Extension Springs

Chapter One – What is an Extension Spring?

"A helical shaped closely coiled wire that offers resistance when extended, and gets back to its original shape is called an extension spring."

Extension springs are helical wound springs that are so closely coiled together to create initial tension in the coils. This initial tension creates resistance against the force applied to its ends for extension. The initial tension helps determine how closely and tightly the coils are attached. An extension spring has end coils, hoops, or loops that are usually present at both ends to help in the extension of the spring when any other object is attached to it.

These springs are good at absorbing and storing energy and also to resist a pulling force. Each end of the spring is attached to any component and when a force is applied to move these components apart, the tensile strength of this spring brings them back to its original point. The purpose of an extension spring is actually to provide an extended force when a spring is pulled apart from its original position. A wide range of extension springs are used in everyday life for different purposes such as hardware, washing machines, doors, toys, aircraft landing gears, hoods of heavy trucks, and many more. Extension spring is shown in the figure below:

Chapter Two – Extension Spring Materials

There are many different types of material for spring production, and the choice of material depends on its application. Choosing the right material for spring production can make a difference in its performance. Some common elements are needed to be considered before spring production and that are:

• Spring parameters
• Corrosion resistance requirements
• Operating temperature

For spring production, the most common type of material that is used is steel. However, many other types of materials are used for its production like:

High Carbon Steels

High carbon steels are the ones with a high amount of carbon content in them which provides a great strength and hardens when it comes in contact with heat.

High carbon steel is further classified into three types:

• Music Wire

• Hard Drawn MB

• Oil Tempered MBM

These are highly magnetic with great tensile strength, easily cold drawn to desired properties and are used for low to average stress application. There are some other properties like available specifications, nominal chemistry, density, minimum tensile strength, modulus of elasticity, modulus in torsion and maximum operating temperature of all these three high carbon steel extension springs which are shown in the table below:

Stainless Steels

Stainless steel extensions spring are widely used for production because they are economical, exhibit good to average corrosion resistance and 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

These stainless steel types of extension springs are usually used as general heat resistant, but type 316 shows better corrosion properties than 302 and 304. Stainless steel type 304 has less carbon content than 302, but they are considered equally in the market. All four types of stainless steel extension springs are slightly magnetic and in order to achieve desired properties material is first spring fabricated and then precipitation hardened. Like high carbon steel, stainless steel also exhibits properties like nominal chemistry, density, modulus in torsion, modulus of elasticity, minimum tensile strength and maximum operating temperature. All these properties with values are shown in the table below:

Alloy Steels

It is also a type of steel, but it is alloyed with other elements like manganese, molybdenum, nickel, chromium, vanadium, etc. This process is done by adding the alloying element varying from 0.1% to 50%, and it is done to add strength, hardness, and toughness to the steel. Alloy steels for extension springs production are further classified into two types that are:

• Oil Tempered Chrome Vanadium Spring

• Oil Tempered Chrome Silicon Spring

Oil tempered chrome vanadium and silicon; both are highly magnetic and are used for shock loads with moderately elevated temperatures. Oil tempered chrome vanadium is available in valve spring quality in accordance with ATSM A232 whereas, oil tempered chrome silicon is available in valve spring quality under ATSM A877. Other properties of these alloys steel are shown in the table below:

Copper Alloys

Copper alloys are well known in the production of extension springs because of their better electrical conductivity and high corrosion resistance. Copper alloys are widely used for marine, low temperature and electrical spring applications.

Copper alloys are further classified into five (5) different types:

• Phosphor Bronze

• Beryllium Copper

• Brass

• Monel 400

• Monel K 500

As stated earlier, copper alloys exhibit better conductivity and good heat resistance properties, so all these work well in elevated temperatures. Monel 400 and Monel k 500 are good for subzero and cryogenic temperature applications, whereas Monel 400 is generally resistant to hydrosulphuric, sulfuric, and hydrochloric acids. And it is typically used for marine and chemical processes. Other than these properties, Beryllium copper and phosphor bronze are non magnetic and work well in cold temperatures and can achieve desired properties by hardening the precipitation after spring fabrication. Other properties of copper alloys are shown in the table below:

Nickel Alloys

Nickel is the most versatile element of all and can be easily alloyed with most of the metals. It has an excellent heat resistance and can withstand harsh temperatures.

These are highly durable, strong, and reliable elements, and because of these properties, nickel elements are in high demand for extension spring manufacturing.

Nickel alloys are further categorized as:

• Inconel 600

• Inconel 718

• Inconel X750

• Elgiloy

• Hastelloy (C276)

These materials have good heat resistance and work better in a chemical environment. Hastelloy is known for its best resistance to sulfur and chloride compounds and used widely in marine and waste treatment industries. NiSpan C is magnetic and is used in timing devices, weighing, and geophysical industries. To achieve desired properties, the material is precipitation hardened after spring fabrication. Inconel 600 works best in elevated temperatures and cryogenic environments. It is widely used in aerospace and heat treating processes. Other important properties of nickel alloys are mentioned in the table below:

Plastic Composite

Plastic composite has exceptional thermal properties and great chemical resistance. It is highly magnetic and is low inflammable.

One type of plastic composite is ultem. Ultem is a plastic composite, highly magnetic, and is low inflammable. It also shows great heat resistance and good thermal properties. Other features of ultem are given in the table below:

Chapter Three – Spring Finishes

Extension springs are further treated with different types of coatings to enhance their existing properties such as conductivity, heat and corrosion resistance, etc. The different types of extension spring finishes are:

Black Oxide:

Black oxide is the most inexpensive spring coating that is used widely. It is inexpensive and the best way to put off corrosion and also gives a finish that is durable.

