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Airframe ground connection

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Louie928

Joined: 15 Nov 2009
Posts: 32
Location: Mosier, OR USA

Posted: Thu Jul 21, 2011 2:08 pm Post subject: Airframe ground connection

I'm building a Zenith 601XL. The airframe is constructed of mostly 0.025" 6061. The battery will likely be mounted behind the seat and the ground connection will be made to the nearby rear spar carry through which is 0.032" 6061 riveted to the floor and side skins. The firewall is 26 ga., 0.018", galvanized steel. I've seen the battery ground connection #4 AWG terminal bolted directly to the 0.032" sheet, and the engine connected to the firewall by a short #4 or braided copper. It seems to me that the current density at either of these connections is going to be quite high during engine start. Is it recommended to have a thicker metal section under the terminal at these connections? Something like 1/8" aluminum 2" or 3" in diameter riveted with, say 8 #4 rivets, to the underlying thin aluminum or galvanized steel to distribute the current? Is a couple of thick washers on each side of the sheet metal under the bolted through terminals enough?



Or, run a +12V #4, and a ground #4, to the starter and engine thereby bypassing the airframe from the large starter current. The wire length from battery to engine is about 7 feet.

_________________
Louis W. Ott

601XL-B Corvair

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jul 22, 2011 4:36 am Post subject: Airframe ground connection

 	  Quote: 	
		  

Or, run a +12V #4, and a ground #4, to the starter and engine 

thereby bypassing the airframe from the large starter current. The 

wire length from battery to engine is about 7 feet.


	


    That's the 'clean' way to do it. #4 welding cable

    for battery wires. Take battery (-) to firewall

    feed through bolt, battery (+) through battery

    contactor and then firewall to starter contactor.



   Bob . . .

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Fri Jul 22, 2011 6:23 am Post subject: Re: Airframe ground connection

 	  Quote: 	
		  Something like 1/8" aluminum 2" or 3" in diameter riveted with, say 8 #4 rivets, to the underlying thin aluminum or galvanized steel to distribute the current? 	


Rivets do not make very good electrical connections unless there are many of them in parallel (electrically).  Certain brands of switches are prone to fail because they use rivets to carry the current.

Joe

_________________
Joe Gores

Neal.George(at)hurlburt.a
Guest

Posted: Fri Jul 22, 2011 7:23 am Post subject: Airframe ground connection

The switches that we've seen fail at riveted joints didn't fail because

of the rivets, per se.  They failed because rivets were used to join

grossly dissimilar materials - Bakelite/plastic sandwiched between metal

tabs - AND the leads were not supported adequately.  In service,

movement of the wire leads relative to the switch body flexed the

terminal tabs, eventually loosening the riveted joint.  As the riveted

joint works and loosens, the gas-tight electrical connection is

compromised.  The less-than-ideal electrical connection introduces

electrical resistance which manifests as heat.  The heat further damages

the loose rivet/plastic/copper interface thru corrosion and differential

thermal expansion rates, and a destructive cycle of rising resistance,

higher heat and accelerated damage results until the Bakelite cooks and

crumbles or the copper bits inside anneal and deform to the point that

the actuator can't move the rocker far enough to make/break the

connection.    



Neal



 	  Quote: 	
		   Rivets do not make very good electrical connections unless there are

many of them in parallel (electrically).  Certain brands of switches are
	


prone to fail because they use rivets to carry the current.

Joe Gores



 	  Quote: 	
		  > Something like 1/8" aluminum 2" or 3" in diameter riveted with, say 8

#4 rivets, to the underlying thin aluminum or galvanized steel to
	


distribute the current?

