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Old 10-18-2017, 11:24 AM
longyard longyard is offline
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Default Double Twisted Springs

I first saw a double twisted spring inside an AK-47 I owned. It was used to spring the hammer. It occurred to me that this type of spring is less likely to break, kind of like wire rope versus plain wire. If that were the case, it seemed to me a brilliant innovation by the untrained engineer Kalishnikov, whose design dates from, of course, 1947.

Years later, I looked inside a German MG-42 machine gun and saw the same type of double twisted wire spring also used on the hammer. Okay, so Kalishnikov copied the ideas from the Germans because the MG-42 obviously came out in 1942.

On a visit to the Technical Museum in Dresden, Germany, I saw a bicycle seat that also used the double twisted springs. The bike was produced as early as 1940, and probably earlier.

Can anyone comment on these types of springs? I'm I correct in believing it is a more durable way to spring a given load than having a single stranded wire spring?

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Old 10-18-2017, 12:39 PM
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I don't believe it makes them more durable but it will have a damping effect on the spring. It will also harbour moisture and make them more prone to corrosion-no good for a highly stressed spring.
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Old 10-18-2017, 08:40 PM
cliffrod cliffrod is offline
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How Would the thinner cross section affect the rate of fatigue or point of fracture/failure? Would each strand only need to provide a fraction (1/3, 1/2 or however many strands were used) of temper to provide the same cumulative load capacity when wound together to bear a load? A less hard or brittle wire would seem to be less prone to breaking when exposed to the same number of flexing cycles. Maybe someone here understands more and can explain.

All I know is that granite makes poor springs, although it does flex a little bit...
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Old 10-18-2017, 09:08 PM
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I'd tend to agree with Paul. Coulomb damping is used in springs to absorb energy. In this case probably to prevent resonant float. The bicycle seat, probably as a shock absorber-marketing gizmo.
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Old 10-18-2017, 09:21 PM
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Jack 1957 Jack 1957 is offline
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Stranded wire springs are especially suited to repetitive impact loading conditions. They are frequently used when impact stresses and velocities are so high that a single wire spring would not provide a long life cycle. Stranded wire consists of three to seven strands of wire that are machine-twisted or woven around each other, or around one wire that serves as a core, to form a single strand.
The stranded wire can be made of traditional music wire, which is manufactured out of tempered high-carbon steel, also known as spring steel, or from high tensile rocket wire, which can withstand even more compression. Stranded wire compression springs can damp migratory waves that traverse the spring under shock loading. If one strand breaks, the spring will still be functional and will continue working under stress even with some fracturing. Stranded wire springs are recommended when fatigue is the primary concern.
Military firearms have the most stringent requirements, and design specifications usually call for each component (including the spring) to be able to withstand 20-30,000 rounds of firing before it fractures and fails. The wire needed to achieve this performance standard in the compression spring may be significantly more expensive (almost double), but the life and death consequences of using the weapon in battle mean designers can easily justify spending a few cents more for a spring. This will avoid overstressing the spring or getting right to the spring's performance limits, and may provide as much as 15 percent more performance from the weapon.

By contrast, commercial manufacturers may be seeking only about 10-15,000 rounds and may opt for a spring that will not wear as long, but will be more cost effective. Typically, military/government weapons are designed around the springs and their requirements, while commercial entities frequently try to get springs to work later in the design stage after sourcing other longer lead time components.
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Old 10-19-2017, 01:18 AM
longyard longyard is offline
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Wow Jack! Thanks for that detailed explanation.
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Old 10-19-2017, 03:20 AM
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One company making and selling this type of spring is ROHRS and they list as the benefits for this design....

(As with all "claims" for people selling anything they are held in suspect until independent testing confirms them.)

ROHRS list:

The essential advantages at a glance:

Absorption of high impact velocities and peak loads
Absorption of high dynamic frequencies
Excellent damping features
Optimization of damping and gliding features as well as durability by means of thermoplastic coating
Optimum utilization of material in the cross section of the wire strand by converting torsion into tensile stresses
High operational safety, as the spring force is virtually unaffected by breakages within the strand
High deflections possible, even given small outer diameters
Flat spring characteristic, increasing progressively when close to solid length
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Old 10-19-2017, 05:11 AM
longyard longyard is offline
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Thank-you Kirk. The factor of converting torque into tension is interesting. I'll have to ponder that one.
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Old 10-19-2017, 05:56 AM
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Fwiw: I used to have twisted springs on a leather bicycle seat and they are the only seat springs that I managed to break. The wires constantly scrape against eachother which causes a lot of aditional wear.
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Old 10-19-2017, 07:46 AM
cliffrod cliffrod is offline
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Quote:
Originally Posted by Jack 1957 View Post
Stranded wire springs are especially suited to repetitive impact loading conditions. They are frequently used when impact stresses and velocities are so high that a single wire spring would not provide a long life cycle. Stranded wire consists of three to seven strands of wire that are machine-twisted or woven around each other, or around one wire that serves as a core, to form a single strand.
The stranded wire can be made of traditional music wire, which is manufactured out of tempered high-carbon steel, also known as spring steel, or from high tensile rocket wire, which can withstand even more compression. Stranded wire compression springs can damp migratory waves that traverse the spring under shock loading. If one strand breaks, the spring will still be functional and will continue working under stress even with some fracturing. Stranded wire springs are recommended when fatigue is the primary concern.
Military firearms have the most stringent requirements, and design specifications usually call for each component (including the spring) to be able to withstand 20-30,000 rounds of firing before it fractures and fails. The wire needed to achieve this performance standard in the compression spring may be significantly more expensive (almost double), but the life and death consequences of using the weapon in battle mean designers can easily justify spending a few cents more for a spring. This will avoid overstressing the spring or getting right to the spring's performance limits, and may provide as much as 15 percent more performance from the weapon.

By contrast, commercial manufacturers may be seeking only about 10-15,000 rounds and may opt for a spring that will not wear as long, but will be more cost effective. Typically, military/government weapons are designed around the springs and their requirements, while commercial entities frequently try to get springs to work later in the design stage after sourcing other longer lead time components.
Very cool. I like smart people. Thanks, Jack.
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