Gold Iridite:

Gold Iridite is a chemically processed coating that is done to the spring to offer high conductivity and durability.

Passivation:

Passivation is a process to coat a thin layer of oxide or nitride on the spring, and it is done to boost the corrosion resistance of the metal.

Zinc:

The metal is coated with a galvanized layer of a shield on the coils. It is done to protect the coils from corrosion.

Chapter Four – How Extension Springs Are Made?

Key Parameters for Designing Extension Spring:

There are some key parameters and specifications that are needed to be considered before designing extension springs. These are crucial for designing a resilient and effective extension spring. These are:

• 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 everywhere and used in our daily life. The process of manufacturing extension springs is simple and straightforward, but sometimes there are variations because of the type of spring made. In this chapter, we will discuss how extension springs are manufactured. The process consists of three steps:

• Winding
• Heat treating
• Coating and finishing

Spring Winding:

This is the first step in which the spring wire is fed to the mechanical machinery, which straightens up the wire coming from the coil and then moulds it into any desired shape. The steps involved in straightening and coiling of the spring wire are discussed below:

Spring Coiling:

The wire is fed to a set of rollers to the spring coilers or computer numerical control (CNC). The wire is guided to the coiling points where it is coiled backward to form a spring.

Forming Machinery Use:

The spring is then sent to the forming machine which has six to eight tooling sides responsible for forming different spring shapes.

Wire Bending:

It is then transferred to the CNC wire bender, which guides the wire to a movable tooling head responsible for bending the wire into desired shapes.

Heat Treating:

When the spring is formed, the next step is a stress relieving process. The spring is then sent to the conveyor belt oven and heated for maximum time and temperature. It depends on the type of spring and material that is used for its production. It is heated and cooled down for an appropriate amount of time. Once the spring is heated, it is sent to a receiving box to cool down for the next step.

Coating and Finishing

The final step for spring production is the coating or finishing of the spring. In this process, the spring is coated with additional layers of some useful elements to enhance its properties, such as heat resistance, corrosion resistance, or durability. Some common coating processes are:

• Shot peening
• Plating
• Powder coating

Chapter Five –Extension Spring Mechanism

Extension springs are very well known for storing mechanical energy in its coils and then releasing it in the form of expansion. The extension springs are loaded with tension. Extension springs store and absorb the pulling energy and are designed to create resistance to a pulling force. Most of the time, the extension springs are attached to components on both ends; hooks or loops are attached to it, allowing a pulling force to be applied to the spring. When a force is applied, and the components pull each other apart, this spring tries to get back to its original position. The initial tension present in the spring determines the tightness of the coils of the spring. The manipulation of this initial tension is necessary for load assessment for any particular application. As long as the spring is neither stretched or compressed, it obeys Hooke’s law, but when a mass or load is attached to its end, simple harmonic motion is obeyed. There are two mechanisms going on at the ends of the extension springs. There is way more tension in the end loops or hooks because of the energy stored in them, and the spring coils are stress free when they are tightly coiled and hence more energy in the hooks and less in the coils can affect the working of extension springs.

Chapter Six –Types of Extension Spring Ends

Extension springs come with many different types of spring ends or hooks that are used to attach the spring to the source of force. Therefore, these ends are to be considered keenly to avoid any stress in the extension spring due to initial tension. Many types of hooks or loops are designed according to the application, but the most commonly used are machine loops and crossover loops. The loop ends of the extension spring are categorized on the basis of special tools requirements. The loop ends that don’t require special tools for production are shown in the figure below:

Whereas the loop ends that do require special tools are shown in the figure below:

Chapter Seven – Applications of Extension Springs

A variety of hoops and loops are used in extension spring configuration for different functions. They are used widely in our everyday life, from our car garages to automobiles, toys, electronics, and many more. Some of the uses of extension springs in our daily life are discussed below:

Automotive Uses:

Extension springs are used in our car’s interior as well as its exterior. Modern age cars do not use extension springs as much now, but extension springs were a major part of the vehicle’s carburetor.

Garage Doors:

Extension springs are also used in our garage doors. They are fixed at the sides of the doors and help in raising and lowering the door. These extension springs help to reduce the strength that might be used in lifting and closing of such doors.

Springs in Trampolines:

Trampolines are very popular for the use of extension springs. The bouncy effect of trampoline is due to the presence of many extension springs at its base. These extension springs help people bounce back to the trampoline and keep jumping without any effort. The more the extension springs in the trampoline, the bouncier it is.

Toy Applications:

The toy industry uses a wide range of extension springs in their products. Mostly these springs are used for toys that need projectile motion such as shooting games or cars etc. Extension springs are found in almost every fast motion or throwing action toys.

Vise Grip Pliers:

When we grip something with vise grip pliers, these pliers not only grip that but also keep it locked in that position firmly with the help of extension springs present in these pliers.

Washing Machines:

Extension springs are an important part of the washing machine that is commonly used in our homes. The drum of the washing machine, which does all the spinning and washing, has extension springs attached to its sides. The function of these springs is to hold the spinning drum at one place. Otherwise, the drum would bang to all the sides of the machine, making the washing process too noisy.

Medical Devices:

Almost all the medical devices have extension springs like stretchers, surgical lights, instruments etc.

Farming Equipment:

Like many other industries the farming industry also uses extension springs in their equipment. Because it is a very heavy work therefore, harvesters, tractors, and ploughers all use extension springs to make their work easier.

Baby Carriages:

The bouncing movements of baby carriages are due to the application of extension springs in it. Just a gentle push and the carriages will go on rocking on its own.

Fence Gates:

We have gates in our homes that shut on their own after opening them due to the wonders of extension springs that are placed on its sides that help the doors to shut on their own.

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.