rhmc24
Absent Comrade
I found this correspondence in my archives, my question replied to by a Professor of Metallurgy in a Canadian university, an avid gun collector himself -- here veratim -----
Good Morning D-----.
>
> I would appreciate your thoughts on this item. .
>
> A subject seemingly perennial in the gun forums is the
> issue of spring life. Many of those who have any opinion
> contend that periodic replacement of gun springs is
> important and to some, necessary. The defenders of the
> idea have their own ideas of whether seasonal, cycles,
> phases of the moon or other. ---->
----------
Hi Bob:
I'm in awe of your gunmaking abilities but now you've asked about
something I really know about. For the last 40 years I've done, amongst
other things, failure analysis of mechanical components. I've even done
a number of projects on old gun parts, including springs. Procedures
have involved hardness tests, chemical analysis, metallography,
electron microscopy, etc. Sooooo, if you promise not to fall asleep, I
will try to answer your questions in a relatively simple
(non-mathematical) way and try to avoid too much jargon.
I'll begin with commenting on your statement "It would seem a gun
spring properly designed and manufactured of the best alloy steel,
tempered, etc should last as long as the rest of the gun". You are
absolutely correct and that explains why so many early guns still have
their original springs, many hundreds of years old. Clearly, their
'design' was by experience (they had no theoretical understanding) so
they had an excuse for failures. That no longer exists since we
understand the loading, materials, manufacturing processes, stress
analysis..our theoretical predictive capabilities are outstanding and
we can design components to last forever (or however long we would
like).
That said, lets look at putting what we know into some kind of
framework.
Endurance or Fatigue Limit:
Failures in metals are related to stress level. All ferrous alloys have
a stress level below which they may be cyclically loaded for an
infinite number of cycles without failure. This stress level depends on
a number of things, including the particular alloy, and is called the
"endurance limit" or "fatigue limit" for the particular alloy.
Typically, this stress level is associated with 1,000,000 cycles of
loading. Gun springs can be designed in this way and, while they would
not usually be loaded (cycled) that many times, if they are properly
designed they should last the life of the gun.
Finite Life:
Typically, airplanes would not fly if their main components were
designed for infinite life because the plane would be too heavy (for a
given loading, smaller components will have higher stresses and
therefore be more prone to failure). Thus we often design aircraft
compnents to survive for a limited specified time and inspect regularly
to look for crack initiation. At the specified time, the component is
replaced. This is "finite life" design.
S-N Curves:
We know that, for a given component, the higher the stress level, the
shorter the life of the component (the fewer number of cycles the
component can loaded and survive). The development of fatigue cracks is
very complex and we cannot predict this purely theoretically so we rely
on laboratory tests to develop "fatigue life curves" or S-N Curves".
These are graphs in which the cyclic stress level (S) is plotted on the
vertical axis and the number of cycles (N) on the horizontal axis. The
failure line starts at the fracture strength (failure after 1 cycle of
loading) and slopes down to the right until the endurance limit is
reached at 1 million cycles. Such an empirical curve is created for
every alloy so that we can design for infinite life or finite life. (if
you like, I can scan and email you such an example curve).
Low Cycle Fatigue:
When a component survives for only a few cycles (less than 1000) the
analysis is fairly complicated and nobody in their right mind designs
components to survive so few cycles (uncertainty is too high in
precisely predicting life). It may be that some components fail from
low cycle fatigue but this is most often simply poor design (or totally
ignorance of stress levels and failure modes).
Fatigue Life Limiting Factors:
There are many things which affect the fatigue life of a component.
Poor forging practice leaving slag inclusions or voids (not unusual in
poorly made antique firearms), poor heat treatment causing
overhardening, poor tempering leaving the material too brittle, poor
surface finishing which causes stress concentration, etc. Such factors
reduce the endurance limit for the component.
