Weighing Freight Car Models with Liquid Gravity
Peter Ness
Hi Jim,
All good points for closed cars and hoppers. Most flat cars and some gondolas pose interesting challenges where “more is more” instead of “less is more” J
Peter Ness
From: main@RealSTMFC.groups.io [mailto:main@RealSTMFC.groups.io] On Behalf Of Jim Betz
Dan/all,


Carl Gustafson
On Mon, Oct 29, 2018 at 10:35:54PM +0000, Andy Carlson wrote:
I would guess the mention of Titanium was a mistake, the intended metal perhaps Platinum, which is as heavy (Or heavier) than Tungsten. Platinum, though, is incredably expensive, though its weigh rivals Tungsten and depleted uraniumSo does gold, for that matter. Use that or platinum and you can have "investment grade" cars. Carl Gustafson


Ed
You also might try golf supply stores for the pourable shot they use in golf clubs. Ed Robinson


Peter Weiglin
Given that the smaller the shot, the denser the lead weight, I reasoned that lead powder might offer the greatest density.
And I found that lead powder was indeed available from a golf supply house. Seems they use lead powder to weight golf clubs. Not available or shippable to California, I was told. So we moved to Ohio. (Well, there were other reasons.) Handle with care  but it does fill the nooks and crannies in hopper cars, etc. Peter Weiglin


Dave Parker
Peter Weiglin wrote:
"Given that the smaller the shot, the denser the lead weight," This was also discussed previously and, unless the shot are sufficiently large to preclude close packing in the available space, the weight you can gain does not depend on the diameter of the spheres, only on the density of the metal. See post #139844. Dave Parker Riverside, CA


Tony Thompson
Dave Parker via Groups.Io wrote: This was also discussed previously and, unless the shot are sufficiently large to preclude close packing in the available space, the weight you can gain does not depend on the diameter of the spheres, only on the density of the metal. See post #139844. Or to put it another way, the proportion of space occupied by spheres, even in the closest packing, is independent of the size of the spheres. Of course, if you have a range of sizes of spheres, the little ones pack between the big ones. But if they are all the same size, using smaller ones changes nothing. Tony Thompson Editor, Signature Press, Berkeley, CA 2906 Forest Ave., Berkeley, CA 94705 www.signaturepress.com (510) 5406538; email, tony@... Publishers of books on railroad history


Tim O'Connor
Tony and Dave
There HAS to be a flaw in this logic. Although Tony has expressed that size DOES matter. Let me explain. I think the flaw is to assume perfect packing that yields the magic 26% open space regardless of the size of the spheres. That level of packing would only occur if the dimensions of the space were an INTEGRAL multiple of the diameters of the sphere, and if there were no wasted space above the spheres! Think of the case of a POWDER  essentially very very tiny spheres, packed into a 100x100 format (i.e. their diameters are 1/100 of the dimension of the space) versus large spheres in a 1x1 format. CLEARLY you're going to get more stuff into the space with the powder. In other words, in a 1x1 format, the "empty space" is actually 1.00  4.19/8.00 = .47625 or 47% empty space! The 26% empty space is a BOUNDARY CONDITION of maximally packed spheres. In any case, it is subject to experiment to determine whether this is true, or not. Tim O' Or to put it another way, the proportion of space occupied by spheres, even in the closest packing, is independent of the size of the spheres. Of course, if you have a range of sizes of spheres, the little ones pack between the big ones. But if they are all the same size, using smaller ones changes nothing. Tony Thompson  Tim O'Connor Sterling, Massachusetts


Dave Parker
Tim:
My comments on this have always included the caveat that spheres the have to small enough to achieve something at least close to idealized spherical packing. If you had a centersill space with a 1 x 1 cm crosssection, and you carefully filled it with 1cm diameter spheres, you would indeed have about 52% sphere and 48% void space (just the ratio of the volume of a sphere and a cube of the same size). Obviously, if the spheres are say 0.75 cm in dia, then you can't pack them in there efficiently, and you'll get less weight. But if the spheres were say 1 mm, or even 2, then you are going to get something at least close to idealized packing (7426). Going to 0.1 mm spheres is only going gain you something on the order of one or a few percent (maybe), hardly worth worrying about. Concerning Tony's comment, I agree but only in theory. If you start mixing smaller spheres in with bigger ones, then the little ones can get in the way of the dense packing of the big ones. The gain in density will likely be rather minimal in most cases, and it may even reduce the density. This is a very common observation in soil science  things like dune sands usually have higher (dry) densities than do other soils that have a range of particle sizes. Dave Parker Riverside, CA


