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Designing Auger Geometry for Materials Handling
  • Augers are a version of Archimedes' Screw, useful for moving materials from water to grain to soil to whatever.
    I notice that materials-handling augers are part of the GVCS. I've been thinking about designing a general-purpose auger system for my own use, and I figured it would be a great thing to have for OSE/GVCS too.
    So I started noodling around, trying to figure out how to design the auger geometry. It turns out that each section of the auger's spiral (a "flight") is a very complex mathematical surface which is pretty hard to model exactly accurately.
    But fortunately for us, I think real-world sheet metal is a bit more forgiving than cold, hard mathematics, and we can fudge the factors to work for us.
    So here's what I've found for designing an auger flight.
    Let's say we need a 10" diameter auger to move soil from ground level up into the hopper of the CEB. The auger will have a 10" inner diameter (ID) and will have a 1.5" steel pipe as the central axis.
    We need to decide on the "pitch" of the auger, or how far the screw moves the material with each revolution. Since we're moving heavy soil, let's keep the pitch relatively short and say 4".
    The auger is essentially helical in shape. A helix is a right triangle wrapped around a cylinder. The horizontal leg of the triangle is the circumference of the finished auger (10" * pi, or 31.416" in our example). The vertical leg of the triangle is the pitch of the desired auger, or 4".
    The hypotenuse or long side of the triangle represents the outside edge of the auger's flight. This is the outer circumference required of the flat-pattern to create the desired auger. (Here's my SW equation: "Section_Circumference"= sqr( "Auger_Circumference" * "Auger_Circumference" + "Auger_Pitch" * "Auger_Pitch" ) which is basically the Pythagorean theorem A^2 + B^2 = C^2, and solves to 32.969"). Dividing that circumference by pi gives the diameter of the pattern circle.
    As you can see, the outer circumference of the flight's pattern is slightly bigger than the auger's final size. That's because as the flight wraps around the central axis and gets "stretched" out into the spiral shape, it will contract in diameter.
    We determine the circumference for the inner diameter the same way, and get 1.968".
    Now all we have to do is lay out the two circles on our sheet metal, and cut one slit from the outer to the inner circle.
    To build the auger, we place one flight on top of the previous and spread the edges with a spacer, and weld up the seam between the two adjacent flights. We keep adding flights, welding the seams as we go, until the required number of flights is reached (desired length / pitch = number of flights).
    The stack of connected flights is then slid onto the central axis shaft, clamped at one end and spread out the desired distance, then tack welded at each end. Once it's spread evenly, it can be tacked in the middle, then the quarters, then the eighths, and so on, until it's firmly tacked in place, then welded up solid.

    I hope I haven't made you go cross-eyed trying to follow my method here.
    I posted this to get some feedback on my proposed design before I post it on the Wiki.
    I have created a tool in SolidWorks for designing the flight for any desired auger. If someone wants to try my method and actually build one of these, I'd love to be involved.
    I look forward to hearing what you think about what I've described.
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  • I think you are right on there Rick. I have done this the trial and error way. It took 2 flights before I got it right. I had the shaft and outer tube already. I marked off the starting and ending points of the helix I wanted and used a string to follow that path. This gave me good starting diameters.

    It's amazing to see it all happen once you get to the stretching part. We put the drive shaft in the middle of the "pan cakes" as a guide and fabricated a plate to weld on to the leading edges to provide eye holes. Then simply used a come-a-long cable winch to do the stretching. Voila! an auger.

    There was lots of hammer and dolly tweaking while keeping the tension on. That seemed to work the best. And it was quite springy too. You had to pull much further than you intended to allow for spring back.


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