way back, we moved from star washers, inside or outside teeth to serrated
heads, and in all cases, the surfaces had to be masked from any painting or
coating process, that was not part of the metal passivization.
Several vendors supplied parts painted with powder coat epoxy and no parts were
available to break thru that.
A dremal took way to long to grind through the epoxy.
I seem to recall a 40 amp test across the joint, and some voltage drop to be
measured, I think under a 1 volt?
And I used to complain that this test at DC or 50/60/400 Hz, does not account
for RF effects.
The mechanical theory being to form a gas tight metal to metal deformation.
ps. and use compatible metals that are less than ?? emf volts apart, and thick
enough plating that the emf was not exceeded.
A stack up of various materials, with each junction much less than the emf
limit required.
So just imagine your local mechanical engineer going into this detail and
calling out the correct 'stackup'
And so the local EMC engineer. and physicist and chemical engineers added to
the evaluation, to fully and correctly describe the electro-mechanical
parameters for the joint.
From: Ed Price <edpr...@cox.net>
To: EMC-PSTC@LISTSERV.IEEE.ORG
Sent: Saturday, February 28, 2015 3:10 PM
Subject: Re: [PSES] Serrated head screws for grounding/bonding
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first learned about the non-intuitive value of metal displacement from the now
almost forgotten technology of wire-wrap, and then I found it had been in use
for quite a while on telephone “push-down” wire connectors. Both of those
technologies yielded long-term reliability, so I can’t see why that wouldn’t
work for star washers too. OTOH, there is still that possibility of a heavy
fault current blasting away tiny point-contact conduction paths. Hmmm. Maybe
commercial standards have been overly influenced by arguments of cost, as it’s
certainly cheaper to just use a serrated skirt bolt in place of an assembled
stack of components. We might be in the land of “maybe not the best, but good
enough for us.” BTW, the military system also emphasizes that you are not done
when you torque the fastener; you still have to protect the assembly with some
coating (like varnish or liquid plastic or paint). I suppose this is
acknowledging the issues of joint capillary action and not being gas-tight. I
never cease to be amazed at how many EMC questions depend on answers from
mechanical engineers. Ed Price
WB6WSN
Chula Vista, CA USA From: Doug Powell [mailto:doug...@gmail.com]
Sent: Saturday, February 28, 2015 9:18 AM
To: Ed Price
Subject: Re: [PSES] Serrated head screws for grounding/bonding Ed,The idea of
not using a star washer for military use may be at odds with some of the
international standards I use. I fully agree that clean non-painted surfaces
are important but restrictions on the use of stars or serrated bolts may be the
issue. From your mil-std quote and the description of flat smooth surfaces, it
would seem that the goal of these statements is to achieve a high conductivity
connection. For international standards work I am involved in doing the goal
is to achieve adequate conductivity for safety even after years of non-current
carrying operation. One of the issues with flat surfaces is corrosion control.
The star washer is a way of "biting" into metal connections with an oxygen
free connection. This cannot be guaranteed with smooth metal surfaces.I would
be interested to hear other thoughts on this.Doug On Fri, Feb 27, 2015 at
7:47 PM, Ed Price <edpr...@cox.net> wrote:Constantin: The military practice is
to never use a star washer or ribbed underside bolt to cut into a painted,
plated or unknown surface to achieve a ground bond. I would show you a nice
view of the fastener stack from the very old MIL-B-5087 Bonding standard, but
the server does not allow graphics within posts, so we’ll just have to imagine
it. MIL-HDBK-1857 is much more current; it has a number of examples of bonding
and says essentially the same thing: 3.2.6 Grounding requirements. A ground
stud shall be provided on equipment. The ground stud shall provide the
electrical ground connection to the chassis or frame and shall be mechanically
secured to insure low resistance joints by soldering to a spot welded terminal
lug or to a portion of the chassis or frame that has been formed into a
soldering lug, or by use of a terminal by a screw, nut and lock washer. The
ground stud shall be of a size to allow electrical connection of size AWG 10
wire. All hardware used for grounding or other electrical connections shall be
made from copper or copper alloys. Terminal luge shall be tin plated or hot tin
dipped. Paint, varnish, lacquer, etc., shall be removed from the vicinity of
the fastening point to insure metallic contact of the two surfaces. Corrosion
protection shall be provided for all ground connections.Internal or external
lock washers shall not be used on any grounding or other screw type electrical
connections. Lock washers shall not be located between the metal plate and
terminal lug or other part being grounded, so as not to interfere with the full
and direct contact between these two members. Neither locking terminal lugs nor
self—locking nuts shall be used for grounding. Flat washers shall be inserted
next to any part having insufficient contact area with its adjacent part. The
way I read all this is that the military does not consider any ground or bond
proper if it doesn’t start out with surfaces prepared for good conductivity. If
you have a painted chassis or box, you should either mask the intended ground
location before painting, or remove that paint in a separate operation before
assembly of the fastener stack. Maybe the commercial codes (NEC?) allow for
fasteners which cut their own bond path (I see that a lot in appliances and
residential wiring). Remember that I’m thinking of a bond as doing two things;
first, providing a low-impedance RF path, and second, providing a low-impedance
and high current path, so as to pass heavy fault current to allow protective
devices to activate. I agree that multi-toothed star washers, when really
torqued down, seem to do a good job of cutting through coatings and oxides;
it’s just that the military doesn’t see that as good enough. Maybe another
thing the military has against star washers is that, by cutting their own bond
path, they are actually doing a small machining operation, and the paint and
base metal micro-debris could be considered FOD. I can’t cite any prohibitions,
but I can’t recall military products using any lock washers other than the
split-ring style (not getting into locknuts here J). Hope this helps! Ed Price
WB6WSN
Chula Vista, CA USA -
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