critical speeds but it should get you going.

I hope this helps and I would be grateful for other methods of going about this.

Best Regards
Bob


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## Tin Falcon (Apr 13, 2010)

I have used anything from brass to plastic and aluminum for flywheels. in a model that runs for fun it does not seem to make a huge difference. 
You can do serious moment of inertia calculations bun not needed for the small models in most cases. 
Tin


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## itowbig (Apr 13, 2010)

well thank you guys . i was just curios. i dot have a particular engine or anything. i was just sitting in the garage looking at my little engines wondering how and if there was a simple way of getting a flywheel made for a particular type of engine .
ive started a while back on one that has a 1 inch bore +/- and 1.5 stroke +/- but have not decided what to do with it yet(steam or hit miss ect) ive seen a lot of engines here all very nice work ans some have large flywheels and some have smaller ones. i understand it has to do with momentum ect just dont quite get it in my head as to figger out a simole way of getting the size close ie 1/2 bore by 1" stroke and a flywheel thats close enough like maybe 5" or so. maybe 5 or so times the bore and or stroke . 
if any of this makes sence. any who i find this all interesting and you guys "n"gals make this a wonderful place :bow:


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## itowbig (Apr 13, 2010)

i almost forgot. thank you Brian for the offer of help it is very nice of you. 
(just thought i would thank you on here ) hope that is ok


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## deadin (Jul 14, 2010)

Sorry to resurrect this older thread, but I have a related question...

 I'm getting ready to build Brian's Horizontal Twin which calls for two 4" brass flywheels.
I just priced brass stock . OMG!
 The design doesn't lend itself to increased diameter, so it looks like I'm stuck with steel, CI or aluminum. I would prefer not to use steel as my lathe is pretty small and I need to make two of them. (I can't chuck up 4" and will have to use either an arbor or build a fixture of some sort.)
I've never worked with CI and couldn't find a source for 4" CI flywheels. Besides, from the prices of CI flywheel castings I did find, I might as well buy brass. 
So, it looks like it will be aluminum but I am concerned with the lack of weight when compared with brass. 
Should I just go ahead and try it with aluminum and see if it works? (with the plan to add lead inserts in the rim if necessary)


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## Brian Rupnow (Jul 14, 2010)

The price of brass kinda makes you quiver all over, doesn't it!!! That engine really does need heavier flywheels to function smoothly. Take a look at the thread I did on building my Webster engine. On it I used aluminum for the flywheels, with brass inserts. Not quite as heavy as steel or brass, but much better than aluminum alone.---Brian


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## deadin (Jul 14, 2010)

Brian,

That's what I had in mind, only with poured lead in the holes.
(Brass may be flashier, but lead is cheaper... ;D)

Dean


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## Jasonb (Jul 14, 2010)

I've always thought CI flywheels were relatively cheap in the US, $20 for a 4" one is reasonable.

http://martinmodel.com/MMPflywheelslist.html

Another way to add weight to an alloy flywheel is to cut a groove all round the rim, fill with lead and then loctite on a rim turned from thick wall steel tube, have a look at this video

[ame]http://www.youtube.com/watch?v=m_-iPpEuWRA[/ame]

Jason


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## deadin (Jul 14, 2010)

Jason,

Thanks for the link to martinmodel. I may just try them out.

I'm not quite ready to try your video process.(but I'm impressed) ;D
My problem with being able to machine larger diameters isn't mounting over the bed, but rather getting my tool post far enough away from center to be able to turn the OD.

BTW, I do like your "riser". Any plans available for it??

Dean


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## Jasonb (Jul 14, 2010)

Its not my video. Myfordboy does sometimes post om ME forum under the name of Enginebuilder you may be able to contact him there.

http://www.model-engineer.co.uk/

If you look at the video you can see he has his toolpost mounted on a couple of angle plates to get it up & out.

Jason


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## Lew_Merrick_PE (Jul 14, 2010)

Itowbig,

If you go to http://www.scribd.com/Lew Merrick, you will find a paper posted under the title _Rotational Dynamic Design Formulae_ that will give you more information on the subject than you probably want. It covers rotary dynamics in a manner much simplified from the _Dynamics 211_ engineering class that left most of my peers totally confused. There is a section in it on energy make-up flywheels. The solution to these is iterative, but spreadsheets making solving such things trivial these days. Those of us who had to find solutions to such problems with paper and a slide rule are horrified by the way _kids these days_ have it so "soft..."


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## Brian Rupnow (Jul 14, 2010)

Deadin---another way of approaching this is to go to your local largest machine shop, and try to beg some "off-cuts" of heavy wall 4" o.d. pipe.---Something with about 1/2" to 3/4" wall. If you can hold it in your chuck for a clean up of the inner diameter, then you can press and Loctite it on over an aluminum flywheel. The weight of a flywheel only accounts for anything out at the rim anyways.


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## deadin (Jul 15, 2010)

Brian,
Found some 4" OD 3/4" wall pipe, so I'm off and running!! ;D

Thanks,

Dean


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## walnotr (Jul 16, 2010)

So much for the flywheels, I was impressed by the non-geared forming rolls he used to make the rims. That was a new one for me.  

Steve C.


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