Scaling up

[MikelkieParticipant @mikelkie]

[MikelkieParticipant @mikelkie]

Iv'e been asked the question how to scale up the cylinder bore of a 5" to 7 1/4 loco. My method was simply multiply the area of the bore by 1.5 (1.45) to arrive at a scaled up figure. Eg. Bore 40mm apply R x R x 3.142 = 1257 Scaled up by 1.5 = 1885 However i'm told by another ME that i'm wrong for i should multiply the diameter of the bore by 1.5. This now produces a inflated figure of 2828. I'm no expert in ME although i built a number of locos in my day. Am i missing something or has my maths rusted up?

[blowlampParticipant @blowlamp]

Are you trying to obtain a scaled power output from the model?

I'd just multiply the bore by the required scale factor. So 40mm x 1.45 becomes 58mm.

I think if you used your current method to scale a 5" model cylinder bore back up to full size you might end up with something different from the original.

Martin.

[JasonBModerator @jasonb]

Just multiply lengths and diameters by 7.25/5 and let areas and volumes take care of themselves.

[Clive FosterParticipant @clivefoster55965]

Time to be pragmatic about things. When it comes to models the outside, visible, part needs to be the right size and the inside part just needs to work.

So scale the outside. Compare the resulting bore / stroke / piston area with that of a similar model thats known to work at least reasonably well. If your scaled result is close enough for government work call if good and get on with the important stuff.

Clive

Edited By Clive Foster on 10/02/2018 20:17:51

[Marcus BowmanParticipant @marcusbowman28936]

Clive's answer makes good practical sense. But if you are scaling the bore, will you not also be scaling the crank throw, along with all the other dimensions? In that case, the swept volume will change as you scale the bore. It will change again if you scale the crank throw. Swept volume = 3.14 x diameter x stroke (or throw), so scaling by 1.5 multiplies the swept volume by 1.5 x 1.5 = 2.25

If you just want to scale the volume by 1.5, you would either need to keep the stroke the same as the 5" model, and multiply the bore by 1.5 or leave the bore the same and multiply the stroke by 1.5 (which you probably need to do if you are scaling the other linear dimensions). Intuitively, it seems wrong to scale one dimension without scaling the other. Then what about the dimensions of the valve gear? Surely that requires everything, including the stroke, to be scaled by 1.5. Then there is the power, which must depend to some extent on the swept volume, suggesting the bore and stroke should both be scaled. Surely the volume of the boiler will increase as its diameter and length are both increased, suggesting you would need a scaled bore and stroke to cope. It starts to get complicated.

Which is why Clive's advice seems sound.

Marcus

[Marcus BowmanParticipant @marcusbowman28936]

There is an error in my last post, and I can't see how to edit it (logged out, then back in, and as noted by others this removes the chance to edit my post).

The area of a circle is pi x (radius squared). Swept volume = 3.14 x r x r x stroke. So scaling dimensions by 1.5 scales volume by 1.5 x 1.5 x 1.5 (one for each occurrence of radius and one for the stroke).

Given that the boiler volume scales in the same way (and I don't know if that's the best way to scale a boiler, fire grate etc), the cylinder and boiler volumes might still roughly correspond. Clive's answer is still the most practical, I think.

Marcus

[MikelkieParticipant @mikelkie]

Hi all. Thanks for all replies and inputs, yes the engine is scaled up by 1.5, my question arise from scaling up the bore of the cylinder since scaling up the area of the smaller one comes to 1885 mm sq. now scaling up the bore by 1.5 results in an area of 2828 mm sq which is 50% bigger. I guess is should not make a big difference when it comes to steam supply from the scaled up boiler.I never built a scaled up something before but was asked to make the scaled up pattern for the cylinders. i must take measurements and work from the cylinders of a running model that was completed in 1959 by one good ME since the workmanship is outstanding.I would not like to make a "boo boo"of the pattern. I will ask the owner for pics and post it soon. I will follow Clive's advice. Thanx to all for replies and inputs once again.

Greetings to all Mike

[JasonBModerator @jasonb]

When scaling anything up areas will increase by the square of your scaling factor eg 1.5 x 1.5 = 2.25 times larger

Volumes will increase by the Cube of the scaling factor eg 1.5 x 1.5 x 1.5 = 3.375 times larger.

Having scaled up or down 11 models in total all of which run fine do what I said and forget the over complicated stuff. just multiply lengths, radii and diameters by your scaling factor and let the areas and volumes take care of themselves. You may need to make slight adjustments for stock sizes and threads but nothing more.

The only time this will vary if is rather than a lineer scaling (which going from 5g to 7.25g is) will be if you want to scale the model by a different factor say take the capacity from 3cc to 5cc where you need to work back from the volume to get a scaling factor that can be applied to all the lengths etc on the drawings to end up with a model in the correct proportions but with the larger capacity. been there done that too.

[John HinkleyParticipant @johnhinkley26699]

Let me start by saying that I have no particular interest in steam engines – of any scale – so feel free to ignore the rest of this post! It doesn't stop my enjoyment of pondering the engineering solution to a problem, however, so here goes……

I think I would approach the problem from an alternative direction. That is to say, I would take the bore/stroke dimensions from a proven 7¼-inch loco design and see how that fitted into the scaled-up drawings of the 5-inch model. Or is that too simple? Like I said, I've never delved into steam-powered models, so I may well be way off the mark.

John H

[JasonBModerator @jasonb]

I think the problem is that it is not just the cylinder proportions that may make a particular loco a good runner. You also need to figure in things like boiler size, grate area, heating area (size & No of tubes) number of driven wheels, tractive weight etc and I'm not a loco man either.

There is also the fact of why the person wants a larger version of a particular engine, most likely because they like the look of a particular one otherwise they would just choose something already available in the bigger gauge. So if it is the look they are after then having cylinders or a boiler that does not look right may not be they way to go. Also do they just want to do a few loops of the local track a couple of times a year in which case it does not matter too much if performance is not 100%, but if they want to be hauling passengers every weekend than looks may have to take second place to practicality and performance.

[John HinkleyParticipant @johnhinkley26699]

I bow graciously to your superior knowledge, Jason, and I appreciate your comments and reasoning.

John H

 

Edited By John Hinkley on 11/02/2018 13:22:11

[Author]

Posts