I hope last week’s introduction of Bulk Material Handling helped you think about the amazing hack, and we’ll be reporting on a lot of DIY Cap’n Crunch Robots soon. This week we will look at how to measure particle size, separate particles and even crush them when you need them.
Measurement of material properties
Last week we talked about integrated energy. The bulk material behaves somewhere between a solid and a liquid – if you have finished your homework, it flows down the funnel. But if you don’t have it, it sticks together and holds the rest of the ingredients. Combined strength is a measure of how much weight the component below the funnel can hold.
You can get a rough measure by packing the material in a box with a square hole at the bottom. There should be a retractable slide on one side of the hole. Slowly withdraw the slide, making the hole rectangular. There will be bridges over the material, and then at some point a large chunk will fall out. It is about the minimum opening size that will not arch, and a practical measure of the composite strength of the material.
Many ingredients combine well when wet. To determine the drying of a sample in a microwave Percent moisture Weighs before and after.
Integrated energy is closely linked Shear power. If you want to measure shear strength, cut two 1 cm wide rings of 5 cm diameter PVC pipe, stack them, pack with material, place a disc on top of the material and load it, then pull the top ring down with a spring scale. . . The force per unit area is the shear force at that pressure. You will see it in the curve when it starts packing.
Packing factor Another useful measure. Gently shake the material and weigh it to fill in a rigid container. Now empty the container and refill, pack the material as tightly as possible with a length of 1 ”dowel. Re-weigh, and the ratio of the two weights tells you how well the material packs.
Actual bulk material is almost always composed of particles of different shapes, sizes and compositions. Dirt is a particle of a variety of minerals and organic matter that varies directly from rock to sub micron soil particles. If you have problems, get one Material size distribution graph Can be helpful.
For particles above about 75 M, you can measure the size with a sieve. If you want to be fancy, they sell a nice set of metal sieves with wire mesh at the bottom. Screen storage is cheap. Below 75 μM, you need to use a hydrometer. It is messy and takes some time, but works.
The idea is to mix the ingredients with soapy water and then use a hydrometer from an auto parts store to measure the concentration. Particles fall by Stokes law, big ones first. Stokes law is the proportion of the square of the radius of the force drawn on a sphere. The mass will go up as a cube of radius, so the larger particles fall faster than the smaller ones. As these fall, the density of the fluid decreases. This page describes how to do this and has a simple calculator to explain the results.
You can change the particle size in your mixture. If the particles are too large, they may be crushed or ground. You can sort by size and grind only some shapes or discard some ingredients. Grinding is a complete science. The more you grind, the harder it will be. Cosmetic and pharmaceutical companies specialize in grinding.
In general, there are three ways to make something smaller – crush it, cut it or hit it.
Crush straight. Use a roller or jaw, a rolling pin or a rock crusher. Do not ignore the vision. A jaw crusher only crushes particles larger than the jaw space, suitable for creating a certain size. Rock crushers have a complex motion (video) that should still be easily imitated by a hacker project. Amateur / Hobby Gold Prospectus has an accessible community.
The crushing action on the roller only works until the particle is small enough that the surface of the roller is deformed instead of the particle. For most of history, stone has been used to grind grain into flour.
Cutting is best for soft things and hard things like sticky worms (video.). Make sure the cut material has an easy way out. Think of an old fashioned kitchen meat grinder. .
If you want smaller particles, you need an impact grinder. A coffee mill or blender works by hitting the particles with a fast moving impeller. It can be a blade – useful if the ingredient needs to be cut first, like a coffee mill – or blunt. Many industrial mills use two pivoting weights in a shaft and this unit uses only chain (video).
Another impact mill is the ball mill. Rotate a drum with steel balls and components. The balls travel sideways upwards, then hit the material, falling to the bottom.
All this works by breaking down the material. If you try to powder something that doesn’t fracture, say rubber and rings? For that, there is Cryogenic grinding.
Many rubber components are actually glasses – components that are a glossy liquid at high temperatures, often brittle at cold temperatures, and soft in between. The glass you are thinking of is probably a brittle, brittle material at room temperature, but at high temperatures it is a liquid. The transition point is the ‘glass transition temperature’.
So what about our O-ring? If they are natural rubber, their conversion point is about -70 ° C. At temperatures below they are brittle and can fill the soil. Unfortunately, grinding is going to bring back the heat So consider grinding slowly – some labs grind biological material like skin samples with a special mortar and pre-cool with liquid nitrogen. Just make sure everything in contact with the material has cooled, and use a thick-walled pot with plenty of heat mass.
Separation From husk to wheat
Sometimes you have a mixture and need to separate it. Your roommate throws all the sticky bears and those weird ginger candies in a bowl or something. Last week we introduced particle segregation as a bad thing. But when you want to un-mix a mixture, it can be a good thing. Any strategy from last week can be helpful.
Filters and screens work to separate by size. They get stuck unless the material continues to move on them. An easy way to do this is to flow the material over the sieve on a diagonal board, not the best sieve at first. Another is to vibrate the screen mechanically. Paper filters are just fine screens, and don’t get stuck.
A Drums A slowly curved cylinder with screen walls of different sizes along its length. The material is fed to the edge of the fine screen and slowly moves to the other side.
The Stokes law we saw above provides another way to separate materials. Draw an upward wind on a vertical pipe. The pipe part distributes the material in the way up. The material that is pulled more than the weight will increase, the larger material will go down. You can use wind speed to control particle size. An industrial machine called an air classifier does this with high-velocity air-flowing material on the rim of a spinning set of blades.
It could be the air (or other gas) that you want to remove There are several ways to do this. The first is a cyclone familiar to the wood shop. The second is simpler – insert a mixture of air / matter with a tube on top of a long, thin vessel that extends halfway up the vessel. Allow air to escape from an outlet pipe on the roof. The air flow expands, slows down and the material falls off.
You can just blow the ingredients aside – this is how the old method of threshing wheat works. Wheat comes from the plant with a husk, you beat it with a flail to loosen the husk, providing you a mixture of wheat granules and bran. Place the mixture on a blanket and toss it repeatedly with four farmers. Husk flies in the wind.
A highly sensitive separation technique is inert separation. Here is a mixture of sticky cola and jelly beans. We separate them by tilting and gently shaking the sheets. An element moves to a sheet in one place until the acceleration exceeds some critical value. Then roll or slide it.
If your material is dirty or such, run a magnet through it. Lots of soil contains iron ore and man-made iron bits. It can get motor and such. Run the material past magnet if you need it. An AD current separator uses AC magnetic effect to do the same thing with non-ferrous metals.
You can separate the materials by dissolving them. A mixture of table salt and white sand may seem impossible, but if you shake it in water, clear and boil the water, salt and sand can be recovered separately. But we think we are entering chemistry now and we should stop.
Next time we will end by looking at the movement controls: building gates and contraptions that move your bulk elements without getting stuck.