A Brief Diversion – The Archimedes Screw

Thu, 10 Mar 2011

This morning I had an Google chat message (I'm usually around on [email protected] if you want to call) from subscriber Jay asking if I had any thoughts about how his students could approach the Archimedes Screw. It so happened that I had had a look at this when I was making the worm gear so had a few pictures and prototypes. Always the easily distracted type (Ooh look! There's and interesting idea for a mechanism) I dug out the photos and prototypes and had a quick reprise. 

The Archimedes screw was originally used as a way of raising water for example in irrigation. At the centre of the screw is a spiral like the one above – an extension of my worm gear

This is wrapped in a sheath, the sheath is fixed into place, there is no need for the screw to turn inside it, the whole thing rotates as one fixed unit.

Then it is just a case of dropping one end into liquid, or in this case salt, and turning it. the liquid is trapped within the screw and moved up the tube as it turns, emerging from the top in an intermittent stream.

I've made a new one today and fitted it into a box with an bearing at the bottom. Now with the box filled with salt, the screw works really rather well! I'm going to try the same but with a steeper pitch to th thread and a slightly wider outer tube o see if I can get it to lift more salt with each turn. I think this will make a nice kit!

Comments (6)

  • William Hornbaker March 11, 2011 at 2:13 am

    Unless I am badly mistaken

    Unless I am badly mistaken the optimum angle for pumping the maximum amount of fluid (salt) is at   45 degrees with the horizontal. Low angles pump little and angles greater than 45 spill across the center shaft to be recycled and wasting driving power that turns the screw.

    With regard to the screw itself I am of the opinion that the pitch will be optimum at 45 degrees.

  • najmanjay March 11, 2011 at 4:12 am

    I happen to have come across

    I happen to have come across an article that just happens to address this issue of optimal pitch.

    http://www.scribd.com/doc/17955689/Screw-Optimal-Design

    I don't really understand all the math, but it seems that the optimal values are a fairly complex function a relationship among the following factors: the pitch of the screw, the number of blades, the pitch of blades, and the inner and outer diameters of the cylinder.

    Also, although I can't be sure, it seems that an optimal pitch for the screw would tend to be closer to 30deg. 

    Of course, none of this has very much to do with Rob's awesomeness for dredging up and posting his model, but it's fun, nevertheless.

  • trishjoyce March 11, 2011 at 3:34 pm

    mmmmmmmmmm think I need to

    mmmmmmmmmm think I need to stay in the library longer..

    • robives March 11, 2011 at 7:55 pm

      Oh wow! Science is a

      Oh wow! Science is a wonderous thing! I'd missed out on the double blade thing on my original model, now working on a two bladed screw with 45 degree pitch. 

      • najmanjay March 13, 2011 at 4:40 pm

        when you say 45deg, do you

        when you say 45deg, do you mean angle of the blades on the screw, or angle of the screw mechanism, as hypotenuse?

        • robives March 15, 2011 at 5:00 pm

          @najmanjay: The angle of the

          @najmanjay: The angle of the blade on the screw.

Comments are closed.