[Milsurplus] Naval radio experts: captive screws and other mysteri es

[WA5CAB at cs.com]

George and Todd,

All captive screws are basically of the same design.  The diameter of the 
head is larger than the major diameter of the thread.  The diameter of the shank 
(that portion of the screw between the head and the threads is smaller than 
the minor diameter of the threads.  When it comes to implementation in a 
practical assembly, there are far more than two versions.  The simplest is the tapped 
panel.  This requires that the female threads in the other component 
(cabinet, chassis, heat sink, etc.) be counterbored or otherwise arranged to be 
farther away from the panel than the length of the threads.  Probably the most 
common implementation in WW-II and Korean War military radio and test equipment is 
a shouldered cylindrical fitting counterbored about half way through and 
tapped the rest of the way.  This is swaged into the front panel or the lip of the 
cabinet.  And the lip of the cabinet or the panel is tapped (or has a swaged 
in threaded unit) to either engage or captivate the screw.  I don't have any 
here handy to look at but memory says the RBA/RBB/RBC and associated power 
supplies used the former approach and the AN/URM-25 the latter.

Later (i.e., cheaper) variants (I'm looking at one right now) used a stamped 
and formed cylinder with a through hole larger than the thread major diameter. 
 The captive screw was dropped through it and one of the thin split 
lockwasher looking thingies run up over the threads to perform the captivation.  This 
assembly was then swaged into the panel or other removable component and the 
tapped hole was in the cabinet or fixed component.

The BC-611 top cover used another approach.  Pretty standard captive screw, 
but the captivating method was a berillium copper strip riveted to the top 
cover with one of the battery ground springs.  In line with the hole in the center 
of the top cover, there is a tap drill diameter hole through the strip, and a 
radial split from the hole to allow it to operate lick the thin split 
lockwasher looking thingie.



In a message dated 5/12/2005 10:21:11 PM Central Daylight Time, 
gl4d21a at juno.com writes: 
> There are two or more versions of those captive screws.  Keystone 
> (keyelco.com) sells just the screws which are used with a simple tapped hole in the 
> panel.  They have a unthreaded shank and threads on the tip.  The more 
> complicated version has a sleeve which is "riveted" into the hole, and the screw is 
> captive inside the sleeve from the factory.  It sounds like you have the 
> latter.  These can be removed, but it is like trying to remove a rivet in 
> salvagable form.  Possible, but not very easy.  I am sure that that hardware is 
> available somewhere in small quantities, as it was used where I used to work.  
> Sometime in the next few weeks, I will determine the source and post it here.  
> In the meanwhile, what size (thread like 10-32) are you seeking, as I have a 
> few somewhere left over from one of those projects?
> 

Robert Downs - Houston
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