Determining a flywheel diameter.

Hi all,

I just wonder if there are any rough and ready guidelines than can assist a beginner in determining approximately the size / weight of flywheel needed on a small model steam engine ?

I appreciate all engines are different and even 2 built to the same plan can vary in efficiency but is there any sort of general 'rule of thumb' that can give an approximate idea of what may be needed.

To give a working example, my first engine was single acting with a 13mm bore and 15 mm stroke, by guesswork I reasoned a 50mm dia steel flywheel seemed about right and sure enough it worked fine.

I now want to make another engine, this time double acting and with approx. twice the bore and stroke (say 25mm x 30mm).

If I double the linear dimensions is it a fair assumption to think the flywheel also needs to be approx. double the diameter and possibly a bit more for a double acting engine ? (I'd also hope my second engine will be more accurately made and be generally more efficient).

That gives me 100mm + size of wheel which my gut instincts say will be to heavy.

I realise by doubling dimensions the engine cc is 8x larger and the same applies to the flywheel weight but somehow it doesn't seem right.

What do others think ?

Many thanks

Peter.

Edited By Peter Nichols on 19/08/2015 16:30:27

Its not just diameter you need to consider, thickness will also play a part as well as the profile of the flywheel eg is the weight even across the radius or all on the outer edge where it will have more effect.

For example look at a similar bore/stroke stationary engine and a marine engine or equal capacity. The stationary engine will tend to have a larger dia spoked flywheel than the more compact solid marine engine one as there was not room below the propshaft for anything larger to swing.

Hi Peter,

If you want to calculate the size there is a calcualtor here.

Thor

Bear in mind that the energy stored by a flywheel increases as square of the diameter so a 'double size' flywheel of eight times the volume and mass will store sixteen times the energy.

This is why big engines with the same proportions can run slower and more smoothly, and why, if you look at full size engines, the rims are often rather slimmer than those of model engines.

Neil

Minor hijack - I have some bits of an old steam engine, the flywheel being one of the missing components. The cylinder is 4inch stroke and 2inch diameter. What flywheel dimensions would look 'right' if I ever get round to re-building it? It was meant to be a model of a mill engine, so pretty sure it would have had a single flywheel.

S

Steve I would say 10" minimum anything less would look out of proportion to the cylinder.

J

Many thanks for the replies

I must say I wasn't aware that a flywheel of double the size in all dimensions actually store 16x the energy - I just assumed it was 8x (in direct proportion to it's weight increase).

I guess that kind of answers my question - I had thought if I double the measurements of everything although the cc and the flywheel mass are 8x they are still in the same proportions, however it now seems the flywheel energy stored is twice as much again.

Presumably that means all things being equal it's not a simple matter of scaling up in the same proportions.

.

Peter,

I don't know whom to credit; but there is a very wise expression

You can't scale Nature

MichaelG.

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Edit: This use is from 1949, but I suspect the wisdom is much older.

Edited By Michael Gilligan on 20/08/2015 08:50:55

Bear in mind that assumes the same RPM. If you ran a large engine at small engine speeds the flywheel would burst.

The safe maximum speed for a spoked cast iron flywheel is roughly 570,000/diameter in millimeters

So for a model with a 4" flywheel it is approaching 6000rpm*, but for a full-size engine with an 8-foot flywheel the safe maximum speed is just about 240rpm.

Big engines therefore have to have slower maximum speeds and this will cancel out the effect.

So, if you are scaling a model and you want it to run at scale rpm, then a small model may need an over-scale flywheel (usually a thicker rim) to achieve slow running, although if the model has more than one cylinder or runs unloaded then this will help.

If you have a small model and are making it bigger but want to run at the same rpm, then assuming the rpm are safe for a larger flywheel, you can just make the rim somewhat lighter.

In your case you made a small engine run OK by using a relatively heavy, virtually solid flywheel. I would double up the size, keep the same rim thickness, but consider making bigger holes, using a spoked casting or fabricating a spoked flywheel. Other things being equal you will probably find that this allows the larger engine to still run slower and smoother.