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jul 22, 2011 8:15 am Post subject: Airframe ground connection

At 10:19 AM 7/22/2011, you wrote:

 --> AeroElectric-List message posted by: "George, Neal Capt 505 TRS/DOJ" <Neal.George(at)hurlburt.af.mil>



 The switches that we've seen fail at riveted joints didn't fail because

 of the rivets, per se.  They failed because rivets were used to join

 grossly dissimilar materials - Bakelite/plastic sandwiched between metal

 tabs - AND the leads were not supported adequately.  In service,

 movement of the wire leads relative to the switch body flexed the

 terminal tabs, eventually loosening the riveted joint.  As the riveted

 joint works and loosens, the gas-tight electrical connection is

 compromised.  The less-than-ideal electrical connection introduces

 electrical resistance which manifests as heat.  The heat further damages

 the loose rivet/plastic/copper interface thru corrosion and differential

 thermal expansion rates, and a destructive cycle of rising resistance,

 higher heat and accelerated damage results until the Bakelite cooks and

 crumbles or the copper bits inside anneal and deform to the point that

 the actuator can't move the rocker far enough to make/break the

 connection.    



 

     An excellent recap of some extensive discussion

     shared here on the List some time back.



 [img]cid:.0[/img]



    Rivets have been used to assemble gazillions

    of switches for decades. The designers of such

    devices strive for rivets not to be the "weakest"

    link in the chain of stresses on service life.



    Let's look at the inner workings of a Carling toggle

    (common to tens of thousands of Cessnas and certainly

    thousands of OBAM aircraft). We observe TWO features

    in the current carrying path that are more vulnerable

    to wear-out or environmental failure. One is the

    contacts. Resistance across the contacts is a function

    of spring pressure and contact surface condition.

    Resistance across the pivot-saddle is also a function

    of spring force and surface condition . . . but at least

    it gets "scrubbed" each operating cycle. Contacts just

    get "hammered". Contacts also benefit from the cleansing

    effects of electrical arcing to burn away products of

    corrosion.



    When installed with some attention to stresses on riveted

    terminals, one can expect failure of the low-pressure,

    moving-components to precede failure of the rivets by

    perhaps an order of magnitude of service life.



    Rivets in the switches are used in tension and they

    join a stack of materials that includes a plastic.

    Rivets in sheet metal are used in sheer. Further, if

    properly drilled and set, they swell up in the hole to

    form a metal-to-metal, gas tight fit . . . not unlike

    the gas-tight fit of wires properly mashed in the

    barrel of a terminal.



    Riveted doublers to reduce sheet current density

    around a high-current ground to airframe is probably

    not a 'bad' idea . . . but if there are concerns

    for getting a good ground in a thin-metal assembly,

    perhaps going around it entirely with a bundle of

    copper "cat-hairs" is a 'better' idea.



    Bob . . .

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Louie928

Joined: 15 Nov 2009
Posts: 32
Location: Mosier, OR USA

Posted: Fri Jul 22, 2011 9:01 am Post subject: Re: Airframe ground connection

 	  Neal.George(at)hurlburt.a wrote: 	
		  The switches that we've seen fail at riveted joints didn't fail because

of the rivets, per se.  They failed because rivets were used to join

grossly dissimilar materials - Bakelite/plastic sandwiched between metal

tabs - AND the leads were not supported adequately.  In service,

movement of the wire leads relative to the switch body flexed the

terminal tabs, eventually loosening the riveted joint.  As the riveted

joint works and loosens, the gas-tight electrical connection is

compromised.  The less-than-ideal electrical connection introduces

electrical resistance which manifests as heat.  The heat further damages

the loose rivet/plastic/copper interface thru corrosion and differential

thermal expansion rates, and a destructive cycle of rising resistance,

higher heat and accelerated damage results until the Bakelite cooks and

crumbles or the copper bits inside anneal and deform to the point that

the actuator can't move the rocker far enough to make/break the

connection.    



Neal



 	  Quote: 	
		   Rivets do not make very good electrical connections unless there are

many of them in parallel (electrically).  Certain brands of switches are

	


prone to fail because they use rivets to carry the current.

Joe Gores



 	  Quote: 	
		  > Something like 1/8" aluminum 2" or 3" in diameter riveted with, say 8

#4 rivets, to the underlying thin aluminum or galvanized steel to

	


distribute the current? 	




