Extension Springs

Chapter One – What is an Extension Spring?

An extension spring is a type of helical spring that is tightly wound to offer resistance when stretched, and it returns to its original shape once the force is removed.

These springs are characterized by their tightly coiled design, which creates an initial tension within the coils. This initial tension provides resistance when a force is applied to stretch the spring. The tension determines the coil's tightness and attachment. Typically, extension springs have loops, hoops, or coils at both ends to facilitate attachment and extension.

Extension springs are effective in storing and absorbing energy while resisting pulling forces. When a force causes the attached components to move apart, the spring’s tensile strength pulls them back to their original position. They are commonly used in various applications, such as in hardware, washing machines, doors, toys, aircraft landing gear, truck hoods, and more. A diagram of an extension spring is provided below:

Chapter Two – What materials are used for extension springs?

Springs can be made from various materials, and selecting the appropriate one is crucial for optimal performance. The choice of material largely depends on the intended application of the spring. Key factors to consider when choosing a material for spring manufacturing include:

• Spring parameters
• Corrosion resistance requirements
• Operating temperature

Carbon steel is one of the most frequently used materials in spring manufacturing. Nonetheless, several other materials are also utilized, including:

High Carbon Steels

High carbon steels contain 0.60% up to 1.00% carbon, which gives high carbon steel greater strength and allows it to be heat treated to increase its hardness and toughness. Various alloying elements are added to high carbon steel to increase its tensile strength over low carbon steel, making high carbon steel the most common steel for making springs.

To enhance the strength and operating temperature range of high carbon steel, alloying elements such as chromium, manganese, molybdenum, nickel, silicon, and vanadium are added. The concentration of these alloying elements can range from 0.1% to 50%.

During the smelting of high carbon steel, base metals are separated, and alloying elements are introduced in varying proportions based on the desired properties of the steel. For instance, vanadium is added to increase toughness, while silicon and chromium are used to enable the steel to withstand higher temperatures.

Alloy steels used for extension springs are generally categorized into two main types:

Chrome Vanadium alloy offers high strength and toughness, making it ideal for applications involving high impact forces and shock loading.

Chrome Silicon alloy provides superior tensile strength and can perform at elevated temperatures compared to high carbon steel or Chrome Vanadium steel.

For specialized uses, Chrome Silicon Vanadium alloy combines the benefits of both Chrome Vanadium and Chrome Silicon, delivering exceptional strength and impact toughness in a carbon-based steel.

High carbon steel is further classified into three types

• Music Wire

  

  
  
• Hard Drawn MB

  

  
  
• Oil Tempered MBM

  

  
  

These materials are highly magnetic and possess excellent tensile strength. They can be easily cold drawn to achieve the desired properties and are suitable for low to moderate stress applications. Additional characteristics, such as available specifications, nominal chemistry, density, minimum tensile strength, modulus of elasticity, modulus in torsion, and maximum operating temperature for these high carbon steel extension springs, are detailed in the table below:

Stainless Steels

Stainless steel extension springs are commonly used in production due to their cost-effectiveness. They offer good to moderate corrosion resistance and exhibit specific magnetic properties.

Stainless steel expansion springs are further classified into four types:

• Stainless Steel 302

  

  
  
• Stainless Steel 304

  

  
  
• Stainless Steel 316

  

  
  
• Stainless Steel 17-7 PH

  

  
  

Stainless steel extension springs are generally used for their heat resistance, with Type 316 offering superior corrosion resistance compared to Types 302 and 304. Type 304 stainless steel contains less carbon than Type 302, but both are similarly valued in the market. All four types of stainless steel extension springs are slightly magnetic and are initially fabricated as springs before undergoing precipitation hardening to achieve the desired properties. Similar to high carbon steel, stainless steel also possesses attributes such as nominal chemistry, density, modulus in torsion, modulus of elasticity, minimum tensile strength, and maximum operating temperature. These properties and their respective values are detailed in the table below:

Alloy Steels

Alloy steel is a type of steel that is enhanced with elements such as manganese, molybdenum, nickel, chromium, and vanadium. These alloying elements are added in amounts ranging from 0.1% to 50% to improve the steel's strength, hardness, and toughness. Alloy steels used in extension spring production are typically categorized into two main types:

• Oil Tempered Chrome Vanadium Spring

  

  
  
• Oil Tempered Chrome Silicon Spring

  

  
  

Oil-tempered chrome vanadium and silicon steels are magnetic and suitable for handling shock loads at moderately high temperatures. Oil-tempered chrome vanadium is available in valve spring quality under ASTM A232, while oil-tempered chrome silicon is available in valve spring quality per ASTM A877. Additional properties of these alloy steels are detailed in the table below.

