Craft Gunmaking, No compromises, No corners cut, EVER

Tuesday, March 24, 2020

Spring Has Sprung

So it's as good a time as any to discuss those flexible, boingey bits, without which guns couldn't function.  I'll cover each of the different types of spring most commonly found in various gun designs, the materials that they're made from, their design and application.  I will not get into any of the non-ferrous alloys such as stainless steels, copper alloys or brass alloys since they, for the most part, are uncommon in gun mechanisms.  For the same reason, there will be no discussion of the more "exotic" materials from which springs have been and are made such as wood and fiberglass.  Belleville and air springs will also not be discussed, since their appearance in firearms is quite rare.  The explanations herein will be very simplified for the sake of brevity and to prevent any deaths due to terminal boredom.

Springs in gun mechanisms serve to return a part or parts to their original position by releasing the potential energy that is stored within the spring, when it is deflected by the movement of the part that it drives.  In simple terms, this happens because the amount of deflection in the spring remains well within the elastic range of the spring's material.  This range will vary based on such variables as the material from which the spring is made, the heat treatment of the material and the actual design of the spring.  Depending upon those variables, if the spring were loaded beyond its elastic range, it may simply deform or it may actually fracture.  Most leaf and "V" springs in gun-specific applications are made from high-carbon plain steels with a carbon content of 0.70 to 0.95 percent.  Coil springs in gun applications are generally made from music wire, which has a carbon content of 0.80 to 0.95 percent and very high tensile strength, which comes from the cold drawing process by which music wire is formed.

Generally, the springs found in firearms will experience either of two types of stress:

bending (as a beam)

or torsional (twisting).

Which type is not always intuitively obvious which you will soon see.

The term "spring rate" will also come up.  What this means is that for X amount of force applied, the spring will deflect X amount.  For example, assume a straight-rate coil spring of 10 pounds per inch.  What this means is that 10 pounds of force applied to the spring will cause it to deflect 1 inch, 20 pounds will be 2 inches, 30 pounds will be 3 inches and so on in that fashion.  That would be what is referred to as a "straight-rate" spring, easily recognized by its equally spaced coils.  There are also what are referred to as "variable-rate" springs, easily identified by their more closely spaced coils at one end.  In a variable-rate spring, the more closely spaced coils make up the softer portion of the spring and when those closely spaced coils stack solid, then the rest of the spring comes into play.  The reason that the area of more closely spaced coils is "softer" (it has a lower rate) is because that area of the spring is actually longer, which is why there are more coils and why they must be more closely spaced.  Variable-rate springs are rarely found factory installed.  They are common in the aftermarket as recoil springs for semiauto pistols.  Their value in these applications is dubious.

You may have noticed that dashed line in the center of the beam in the first drawing.  It is what is known as the neutral axis, which is the cross-sectional point where the opposing forces of bending (or twisting, in a torsion bar) change direction and cancel each other.  In the case of the beam in the following drawing, the upper surface is experiencing compression and the lower surface is experiencing tension.  Since these forces cancel each other out at the NA, it should be obvious that they are greatest at the outer surfaces and diminish toward the NA.  The NA is not always at the centerline of the stressed portion of a beam or spring.  It will coincide with the centerline only in a beam of symmetrical cross-section.  Calculating the NA for a non-symmetrical part is a bit beyond the scope of a blog post but it generally moves toward the more massive side.  It should also be obvious, now that you know this, why tool marks (especially across the surface) would shorten the lifespan of a leaf spring.

Now, on to different types of springs:

The torsion bar, not to be confused with a torsion spring:  A torsion bar is a spring that acts by being twisted about its long axis when a torque force is applied.  For a given material, diameter and load, the longer the torsion bar, the lower its spring rate will be.  This means that the longer it is, the more it will flex under a given load.  Torsion bars are not commonly encountered in firearms but understanding its function is essential because it is identical to its first cousin, the coil spring.

The coil spring:  The single most commonly used type of spring in all firearms is the coil spring.  They will be found in both compression and extension types.  It is commonly assumed that when a coil spring is compressed (or extended), that the coils bend, this is not the case.  When a coil spring is deflected, the entire spring (assuming a straight-rate) experiences torsional strain through the cross-section of the wire diameter, throughout its entire length.  This is why a coil spring gets shorter when compressed (or longer when extended).  Take a torsion bar and wind it perpendicular to its long axis into a spiral, you now have a coil spring.  As with the torsion bar, for a given diameter and material, the more coils, the lower the spring rate (it will be softer).  The coil spring has the possible advantage of still functioning even when broken, assuming it is sufficiently guided, either internally (with a guide rod) or externally (in a tunnel), and the broken ends do not "thread together" and induce coil bind sufficient to restrict movement of the mechanism.  Coil springs that have a uniform coil pitch are straight-rate springs while coil springs that have an unequal coil pitch are of variable rate.

The leaf spring:  This type of spring is commonly used as the mainspring (hammer spring) in many repeating rifles, single shot rifles and shotguns and a number of revolvers.  It is basically a deflecting beam.  It is invariably tapered as well, in either plan view or elevation, sometimes both, with the thickest point being at the anchor.  The reason for the taper is to load as much of the free length as uniformly as possible, without stress concentrations that would affect its rate and service life.

