Fly wheel calc

[Ripcrow]

Is there a standard way of working out diameter and weight needed in a fly wheel.i have built a air motor that has standard valves for inlet and outlet and I know my outlet needs some modification to allow proper airflow but also thinking fly wheel may be to heavy or large or maybe not large enough,my stroke is 40 mm with a bore diameter of 38 mm.

 

[Tin Falcon]

Is there a standard way of working out diameter and weight needed in a fly wheel.

To answer this question YES. Any good engineering book will give the formulas.
I recommend a copy of the machinery handbook. You should be able to get a used one a couple editions old for about $ 20. Or Google and download a pdf. I recommend getting one out of copyright if you do the PDF.

thinking fly wheel may be to heavy or large or maybe not large enough,my stroke is 40 mm with a bore diameter of 38 mm.

In most cases just pick a fly wheel that looks right for the size of your engine and the engine design/style.

Moment of inertia is one of those things that just does not scale well.

IMHO every model engine the flywheel is mathematically too small.


That said most engines are not powering anything so no power is being drawn from the system it is easy for the flywheel to keep going as it does not need to store much energy.

IMHO KISS look at other engines of the same stoke bore and style as yours and pick flywheel of same or similar size.

More info please what type of engine is this.
What size flywheel do you have. It could just be a timing issue.




Tin

 

[Terry_S]

Traditional wisdom is that you really cannot "oversize" a flywheel as long as you do not exceed the wheel strength vs centrifugal force issue. (Usually not an issue on model sized engines). If the engine is rough, you might need more inertia, not less.

Having said that, like Tin Falcon says, look at other successful model engines and scale the flywheel based on them.

Here's a staring point. For single cylinder, double acting engines: flywheel diameters often end up being around 3X of the stroke. So, for a 40 mm stroke, a wheel diameter of around 120 mm is a good starting point.

The metal near the OD of the wheel contributes most of the inertia. So, you can thin down the portion between the hub and the OD if you want. Or you can make it with spokes or holes. On larger engines, the thickness of the "rim" is usually in the range of 20-30% of the width of the rim.

Terry S.

 

[Tin Falcon]

If i had to throw a number out based on the original question I would have said 150 mm /6" .
from what I have seen in general marine engines have smallish wide rim fly wheels . Mill engines have good sized fly wheels and corlis engines, pumping engines and walking beam engines have large diameter wheel with a relatively narrow rim.

I try not to discourage new folks from doing there own design but IMHO there is a lot of value in building a few engines of different types of proven designs before one attempts there own. I also see nothing wrong with taking a part from different engines and combining them often one looks at a part of an engine and sees a better way(to them) of making it.
tin

 

[Ripcrow]

I don't have the equipment or the ability to build the engines that are mostly built here,I have a lathe angle grinder welder oxy kit so I am limited. The engine is simple and I know my outlet timing and opening distance does need attention .tried it without a fly wheel and was getting 3/4 of a rev out of it so lacking material I quickly attached a piece of 150 mm /5 mm steel plate 30 cm long and got it it do about 10 revs before always stopping on the exhaust stroke.my " valves " are set of air taps that are operated by a normall system of cam pushrods.I tried 3 times to build a head but was unsuccessful at getting valves to seal so I cheated and used taps.today I am going to revamp my valve arrangement as they do take a lot of force to operate and hopefully with this done and a flywheel of the right weight we may gave a runner