Neil

*These speeds also apply to things like lathe faceplates.

Edited By Neil Wyatt on 20/08/2015 13:17:04

regarding "You cannot scale nature" what about Great Danes and ...Chihuahuas.

What is true is you cannot scale dirt.

Martin

.

What about them, Martin ?

Whilst there is some superficial similarity ... one is not a scale model of the other.

Things like heart rate, lung capacity, et. etc. differ disproportionately to the scale.

MichaelG.

It is interesting to note that in Machinery's Handbook flywheel design is framed in terms of stored energy. Flywheel diameter is secondary and is dismissed as usually being set by other considerations. The energy in a flywheel is proportional to the mass of the rim and the square of the rim speed. So for a given diameter you can play about with the stored energy by altering the mass of the rim. Machinery's Handbook also has tables of safe speeds for various types of flywheel, although not necessarily that useful as they start at 1ft diameter and work upwards to 30 feet.

Both 12" and 18" faceplates on my lathe have max rpm labels on them, as does the 4-jaw chuck.

Andrew

> Flywheel diameter is secondary and is dismissed as usually being set by other considerations.

Perhaps there are some simple rules of thumb on the effects of different changes, with all other things being equal:

Doubling diameter halves the safe top speed but doubles the lowest speed the flywheel is effective at (bigger machines can run slower).

If you can't increase diameter, add mass at the rim.

'Filling in' a spoke flywheel byrepoalcing it with a solid one makes surprisingly little difference.

If you need to make a flywheel slightly more effective (e.g. halve the minimum speed an engine will run at) a flywheel 1.2 times larger in all dimensions will store just over twice as much energy. (fourth root of 2)

If doubling all the linear dimensions of an engine, but wanting it to run at the same speed then the flywheel needs to be increased in size by about 1.7 times. (fourth root of 8)

Neil

Edited By Neil Wyatt on 20/08/2015 10:22:19

My point was "You cannot scale nature " is a bit of an ambiguous statement.

Martin

Sorry should be 570,000/diameter in mm

So 100mm flywheel it's 5,700rpm

1m flywheel it's 570 rpm

2.4mm flywheel (8 foot) it's 237 rpm.

I changed from centimetres to millimetres as cm are persona non grata these days but obviously forgot to add the 0. I'll edit my post to avoid confusion for those who come after.

Neil

I've edited this post so it look like you got it right rather than having the 8ft flywheel doing 327rpm (JasonB)

Ah well, that explains why my daughter has done better at maths than I did You could have put in my missing apostrophes at the same time.

(Neil)

Edited By JasonB on 20/08/2015 13:23:01

Edited By Neil Wyatt on 20/08/2015 15:51:16

This is one made of hot rolled steel plate, in two halves and bolted together, the diameter is 4", and about 7/8" across the rim. It's designed for one of my hot air engines, and is run at between 600 and 1200rpm. It weighs a bit over 800 grams. The total thickness of the thinned part is 1/8", the main advantage of reducing the weight is the reduction of weight/friction on the bearings. Since this photo, the holes have been enlarged from 1/2" to 5/8", with a surprisingly little weight reduction. Ian S C

Edited By Ian S C on 20/08/2015 13:47:29

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I have to disagree, Martin

To my mind; those four words are a remarkably concise statement of a fundamental law.

... but let's not take Peter's thread off its practical topic.

MichaelG.

Thanks for all the replies, there's obviously a lot more to flywheel design than meets the eye.

Neil, as you say the first engine had a small but relatively heavy flywheel with just 6 radial holes to lighten it and make it a bit more attractive, if I thought it was possible I'd try to make a spoked one but with the limited tools and experience I have I think that's beyond my skill.

Realistically I think I'll have to stick with the 6 radial holes method again, it's far from ideal but at least it's easy to make.

Peter.

Edited By Peter Nichols on 21/08/2015 08:46:31

I have a number of ways of determinating the size of a flywheel,(1) what's in the scrap bin. (2) does it look right. (3) is it about the right size.

Make it on the larger size, it can be made smaller/lighter. If it's a casting, you'r stuck with what you have.

Ian S C