The "V" spring:  Maybe the second most common type of spring in the gun world and the single most common where my work is concerned.  In most (non-American) doubles, the hammers, ejectors and toplevers are powered by springs of this type.  It's basically two opposing leaf springs, joined at the fat end.  The limbs always work opposite each other.

The final commonly encountered spring type that I'll touch on here is the torsion spring.  Contrary to its name (and non-intuitively), it does not work in torsion at all.  Its name is derived from the way in which it drives whatever part it is driving.  That is, around a shaft or pivot pin.  This type of spring experiences beam-type bending stresses, from one end, around all of its coils and all the way to its other end.  For a given wire diameter and number of coils, the longer the "legs", the lower the rate.  Similarly, for a given wire diameter and leg length, the more coils, the lower the rate.  This is because in both cases the spring is longer.

Now that the boring stuff is out of the way, let us debunk some more "gun-world lore" because, like it or not, the gun world is not exempt from the laws of physics.

Cutting springs to make them softer:  Shortening the length of any spring INCREASES its rate.  That means it is stiffer, NOT softer.  "Bullshit!" cries the kitchen table trigger jobber, "I cut the rebound spring in all of my Smith action jobs and it works."  No Cleetus, it doesn't.  What happens is that, by cutting the spring, the installed preload is less because of the spring's now-shorter free length.  It therefore exerts less pressure when installed and because the spring is now shorter, it also does not compress as far during its cycle.  The only thing you've lessened is the reliability of the trigger return.

If the logic of cutting springs were sound, the short piece of spring that was cut off would be much softer that the rest of the spring.  This is easily proven to be false:  Take an old 1911 recoil spring (or any coil spring) and cut 4 coils off of it.  Now try to compress those 4 coils.  Let me know how that works out for you.

Leaf vs. Coil Mainsprings:  If I had a buck for every word of flowery prose written by gunwriters who've never competently operated a screwdriver about the virtues of V springs over coils, I'd be pretty well off.  Apparently, V springs are "snappier" (whatever the hell that means), faster, stronger, less filling, taste better and are generally superior in every way to coil springs.  It is a bit hard to argue while being intellectually honest, since (with one exception) there are no guns that are identical in design except for their form of mainspring.  With so many differing designs and lockwork geometry encountered, an apples-to-apples comparison isn't really possible.  It's a bit like arguing that purple is better than Ferrari.  The above referenced exception to this case is The Perazzi MX-series over/under.
These guns utilize a detachable trigger group that is available in both V and coil spring form.  So in that case a direct comparison is possible. 

The mainspring's job is to accelerate the hammer with sufficient speed to deliver an impulse to the firing pin, sufficient to detonate the primer.  Ideally, this will be accomplished in the shortest possible span of time.

Let us assume that the mass of the hammers is the same for both spring types, that the energy stored in the deflected springs is the same and the hammer travel is also identical.  At that point the most important variable is the weight of the springs.  I don't mean the weight, as in the "pressure" that the springs exert, we've already assumed that that is identical.  I mean the actual physical weight of the parts themselves.  This is important because the spring, regardless of type, must not only accelerate the hammer but its own mass as well.

Obviously the spring that has more mass will be slower to accelerate, so the lighter one wins.  Is the coil spring and its guide rod (which must also be taken into account) lighter in weight than the v spring and its swivel link?  I don't know because I've never compared the weights of these parts but it seems reasonable to think so.  Let's assume just for the sake of argument that it is slightly lighter.  Can most people discern the microseconds of difference in lock time?  That seems unlikely since most shooters can't discern a 4 pound trigger pull from a 5 pound pull.  What about the much-touted "snappiness" of the v spring?  If the hammers in both cases are accelerated at a rate that differs in microseconds, how can the claim that one is "snappier" than the other stand?  The reasonable conclusion seems to be that the vast majority of shooters would never know which trigger group that they were shooting, if they didn't already know beforehand.  Now there are some firearms whose lock time is so glacially slow (Colt SAA and 1911) that it is readily apparent to the shooter, Perazzi MX guns are not among them.

Then there are those who fetishize the v spring because they imagine Luigi the white-haired gunsmith, looking over his wire-rimmed glasses, hand filing and polishing their beloved v springs to perfection before heat treating them in the last remaining cup of whale oil on the planet and finally hand polishing them again before installation.  The reality is a bit different.  Perazzi wire EDMs the v springs by the hundreds, they are mechanically polished and heat treated in huge batches and then polished again, mechanically.  Each one is identical to the last and the next.   This is how Perazzi's v springs can be so supremely reliable.  If it's as reliable as a coil spring and offers little to no tangible advantage, then why would they offer it at all?  Well, for the same reason that they still make things like barbecue-flavored potato chips and tarted-up Toyotas called "Lexus": because people will buy them. 

My verdict:  Find something more important to argue about, like how many angels can dance on the head of a pin.


  1. Dewey,

    Everything I ever wanted to know about springs but was afraid to ask. Interesting post!
    I suppose you noticed your post about barrel wall thickness measurement on the Doublegunshop forum has brought out a few "internet experts".

    Keep the posts coming.


  2. Yeah, you can lead the horse to water but you can't make him think, that's what Google is for.