You make a valid point regarding the riveted connectors on the switches. I've come across the exact problem you describe in my work. It can lead to intermittent problems before identification of the actual trouble spot. I bought all my switches from B&C and was dismayed to see they have the riveted connection to the push on spade connector. It takes little effort to move the connector under the rivet. These have apparently been used in a lot of airplanes over a long time so maybe no problems have surfaced. I'll need to be sure I have the wires well supported near the switch with no strain on the riveted connector.



Yes, distribute the current over a wider contact area of the thin aluminum, or steel. A simple bolted connection to 0.032" aluminum or the 0.018" galvanized steel firewall has a ridiculously small cross sectional area for the current to pass through. If it was wire it would be a very small CMA, like a fuse link. True, it is a short distance and the voltage drop won't be a lot. Heat will be generated which can contribute to problems down the road. If the bolted connection can be made through a thicker piece of material of 2 or 3 square inches area, and that thicker piece electrically connected to the thinner piece, the current will be distributed out to a much larger area of the thin metal and the current density won't be so high.

_________________
Louis W. Ott

601XL-B Corvair

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jul 22, 2011 10:30 am Post subject: Airframe ground connection

It takes little effort to move the connector under the rivet. These 

have apparently been used in a lot of airplanes over a long time so 

maybe no problems have surfaced. I'll need to be sure I have the 

wires well supported near the switch with no strain on the riveted connector.



    Good practice . . . whether the terminals are riveted

    or not.



Yes, distribute the current over a wider contact area of the thin 

aluminum, or steel. A simple bolted connection to 0.032" aluminum or 

the 0.018" galvanized steel firewall has a ridiculously small cross 

sectional area for the current to pass through.



    If you study the three sheets of Figure Z-15 found

    at http://www.aeroelectric.com/PPS/Adobe_Architecture_Pdfs/

    You will see a directed effort to minimize the use of airframe

    for any ground return currents.



If it was wire it would be a very small CMA, like a fuse link. True, 

it is a short distance and the voltage drop won't be a lot. Heat will 

be generated which can contribute to problems down the road. If the 

bolted connection can be made through a thicker piece of material of 

2 or 3 square inches area, and that thicker piece electrically 

connected to the thinner piece, the current will be distributed out 

to a much larger area of the thin metal and the current density won't 

be so high.



    Assume a firewall ground feed-through stud is the

    central, single-point ground. Assume further that only

    a limited number of low-current remotely mounted accessories

    enjoy local airframe grounds, then current density and bonding

    issues go away. The firewall sheet carries a small percentage

    of the total system currents and never cranking current.

   Bob . . .

Louie928

Joined: 15 Nov 2009
Posts: 32
Location: Mosier, OR USA

Posted: Fri Jul 22, 2011 11:14 am Post subject: Re: Airframe ground connection

Thanks very much for helping clear this up.

_________________
Louis W. Ott

601XL-B Corvair

user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Sat Jul 23, 2011 5:28 am Post subject: Re: Airframe ground connection

Neal,

That is very well put and explained.  I agree with everything that you said.  I wish that I could write that well.

Joe

Do not archive

 	  Quote: 	
		  The switches that we've seen fail at riveted joints didn't fail because

of the rivets, per se. They failed because rivets were used to join

grossly dissimilar materials - Bakelite/plastic sandwiched between metal

tabs - AND the leads were not supported adequately. In service,

movement of the wire leads relative to the switch body flexed the

terminal tabs, eventually loosening the riveted joint. As the riveted

joint works and loosens, the gas-tight electrical connection is

compromised. The less-than-ideal electrical connection introduces

electrical resistance which manifests as heat. The heat further damages

the loose rivet/plastic/copper interface thru corrosion and differential

thermal expansion rates, and a destructive cycle of rising resistance,

higher heat and accelerated damage results until the Bakelite cooks and

crumbles or the copper bits inside anneal and deform to the point that

the actuator can't move the rocker far enough to make/break the

connection.



Neal

_________________
Joe Gores

n8zg(at)mediacombb.net
Guest

Posted: Sat Jul 23, 2011 6:53 am Post subject: Airframe ground connection

Thanks Joe - 



Bob's an exceptional teacher, and I'm a poor student, but I'm trying...



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