Re: Weighing Freight Car Models with Liquid Gravity


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 non-homogeneous. 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
Sent: Tuesday, October 30, 2018 9:45 PM
To: main@RealSTMFC.groups.io
Subject: Re: [RealSTMFC] Weighing Freight Car Models with Liquid Gravity

 

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 center-sill space with a 1 x 1 cm cross-section, and you carefully filled it with 1-cm 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 (74-26).  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

Join main@RealSTMFC.groups.io to automatically receive all group messages.