ASTM A232 Steel - ASTM A232 is a chromium-vanadium spring steel known for its high tensile strength among wrought alloy steels. It features low ductility and high thermal conductivity, making it a top-quality valve spring material that maintains uniform quality and temper. This steel is ideal for valve springs that require high fatigue resistance and are exposed to moderately high temperatures.

ASTM A877 Steel - ASTM A877 is a chromium-silicon steel wire designed to endure shock loads, stress, and high temperatures. It undergoes cold drawing and heat treatment to enhance its strength and durability. This steel is renowned for its ability to withstand rapid and repeated loading without deforming or compromising performance.

Copper Alloys

Copper alloys are commonly used in the production of extension springs due to their excellent electrical conductivity and high corrosion resistance. These alloys are particularly suited for applications in marine environments, low temperatures, and electrical components.

Copper alloys are further divided into five distinct types:

• Phosphor Bronze - Phosphor bronze is one of the most used of the copper alloys for the manufacture of springs. It is resistant to chemical corrosion, wear, and fatigue. The resilience of phosphor bronze is the reason that it is so often used to produce springs. It is made of a combination of tin, zinc, iron, lead, and phosphorus with a tin content of 5% to 7%.

  

  
  

• Beryllium Copper - Beryllium copper meets the standards of ASTM B194 and is referred to as BeCU or Alloy 25. Like copper, BeCu is very ductile, which makes it easy to shape and form. Its resistance to corrosion and oxidation makes it an excellent choice for manufacturing small extension springs.

  

  
  

• Brass - Brass is a zinc copper alloy and is ideal for making strong flexible springs that can store high amounts of potential and mechanical energy. It is the least expensive of the copper based alloys and retains its strength at subzero temperatures.

  

  
  

• Monel 400 - Monel 400 springs have enhanced strength, corrosion resistance, and torsion. They are heat treated to increase their hardness, which significantly improves their performance. Monel 400 is a nickel copper alloy that is resistant to the effects of acids, especially hydrochloric and hydrofluoric. The springs produced from monel 400 are commonly used in brackish and seawater conditions.

  

  
  

• Monel K 500 - Monel K 500 is a precipitation hardened nickel copper alloy that has corrosion resistance combined with exceptional strength and hardness, properties that are increased by the addition of aluminum and titanium using a thermal processing method. It maintains its strength and properties up to 1200°F.

  

  
  

Copper alloys offer excellent conductivity and heat resistance, making them suitable for high-temperature applications. Monel 400 and Monel K-500 are effective in subzero and cryogenic environments, with Monel 400 also showing resistance to hydrosulfuric, sulfuric, and hydrochloric acids, making it ideal for marine and chemical applications. Additionally, beryllium copper and phosphor bronze are non-magnetic and perform well at low temperatures. These alloys can achieve desired properties through precipitation hardening after spring fabrication. Further details on the properties of copper alloys are provided in the table below:

Nickel Alloys

Nickel is an exceptionally versatile element, easily alloyed with many metals. It offers excellent heat resistance and can endure extreme temperatures. Nickel alloys are known for their durability, strength, and reliability, making them highly sought after for extension spring manufacturing.

Nickel alloys are further categorized as

• Inconel 600 - Inconel 600 is a nickel chromium iron alloy that has good corrosion and oxidation resistance with high strength at high temperatures. Its high nickel content protects it from corrosion cracking when exposed to chloride ion stress. Inconel 600 maintains over 70% of its strength at 1100 °F.

  

  
  

• Inconel 718 - Inconel 718 is an alloy of nickel, chromium, columbium, and molybdenum. It has exceptional strength and maintains its strength at - 423°F up to 1300°F. The excellent fatigue, creep, and rupture strength of Inconel 718 makes it ideal for the production of springs.

