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Tragedy of my trajectory calculations for rotary broadcast speaders

  1. Dec 2, 2013 #1
    Hello Everyone,

    First time posting on this forum and looking forward to see what you guys can teach me! I am working on a program for a Raspberry Pi as a controller for a mechanical rotary and need some help from you physics enthusiast out there. Looking for some insight on the calculations that I should consider for calibrating the spread and "swath" of a rotary broadcast spreader. The spreader will have a similar build to that of a lawn broadcast spreader for fertilizers. If your not familiar, a close comparison would be the "Scotts Turf Builder Edge Guard Mini Broadcast Spreader". This spreader will be stationary with heights ranging from 1 - 30 feet off the ground and will have an electric motor with variable speeds.

    From my point of view, I think I will need to account for the weight/dimensions of what is being spread (considering something like wood pellets or granular rocks sock), radius of the chamber the widget falls into, rotation of the spin, angle of the spreader, period of time the widget is in the chamber and spun before release, and of course the speed at which the spreader spins.

    Is there any other major object/formulas I need to account for to achieve a decent calculation (>=85% confidence interval) of what I can expect given the widgets I decide to use and the speed at which the spreader spins?

    I don't have a problem with my numbers considering a vertical/catapult type launch of the widgets, but I'm trying to achieve the spread/single-motor/gravity-feed benefits of the spinning the mechanism horizontally with no more than 20 degrees of vertical play. I'm aiming for distances of 20 ft+ peak and a width of 10 ft min at the half way mark. Also any easier alternative ideas to getting the spread I need would be great as well.

    I have to admit that I still have much to learn about physics so making formulas/concepts amateur proof would be nice.
  2. jcsd
  3. Dec 2, 2013 #2


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    Hello and welcome to PF.
    The problem splits into two parts, basically.
    There is the dynamics of the trajectory of the stuff that's being spread, once it's left the spreader and then there's the question of how to get it to the appropriate speed and direction (i.e. velocity). There are many ways of killing that particular cat.
    Presumably you want a reasonable even coverage (is that what your "85% confidence level is for?). The spreaders in your link are for much smaller areas than you want, it seems to me. They achieve wide coverage by pushing the machine and the spreader is very rudimentary. The coverage of a simple rotary spreader would be very approximate and you may need to think in terms of some 'howitzer' style, ballistic launcher with a controlled / steerable nozzle / chute or you'll end up with a disc shaped coverage. This would not be a trivial exercise. How intricate are you prepared to go in your control system?
  4. Dec 4, 2013 #3

    Thanks for the reply. I am aiming to keep this project simple in terms of mechanics and moving parts.

    Considering how you divided the problem into two parts. I think I have just been over-thinking everything as I was trying to incorporate angular velocity and displacement of the material as they enter the spreader.

    Going back to the basics, I am leaning towards using an impeller type blade and having more control of how the material come into contact with the blade. I could have the material fall directly down from a container that has a bunch of holes in a straight line. I would then only need to focus on the speed of the blade as the point of contact and angle of the blade will be considerably fixed.

    Do you think that will work?
  5. Dec 4, 2013 #4


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    If you want to use a 'blade' stye dispersion then, as you say, dropping the stuff at different parts of the blade might achieve a speed distribution but you would need to make sure that the slow stuff didn't get in the way of the fast stuff. A rough sketch may help to get your idea across.
    Also, if you use an impeller on the stuff you are spreading, it may chop it up(?).
    I was wondering if a blower system would be suitable (a large, low pressure 'air gun'). It would be easy to get the air speed right by choice of nozzle and blower power, over a long enough barrel, the speed of the projectiles could be made what you want. The load could be let into the barrel at a constricted section where the Bernouli effect could give a local low pressure. This would handle the projectiles gently and keep them out of contact with any moving parts. They could fall down from a hopper, into the gun itself, which could rotate or oscillate from side to side like a lawn sprinkler. But that's just an idea I came up with and you may not find it attractive - there are so many possibilities. In the end, you would probably be best to go with an idea of your own and use what materials you have to hand. (I don't actually have to make it work!)
  6. Dec 6, 2013 #5
    Haha I actually came across a cool idea i might be able to use with an air compressor.

    Here is the link

    The only problem is the material I will be using will be like a granular pebble so pressure will be a problem. I def want to try it out though. Ill let you know how it goes.
  7. Dec 7, 2013 #6


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    Dear Father Christmas. If I am good for the rest of the year, can I please have an Airsoft Machine Gun? I have this squirrel that keeps pinching the birds' food. . . . . . . .

    Air propulsion would depend upon the mass of those pebbles. You could, perhaps, consider an alternative system, driven by water from a mains pressure hose. No motor needed, in that system.
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