# Screw Conveying: Illusion or Not?

**URL:** <https://boards.straightdope.com/t/screw-conveying-illusion-or-not/538702>\
**Category:** Factual Questions\
**Created:** [May 8, 2010, 12:08am UTC](https://boards.straightdope.com/t/screw-conveying-illusion-or-not/538702 "2010-05-08T00:08:36Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\
**Post date:** [May 8, 2010, 12:08am UTC](https://boards.straightdope.com/t/screw-conveying-illusion-or-not/538702/1 "2010-05-08T00:08:36Z")

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This has bothered me for a long time. Like rotating stripes on a barber pole, the stripes appear to advance along the length of the pole. But, we know this is an illusion. And yet, rotating screws are used to advance materials along. How can this be when the screw is not really advancing?

Is it perhaps not the screw, but the cavity itself that IS advancing??? I guess this should be obvious, but knowing the advancement of the screw is really an illusion is really throwing me here.

Can the SD perhaps explain how they understand this? 😕

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**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\
**Post date:** [May 8, 2010, 12:17am UTC](https://boards.straightdope.com/t/screw-conveying-illusion-or-not/538702/2 "2010-05-08T00:17:22Z")

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[Archimedes Screw](http://en.wikipedia.org/wiki/Archimedes%27_screw). The animated graphic at the top left of the web page is very helpful.

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**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\
**Post date:** [May 8, 2010, 12:57am UTC](https://boards.straightdope.com/t/screw-conveying-illusion-or-not/538702/3 "2010-05-08T00:57:55Z")

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I think it’s clear the cavities are advancing.

If by “stripes” you mean the portions of paint helices that you can see, those ARE advancing.

Water in the ocean isn’t rolling toward the shore, but waves are. We can see things move that aren’t themselves a unique body of matter, but rather a passing tendency through the matter. Which is a reasonable thing to do - you have to plan for when a wave goes by you if you want to jump over it, whether the water is moving en masse or not.