Understanding Spring Tension: Definition & How to Measure It
Spring tension is a simple term that describes the load applied to an extension (also called tension) spring in an axial direction. Extension springs have tightly wound helical coils that touch each other, don’t allow light to pass through when the spring is unloaded, and resist applied tensile forces that pull the helical coils apart during loading. Conversely, compression springs’ helical coils are compressed together during loading.
Furthermore, the spring tension is related to the extension spring’s initial tension, which is the tension force required to separate the helical coils just enough to observe light passing through them. It represents the initial tensile force that must be overcome to allow the spring to absorb further displacement and load. In any case, initial tension and tension resulting in visible coil separation are critical spring parameters you need to understand to choose a spring that operates efficiently and reliably in its intended application.
A few principles will help you better understand spring tension and how it’s measured. We will then show you a step-by-step guide on calculating spring tension and explain why careful quantification of spring tension and related properties is vital for spring design and manufacturing.
Understanding Spring Tension: Definition and Importance
Understanding your spring’s initial tension is critical because it represents a load that must be overcome before measurable spring extension occurs and the helical coils separate. As such, this represents an important design parameter for which designers must account.
Most spring manufacturers will typically provide initial spring tension as a specification value for stock springs. However, verifying the value by measuring the initial tension in the spring is valuable if you do not have the specification parameters or want to verify them for quality control purposes.
Other times, when shopping for extension springs, design engineers will work from a known spring tension force they know they need for their spring to work in an application. In such cases, spring tension is an important parameter for spring manufacturers because it represents a manufacturing target that the manufacturer can use to determine the extension spring’s wire diameter, outer diameter, free length, and initial tension. From here, the manufacturer can evaluate whether a stock or custom spring best fits the application. If a custom spring is needed, the manufacturer uses the initial tension and working spring tension to evaluate the manufacturability of such a spring and propose alternative spring geometries if needed.
Furthermore, it is critical for both the design engineer and manufacturer to calculate spring tension for a given displacement to verify that the extension spring will not be overextended under your mechanism's operational loads, which may lead to premature spring failure.
Finally, it’s important to note that spring tension almost always refers to extension springs. Still, the term is occasionally used to describe torsion springs under torsion loads because, technically, the wire diameter is under tension as the torsion load deforms the spring.
How to Measure Spring Initial Tension
However, the example here will detail how to measure the spring tension in a few simple steps. Spring tension is measurable during visible spring deformation. However, following these steps, you can measure an extension spring’s tension under loading conditions.
1. Prepare the Equipment:
You will need the following:
Calipers or high-resolution ruler
Spring scale
2. Measure the Spring’s Free Length:
Place the spring flat on a table for measurement. When measuring with calipers, place the large outer caliper around the helical spring coils with the jaws inside the ends of the extension spring. Record this value as the spring’s Free Length for later reference.
3. Secure the Spring to the Spring Scale
Attach one of the ends to the scale hook to secure the extension spring within a tension gauge or dedicated spring scale. The free end should hang below freely with enough space to apply a load.
4. Measure the Initial Tension Load:
Slowly apply a load to the end of the spring until you can visualize light coming through from the helical spring coils. While holding the end of the spring steady, record the load on the spring scale. This load represents the spring's initial tension.
5. Apply a Force to the Spring:
Next, apply additional force to the free spring end until there is visible coil separation. Depending on the size of the spring, this load might require additional equipment to facilitate the application of a load of significant magnitude to produce a measurable spring displacement.
6. Measure the Load and Displacement:
While holding the spring in its displaced position, read and record the load on the spring scale. Then, measure the extended spring length by placing the large outside jaws around the body of the deformed helical coils inside the extension spring hooks. Use the same measurement technique as was done to measure the spring’s Free Length.
7. Analyze Data:
Be sure to record the equipment's displacement and force reading carefully. Then, you can subtract the spring's free length from the extended spring length to determine the spring's displacement under load.
You may also use spreadsheets or software programs to calculate and plot the spring extension as a function of the applied force on the spring. A Force vs. Displacement graph should be a straight line where the slope is the spring rate or stiffness. You can also calculate the spring rate using the equation in the next section.
Additional Measurement Considerations:
Check that all equipment is properly calibrated before beginning measurements.
Check that the spring gauge is zeroed before applying any load to the spring.
Advanced digital spring force scales are available should you need additional accuracy in measurement.
Take multiple measurements to ensure that all values are properly measured.
Spring Tension Formula and Calculations
Once you know the force you’re applying to the spring and the resulting displacement, as you have carefully measured both in the steps above, you can calculate the most critical property of extension springs, the spring’s Spring Rate, also called the spring stiffness.
In simplest terms, the spring rate parameter (k) represents the force (F) required to extend the extension spring by a designated length unit (x). In engineering, this general equation is commonly known as Hooke’s Law, and it applies to extension springs as follows:
For example, this equation indicates that the spring rate is formally defined as the load required to compress the spring 1.0 inch, typically measured in pounds (lb). It is typically denoted in units of lb/in, meaning the force required to extend the spring by 1 in. The larger the force required to extend the spring a given distance, the larger the spring rate. For example, if you measured a force of 20 lb while displacing your spring 2.0”, then your spring rate would be calculated as follows:
However, when calculating the spring rate of an extension spring, you must consider the spring’s initial tension (IT) and its relationship to the spring rate.
Because the initial tension is the force required to begin the separation of the helical coils, in an extension spring, the true spring rate is calculated as follows:
For example, reconsider the example above, but this time with an extension spring with an Initial Tension of 10 lb. This means you must apply 10 lb just to get the extension spring coils to begin to separate. In this example, it takes 20 lb to separate the helical coils by 1.0 in.
The equation becomes:
This means it takes 10 lb of force to produce a 1.0 in spring extension.
The Role of Spring Tension in Extension Springs
For some additional background, our extension spring selection features many sizes of mechanical springs designed to deform elastically under tension loads. Naturally, these tension loads cause the spring to extend in length. Like other springs, their primary function is to store and release energy by resisting the external pulling forces applied to the spring ends. This resistance stores energy and creates a force that acts opposite the direction of the spring extension. As we have discussed, the name for this resistance is spring tension.
As such, extension springs are attached to components that move away from each other. The extension of the spring stores energy that is then used to bring the components back together. Observing extension springs in consumer products like trampolines allows you to visualize this mechanical action. Under loading, the extension springs around the perimeter of the trampoline and then produces a counterforce upon unloading that propels the trampoline mat upward.
To further illustrate the principle of spring rate and spring tension in spring selection, designers typically operate with a known load that the spring must absorb, as well as a space or deflection envelope in which the spring must operate. You can use a particular spring’s spring rate and size to calculate the deflection under design loading conditions. This information allows for an easy determination of whether such a spring will meet your space envelope design requirements and allowable deflection envelope.
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