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 Post subject: Re: Fundraiser For Severely Burned Cass Employee
PostPosted: Tue Apr 17, 2018 8:57 am 

Joined: Tue Sep 14, 2004 7:52 am
Posts: 2561
Location: Strasburg, PA
Overmod wrote:
Given the known issues with separation of the threads from the body of the plug, why does the thread on the example pictured have so many apparent stress raisers 'machined' into the root, rather than a reasonably smooth radius from the body up to the engaging portion of the square threading? Seems to me there might even be advantage to roll this radius after machining to form it up against lateral bending stress when installed.

I'm not sure I understand your question. There is one stress riser, in a spiral shape around the plug.

The root diameter of the female square thread is .020 larger than the root diameter of the plug, so a radius of .010 is all that the design will allow.

The bottom line is that someone at Huron made a bonehead move in designing the plugs with square threads instead of Acme or buttress threads. Square threads were obsolete in the 1890's for being weak and hard to manufacture.


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 Post subject: Re: Fundraiser For Severely Burned Cass Employee
PostPosted: Tue Apr 17, 2018 11:16 am 

Joined: Thu May 24, 2012 1:37 pm
Posts: 2213
Quote:
"I'm not sure I understand your question. There is one stress riser, in a spiral shape around the plug."


I was not so much addressing the geometry as the visible construction.

It appeared to me that there were a great many small dark imperfections at the root of the square thread, almost like chatter marks or casting voids, and it was those small points that I was referring to as potential stress raisers.

I have not calculated the loss of engaging-face area between the square contact faces of plug and bushing that would result from 'rolling' the root of the thread in the plug to a sensible radius (and relieving the nose of the bushing threads to clear). But I can't imagine it would be more than a few percent, and of course at the distal edge of where steam under pressure comes in contact with the sealed plug assembly. Surely this is a reasonable design change to consider?

I agree with all the other points made.

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