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The Importance of Spring Tension in Extension Springs

Many people do not understand one of the most crucial spring design elements: spring tension in extension springs. In most cases, designers only need to guess the precise force required to overcome a one-time load or disregard it altogether.

The Importance of Spring Tension in Extension Springs - KB Delta

Two springs may look the same, i.e., they look identical physically, but they may vary considerably in load at a particular length. This depends significantly on the amount of initial tension.

Therefore, what is initial tension, and why is it so important? Initial tension is a crucial term used in relation to extension springs. This article answers this question as much as possible.


What is Spring Tension in Extension Springs?

Spring tension signifies the tension stored between the coils of an extension spring and keeps them together. In other words, it is the force that ensures the coils of an extension spring stay the way they are primarily designed without becoming lax. In addition, the initial tension ensures the coils remain closed, and only an outside force exceeding it can open or separate the coils and flexes the spring.

As soon as the tension in the spring gets released, the coils separate accordingly. But when this force lets up, the coils come back together immediately. This allows the extension spring to compress once more by returning to rest.

The initial tension of extension springs is only released when the springs are extended until one barely sees the light passing through the coils. The value of this initial tension is directly proportional to the load required to pull an extension spring. This is why designers are expected to keep this crucial information in mind when designing, creating, or buying tension springs.

Extension vs Compression Springs

Unlike the compression spring, an extension spring can be pre-loaded. Therefore, the amount of force a well-designed extension spring generates is a crucial part of spring design and production as it significantly influences overall performance.

Compression springs differ from extension springs, especially regarding how they work. Extension springs typically become longer under a load. But in the case of compression springs, they become shorter. Compression springs are only designed for use in applications where 2 components try to push toward one another.

A spring exerts a constant force, which is the rate per unit of distance traveled. This is why it is usually expressed in newton per millimeter or pounds per inch. Since springs already have initial tension before they are deflected, the value is added to the load the spring rate determines.


How You Can Measure Spring Tension

Calculating the spring tension of extension springs is not as complex as you think. You can do so using a simple formula:

P1 = P – R (T), where the letters represent the following variables:


  • P1 = Spring Tension
  • P = Load
  • R = Rate
  • T = Travel or distance traveled


For instance, let’s assume R = 20 pounds per inch; P = 40 pounds, and T = 1.5 inches. You can calculate the pounds of force for the initial tension of a particular extension spring by doing the following math:


  • P1 = 40 – 20*1.5
  • Which equals 40 – 30
  • P1 = 10lbf (pounds of force)


Things You Should Consider

As highlighted in the previous section, you have seen the importance of initial tension in extension springs. This is essential, especially if you need to replace a few springs in your machine or application. In addition, calculating the initial tension will let you know the exact type of new springs you should purchase and will allow you to test your spring.

This will save you a lot of time and resources and prevent unnecessary downtimes due to fixing and using improper extension springs in applications.

Moreover, you need to also consider that extension springs have zero hard lines

Extension springs don’t have a specific hard line where they stop in order to prevent an overload, unlike compression springs. If you continue adding more load to the extension spring, the spring responds likewise by extending.

Despite this, extension springs with stopping points exist. It is possible to come across extension springs with stopping points, also known as drawbar springs. Watch out for them and ensure you know their properties, what they are suitable for, etc., as this helps you determine whether or not you need them for your application.


What are Extension Springs?

Extension springs are wires that react to forces that pull the ends apart. This action physically extends the shape of these springs even as they try to offer resistance and return to their original shape, i.e., a compact coiled shape.

An extension spring’s use is to store and absorb energy and create resistance to pulling forces. Therefore, the spring tension determines how the tightness of an extension spring.

The spring tension of an extension spring can easily be manipulated in order to attain specified load requirements for your particular application. This specific type of resistance is the primary reason extension springs exist in a wide variety of applications, such as:



Extension springs are manufactured and designed to accommodate different applications, loads, and specific conditions by varying the extension spring’s length, the wire’s size, and the coil’s size. They are usually manufactured with round wire, using initial tension to close-wound them.

The usual problem has to do with custom extension springs. Manufacturers of stock extension springs can usually produce large quantities within a relatively short turnaround period. But they mostly cannot provide the ideal customization required to best fit one’s application or product.

This is why you should look for manufacturers who don’t mind creating custom pieces of extension springs for their esteemed customers.



Knowing your extension springs’ initial tension is essential, and its importance cannot be overstated. This makes it easier to determine the type of springs you need that will meet your requirements for what you want to use them for. In addition, this helps prevent costly production errors and downtime that may result from using a defective extension spring with unknown values of initial tension.



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