🚀 go-pugleaf

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.

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.  But you're the 
big-iron transmitter guy.   Gotta run, I have a bunch of guys coming to 
the lab in a few minutes, and I have to start the coffee pot!

Cheers

Phil  Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net