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nuckolls.bob(at)aeroelect
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 Posted: Fri Jan 02, 2009 6:59 pm Post subject: Follow-up on Carling switch failures. . . |
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At 06:42 PM 1/2/2009, you wrote:
| Quote: | Bob,
While I can't disagree with your findings, if I remember the
discussion correctly, most of the failed B&C S700 switches were on
strobe systems. I think I kicked off the recent discussion with a
failed switch on my strobe system. I have several other B&C S700
series switches, all are wired similar to the strobe switch, and
none of them have failed or showed signs of heat stress. Why should
the switches on a strobe system be more failure prone?
|
Certainly switches carrying higher currents
are more likely to exhibit effects of heating as
part of the trail of evidence. Loosening of
joints increases resistance joint which means
makes it get hotter. Heating piled on
top of loosening will exacerbate the rate of
failure . . . and cause more damage during the
transition from satisfactory to unsatisfactory
service.
The effects of current is a squared function.
For every doubling of current through a given
value of resistance, power dissipated in that
resistance goes up by a factor of 4. This means
that the strobe switch with say 7A of current
flowing through it is under 4x the stress of
same switch having 3.5A of current.
This stressing of rivets thing is not easy
to predict based on similarity of wiring. Also,
the hypothesis does not preclude quality assurance
issues with switches (although the spectrum of
reported failures spans years of date codes).
When looking for a place to "hang the hat" we
need to keep in mind that these switches and
their close cousins have millions of service-hours
dating back the time when fast-ons were incorporated
into the Cessna single-engine product line . . .
not to mention perhaps 100-1000 times that service
record throughout the total customer base for these
switches.
Manufacturing errors do happen but they tend to be
single lot issues that have a low probability of
finding their way into our exceedingly low-volume
market.
This leaves us with considering mis-application of
the devices (failure to observe limits/ratings)
or errors of installation.
Loose rivets a common to ALL the observed failures.
Given that several failures occurred with no
signs of heating, we're on pretty solid ground
to suggest that loosening is the the first event
in the failure chain followed by heating (where
current levels and duty-cycle are sufficiently
high to offer contributing stress).
Gee, why didn't Carling warn us about this? Where's
the prohibition for using rows of these switches
in front of tightly bundled wires? We're
less than 1% of their total market. Occurrence
of failures we've seen is a tiny fraction
of that market. It has probably never occurred to
their tech writers that such a prohibition is necessary/
useful. It also occurs to me that the tabs could be
subject to some side loads during installation of
those really tight PIDG fast-on terminals!
Now, we've been here before . . . thinking that we
understood all of the simple-ideas that fit the
observed facts. But until someone has new data and/or
an alternative logic, that's my story and I'm sticking
with it!
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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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Jan 03, 2009 5:48 am Post subject: Follow-up on Carling switch failures. . . |
|
|
At 08:34 PM 1/2/2009, you wrote:
| Quote: | Vern,
When you say "Carling switches with loose rivets". How loose? Do you mean wobbly terminals?
I've checked all mine and most of the terminals can pivot around the rivet until it hits a stop molded into the switch. I wonder if this is tight enough. I would expect them to be tighter. |
Interesting! The riveted-tabs are obviously not
as rigid about the rivet axis. Some versions do
drop the tab base into a "pocket" molded into the
back of the housing. See:
http://tinyurl.com/96yqmg
I'm away from home for a week so I can't shuffle through
all the switches in my shop. But even with the recesses
shown, the tab isn't really "captured".
| Quote: | | I have a couple Mil-spec switches and I don't think they pivot like this but will check again next time I'm at the shop. Maybe this pivoting action is what is not compatible with a high vibration environment. |
I'm aware of no mil-qualified switches that do not feature
terminals molded into the housing. Here's an exemplar
design (Microswitch) . . .
http://tinyurl.com/842eos
| Quote: |
I've had two strobe switch failures, one landing light switch (actually a wig-wag) failure, and one master switch failure (loose rivet but no charred terminals). I have also received brand-new Carling switches with loose terminals.
|
In retrospect, the only thing that resists rotation of
the tabs on the rivets is friction between tab and
housing established coefficient of friction between
plastic/brass and force of the riveted joint. Obviously,
there are limits to the force you can put on the plastic
housing . . .
| Quote: |
As an experiment, I replaced several Carling switches with Honeywell switches and have been running them, monitoring for failures. I proposed about 100 hours of testing before any conclusions could be made. This will probably take several more months. I have check them in the interim, with no problems.
