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  • Need help with soil sifter

    Discussion in 'The main mechanical design forum' started by Medic7594, May 31, 2013.

    1. Medic7594

      Medic7594 Member

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      Thanks Chris,

      You have been very helpful in making this come to furition.

      After I posted my message, I was tinkering with the online calculators and the pullies I have. If I understand your post correctly, increasing the speed will be of better benefit than changing the stroke.

      Here is my plan.

      I plan to replace the 9" pulley on the long axle with a 5" pulley and keep everything else in place, This will bump my RPM's from 77 to 140. Hopefully this will increase the G's enough to give me the desired effect.

      What do you think? I have everything pretty tight and well supported. 2G's should not impact it much.
       
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    3. Medic7594

      Medic7594 Member

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      Tonight I replaced the 9" pulley with a 5" pulley. This should increase me to 140 RPM's. the machine works but it needs a little more kick. Perhaps I need to move the connecting rod to the rim?

      take a look and let me know what you think.

      [​IMG][/URL] image by medic7594, on Flickr[/IMG]

      Here is a video of it in motion.

      [video]http://www.flickr.com/photos/[email protected]/9012126553/[/video]
       
      Last edited: Jun 11, 2013
    4. ChrisW

      ChrisW Well-Known Member

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      Nearly there now! Good action and doesn't look too distressed, moving the linkage towards the rim will give it a bit more action as you suggest. You're now running 1.3g or thereabouts. The full 10" will raise to 2.6g.
      By way of comparison, when you were running at 375rpm the peak acceleration would have been round 10g. Far higher than commercial screens run.
      Only problem now is my wife wants one for her veg garden.
       
    5. Ligier

      Ligier New Member

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      I think the problem of using a crank the way you are is that, starting at the Max. back stroke, the crank motion has stopped. The crank gradually accelerates until the crank wheel moves through 90 degrees. In the next 90 degrees the crank acceleration reduces until at 180 degrees, the crank has again stopped and then everything reverses. This produces a relatively smooth acceleration/deceleration which I do not think is the motion you need in a soil sifter. I would have thought you needed a good shake to sift the soil. This would be achieved with a high speed crank but would put a lot of strain on the mechanism.
      Two methods of shaking come to mind. One can be very easily adapted from what you already have. The other involves quite a few rotating/sliding actions but I can describe it if you ask me to. I tried to send you a picture of the first idea but was unable to add it to this supply. If there is a way of getting in touch with you, external of this reply system then I can e-mail you the idea.
       
    6. Medic7594

      Medic7594 Member

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      Chris,

      I'll extend the connecting rod when I get home. As for your wife... build her one. It should be relatively easy for you. You have seen my kinks and now know what not to do. LOL
       
    7. Medic7594

      Medic7594 Member

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      Ligier,
      I agree, a good shake is needed for a good sifting. Not being an engineer, with enough speed and torque, I figured this would be the best alternative. When I was experimenting, I would roll the sifter vigorously back and forth and think of how I could reproduce the same motion with too much difficulty. hence the current design.

      However, I am always up for learning a new method to do something. What are your suggestions?
       
    8. ChrisW

      ChrisW Well-Known Member

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      Best of luck Medic. Don't think my wife will be getting one though, I've been designing screening and crushing plant for 15 years and giving the odd word of advice is as near as I want to bringing the job home lol.

      Do remember though, vigorous = high "g" and will destroy a timber structure. Changing travel will proportionally change acceleration, changing speed will give a squared response i.e. doubling the speed gives a 4x increase in acceleration. If you have numbers in mind I will be happy to calculate the result for you.
       
    9. Ligier

      Ligier New Member

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      My suggestion is to tether the top box to the bottom box with a bungie cord to each side which will pull the top box forward. Maybe put a series of holes along each side of the top box so you can adjust the tension if required (Big load/small load)
      Remove the crank arm and where you have attached your crank to the top box, replace it with a spacer with a wheel on the end. Say 2" dia. wheel on a stick.
      Make a cam, out of wood, possibly external ply, I am finding it difficult to explain the shape of the cam but here goes. Start off by drawing two concentric circles, the smaller one would be slightly larger than the crank wheel, the second one offset by whatever movement you want to move the top tray plus a bit. (I will explain the plus bit later) Draw a tangent line from the inner circle to a point on the outer circle and then draw a line from the intersection point of tangent line to outer circle back to the center of the circles.
      What I am trying to explain is that when the cam is cut out we need to have it mostly round to the inner circle diameter then raise up the tangent line and then fall back to the inner circle. It will make sort of a one tooth saw blade shape. The plus bit I mentioned earlier is that the point of the saw tooth should be rounded. You could of course add more lifts if you need them. you could also add a radius, slightly larger than the wheel radius, where the wheel would drop back to the base circle. This cam would be bolted concentrically to your crank wheel and in line with the wheel on the stick. The wheel spacer (the stick) should make up the distance between the top box and the lower cam diameter but the shorter you can make it the better.
      That's a long winded explanation that a picture would have shown with more clarity but I hope it makes sense.
      The idea is that when the crank wheel is turned slowly, the top box will be pushed back slowly, tensioning the bungies when the wheel drops off the peak of the cam, the bungies will move the top box rapidly hence the shake.
      One other thing comes to mind is that some external buffer arrangement could be provided so that the wheel on a stick does not constantly thump the inner circle of the cam and thus possibly damage the wheel.
       
    10. Medic7594

      Medic7594 Member

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      Hey guys,


      I tinkered some more and think I have it 99% complete.


      As discussed, I moved the connecting rod to the rim of the 10" pulley. When I ran the sifter it was very jerky and the sifter tray was jumping quite vigorously. I noticed the axle was loose. I took a rasp and opened up the washers which were holding the bearings in place. This helped the seating of be axle. I then moved the connecting rod to the mid spoke position on the 10" pulley.


      Overall it works. The sifter tray still jumps a little and the sifting action seems to be better.


      The video below shows how it turned out. The sifting does slow down when you put a lot of dirt in at once. For my purposes, it isn't a big deal.


      What do you guys think?p


      [video]http://www.flickr.com/photos/[email protected]/9021868700/[/video]
       
    11. ChrisW

      ChrisW Well-Known Member

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      Looks good to me, the sieve has to move faster than the mass of soil can accelerate and cause a shear in the lower levels. You have just got there within the limits of a wooden structure so well done.

      Ligier, a very ingenious idea and not unlike some percussion hammer mechanisms. However, giving it a shock in one direction only will drive the fill to one end where it will be unable to shear. You may find it practical if you incline the screen so as to try to drive the soil uphill with the shock but I don't know how easy getting the right angle would be. If you give it a try let us know how it works.

      The best way overall to screen soil is with a trommel or drum screen ( a recent post discussed this) but it is way beyond the scope of a competent handyman or joiner as it requires metalwork and fabrication, as well as the transmission.
       

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