spring strength: elasticity vs plasticity

fatcorgi

fatcorgi

Joined
Nov 12, 2011
Messages
658
for the engineers/knife makers/general metallurgy nerds: intuitively we imagine slipjoint springs getting softer with age, and a lot of us talk about breaking in a knife with a strong pull by working it open and shut continuously. but how true is this really? I have heard the terms elasticity and plasticity mentioned in relation to the temper of springs, and as long as the spring is kept within its range of its elasticity and doesn't go over into plasticity, it should last indefinitely. The breaking in of the archetypical heavy GEC springs in front of the TV speaks against this, but then we marvel at the strength of the springs of some old traditionals, and I think of SAK knives on display that spend 30 years continually in a half open position and still have perfect snap when they get sold.
 
I agree.
I think "breaking in" a new knife is more a case of smoothing some tiny burrs & smoothing/loosening between the liners & tang.
The spring shouldn't soften with use, certainly not over 100 cycles.
 
fatcorgi (awww) you are right. Under no circumstances should a backspring get softer simply because the knife is left partially open for a length of time- unless the design or spring is a poor one.

The ease of opening may however be improved by rounding of corners on the blade tang from wear, due to decreasing the distance the backspring is deformed during opening.
 
As Greg said, slip joints break in because of erosion of the contact surface between the blade and the spring.

As far as springs go, theoretically they will last forever if the load is kept within the elastic range. However, in reality a spring will eventually fail due to cyclic loading. The failure of springs is due to crack formation and propagation. This is called fatigue failure, and it occurs well below the yield strength of a material. Fatigue failure generally occurs on the order of several million cycles. Springs also do not lose their strength over time while under constant load. For a carbon steel spring, the ambient temperature is not high enough to cause creep to occur. Creep occurs in materials at one third the melting temperature. So in order for a carbon steel spring to lose its strength under loading, the ambient temperature would have to be around 850 degrees F
 
Goblin is spot on with the engineering /metallurgy analysis and discussion. Of course there can be defects that cause premature fracture or creep.
 
thanks, all very informative replies :)
I guess the smoothness of walk and talk of some older knives might be explained by the wear of minor burrs etc. and as I understand it from the metallurgy point of view, this means that cyclic loading is not a practical issue for any knife, unless we are an astronaut spending 1000s of years on planet Venus, with only one carbon steel pocket knife.
 
I'm thinking that "breaking in" also works residue from the manufacturing process out of the joints. A thorough cleaning is always something I recommend before returning a knife to the Manufacturer.
 
I do know that magazine springs soften if you keep them loaded all the time. So do recoil springs after many rounds soooo.....
 
Different situation there though--the magazine springs are in a static position, while the recoil springs are going through repeated cycles of compression and expansion.

I'm unconvinced mag springs soften as a general rule. I've seen too many quality (not wartime-manufacture) magazine springs that had been compressed for 40-50 years (or even longer) that are still perfectly functional. On the other hand, I've seen some not-so-good mag springs fail during their first few cycles. Last time I lucked into a batch of new-in-paper WWII-contract mags for the 1911, almost 1/3 of them needed new springs in order to function reliably.
 
a leaf spring (in slip joints) demonstrates flexibility of the metal. a helical spring also demonstrates flexibility of the metal. it is the helix shape that is elastic as a result. i can't believe that up to now, elementary and high school science books try to show "elasticity" in metals.
 
Magazine springs that are heat treated and designed properly should be able to stand being loaded for 100+ years without losing their tension. Obviously reality isn't that perfect but it shouldn't come up with a non-defective part in normal service. Same goes for pocket knives.
 
coffeecup said:
Different situation there though--the magazine springs are in a static position, while the recoil springs are going through repeated cycles of compression and expansion.

I'm unconvinced mag springs soften as a general rule. I've seen too many quality (not wartime-manufacture) magazine springs that had been compressed for 40-50 years (or even longer) that are still perfectly functional. On the other hand, I've seen some not-so-good mag springs fail during their first few cycles. Last time I lucked into a batch of new-in-paper WWII-contract mags for the 1911, almost 1/3 of them needed new springs in order to function reliably.
Click to expand...

I do understand the differences. Slipjoint springs are under tension always as well. Also, the act of opening and closing them compresses and expands the spring. I routinely change my recoil springs. I have never had one fail, just told that its always a good idea. Also my mags either work or dont. Changing the spring has fixed them but I also have no evidence that the spring failed over time. Again its one of those "Common Wisdom" things that may be incorrect.
 
You must log in or register to reply here.
Back
Top