Flywheel material - ok to subsitute aluminum for steel ?

[David Morrow]

I'm building Jan Ridders' Atkinson engine "Wouter" and need to make a flywheel - 6" diameter x .750" thick and the plan calls for steel or brass. Due to equipment and material issues, I'd like to use aluminum. My question is, with aluminum being lighter than either steel or brass, will this cause me any problems in running the engine ? I'm thinking that I could make up for weight difference by making an aluminum flywheel 1" thick.

Any thoughts on substituting the thicker aluminum ?

 

[Swifty]

I've no experience with aluminium flywheels, but I don't like your chances of it working. The flywheel is there to provide momentum and relies on the weight. Even going from 3/4" to 1" thick won't help.

Paul.

 

[RonGinger]

Can you find a piece of thick wall steel pipe to make a rim for the flywheel? Weight in the rim is most important- note how many flywheels have spokes or holes in the center. Even a 1/2" thick rim, on kind of like a tire, would be much more likely to work.

 

[dave-in-england]

I'm building Jan Ridders' Atkinson engine "Wouter" and need to make a flywheel - 6" diameter x .750" thick and the plan calls for steel or brass. Due to equipment and material issues, I'd like to use aluminum. My question is, with aluminum being lighter than either steel or brass, will this cause me any problems in running the engine ? I'm thinking that I could make up for weight difference by making an aluminum flywheel 1" thick.

