Scale weight

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Mr Ron
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Scale weight

Post by Mr Ron »

When comparing a scale locomotive (any kind) to a full scale prototype, is there any proportionality when it comes to weight? In other words, can weight be scaled down?
Mr.Ron from South Mississippi
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NP317
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Re: Scale weight

Post by NP317 »

It's a cubic function.
~RN
pete
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Re: Scale weight

Post by pete »

Think of scale weights like this. Lets say a 1" x 1" x 1" solid steel block weighs one pound. Double it's size so then how much does it weigh? Two pounds? Nope 8 pounds. Doubled in size is 2" x 2" x 2" so 8 cubic inches. It works the same in reverse when scaling down. Let's say full size scaled to 1/2 size = 1/8th the weight of the full size if everything was exact.
Magicniner
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Re: Scale weight

Post by Magicniner »

Do friction and traction scale down with size and weight?
Is weight greater than scale required for models to generate optimum traction?
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NP317
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Re: Scale weight

Post by NP317 »

Coefficients of friction remain the same regardless of weights involved. (I think...)
They are a function of the specific materials in contact.
Traction results from weight and that friction.
So yes, greater than scale weight increases traction.
~RN
Magicniner
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Re: Scale weight

Post by Magicniner »

I didn't mention Coefficients as I'm aware that they are physical constants but frictional force between materials is affected by contact area as well as weight, I wondered if it scaled making scale weight adequate for scale traction in a scale engine, the answer is yes or no though, indicating you know as much as me about it :D
John Hasler
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Re: Scale weight

Post by John Hasler »

To a first order static friction is independent of contact area. Beyond that you get into tribology and it gets really complicated.

However, I would think that there would be a lot of literature on locomotive traction with lots of empirical formulas, tables, and nomograms that could be extrapolated.
Mr Ron
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Re: Scale weight

Post by Mr Ron »

I really wasn't concerned about traction, only the actual scale weight compared to a full size engine. Obviously if the model were made of cardboard or wood, the weight would not scale the same; but if the materials used were the same as in a full size engine, I can't see how a model would scale down proportionately.
I really answered my own question after giving it more thought. For example take a 50 ton loco and scale it down to 1/16 size; the result would be 3.125 tons; obviously a model built to 3/4" to the foot scale couldn't weigh 3.125 tons. Case closed. I appologize for wasting your time on something so obvious.
Mr.Ron from South Mississippi
mikeehlert
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Re: Scale weight

Post by mikeehlert »

Just to make the discussion a bit more obtuse. Coefficient of friction is "constant" for a given materials pairing. In the case of railroads we are dealing with concentrated loads and things aren't so "constant". A round wheel on a rail starts out a a point load and progresses to a line load which widens as the load increases flattening the wheel and indenting the rail. Iron is very stiff but at these loads it is elastic enough to deflect. Short of cold welding, the high points of one embed in the other and give a higher than calculated coefficient of friction and thus higher traction, and then they slip and spin.

Loads in real life are high enough that you'll sometimes see flaking of the track. IIRC it's Von Mesis stress exceeding the strength of the rail below the surface. Guess this was a whole lot of rambling to say you can go too far in your search for traction.
K. Browers
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Re: Scale weight

Post by K. Browers »

Hello Mr. Ron,
I think you need to consider the scaling factor in another way the 50 ton locomotive is not divided by the scale factor but the scale cube factor. Imagine you have a 1 inch cube of material and scale that cube to 1/16 scale - 3/4 inch to the inch. Your scale cube would be 1/16 of an inch to a side so you see it will weigh much less than the assumed 1/16 of the original weight so taking the weight of the 50 ton locomotive and dividing by the scale factor of 16, 3 times you will get your theoretical locomotive weight( 100,000/16/16/16 ) of 24.41 lbs.. Now the actual model will weigh more than this exact scale parts would not have the strength needed for a working model. It could be assumed the parts especially the boiler would be twice or more scale thickness. this would make the model weigh about 50 to 70 lbs well within the range of 3/4 scale models of that size. I hope this helps
Cheers
Karel
Mr Ron
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Re: Scale weight

Post by Mr Ron »

K. Brouwers wrote: Thu May 31, 2018 3:32 am Hello Mr. Ron,
I think you need to consider the scaling factor in another way the 50 ton locomotive is not divided by the scale factor but the scale cube factor. Imagine you have a 1 inch cube of material and scale that cube to 1/16 scale - 3/4 inch to the inch. Your scale cube would be 1/16 of an inch to a side so you see it will weigh much less than the assumed 1/16 of the original weight so taking the weight of the 50 ton locomotive and dividing by the scale factor of 16, 3 times you will get your theoretical locomotive weight( 100,000/16/16/16 ) of 24.41 lbs.. Now the actual model will weigh more than this exact scale parts would not have the strength needed for a working model. It could be assumed the parts especially the boiler would be twice or more scale thickness. this would make the model weigh about 50 to 70 lbs well within the range of 3/4 scale models of that size. I hope this helps
Cheers
Karel
Thank you. You have set me straight. Learned something new today.
Mr.Ron from South Mississippi
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