Peter Ness
Hi Tony,
“Of course, if you have a range of sizes of spheres, the little ones pack between the big ones. But if they are all the same size, using smaller ones changes nothing.”
I may not be reading this the way you intended, so apologies in advance if that is the case. Not quite correct or true to my understanding and experience:
The first part of the statement is true  smaller particles fill the interstitial space. Assume we are talking about a homogenous material – meaning it’s either all Lead spheres or all Tungsten spheres and for purposes of clarity spheres means a round if not uniformly so (i.e. not a perfect sphere) particle geometry.
So, using a range of particle sizes that includes what we may term shot or powder, because, as you state, the little ones pack between the big ones, will increase the mass (weight) contained inside the identical volumetric space (cavity). To be clear, if one fills the cavity with Lead shot and then adds Lead powder to fill the spaces, the mass (weight) of the loaded cavity will increase.
For the second part of the statement; If one has two identical cavities and fills one volume with Lead shot and the other with Lead powder, the cavity filled with Lead powder will have more mass than the one filled with shot. Why? The smaller the particle, the higher the surface area and among homogenous material, the smaller the interstitial spacing – all to say there is more mass of powder in the cavity than mass of shot in the other cavity.
Back in my day when working with metal powders of varying particle size, the key physical property that applies here was tap density and there was/is an ASTM standard to quantify this material property. Similar to specific gravity, tap density is a measurement of mass per unit volume. The smaller the particle size, the greater the tap density and the closer this value approaches the theoretical density or specific gravity of the material.
So in both cases, using powder to fill the space between shot or using a uniform small particle size powder, will increase the mass – weight of that part. And if the cavity is filled with a 2 micron average diameter particle size powder rather than a 15 micron average diameter particle size powder, the mass of the part will be greater as well.
Again, if I misunderstand your statement, apologies. Now, my head hurts from digging back more than 20 years in memories, so with permission I’ll go back to modeling freight cars.
Peter Ness
From: main@RealSTMFC.groups.io [mailto:main@RealSTMFC.groups.io] On Behalf Of Tony Thompson
Sent: Tuesday, October 30, 2018 8:11 PM To: main@RealSTMFC.groups.io Subject: Re: [RealSTMFC] Weighing Freight Car Models with Liquid Gravity
Dave Parker via Groups.Io wrote:
Or to put it another way, the proportion of space occupied by spheres, even in the closest packing, is independent of the size of the spheres. Of course, if you have a range of sizes of spheres, the little ones pack between the big ones. But if they are all the same size, using smaller ones changes nothing.
Tony Thompson Editor, Signature Press, Berkeley, CA 2906 Forest Ave., Berkeley, CA 94705 www.signaturepress.com (510) 5406538; email, tony@... Publishers of books on railroad history


Tony Thompson
Peter Ness wrote:
If you use powder, with a range of particle sizes and shapes, sure, no argument. Then you would pack little ones between big ones, and oddshaped ones into spaces. But if you use actual spheres, the size CANNOT matter. That's all I said. Tony Thompson Editor, Signature Press, Berkeley, CA 2906 Forest Ave., Berkeley, CA 94705 www.signaturepress.com (510) 5406538; email, tony@... Publishers of books on railroad history


Peter Ness
HI Dave,
“The gain in density will likely be rather minimal in most cases, and it may even reduce the density. This is a very common observation in soil science  things like dune sands usually have higher (dry) densities than do other soils that have a range of particle sizes”
I think the statement you are substantiating here regarding reduced density is true because the material is nonhomogeneous. I am not a dirt guy, but to me I see this saying sand has a different density than potting soil, which I expect may be true even if I chose poor examples.
“If you start mixing smaller spheres in with bigger ones, then the little ones can get in the way of the dense packing of the big ones.”
Not true in general for homogenous materials which behave very much in agreement with physics. The condition you are defining is when the small particle interferes with the direct contact points of the larger particle with adjacent larger particles and the area surrounding this point contact that is less in distance than the diameter of the smaller particle.
Overall, this is a very small percentage of the surface area and, since most commercial particles are within a range of particle sizes and not perfectly uniform, the percentage decreases from theoretical.
This does not apply if the larger and smaller particle sizes are similar (i.e. 5 micron and 2.5 micron) because the range of particle sizes in both powders will probably overlap in the tails of the particle size distribution. Also, if some new high falutin’ technology is used that does create extremely uniform particle size with minimal or no distribution this does not apply as well.
“Going to 0.1 mm spheres is only going gain you something on the order of one or a few percent (maybe), hardly worth worrying about.”
True enough for us in this group, but still a relative statement. Not so true if you are making a space shuttle tile, turbine blade or ceramic gun barrel liner. :D
Peter Ness
From: main@RealSTMFC.groups.io [mailto:main@RealSTMFC.groups.io] On Behalf Of Dave Parker via Groups.Io
Tim:

