How different in size can my flywheel be?

[HenryBanjo]

Hi there,
I am currently working on a Quayle 16-16H steam engine, but I am having trouble sourcing a flywheel to fit the engine. Its a 16-bore x 16-stroke steam engine designed for making from bar stock. The drawings call for a 59mm steel flywheel. I have a cast iron flywheel a bit larger than 50mm that has a pretty big rim. Would this do for as replacement?

 

[Andrew Stroud]

A bit larger is good and will not cause any issue, in fact it will tend to improve the slow speed running of the engine. Points to consider are the aesthetics, does it look in keeping with the rest of the engine and also are you able to machine the rim and bore the hub in one setting to eliminate any wobble when running.

 

[Jasonb]

If you make it a bit wider that will help cancel out the lack of diameter.

It will likely work with a lot less but depends what you want to do with the engine, if it's for display with no load then a larger mass flywheel will help it to run smoothly at low speeds. If you are going to put it to use in say a small boat hull then it will be running a lot faster so a smaller flywheel is OK and will also help get the prop shaft angle line lower down

I'm just making a 16mm bore x 15mm stroke engine at the moment that will have an 80mm flywheel but I also have 16 x 16 with 38mm flywheels that runs smooth at slow speed and will also rev to 1800rpm as well as 16 x 40 with 200mm flywheel

 

[djc]

If there is a flywheel of particular shape and material specified for the engine, you can work out its mechanical/dynamic properties (a function of geometry and density). Any flywheel you then substitute that has the same properties should perform the same.

This seems to give the basics:

https://www.engineersedge.com/calculators/flywheel_inertial_energy_15646.htm
It would allow you to put a number on the 'bit wider' suggested above.

I think I read a post long ago by Brian Rupnow about putting a brass rim onto flywheel that was originally supposed to be iron or steel. The extra density of the brass gives a better flywheel effect for the same diameter.

There's lots of possiblities: you could perforate the rim and glue in bits of tungsten rod, for instance. Mill out a channel and fill with lead. I read that depleted uranium is quite dense.