Any thoughts on substituting the thicker aluminum ?

~~~~~~~~~~~


Rotary Momentum relates to mass in a flywheel.

The density of the flywheel material affects the overall mass, but you can
alter the dimensions about to get the mass you need.

A flywheel of 6" diameter and 0.750" thick in Brass has a mass of 6.5 LB.

Same size flywheel in mild steel = 6.0 LB.

For a 1060 grade aluminium flywheel of the same diameter, it would need to be

a thickness of 2.2" for 6.0 LB.

and a thickness of 2.36" for 6.5 LB.

( The shaft hole diameter will make a slight variation )


dave

 

[David Morrow]

Presumably the mass closer to the perimeter is of greater importance than the mass toward the axis. So, I wonder if adding to the diameter and adding steel to the outer-most portion of the flywheel would solve the problem.

 

[dave-in-england]

Presumably the mass closer to the perimeter is of greater importance than the mass toward the axis. So, I wonder if adding to the diameter and adding steel to the outer-most portion of the flywheel would solve the problem.

If you can get the majority of the mass to the outside edge of the flywheel,
you can then have a much thinner flywheel.
A thick outer steel ring will be good.

 

[vederstein]

What you're getting into is in kinematics called the mass moment of inertia, in mechanical engineering circles known as J.

I don't want to get into the formulae of it because it's a pain. But if you want to switch from steel to aluminum and have the same mass moment there are several options, but first start with a baseline:

Calculate J for the steel flywheel to know your goal.

Then work with different geometries with aluminum until you reach the same J.

By running the numbers you will find that you can actually have a larger mass moment with a smaller mass by putting the mass of the flywheel on a larger radius. There are practical limitations to this though.

I got the following graphic from http://www.learneasy.info.

(I know that here they're using I instead of J. When I was in engineering college, I was for the area moment and J was for the mass moment. Perhaps nomenclature has changed in 20+ years.)

...Ved.

 

[Tin Falcon]

As I have said before The Machieries Handbook should be a part of the Home shop machinist library either hard copy or PDF older editions are fine.
There is a section on flywheels and fly wheel calculations.
Tin

 

[David Morrow]

Thanks for the tip Tin. For others who have not seen or heard of this book, here's a free PDF link that I just found for the 26th Edition :

(link removed by tin Falcon)

As to my adventures in flywheel making, I think I have a good solution. I'm going to make the flywheel 1" thick and cross drill close to the perimeter and insert a number of .750" pieces of brass rod. That should add the needed mass and help make it look real nice to boot. I'll likely also make it 7" diameter as well. But first, I'll read the flywheel section of the book and work the math. I might also see if I can work up an Excel spreadsheet using the info from Ved above.

Thank you all for the education. I love to bury my mind in solving design problems and it's a good rainy day here to do just that.

 

[Tin Falcon]

Dave and others I removed the link to the 26th edition.
This is a 2000 edition and therefore under copyright.

I suggest if you want a free copy download an older out of date digitized version. or purchase an older edition hard copy.
I can not prevent others from posting or downloading copyright protected material on the internet on other sites.
It has been the policy to to respect copyright laws on this forum therefore as an administrator am compelled to remove the link. Thanks for your understanding.
Tin

 

[David Morrow]

Oops, sorry. I hadn't thought about that. I assumed since it was so readily available there were no issues. I understand completely.

 

[David Morrow]

This has been a very interesting and educational exercise.

I used the illustration and accompanying table that Ved provided above to get me started. I used the thick ring illustration and formula for my flywheel. I also received confirmation from the Machinerys Handbook that the most inertia will come from the outer most part of the flywheel. Also that, in order to simply the calculations a bit, I ignored the area of the spokes and inner hub. They are inner-most and of the smallest area and therefore provide the least amount of inertia.

Next, I used the Online Metal Store weight calculator to determine that the weight per cubic inch of Brass, Steel, and Aluminum were .307, .283, .098 pounds respectively.

Next, I caculated the mass of the outer "donut" ( new technical term for outside portion of a spoked flywheel ). That was easy - calculate the area of a circle x thickness x weight per C.I. of the metal as noted above. First I calc'd the OD and subtracted the calc'd amount for the ID. That gave me the mass for each metal, OD and ID variation on my table.

Finally, I used the Mass Moment of Inertia ( MMI ) formula for the Thick Ring on the table that Vled provided - A.K.A. the "Donut", using the formula M(r1^2 + r2^2)/2. Substituting the info above, I used Mass x (O.D. squared + I.D. squared ) / 2.

Now back to practicalities. Jan Ridders design, roughly translated from metric to inches is about 6" O.D., 5" I.D and .75" thick. This results in a MMI of 41.95 for steel and 45.50 for brass if my arithmetic and Excel skills are working. I changed the O.D., I.D., thickness, and weight to use aluminum and came up with and O.D. of 7", I.D. of 6", and thickness of .750" resulting in a MMI of 42.53. Sounds just about right.

So, there you have it, at least by my calculations, is the answer. 7" x 6" x 1" using aluminum.

Thank you all for your input and contribution to my continuing education. Please pass the aspirin...

 

[Tin Falcon]

Oops, sorry. I hadn't thought about that. I assumed since it was so readily available there were no issues. I understand completely.

The 1914 Edition is still available on google books.
There was a nicely digitized copy of the "41 edition out there as well.
In more recent years Industrial press has offered digital,PDF,cd-rom versions of the Machinery's Handbook.
The current MSRP for the latest pdf version(29th Edition) is close to $100. So this is not free ware.
The Machinery's Handbook is very often used as a college text book and as such used copies of older editions are widely available you should be able to find a copy pf the toolbox edtion for $20- $30. For us older folk they also have the large print desk edition.

Tin

 

[lohring]

I doubt that it's an issue in this case, but if you turn a flywheel fast, strength becomes an issue. In building an inertial dynamometer, I was worried about that issue. Several checks showed mild steel would be OK for a 5" diameter flywheel at 20,000 rpm. We used heat treated 4340 to be sure. We've since turned over 25,000 rpm. The highest stress is at the bore, so you need to be careful with heavy press fits on the shaft. We used a very light press with Locktite.

Lohring Miller

 

[tornitore45]

If you look at the post on this website of a fellow that made the Dulcos gearless engine you will see he made an aluminum flyweel but studded the periphery with brass dowels pressed in. You can also add thickness and calculate hether you reach close to the steel weight in the flyeel crown.

 

[Wizard69]

I'm building Jan Ridders' Atkinson engine "Wouter" and need to make a flywheel - 6" diameter x .750" thick and the plan calls for steel or brass. Due to equipment and material issues, I'd like to use aluminum. My question is, with aluminum being lighter than either steel or brass, will this cause me any problems in running the engine ? I'm thinking that I could make up for weight difference by making an aluminum flywheel 1" thick.

Any thoughts on substituting the thicker aluminum ?

I look at it this way, do whatever you want as long as you do the engineering. In the end you want the flywheel to be safe.


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