Author Topic: Flywheel & Crankshaft Proportions - How to design?  (Read 2539 times)

Offline Steam Haulage

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Flywheel & Crankshaft Proportions - How to design?
« on: May 22, 2014, 08:07:33 PM »
Probably everyone vaguely recalls that I am (very slowly) building a the MEM Corliss. This question might apply to that, but really is meant to be more generalised.

What factors have to be considered when deciding the diameter of the shaft for a steam engine?

I presume -
flywheel weight,
distance between shaft bearings,
shaft material,
engine speed.

But what else? When I see full size practice it is difficult to estimate the shaft diameter, but if I look at road wheel hubs and lorries it seems that diameter is inversely proportional to the expected road speed. I presume than that the bearing speed in ft/min for any given wheel OD is thus kept as low as feasible.

I am interested in how this is worked out from 'first' principles, or is it all down to experience?

Any comments please?

Jerry :headscratch:
Dogs look up to you, cats look down on you, pigs treat you as equal.

Offline gbritnell

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Re: Flywheel & Crankshaft Proportions - How to design?
« Reply #1 on: May 22, 2014, 09:52:19 PM »
Hi Jerry,
I would think that the first thing designed would be the flywheel. The mass and diameter would be used to give the engine inertia between strokes and to smooth out the operation. Once this has been calculated then an appropriate crankshaft would be created. Naturally the larger the journal diameter the more support and strength there would be but that comes with a price, too large of a journal and the surface speed would be too great and cause too much heat thereby possibly burning up the journal bearing. The configuration of the crank would also have to be taken into account, support on both sides of the flywheel, support on one side of the flywheel, weight of the flywheel etc. I don't think there's really a number ratio like 6 foot flywheel would need a 3 inch shaft.
gbritnell
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Offline Maryak

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Re: Flywheel & Crankshaft Proportions - How to design?
« Reply #2 on: May 22, 2014, 11:21:22 PM »
Hi Jerry,

Non of the relationships are linear. They tend to fall either to the square, the cube or somewhere in between. e.g. shaft size 2" - 100hp, 8" - 1000hp, 24" - 27000 to 35000hp. Also in full size most shafts are hollow by about 1/3 of their OD, this provides for reduced weight and increased torque in the shaft.

When it comes to flywheels the size is dependent on the horsepower and the number of cylinders. A very very basic starting point for a single cylinder is 15lbs of force per horsepower.

Our models are just that; models. Scaling down from full to model sizes has many and varied implications but in the end it's more about the bits you can see being truly scaled and the bits you can't see being modified to allow the model to work or even work with the best possible output. i.e. Look Ma it Goes, or Look Ma the lights are on.

HTH

Best Regards
Bob
Если вы у Тетушки были яйца, она была бы Дядюшкой

Offline kvom

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Re: Flywheel & Crankshaft Proportions - How to design?
« Reply #3 on: May 23, 2014, 12:10:34 AM »
Model steam engines look best when running slowly, and unless there is some load being driven the forces on the crank are very low.  So really it's a question of making it look proportional to the prototype.  If you have a reamer that's close to the size you like then it can be used for the flywheel and bearings after getting material with matching diameter.

My current mill engine project has a 9" flywheel, and the plans call for .75" holes.  Since I have a .75" reamer that's what I'll use.

Offline steamer

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Re: Flywheel & Crankshaft Proportions - How to design?
« Reply #4 on: May 23, 2014, 12:15:19 AM »
Efficient flywheels have most of their weight in their rims.   The purpose of the flywheel is to carry the engine over during the periods near top and bottom dead center.   Knowing the average HP of the engine, it is easy to calculate the amount of inertia you require for the flywheel to be effective.   It's just an energy storage device.

Dave
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Damned ijjit!

Offline old-and-broken

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Re: Flywheel & Crankshaft Proportions - How to design?
« Reply #5 on: May 23, 2014, 02:39:00 AM »
I did some searching and it seems the weight of flywheel can be calculated(hope you remember your trig from school) from common data like RPM and HP

A rule of thumb design as others have pointed out is perfectly acceptable since the flywheel is just a battery that powers the motor during the exhaust stroke
using power it absorbs from the power stroke.  The mechanical equivalent of a capacitor smoothing pulsing DC from a bridge rectifier.  Make it smaller and the speed variations increase, make it bigger and the variations decrease.  To big and your standing inertia will absorb more power trying to start rotating than the motor can produce.

http://www.freestudy.co.uk/dynamics/tm%20diagrams.pdf


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Offline Steam Haulage

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Re: Flywheel & Crankshaft Proportions - How to design?
« Reply #6 on: May 23, 2014, 07:32:54 AM »
The contributions above have gone a long way towards resolving my thoughts. I had already adsorbed the need, purpose and design of the flywheel itself. The flywheel energy store is relatively straightforward to find, but had never really understood why the rotating mass with fluctuating energy input did not eventually cause the shaft to bend or break.
I shall have to study the Edexcel tutorial a bit more.
Thanks to everybody, back to the Calculus and the strength of materials tables.
Jerry  :happyreader:
Dogs look up to you, cats look down on you, pigs treat you as equal.

Offline steamer

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Re: Flywheel & Crankshaft Proportions - How to design?
« Reply #7 on: May 23, 2014, 09:24:41 AM »
The shaft does bend....just not permanently....think like a spring.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline Maryak

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Re: Flywheel & Crankshaft Proportions - How to design?
« Reply #8 on: May 23, 2014, 10:14:33 PM »
The shaft does bend....just not permanently....think like a spring.

Dave

Dave is right they all bend and twist then spring back. For the bend/twist to become permanent the modulus of elasticity has been exceeded.

When I was a young lad my Dad made me climb to the top of the factory chimney, the tallest in Lancashire at the time,(365'............ there will be no bloody Acrophobia in this family). I was, (and still am), amazed at how a seemingly solid, large brick structure swayed in the breeze.

Best Regards
Bob
Если вы у Тетушки были яйца, она была бы Дядюшкой