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  • Soil Screenig mechanics

    Discussion in 'The main mechanical design forum' started by ralphey, May 25, 2013.

    1. ralphey

      ralphey Member

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      HI:

      First off I am not an angineer just a farmer that likes to tinker so excuse this very basic question . I want to make a soil screen. I have the frame set - basically just a rectangle with screen set on springs at an angle.

      Heres my question : I am going to use an 8 hp engine and I want the screen to vibrate I know I can use a cam or some use like a crankshaft arm ( I guess its the very basic design of changing rotary to linear motion) but I need the specifics of exactly how one would construct that ( ie how to hook this into the table ) I know that I would use a belt or chain drive form the engine up to the table.

      At the table I would have a shaft with a sprocket (driven from the motor) held in place by two stable pillow bearings -

      at the end of the shaft then somehow I need to connect into the table to make it vibrate

      What would be the mecshinasm that cause to vibration - how do I make that any? pics references etc woudl be great, aklso I need where to buy supplies I use surplus center but they dont always have everything I need (ie the springs etc) .

      Many THanks (from myself and all our area garden people) to anyone who can help us with this

      Ralph
       
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    3. thebestjake

      thebestjake Member

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      You are asking a very broad question that is difficult to answer without a lot more specific information.

      I would suggest you start drawing this out on paper and calculating as much as you know. From there when you get to specific questions on the calculations then that would be a good post for this forum.

      McMaster-Carr is the best source for general mechanical supplies like this in the US.
       
    4. Michael Ross

      Michael Ross Well-Known Member

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      I saw a shake tester that was simply a motor with an eccentric collar clamped on the end of the shaft. The thing being shaken was pulled against a hard stop by an extension spring (like a screen door spring only somewhat larger). The eccentric collar would come around and push the shaken thing off the stop then move away and the thing would strike the hard stop driven by the spring. Viola! Vibration, and very sharp impact vibration.

      This is easy on the motor if the radial forces that shaft are not too high (pick your spring right and don't stretch it too far). The hard stop might be something like a block or ultra high molecular weight polyethylene (UHMW PE). Nothing to be worried about there all things can be found at McMaster Carr. The UHMW is very frictionless and durable. It can survive in a dirty environment, because the specs get pounded into it and stop having an effect. But you want to keep the motor and shaker mech pretty clean by enclosing it. A totally enclosed fan cooled (TEFC) motor is a good choice, but drip proof might be good enough. (You could use an internal combustion engine if that is handy.) It might be worthwhile to have a hardened collar.

      There are usually country machine shops around that are a great resource for this sort of thing. Also, good people to try are experienced welders (often the dirty back yard type are great), factory mechanics and maintenance guys and gals. If you don't already know someone like this you might contact a bearing supplier in your area - they will know who is who. You want some dirty fingernails people in on this.
       
    5. ralphey

      ralphey Member

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      great idea!

      Here's a real nice system - it answers a lot of my questions - it clearly shows how to make the counterweight and hook it into the system attach it to the table etc etc


      bnpMqiYMzMU

      I see what looks like a homemade double loop coupling and that is being used to prevent the transmission of vibration into the system

      any ideas on how to make that?

      Ralph
       
    6. Michael Ross

      Michael Ross Well-Known Member

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      The scale you are interested in is for coarser soil than I was thinking. A whacker I mentioned would be good for grading sand and powders.

      That coupling: Cut up an old tire and get some nuts, bolts and washers. You can buy them off the shelf like that, too.

      I would want to see that shaker actually sifting my kind of soil before putting welder to steel.

      If you have a land grant university handy, their extension service could be useful. There are several departments that might have good ideas: civil engineering, agricultural engineering, landscape architecture, horticulture, turf science (yes there is such a thing)

      But you hit on a great resource with YouTube/Google

      I like the rotary solution better than a shaker:

      le-Nmg0q9jE
       
    7. ralphey

      ralphey Member

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      trommel

      Yea i looked at making a trommel but thought it would be harder (although some day i want to go gold mining) . I have some great plans i found for making a steel bender to make the loops for a trommel but then I would have to make another tool . ANother interesting idea was a table that hung from pto shafts which was them by a off center weight moved in a vibratory system on a horizontal rather than a vertical axis. Kool I wish I were able to start over and be a mechanical engineer !
       
    8. Michael Ross

      Michael Ross Well-Known Member

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      You can turn your motor so it points up and get that horizontal motion. That's potentially more efficient. Not lifting the soil just scooting it around.

      You could also funnel it down through an air knife over the screen (if you already have a big-assed air compressor that is).

      Nothing better than working with the stuff that is laying around.

      Here's a nice quote you will like:

      To invent, you need a good imagination and a pile of junk.
      Thomas A. Edison
       
    9. ChrisW

      ChrisW Well-Known Member

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      Ralph, what you need to achieve is a speed of 600-1200rpm with a throw (displacement peak to peak) of 4mm - 10mm for efficient screening.
      The example shows the principle very well but it is running very slowly, as a result the vibration isolation is poor as the resonant frequency will be close to the running speed.
      You don't need a linear motion, the shaft with an eccentric weight creating a circular motion will work fine, drive it with a very flexible coupling like the example, a drive shaft or long slack vee-belts. The coupling shown can be found commercially or if you want to diy, two flanges connected by strips of heavy conveyor belt bolted to them will do or sometimes loops of steel cable is used as the driving element.
      If you get a good motion it won't need an inclined deck, the material will flow along a level deck but the acceleration of the deck must be well above 1g otherwise the material won't leave the deck. Commercially 3-4g is normal. A horizontal action may work as a small sieve but screening action will be poor as the material will stratify and only the lower layer will be rolled around on the media.
      One suggestion worth considering, vibrating screens are not necessarily the best for soil, consider finding a concrete mixer and cutting a pattern of holes in the drum to create a trommel screen, which will work better with soil.
       
    10. ralphey

      ralphey Member

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      great info

      Hi Chris that is great info - how would one know what g forces are being applied? Is a trommel better snce it has more ability to basically lift and drop fragments causing them to break apart ?

      [
       
    11. ChrisW

      ChrisW Well-Known Member

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      Soil can be difficult to screen as it is fibrous and likes to pack into the screening media and block the holes. Sometimes it is screened with a tine or finger screen. These have transverse bars carrying tines set apart at the size you want to screen, anything clinging to the tines then slides down to the end and down several rows of tines until it discharges.
      The big advantage of a trommel is that it has a tumbling action which breaks up clumps and also is self-cleaning. Wear isn't a great issue with soil so you could make the drum out of anything cylindrical. A cheap old concrete mixer would be simple by perforating the drum with a plasma cutter, if you need a smaller mesh just bolt a fine mesh panel inside.
      Even a 40 gallon oil drum would probably last some while, support it on 4 small wheels to allow it to rotate, partly cut the bottom out to feed through then drive one of the wheels. The tilt angle only needs to be a few degrees which will give the feed time to work it's way along.
      You can work out basic values for a vibrating screen using simple laws of motion but you will get much better results from a simple trommel. I don't know what prices are like in your neck of the woods, but a used portable cement mixer can be found fo under £100 in UK, only needs a few hours cutting holes and you have the finished item.
       
      Last edited: May 29, 2013

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