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nuckollsr(at)cox.net
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 Posted: Fri Jul 28, 2006 5:29 am Post subject: grounds |
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| Quote: |
I have always been leery of the ground to the firewall on RVs. I think
the real ground is on the inside to the aluminum angle. For this reason I
used a cad plated washer and nut instead of the platenut there. I don't
think that an all metal locknut is necessary.
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???? Really. You don't buy into the concept of high,
sustained force in the fastener for the purposes of
maintaining pressure in the joint over the lifetime of
the airplane? If concerns for loosening of a nut
anywhere else in the airplane is worthy of anti-rotation
technology like lock-nuts, how is it that a firewall
ground fastener is relieved of such constraints? How
does a nutplate (generally an all-metal locking
technology) become inferior to a nut and cad-plated
washer to the achievement of permanent joining short
of welding or riveting?
| Quote: | The threads will contact to the metal part of the nut just fine. I
don't know of any problem of conduction through the platenut. I just
prefer the nut and washer. YMMV
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Threads of a fastener have almost nothing to do with
conductivity of the joint. 99% happens at the surface
of a terminal held in contact with the surface opposite
the nut. To attach a wire to a surface of the airplane,
you'd be just fine with CERAMIC fasteners as long as
the goal of bringing the two critical surfaces togehter
has been achieved.
For the most part, firewalls should not be depended upon
for anything but the most benign of grounds . . . like
perhaps the grounding of a starter contactor coil through
the base of the contactor . . . or grounding the (-) sense
of a "ford" regulator by virtue of it's bolting to the
sheet metal.
The whole intent and purpose of the single point ground
suggested in chapter 15 was to eliminate all uncertainties
both for initial and aged performance of this important
but poorly understood portion of the ship's electrical
system.
One may argue the virtues or evils of various materials
platings and assembly processes at length but in the
final analysis, sustained pressure (for gas tight) and
exclusion of antagonists (by means of platings, silicone
grease, paint, etc.) are the keys to longevity.
Bob . . .
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Hopperdhh(at)aol.com
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| Posted: Fri Jul 28, 2006 6:55 am Post subject: grounds |
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In a message dated 7/28/2006 9:31:56 A.M. Eastern Daylight Time, nuckollsr(at)cox.net writes:
| Quote: |
| Quote: | The threads will contact to the metal part of the nut just fine. I
don't know of any problem of conduction through the platenut. I just
prefer the nut and washer. YMMV
|
Threads of a fastener have almost nothing to do with
conductivity of the joint. 99% happens at the surface
of a terminal held in contact with the surface opposite
the nut. To attach a wire to a surface of the airplane,
you'd be just fine with CERAMIC fasteners as long as
the goal of bringing the two critical surfaces togehter
has been achieved.
|
Bob,
I see your point, but it would be pretty hard to prove where the electrons actually go. Lets say that the relatively large surface area of the terminal and the stainless firewall caused the pressure to be too small to punch through the oxide coating of the stainless steel. Then the path would have to be from the terminal, through the bolt, to the threads, to the nut (self locking by the way), to the washer and finally to the aluminum.
do not archive
Dan Hopper
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Hopperdhh(at)aol.com
Guest
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| Posted: Fri Jul 28, 2006 7:29 am Post subject: grounds |
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Bob,
I should have mentioned that part of what I was trying to accomplish by not installing the platenut behind where the battery ground cable attaches to the firewall was to make a place in the cockpit area to pick up a good ground.
Your expertise in these matters far exceeds mine. So I shall go out and rivet in a 1/4 inch platenut the way the plans show!
Hey, I thought you were supposed to be on vacation!
do not archive
Dan Hopper
RV-7A
In a message dated 7/28/2006 10:58:38 A.M. Eastern Daylight Time, Hopperdhh(at)aol.com writes:
| Quote: | In a message dated 7/28/2006 9:31:56 A.M. Eastern Daylight Time, nuckollsr(at)cox.net writes:
| Quote: |
| Quote: | The threads will contact to the metal part of the nut just fine. I
don't know of any problem of conduction through the platenut. I just
prefer the nut and washer. YMMV
|
Threads of a fastener have almost nothing to do with
conductivity of the joint. 99% happens at the surface
of a terminal held in contact with the surface opposite
the nut. To attach a wire to a surface of the airplane,
you'd be just fine with CERAMIC fasteners as long as
the goal of bringing the two critical surfaces togehter
has been achieved.
|
|
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Bret Smith
Joined: 09 Jan 2006
Posts: 178
Location: Mineral Bluff, GA
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| Posted: Fri Jul 28, 2006 9:27 am Post subject: grounds |
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Dan,
I, and I'm sure many others on this list, appreciate your thought provoking message regarding "grounding". We also appreciate Bob's input. Many has been the time when I have wondered about something but was "afraid" to suggest the idea on list for fear of being "shot down".
Keep posting...keep asking questions... This is how we all learn.
Bret Smith
RV 9A
[quote] ---
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_________________
Bret Smith
RV-9A (Emp) |
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nuckollsr(at)cox.net
Guest
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| Posted: Wed Aug 02, 2006 7:24 am Post subject: grounds |
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At 10:51 AM 7/28/2006 -0400, you wrote:
| Quote: | In a message dated 7/28/2006 9:31:56 A.M. Eastern Daylight Time,
nuckollsr(at)cox.net writes:
> The threads will contact to the metal part of the nut just fine. I
> don't know of any problem of conduction through the platenut. I just
> prefer the nut and washer. YMMV
Threads of a fastener have almost nothing to do with
conductivity of the joint. 99% happens at the surface
of a terminal held in contact with the surface opposite
the nut. To attach a wire to a surface of the airplane,
you'd be just fine with CERAMIC fasteners as long as
the goal of bringing the two critical surfaces togehter
has been achieved.
Bob,
I see your point, but it would be pretty hard to prove where the electrons
actually go.
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Not at all difficult. Let's noodle this out a bit. See:
http://www.aeroelectric.com/Pictures/Terminals/Bolted_Joint_Resistances.gif
the goal is to herd electrons from terminal to the surface of
some other conductor . . . like a fire wall surface or perhaps
a contactor stud . . . what ever.
The resistance in terminal/post joint "B" is where we
concentrate on quality by means of pressure for gas-tightness
and longevity by keeping contaminants out. This is resistance Rb
on the diagram and the majority current flow path.
There's an alternate path that runs through Ra (stud/thread)
+ Re (nut/thread) + Rd (nut/washer) + Rc (washer/terminal) all in
series. I think it's obvious that depending on the alternate conduction
path to become the majority path because of poor preparation or
maintenance of path Rb is exceedingly risky.
| Quote: | Lets say that the relatively large surface area of the terminal and the
stainless firewall caused the pressure to be too small to punch through
the oxide coating of the stainless steel. Then the path would have to be
from the terminal, through the bolt, to the threads, to the nut (self
locking by the way), to the washer and finally to the aluminum.
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This is a surface preparation issue. If the Rb joint isn't
ready to be bolted up and the proper pressures applied from the get-go,
then the ship has left the dock an we weren't on it. You will note that
when the single-point ground system philosophy as described in
the 'Connection is implemented with hardware equal to that supplied
by B&C then there are no electrical joints (other than the few low-risk
items cited earlier) made to stainless. Engine mounts and fire walls
have specific tasks in the design of airplanes that do not include
being major current paths for electrical systems.
Bob . . .
---------------------------------------------------------
< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
---------------------------------------------------------
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