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Best way to connect SS screw to aluminium

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pwmac(at)sisna.com
Guest

Posted: Fri Feb 06, 2009 1:11 pm Post subject: Best way to connect SS screw to aluminium

Al screw & Al washer with blue loctite???

 Or a non conductive washer under the screw head. I frequently use mylar. Make your own with a HF hole punch. 

 Paul

 ===========

 At 09:22 AM 2/6/2009, you wrote:

 [quote]I have a Bob Archer SA-006 antenna (BIG "E"). It is made out of 2024-T4 .016". 



 The shield of the cable is supposed to be connected to the aluminium "E" with a 4-40 stainless steel truss head screw. The only contact to the aluminium is the underside head of the screw. There is pheonelic on the other side.



 The antenna will be permanent bonded on the inside of the rudder with access only through major surgery, thus I want to try and make this connection a good as it can be.



 It happens antenna came assembled and the head is right over some painted markings.



 Should I just scotch bright and reassemble?



 Perhaps put a #4 phosphor bronze star washer under head?



 Or??



 Thx.

 Ron Parigoris



 

 This is not my install but you can see the #4 connection I am talking about, it is the backside of the top one:

   http://www.europaowners.org/modules.php?full=1&set_albumName=album272&id=Archer_Troy_1&op=modload&name=gallery&file=index&include=view_photo.php  

  	  Quote: 	
		  



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nuckolls.bob(at)aeroelect
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Posted: Sat Feb 07, 2009 6:49 am Post subject: Best way to connect SS screw to aluminium

At 11:22 AM 2/6/2009, you wrote:

 	  Quote: 	
		  I have a Bob Archer SA-006 antenna (BIG "E"). It is made out of 

2024-T4 .016".



The shield of the cable is supposed to be connected to the aluminium 

"E" with a 4-40 stainless steel truss head screw. The only contact 

to the aluminium is the underside head of the screw. There is 

pheonelic on the other side.



The antenna will be permanent bonded on the inside of the rudder 

with access only through major surgery, thus I want to try and make 

this connection a good as it can be.



It happens antenna came assembled and the head is right over some 

painted markings.



Should I just scotch bright and reassemble?



Perhaps put a #4 phosphor bronze star washer under head?


	


   Hmmmm . . . my mental image of your words suggests that

   conduction from terminal passes is under a nut on a screw

   through threads of a screw that passes through

   an insulator and then to the head of a screw that's down

   against the aluminum antenna structure? The general rule

   for using threaded fasteners to make electrical connections

   is NOT to depend on the screw or it's threads for anything

   other than mechanical force to keep terminal and substrate

   in good contact. Putting a phenolic (plastic) spacer in the

   mate-up stack does not go toward the maintenance of pressure

   over time.



   Can you move the terminal over to the other side so that

   it's down against the antenna? Can you counter bore the

   phenolic so that the screw clamps up ONLY on terminal

   and antenna. Can the screw be up-sized?



   I'd like to see the phenolic out of the stack for

   maintaining clamp up forces. If you can put in a

   larger screw that would be good too. Clean the parts,

   clamp up to torque recommended for the screw and

   you're in good shape. If there's any chance the

   joint can get wet in the future, coat the finished

   joint with silicone grease before you close it all

   up.



   If the as-designed screw is also depended upon for

   keeping the phenolic block in place, ADD a second

   screw for that purpose so that you can dedicate

   the terminal screw to it's singular task of maintaining

   good conduction pressures over the lifetime of

   the airplane.



   Now, having conducted this little exercise, we'll

   probably find that 20 years from now, nobody has

   experienced any failures due to the concerns we've

   studied. Nonetheless, there are rational reasons

   for doing the exercise and implementing the practices

   suggested. As you've implied, it's a lot easier

   to do some simple things now than to fix some

   really hard things later.

        Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

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