"Noozer" wrote in message
news:OkTce.1178374$Xk.570089@pd7tw3no...
snip

Your voltage and vibration causes for unusually short incandescent bulb
life are correct; but heat and on/off cycles are not. The little bit of
extra heat in a tight fixture makes no difference to a bulb filament
which is operating at 3373 degrees C. For heat to affect bulb life, you
have to either have enough heat to melt the glass, crack the internal
glass seal or destroy the lamp basing cement. Think about incandescent
bulbs in ovens and other high-temperature applications. There's nothing
special about them.

On/off operation might affect bulb life a bit when the bulb is old; but
not when the bulb is new. If on/off made a difference, we wouldn't see
flashing sign lamps on theater marquees.

...but a bulb that is never shut off will last longer than a bulb going
off and on. All my bulbs fail when turned on.

If a bulb is never cycled, its filament can get much weaker before it
fails because it doesn't have to withstand any shock of heating or
cooling.
According to light bulb test data in the lighting literature, turning a
bulb on and off has no effect during most of the bulb life. It is only when
the bulb gets older and the filament starts to crystallize and get brittle
that on/off makes a difference and why bulbs often fail when turned on due
to the current surge and rapid heating. Old bulbs are also more sensitive
to shock and vibration -- the brittle filament again.

Tests with bulbs left on and with bulbs on/off cycled when corrected for
burning time showed little difference in life.

The above are long-held notions, maybe "urban legends", but the data don't
confirm that heat or on/off cycling shorten incandescent bulb life. The
references are in the Illuminating Engineering Society of North America
Handbook, 9th. Ed. Chapter 6.

TKM