Thus endeth your first lesson (remember that you asked for it and
promised to stay awake). I hope that it is understandable and helps. I
will be happy to address any clarifications or further questions.
with best regards
Good Morning D-----.
>
> I would appreciate your thoughts on this item. .
>
> A subject seemingly perennial in the gun forums is the
> issue of spring life. Many of those who have any opinion
> contend that periodic replacement of gun springs is
> important and to some, necessary. The defenders of the
> idea have their own ideas of whether seasonal, cycles,
> phases of the moon or other. ---->
----------
Hi Bob:
I'm in awe of your gunmaking abilities but now you've asked about
something I really know about. For the last 40 years I've done, amongst
other things, failure analysis of mechanical components. I've even done
a number of projects on old gun parts, including springs. Procedures
have involved hardness tests, chemical analysis, metallography,
electron microscopy, etc. Sooooo, if you promise not to fall asleep, I
will try to answer your questions in a relatively simple
(non-mathematical) way and try to avoid too much jargon.
I'll begin with commenting on your statement "It would seem a gun
spring properly designed and manufactured of the best alloy steel,
tempered, etc should last as long as the rest of the gun". You are
absolutely correct and that explains why so many early guns still have
their original springs, many hundreds of years old. Clearly, their
'design' was by experience (they had no theoretical understanding) so
they had an excuse for failures. That no longer exists since we
understand the loading, materials, manufacturing processes, stress
analysis..our theoretical predictive capabilities are outstanding and
we can design components to last forever (or however long we would
like).
That said, lets look at putting what we know into some kind of
framework.
Endurance or Fatigue Limit:
Failures in metals are related to stress level. All ferrous alloys have
a stress level below which they may be cyclically loaded for an
infinite number of cycles without failure. This stress level depends on
a number of things, including the particular alloy, and is called the
"endurance limit" or "fatigue limit" for the particular alloy.
Typically, this stress level is associated with 1,000,000 cycles of
loading. Gun springs can be designed in this way and, while they would
not usually be loaded (cycled) that many times, if they are properly
designed they should last the life of the gun.
Finite Life:
Typically, airplanes would not fly if their main components were
designed for infinite life because the plane would be too heavy (for a
given loading, smaller components will have higher stresses and
therefore be more prone to failure). Thus we often design aircraft
compnents to survive for a limited specified time and inspect regularly
to look for crack initiation. At the specified time, the component is
replaced. This is "finite life" design.
S-N Curves:
We know that, for a given component, the higher the stress level, the
shorter the life of the component (the fewer number of cycles the
component can loaded and survive). The development of fatigue cracks is
very complex and we cannot predict this purely theoretically so we rely
on laboratory tests to develop "fatigue life curves" or S-N Curves".
These are graphs in which the cyclic stress level (S) is plotted on the
vertical axis and the number of cycles (N) on the horizontal axis. The
failure line starts at the fracture strength (failure after 1 cycle of
loading) and slopes down to the right until the endurance limit is
reached at 1 million cycles. Such an empirical curve is created for
every alloy so that we can design for infinite life or finite life. (if
you like, I can scan and email you such an example curve).
Low Cycle Fatigue:
When a component survives for only a few cycles (less than 1000) the
analysis is fairly complicated and nobody in their right mind designs
components to survive so few cycles (uncertainty is too high in
precisely predicting life). It may be that some components fail from
low cycle fatigue but this is most often simply poor design (or totally
ignorance of stress levels and failure modes).
Fatigue Life Limiting Factors:
There are many things which affect the fatigue life of a component.
Poor forging practice leaving slag inclusions or voids (not unusual in
poorly made antique firearms), poor heat treatment causing
overhardening, poor tempering leaving the material too brittle, poor
surface finishing which causes stress concentration, etc. Such factors
reduce the endurance limit for the component.
Thus endeth your first lesson (remember that you asked for it and
promised to stay awake). I hope that it is understandable and helps. I
will be happy to address any clarifications or further questions.
with best regards
