🚀 go-pugleaf

On Sat, 06 Oct 2012 07:06:22 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 10/05/2012 07:47 PM, krw@att.bizzzzzzzzzzzz wrote:
>> On Fri, 05 Oct 2012 19:00:56 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net>  wrote:
>>
>>> George Herold wrote:
>>>>
>>>> On Oct 5, 2:51 pm, Jeff Liebermann<je...@cruzio.com>  wrote:
>>>>> On Thu, 04 Oct 2012 10:03:21 -0400, Phil Hobbs
>>>>>
>>>>> <pcdhSpamMeSensel...@electrooptical.net>  wrote:
>>>>>>> Yep.  As I understand it (possible wrong), AC filaments break in the
>>>>>>> middle, mostly from vibration flexing.
>>>>>
>>>>>> I don't think so, because there's no mechanism for that, as I said.  The
>>>>>> wire is fully annealed at all times, so there's no possibility of
>>>>>> progressive fatigue failure.
>>>>>
>>>>> Oscillating filament light bulb:
>>>>> <http://www.youtube.com/watch?v=t_DwwNVA-7Q>
>>>>> Whether the earths magnetic field is strong enough to induce such
>>>>> oscillations is questionable.
>>>>>
>>>>> While digging for the apparently mythical lifetime test data on
>>>>> incandescent light bulbs, I've found numerous theories on why
>>>>> filaments fail.
>>>>
>>>> Grin,  the internet as a 'fire hose' of information.  I went searching
>>>> for something that contained "Philips tech. rev." and found a
>>>> reference to the following article,
>>>>
>>>> H. Horster, E. Kauer and W. Lechner � The Burn-out Mechanism of
>>>> Incandescent Lamps Philips Technical Review 32,155-164, 1971.
>>>>
>>>> It was referenced in "Illuminating Engineering - Page 32 - Google
>>>> Books"
>>>>
>>>> But nothing about turn on failure... sigh.
>>>>
>>>> Here is a patent by some of the same guys at Philips... lots of stuff
>>>> about the filament getting hottest in the middle.
>>>>
>>>> http://www.freepatentsonline.com/3868159.html
>>>> (Actually a decently written patent.)
>>>>
>>>> George H.
>>>>
>>>> Tungsten evaporation, causing hot spots, is the most
>>>>> common.  One suggested that thermal cycling hardens the tungsten and
>>>>> makes it brittle.  Another suggested that the inrush current causes a
>>>>> mechanical shock if it hits at the 60Hz peak, instead of at the zero
>>>>> crossing.  Yet another speculates that the temperature differential
>>>>> between the hot filament, and the relatively cold mounting structure
>>>>> may cause cracking.
>>>>>
>>>
>>> I can believe that the filament is hottest in the middle.  It's
>>> furtherst from the support, so whatever conductive heat sinking there is
>>> will be less, but more than that, it sees the radiative input from the
>>> rest of the filament on both sides instead of just one.
>>
>> Aren't the filaments welded to the elements at the ends?  It would seem that
>> this would cause a narrowing.  ISTR most filaments broken near the supports,
>> which would be counter to the hotter-in-the-middle theory.
>
>The feedthroughs are made of Dumet, which is basically 42Ni stainless 
>with a borated copper coating to bind to the glass.  It's much 
>lower-melting than the tungsten, so spot-welding them together shouldn't 
>affect the tungsten much.

Welding5 is welding.  Metal has to flow, no?
>
>A necked-down hot spot is a stress concentration point, and ones nearer 
>the support would have more mass hanging off them.  When the filament 
>jumps at turn-on, hot spots at the ends will probably more torque applied.
>
>> My theory is that bulbs tend to fail when turned on because of the thermal
>> shock but only because they were about to fail anyway.  Cycling, itself,
>> doesn't have a huge effect on longevity, certainly not a factor of two.
>>
>>> Sort of similar to the case of a long solenoid, whose B field at the
>>> ends is half what it is in the middle.
>>
>> Do they only burn out when energized?  ;-)
>
>I've never seen one burn out that wasn't energized.

OK, perhaps I sh6ould have said *as* they-re energized (to keep th6e corollary
with light bulbs burning out when turned on).

>But you're the big-iron transmitter guy. 
                         ^^^^^^^^^^^ 

Huh? I think you have me confused with someone else.   

>Gotta run, I have a bunch of guys coming to 
>the lab in a few minutes, and I have to start the coffee pot!

Most important.  Get the good stuff.