  

  
  

• Inconel X750 - Inconel X750 is an exceptionally strong alloy that is widely used in the oil and gas and material handling industries. It maintains its strength at extreme temperatures and is resistant to oxidation and relaxation regardless of the conditions.

  

  
  

• Elgiloy - Elgiloy is a non-magnetic alloy containing cobalt, chromium, nickel, and molybdenum. The combination of its alloys gives Elgiloy exceptional strength, corrosion resistance, and fatigue strength. It is resistant to cracking in the presence of sulfides and is immune to the effects of crevices, pitting, and stress corrosion.

  

  
  

• Hastelloy (C276) - Hastelloy, as with many of the nickel alloys, is exceptionally strong. It is produced by combining chromium, molybdenum, iron, and nickel. To further enhance its properties, Hatelloy may include carbon, tungsten, vanadium, and titanium. There are five different types of Hastelloy, which are B, C, G, X, and N type.

  

  
  

Nickel-based materials are valued for their excellent heat resistance and performance in chemical environments. Hastelloy is renowned for its superior resistance to sulfur and chloride compounds, making it widely used in marine and waste treatment applications. NiSpan C is magnetic and finds application in timing devices, weighing systems, and geophysical equipment. This material is precipitation hardened after spring fabrication to achieve the desired properties. Inconel 600 performs exceptionally well at high temperatures and in cryogenic conditions, and it is commonly used in aerospace and heat treatment processes. Additional key properties of nickel alloys are detailed in the table below:

Plastic Composite

Plastic composites offer outstanding thermal properties and excellent chemical resistance. They are highly magnetic and have low flammability.

Ultem is a type of plastic composite known for its high magnetic properties and low flammability. It also exhibits excellent heat resistance and favorable thermal properties. Additional features of Ultem are listed in the table below:

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Chapter Three – What are the different finishes for springs?

Extension springs are often treated with various coatings to improve their properties, such as conductivity, heat resistance, and corrosion protection. The main types of finishes for extension springs include:

Black Oxide

Black oxide is a cost-effective coating widely used for its durability and ability to prevent corrosion while providing a robust finish.

Gold Iridite

Gold Iridite is a chemically applied coating that enhances both conductivity and durability of the springs.

Passivation

Passivation involves applying a thin layer of oxide or nitride to the spring, which enhances its corrosion resistance.

Zinc

Zinc coating involves galvanizing the coils to provide a protective layer that guards against corrosion.

Chapter Four – How Extension Springs Are Made?

Key Parameters for Designing Extension Spring

Several key parameters and specifications must be considered when designing extension springs to ensure they are resilient and effective. These essential factors include:

• Outside and inside diameter of spring
• Wire diameter
• The type of material that is used
• Initial tension of the spring
• Load that a certain length can get hold of
• Rate pounds per inch of extension
• The length that is installed and maximum extended length in application
• Type of spring ends
• Position of ends relative to each other
• Gap opening in hooks

Steps for Extension Spring Manufacturing

Mechanical springs are ubiquitous in our daily lives. While the manufacturing process for extension springs is generally simple and straightforward, variations can occur depending on the specific type of spring being produced. In this chapter, we will explore the manufacturing process for extension springs, which involves three main steps:

• Winding
• Heat treating
• Coating and finishing

Spring Winding

The first step involves feeding the spring wire into mechanical machinery that straightens the wire and shapes it into the desired form. The process of straightening and coiling the spring wire is detailed below:

Spring Coiling

The wire is passed through a set of rollers to the spring coilers or computer numerical control (CNC) machine. It is then guided to the coiling points where it is wound to form the spring shape.

Forming Machinery Use

The spring is then transferred to a forming machine, which is equipped with six to eight tooling sides that shape the spring into various designs.

Wire Bending

Next, the spring is moved to the CNC wire bender, where a movable tooling head guides the wire and bends it into the desired shapes.

Heat Treating

After the spring is formed, it undergoes a stress-relieving process. The spring is transferred to a conveyor belt oven, where it is heated to a specified temperature for a set duration, depending on the type of spring and the material used. Following heating, the spring is allowed to cool in a designated receiving box before moving on to the next step.

Coating and Finishing

The final step in spring production involves coating or finishing the spring. During this process, the spring is treated with additional layers of various elements to enhance its properties, such as heat resistance, corrosion resistance, or overall durability. Common coating processes include:

• Shot peening
• Plating
• Powder coating

Chapter Five – How does the mechanism of an extension spring work?