If the experiment shows failure of the Honeywell switches, then the problem may be attributed to my installation.
If the experiment does not show failure of the Honeywell switches, then the problem may be attributed to the Carling switches sensitivity to vibration.
|
. . . and I would expect no failures of these devices.
They're exceedingly well designed and tested in environments
our airplanes will never see.
| Quote: | | Bob has concluded that the Carling switches are fine, but should have vibration decoupling loops to minimize vibration stresses on the terminals. Good advice no matter who's switches are installed. Unfortunately, I don't agree that the problem is that my (our) installations are incorrect. Rather, I believe that the problem is that the Carling switches are unusually sensitive to vibration. |
Perhaps 'error' was the wrong term. Lack of understanding
leads to lack of attention to the sensitivities you've
cited . . . and without a doubt, tabs riveted to plastic
housings have a weakness of design where vibration and
temperature cycles are strong.
| Quote: |
In my next project, I am planning to use different switches, plus decoupling loops. In addition, strobe and landing light loads will be isolated with automotive relays which are generally more robust. |
It will be interesting to learn of your experience
with this. Certainly addition of the relays will
eliminate all potential for heating effects.
It occurs to me that a little modification of
the Carling (or similar) switches would add some
degree of robustness. There's a product called
JB Weld that offers a process for beefing up
the riveted joints before the switches are installed.
As soon as I get home, I'll do the treatment on
a few switches and publish some photos.
Bob . . . [quote][b]
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nuckolls.bob(at)aeroelect
Guest
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| Posted: Sat Jan 03, 2009 6:02 am Post subject: Follow-up on Carling switch failures. . . |
|
|
At 01:25 AM 1/3/2009, you wrote:
| Quote: | Yes, I am considering contacting the vendor to see if I can return them, and change to Milspec switches throughout. I don't really want them for a few bucks savings and if the vendor gets enough back, perhaps they will offer/recommend a better product.
Bevan |
If your worried about them I wouldn't discourage you
from upgrading. Keep in mind that electrical system
failures at the component level in a failure tolerant
design is a low risk event. It's unfortunate that the
price ratio of Carling vs. Microswitch is so large
(about 1:4) but in the grand scheme of things, total
dollars difference in the cost of your project for
going with Microswitch is pretty trivial.
I'm still tantalized by the idea that the Carlings
with fast-on tabs have been around for a very long
time. Adding robustness to the tabs before installation
may offer a low cost means by which our observed
shortcomings may be brought to heel.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------- [quote][b]
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fvalarm(at)rapidnet.net
Guest
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| Posted: Sat Jan 03, 2009 1:20 pm Post subject: Follow-up on Carling switch failures. . . |
|
|
RE JB weld.
I had the same thought yesterday. What I don't know is whether the JB weld will also add some thermal insulation to the joint and cause any excess heating which will lead to the same result as what we're trying to avoid. Those big exposed fast on tabs got to be working as radiators, but how much. Perhaps a dab of JB over the rivet only will have no appreciable affect on overall cooling.
Anybody know the co-efficient of thermal conductivity of JB weld as it compares to copper? The fast on tabs on the switches are probably a copper alloy?
Bevan
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert L. Nuckolls, III
Sent: Saturday, January 03, 2009 5:46 AM
To: aeroelectric-list(at)matronics.com
Subject: RE: Follow-up on Carling switch failures. . .
At 08:34 PM 1/2/2009, you wrote:
| Quote: | Vern,
When you say "Carling switches with loose rivets". How loose? Do you mean wobbly terminals?
