Stranded Wire Springs

By Dan Sebastian

We have all seen the graphic images of the Tacoma Narrows bridge, twisting and undulating before it collapsed. If you haven’t, you can look it up online. For most of us, it is the first time we watched an object in its natural frequency.

As a spring engineer, one of the fundamental things we need to check in any design is the natural frequency. Then we must check the operating environment to make sure the spring is not subject to motions that are at or near the natural frequency or its critical nodes (3, 5, 7, 11, 13). This is critical in engine valve springs but may be equally as important in firearms. There are several different things we can do to dampen the frequency wave, thus controlling a potentially critical failure mode.

The firearms industry has used stranded wire springs to control surging in rapid-fire modes for many years.

troops were instructed to only fire the weapon in short bursts. Why have a fully automatic weapon that could only shoot in small bursts?

After many soldiers were found dead on the battlefield with jammed weapons, an investigation was conducted. They concluded that problems with the ammunition, were due to the barrels not being chromed on the inside which allows for a harder and smoother path for the bullets and poor maintenance. The fix, at best, was marginal. A special team looked further into the problem and took high-speed photography of the weapon in fully automatic mode.

A team of engineers from Associated Spring was asked to review the films. I was fortunate enough to be one of the team members. The high-speed photography was amazing in that it revealed the dynamic surging by the weapon in full automatic mode. We submitted our findings, along with a proposal to use a braided wire spring to fix the problem. The design was quickly processed and filed with the “Indiana Jones and the Raiders of the Lost Ark” treasure.

Why do Braided Wire Springs Work?

There are three key issues that make braided wire springs work so well.

A study of the literature on the use of stranded wire springs shows the first large-scale use goes back to the 1930s when they were used in rapid-fire machine guns used by the Spanish during their Civil War. The guns were largely provided to the Communists by the Soviet Union in their struggle with the Fascists.

The earliest published equation I discovered was in a July 1946 edition of Product Engineering. A series of protocols on design with the equation for spring rate and stress calculations can be found in the “Encyclopedia of Spring Design Compression, Extension, Garter, and Torsion Springs,” pages S21-S22.9 The use of braided wire springs proved so successful that when the Soviets introduced the AK-47 as a rapid-fire infantry rifle, they used braided wire springs.

In 1964, the U.S. Army began using the M-16 rifle which was supposed to be a lightweight, rapid-fire automatic gun, ideal for jungle warfare. From the beginning, it was plagued with problems. One problem was the jamming of the gun in full automatic mode which was, initially blamed on poor maintenance and cleaning. Its reliability was so poor that our special operating troops preferred the AK-47 which they secured from the enemy after an engagement. At the operating level, the U.S.

1. Each strand is significantly smaller than the single wire spring, thus yielding very different natural frequencies.

2. The friction between the braided wires is very effective in dampening the potential natural frequency wave.

3. In most cases, the tensile strength (and the fracture toughness) of each individual strand is higher than a single wire. In the end, stranded wire springs are more expensive than conventional springs due to the cost of materials and the complexity of the manufacturing process. The problem, in this case, boiled down to a failure to accept some increased cost of production that would have resulted in a good deal of unnecessary loss of American lives. n

Dan Sebastian is a former SMI president. He holds a degree in metallurgical engineering from Lehigh University and his industry career spans more than four decades in various technical and management roles. He may be reached by contacting SMI at 630-460-8588.