Extension springs are renowned for their ability to store mechanical energy in their coils and release it as they expand. These springs are designed to handle tension, absorbing and storing pulling energy to resist a force applied to their ends. Typically, extension springs are connected to components at both ends, with hooks or loops allowing the application of a pulling force. When a force is applied and the components move apart, the spring attempts to return to its original position. The initial tension within the spring determines the tightness of the coils, which is crucial for assessing the load capacity for specific applications. While the spring adheres to Hooke’s law when not stretched or compressed, it follows simple harmonic motion when a load is applied. The ends of extension springs experience greater tension due to the energy stored in the loops or hooks, while the coils, when tightly wound, are stress-free. This distribution of energy between the hooks and coils can impact the spring’s performance.

Chapter Six – What are the types of extension spring ends?

Extension springs are equipped with various types of end fittings or hooks that are used to connect the spring to the force source. It is important to carefully consider these ends to prevent undue stress on the spring due to initial tension. Although there are several types of hooks or loops designed for different applications, machine loops and crossover loops are among the most commonly used. The loop ends of extension springs are classified based on whether they require specialized tools for manufacturing. The types of loop ends that do not require special tools are illustrated in the figure below:

Chapter Seven – What are the applications of extension springs?

Extension springs come in various hoop and loop configurations, each serving different functions. These springs are widely used in everyday applications, ranging from car garages to automobiles, toys, electronics, and beyond.

Below are some common applications of extension springs in daily life:

Automotive Uses

Extension springs are found both inside and outside vehicles. While their use has decreased in modern cars, they were once a critical component in the vehicle's carburetor system.

Garage Doors

Extension springs are commonly used in garage doors, where they are mounted on either side to assist with the door’s movement. These springs help reduce the effort required to open and close the door.

Springs in Trampolines

Trampolines rely on extension springs to provide their bouncing effect. The springs at the base allow users to rebound and continue jumping effortlessly. A greater number of springs typically results in increased bounce.

Toy Applications

Extension springs are widely used in toys, particularly those involving projectile motion, such as toy guns and cars. They are essential for toys that feature rapid motion or throwing actions.

Vise Grip Pliers

Extension springs in vise grip pliers ensure a secure grip on objects, keeping them firmly locked in place once clamped.

Washing Machines

In washing machines, extension springs are crucial for stabilizing the spinning drum. They help prevent excessive noise by ensuring the drum remains steady and does not knock against the sides of the machine.

Medical Devices

Extension springs are used in a variety of medical devices, such as stretchers, surgical lights, and various instruments, to provide necessary mechanical functions.

Farming Equipment

Extension springs are utilized in farming equipment like harvesters, tractors, and ploughs to assist with heavy-duty tasks and make agricultural work more efficient.

Baby Carriages

Extension springs in baby carriages allow them to gently rock back and forth with minimal effort, providing a smooth and soothing motion for the baby.

Fence Gates

Extension springs in fence gates enable them to close automatically after being opened, enhancing convenience and functionality.

Magazine Springs

Magazine springs provide the necessary tension to push cartridges into the firing chamber of a firearm’s magazine, allowing for rapid and reliable feeding. Made from high carbon steel, these springs are designed to withstand repeated use without losing strength.

Recoil Spring

The recoil spring, situated within the bolt of an automatic or semi-automatic firearm, absorbs the shock generated when a bullet is fired. This helps reduce the recoil experienced by the shooter, making the firing experience more comfortable.

Conclusion

• Extension springs are closely wound coils that store and absorb energy and are bound to resist the force applied to them.
• There is initial tension exists in the coils of the spring, which determines how tightly the coils are attached.
• Many different types of materials are used for the production of extension springs such as copper alloys, carbon steels, stainless steels, nickel alloys composite etc.
• Stainless steel is the most economical and is used widely for spring production.
• Different types of finishes are done to give the spring more durability and conductivity properties.
• Certain Parameters need to be considered in extension spring production such as spring diameter, wire diameter, initial tension, loop ends and many more.
• Many different types of loop ends are also available and are specific in their functions. Some need tools for production, others do not.
• Extension springs are used in our daily life in many things such as garage doors, toys, appliances, fence gates, and many more.

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