I've checked all mine and most of the terminals can pivot around the rivet until it hits a stop molded into the switch. I wonder if this is tight enough. I would expect them to be tighter. |
Interesting! The riveted-tabs are obviously not
as rigid about the rivet axis. Some versions do
drop the tab base into a "pocket" molded into the
back of the housing. See:
http://tinyurl.com/96yqmg
I'm away from home for a week so I can't shuffle through
all the switches in my shop. But even with the recesses
shown, the tab isn't really "captured".
| Quote: | | I have a couple Mil-spec switches and I don't think they pivot like this but will check again next time I'm at the shop. Maybe this pivoting action is what is not compatible with a high vibration environment. |
I'm aware of no mil-qualified switches that do not feature
terminals molded into the housing. Here's an exemplar
design (Microswitch) . . .
http://tinyurl.com/842eos
| Quote: |
I've had two strobe switch failures, one landing light switch (actually a wig-wag) failure, and one master switch failure (loose rivet but no charred terminals). I have also received brand-new Carling switches with loose terminals.
|
In retrospect, the only thing that resists rotation of
the tabs on the rivets is friction between tab and
housing established coefficient of friction between
plastic/brass and force of the riveted joint. Obviously,
there are limits to the force you can put on the plastic
housing . . .
| Quote: |
As an experiment, I replaced several Carling switches with Honeywell switches and have been running them, monitoring for failures. I proposed about 100 hours of testing before any conclusions could be made. This will probably take several more months. I have check them in the interim, with no problems.
If the experiment shows failure of the Honeywell switches, then the problem may be attributed to my installation.
If the experiment does not show failure of the Honeywell switches, then the problem may be attributed to the Carling switches sensitivity to vibration.
|
. . . and I would expect no failures of these devices.
They're exceedingly well designed and tested in environments
our airplanes will never see.
| Quote: | | Bob has concluded that the Carling switches are fine, but should have vibration decoupling loops to minimize vibration stresses on the terminals. Good advice no matter who's switches are installed. Unfortunately, I don't agree that the problem is that my (our) installations are incorrect. Rather, I believe that the problem is that the Carling switches are unusually sensitive to vibration. |
Perhaps 'error' was the wrong term. Lack of understanding
leads to lack of attention to the sensitivities you've
cited . . . and without a doubt, tabs riveted to plastic
housings have a weakness of design where vibration and
temperature cycles are strong.
| Quote: |
In my next project, I am planning to use different switches, plus decoupling loops. In addition, strobe and landing light loads will be isolated with automotive relays which are generally more robust. |
It will be interesting to learn of your experience
with this. Certainly addition of the relays will
eliminate all potential for heating effects.
It occurs to me that a little modification of
the Carling (or similar) switches would add some
degree of robustness. There's a product called
JB Weld that offers a process for beefing up
the riveted joints before the switches are installed.
As soon as I get home, I'll do the treatment on
a few switches and publish some photos.
Bob . . . [quote]
href="http://www.matronics.com/Navigator?AeroElectric-List">http://www.matronics.com/Navigator?AeroElectric-List
href="http://forums.matronics.com">http://forums.matronics.com
href="http://www.matronics.com/contribution">http://www.matronics.com/c
[b]
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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fvalarm(at)rapidnet.net
Guest
|
| Posted: Sat Jan 03, 2009 1:23 pm Post subject: Follow-up on Carling switch failures. . . |
|
|
I agree and adding JB would be the faster and easier way to go. Trouble is, it's all just theory for now, and the airplane is becoming a reality fast.
Bevan
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Robert L. Nuckolls, III
Sent: Saturday, January 03, 2009 6:01 AM
To: aeroelectric-list(at)matronics.com
Subject: RE: Follow-up on Carling switch failures. . .
At 01:25 AM 1/3/2009, you wrote:
| Quote: | Yes, I am considering contacting the vendor to see if I can return them, and change to Milspec switches throughout. I don't really want them for a few bucks savings and if the vendor gets enough back, perhaps they will offer/recommend a better product.
Bevan |
If your worried about them I wouldn't discourage you
from upgrading. Keep in mind that electrical system
failures at the component level in a failure tolerant
design is a low risk event. It's unfortunate that the
price ratio of Carling vs. Microswitch is so large
(about 1:4) but in the grand scheme of things, total
dollars difference in the cost of your project for
going with Microswitch is pretty trivial.
I'm still tantalized by the idea that the Carlings
with fast-on tabs have been around for a very long
time. Adding robustness to the tabs before installation
may offer a low cost means by which our observed
shortcomings may be brought to heel.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------- [quote]
href="http://www.matronics.com/Navigator?AeroElectric-List">http://www.matronics.com/Navigator?AeroElectric-List
href="http://forums.matronics.com">http://forums.matronics.com
href="http://www.matronics.com/contribution">http://www.matronics.com/c
[b]
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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