Olds Elevator vs Traditional Archimedes Screw

Summary
How is this device better than a rotating screw when used for powders and granular stuff?
This device...


... is purportedly superior to the more traditional Archimedes lift where the screw is rotated and the tube is stationary.

The video itself does not explain the comparative physics very thoroughly. From the video itself and from the comments section, three broad types of views emerge :

[1] This device isn't fundamentally different from a traditional rotating screw type, but it's easier to support and drive an outer tube rather than an enclosed screw, and also to keep the material out of the bearings.

[2] Some viewers say there is a basic improvement in the way the input energy is used to drive the material up the tube, and that one can expect significant performance advantages based on the physics going on inside. (I.e., not merely to do with external engineering aspects).

[3] Another point made is that the advantage lies in not propelling air up the tube, and in not crushing the granules -- they seem to believe that this version can tolerate a bigger gap between the screw and the tube, compared with the rotating screw version.

I haven't been able to make up my mind, and haven't been able to fully understand some of the comments from viewers. Maybe someone on this forum can explain whether, and if so, how there is an internal performance improvement from driving the outer tube -- apart from practical engineering/implementation benefits.
 
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By the way, is it possible to include a video link without creating a video box within the post?
 

Tom.G

Science Advisor
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Interesting device. The one drawback I see is for tall lifts there will be a large compressive force on the material entering the bottom of the elevator. It has to push up ALL of the material that is in the elevator... minus any friction drive from the inner surface of the rotating tube.

By the way, is it possible to include a video link without creating a video box within the post?
I haven't tried it with video links, but with other links just remove the initial "HTTPS://".

youtu.be/-fu03F-Iah8

Well I just tried it and it does work with youtu.be
The reader can then copy/paste the 'bad' link into the browser address field and the browser automagically prepends "HTTP://".

Cheers,
Tom
 

jrmichler

Science Advisor
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From the point of view of a teeny tiny observer standing on one of the granules being conveyed, is there a difference between driving the screw and driving the tube?
 
From the point of view of a teeny tiny observer standing on one of the granules being conveyed, is there a difference between driving the screw and driving the tube?
Not really, I guess.

I'm now pretty much convinced that the only considerations that might make this version better would be the external engineering ones like supporting, driving and keeping contamination out of the bearings.
 
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I see two main differences.

One is, that the granular stuff is accelerated by the friction to the wall and slowed by the friction to the screw: the traditional way is the reverse. There might be some difference due this.

The other difference is that there are two small fins at the bottom what makes a pressure upward and feeds the tube, providing a solid base for the lift. This is not common when it is the screw what's moving: that case the feed usually based on gravity alone.

Those fins might also mean a difference in safety.
 
.. the granular stuff is accelerated by the friction to the wall and slowed by the friction to the screw
Thinking about your point.. yes, it makes sense that friction off the wall is probably the dominant driving force for an element of material that is already at the middle of the tube. (I think those scoops at the bottom can only help get the incoming stuff started upwards, but they can't possibly be supporting / driving all the stuff that's already in the tube).

On the other hand, having said that, however, nevertheless.... jrmichler's point of view sounds equally convincing -- how would an element of material at the middle of the tube "know" whether it's the screw that's moving or the wall?

I think in order to understand both points of view we will need to think through the complete force balance for an element of material, and show how it works in both cases (rotating screw versus rotating tube). Since jrmichler's comment is quite convincing, it follows that the force balance for both scenarios would have to be equivalent. The following is possibly a step towards showing that they are equivalent.

Consider a simpler thought experiment where we straighten out the screw and make it a simple ramp / inclined plane. On either side of the ramp are two parallel walls set very close to the ramp. A block of sponge or foam is resting on the incline, with its sides lightly gripped between the walls. If we move the ramp horizontally, the sponge will quite possibly ride up the ramp. (If the relative magnitude of the various friction forces is favorable). But if we keep the ramp fixed and slide the walls the other direction, it's also reasonable that the sponge would still slide up the ramp.
 
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I believe that Rive's Item #2 (fins) is the source of the major performance difference between the 2 conveyors. They allow material to be 'forced' into the conveyor tube, rather than relying on gravity and the 'waiting' material to fill the inlet. The fins may also be 'tuned' for specific material properties / lift geometries.
 

Baluncore

Science Advisor
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First examine the central auger. It has a fixed vertical pitch per turn, call that, P. The flights of the auger are attached to a central tube, call that the inner diameter, Di. The auger is enclosed by an outer tube, call that the outer diameter, Do.

The flights of the auger have a radius dependent slope, S.
Against the outer tube; slope, So = P / (Do * Pi ). Angle = Atan( So ).
Against the inner tube; slope, Si = P / (Di * Pi ); Angle = Atan( Si ).

Notice that as Di approaches zero the slope becomes infinite. Particles that are lifted by the outside of the auger flights can fall vertically back down near the centre. If the central tube Di is made too small the device becomes an auger mixer rather than an auger lift.

Next examine the material being moved. Several parameters are important here.
1. Friction coefficient between a particle and the flight of the auger. If this is too low the particles will slide down the flight and not be lifted.
2. Friction coefficient between a particle and the outer tube. A particle is rolled upwards when it is in contact with the wall and the flight. If wall friction is too low, it will not be lifted.
3. Friction coefficient between two adjacent particles. When sufficient friction is present, a driven particle transfers lift to other particles it contacts.
4. Shape of the particles. Spherical particles like marbles or dried peas will roll down the flight. Flat particles will not roll.
5. Variation in particles size and shape. Notice how the bird seed selected for the demonstrations contains a mix of flat seed that will not roll, to act as chocks for the more spherical seed that would otherwise not be lifted.
To transport spheres you need to add something flatter and smaller that can be removed with a mesh later, to fall back down and be recycled.

So what determines the maximum slope of an auger flight operated with a vertical axis? It is the friction coefficient, Cf, between the particle and the flight. Simply put, when the slope, Si = P / (Di * Pi ) is greater than the Cf, the particle will slide back down faster than it is lifted. You need a gentle slope.

A horizontal axis screw auger will move material horizontally if the screw is turned inside the outer fixed tube. That is not the case when the screw is fixed and the outer tube is turned, unless the auger is more than half filled so particles can fall over the central tube.

A screw auger will not lift vertically unless it is fed in such a way that the particles press against the inside of the outer tube. That explains why the feeder wings are fitted to the bottom of the rotating tube.
To maintain that particle pressure against the inside wall of an outer rotating tube while vertical, requires that it be operated with sufficient material to cover the surface of the flight to some depth. But then it can never completely empty the bottom hopper and the auger.
 

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