Posts: 386
From:McHenry, IL USA
Registered: Nov 2002

posted December 11, 2002 06:50 AM               

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I've recently seen a few photo's of single screws that have been subjected to so much heat they had turned a violet-blue, when cool. I can only guess that this destroyed the chrome finish as well. Can anyone even guess what temperatures they were exposed to - to cause this color change?

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Posts: 737
From:Brodheadsville, PA USA
Registered: Jun 2001

posted December 11, 2002 09:45 AM               

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Simulations of screws which have exhibited the same problem show very high pressures in the areas where they turn blue. On the order of 8-10,000 PSI. The simulations do not indicate high temperatures at the screw surface. Also the materials involved don't show signs of degradation. Some of the evidence indicates high temperatures, some does not. I'm not sure what is going on. Do the screws have any flight wear? I would think screw deflection is likely at these pressures. I've seen screws turn blue while gauling between the flight tip and barrel was happening.

Tom C

[This message has been edited by Tom C (edited December 11, 2002).]

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Posts: 386
From:McHenry, IL USA
Registered: Nov 2002

posted December 11, 2002 02:08 PM               

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Tom C: The pictures I've seen do not have dimentions attached, nor do the owners know what the original dimentions were.

The violet-blue starts just after the feed section - the transition sectio, barrier, and metering & tip are violet blue.

They have not yet inspected the ID of the barrel.

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Posts: 386
From:Christchurch,New Zealand
Registered: Jun 2001

posted December 11, 2002 11:04 PM               

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Whats happening is the feed zone is taking in more than the metering zone can pass- this generates huge increases in pressure and temp in the transition and metering zones- the temps generated cause the blueing, you can expect flight/barrel wear in the blued area.

It occurs when actual output reaches or exceeds the drag flow rate for a screw.

Steve Hodgson

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Posts: 386
From:McHenry, IL USA
Registered: Nov 2002

posted December 12, 2002 06:05 AM               

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Thanks Steve C.: This now makes a lot of sense.

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