How to Test Coil Springs & Recognize Signs of Bad Ones
Spring coil testing is an important yet sometimes overlooked discipline that provides reliable information on how your helical spring will operate or continue to function within its application. Many springs operate hidden from view within mechanisms, and it can be difficult to visually verify components are undamaged and performing reliably.
Spring testing is important for all helical coil springs, such as compression, extension, and torsion springs. This brief survey details how to coil springs in a straightforward process. We will highlight warning signs evident during testing that foreshadow spring performance issues. We will also outline when to replace springs to avoid premature spring failure.
Reliable spring operation is important in safety-critical components used across all industries. For example, helical springs in safety-critical aerospace systems, such as control surface actuation systems, provide the required tension between linkages to maintain stable flight. An unexpected spring failure in such mechanisms may significantly damage equipment or even endanger personnel.
If you follow the procedures outlined here, you can help ensure the safe operation of your spring throughout its operational lifetime.
Why Testing Coil Springs is Essential
Coil spring testing is a relatively easy activity that requires only a few pieces of equipment and the careful following of some simple procedural steps during measurement. You can bring the same quality control disciplines that spring manufacturers employ during spring production.
It’s important to remember that every spring is manufactured to rigorous design specifications, from the stock material specifications to the manufacturing methods and equipment, as well as the spring’s surface finish. As such, inspection and testing of the spring are both important disciplines that you can use to identify and diagnose any defects or imperfections before they affect a spring’s functionality. Spring coil testing verifies that the spring is operating in compliance with the original design and manufacturing specifications, ensuring the safe operation of the mechanisms and equipment in which the spring is employed.
Tools Needed for Testing Coil Springs
You can verify your helical coil spring dimensions using two primary pieces of equipment.
Calipers (Dial or Digital)
Always use a calibrated set of calipers or a similar measurement device to verify all spring dimensions outlined in the section below.
Universal Testing Machine (UTM) with Load Cells and Data Acquisition Software
The UTM can perform load verification testing of your helical coil spring. For context, UTMs are commonly found in test laboratories. The UTM, associated load cells, and data acquisition software provide a versatile load and rate testing platform that allows it to accommodate various specimen sizes, from small material coupons to large components and assemblies.
You should note that you can perform basic spring tension measurements using a spring scale; however, this is not the best way to verify the spring’s performance, as it can be difficult to accurately measure the deformed spring. For additional information on spring tension in an extension spring, check out our Spring Tension and Measurement Guide.
One final note: the steps outlined in the following sections assume you have disassembled your mechanism and removed the spring. If this is not possible, you may still be able to perform a reasonable visual inspection of the spring (Step 1) and take a few measurements (Step 2). The information you gain from these two exercises may be sufficient to allow you to diagnose whether a replacement spring is needed. However, a comprehensive spring test of the component is only possible if you remove the spring from the mechanism entirely.
Step-by-Step Guide: How to Test Coil Springs
Step 1: Visual Inspection for Surface Damage
First, visually inspect all surfaces of the spring, including the OD and ID. Look for cracks, dents, surface corrosion, wear, or scratches that might impact the spring's functionality. Then, methodically review each area of the spring, working from one end to the other on the OD. Since this surface is more difficult to observe, repeat this process as carefully as possible for the ID.
Visually inspecting the surface is critical because spring failure begins on the surface, where defects such as divots or cracks have initiated. As such, it represents the first line of defense against spring failure.
Step 2: Measure Spring Dimensions
Measure the following key dimensions of the spring by following the steps:
Outer Diameter (OD) and Inner Diameter (ID): Hold the spring on the outer surfaces and use calipers to measure the outside of the coil to measure the compression spring's OD. To measure the spring’s ID, insert the inside small jaws of the caliper into the spring's helical coil and record the measurement. For certain spring applications, the ID is important because it may accommodate a mandrel or additional equipment.
Wire Diameter: Locate a straight section of the spring that is not wound into a helical coil. Place the outside large caliper jaws around this wire and record the measurement at this location. The wire diameter of a coil spring is one of the main determining factors of a spring’s strength and is critical to check.
Free Length and Total Coils: To determine the free length of the spring, place the entire length of the helical coil section of the spring between your calipers and record the length of the spring with no load applied. Then, count the total number of coils on the spring, including coil portions in your total count (Total Coils). Measuring the spring’s length and counting the number of coils will allow you to determine if any are broken, damaged, or missing.
After obtaining these and other relevant spring dimensions, check them against the specifications. This exercise will verify that your spring has no gross deformities or malformations. It is important to catch these before moving on to load testing the spring, as outlined in the next step.
Step 3: Conduct a Load Test Using a Coil Spring Tester
After you’ve verified all the applicable spring dimensions, the next step is to test the helical coil spring to verify that it can endure its specification design loads and that the load/deformation profile of the spring matches the spring’s specified stiffness.
Begin by configuring the UTM, software, and data acquisition software for the type and size of the spring and the direction of the applied testing load. At a high level, the load testing process involves placing a known load on the spring and then evaluating its resulting displacement. The goal is to assess the spring’s deformation under a load magnitude below the spring’s maximum rated load limit. You can perform this testing as follows:
Apply a slowly ramping load magnitude of no more than 90% of the spring’s maximum load specification from an unloaded state. Be careful not to exceed the maximum specification load. Hold the load at this value and briefly examine the load and deflection data collected so far. You can do this by using your data acquisition software to plot a graph of the force vs displacement of the spring and then calculate the slope of this line. This value represents the spring rate or stiffness of the spring. This property can be calculated by picking two points on the load-deflection curve within 15-85% of the maximum deflection measured during the test. The data acquisition software should be able to perform this slope calculation using the two data points specified; however, you can manually calculate the spring rate from the two points by using the following equation.
Where: F2 is the force at Point 2, and X2 is the displacement at Point 2. Similarly, F1 is the force at Point 1 and X2 is the displacement at Point 2.
Once you have the spring rate constant of your spring, you should compare it to the specification spring rate value, considering the typical tolerances on the spring constant. Generally, the typical spring rate tolerance is +/- 10%, but the value may be as tight as +/- 5% depending on the spring type. If the spring rate you calculated is within this tolerance range, then the spring meets the specification values.
In most cases, the spring rate constant is the most important parameter to test. However, in some cases, you may want to evaluate the load in the spring at a given spring displacement. This can be easily evaluated using the same general principles. Simply apply a displacement to the spring and then measure the load the UTM requires to achieve that displacement. This value can then be compared to the specification values considering the same general tolerances.
After you have ensured that you have captured all test data, unload the spring completely and remove it from the testing fixture.
Check for Spring Fatigue or Weakness
After you have completed testing, carefully review all data collected, ensuring that the load-deflection curves are similar for each spring and cycle of testing.
You should also compare your calculated spring rate from the load-deflection data to the spring’s specification value to determine if your spring meets its stiffness specification.
After every loading and unloading cycle outlined above, you should remove the spring from the test fixture and carefully verify that the spring has returned to its original length after unloading.
During the load application during the test, pay careful attention to the spring and real-time data as the load is applied. Specifically, observe the load-displacement curve to verify it is a straight line with a constant slope. During spring extension, you are looking for any signs of abnormal spring behavior, such as creaking or cracking sounds in the spring.
Finally, you should repeat the loading procedure a few times to ensure the displacement is consistent and repeatable.
In addition to the slow, static testing procedure outlined here, performing an additional cyclic fatigue test on your spring is important. Specialized third-part testing laboratories are well equipped to perform cycling load testing on your spring under any condition to verify its performance and reliability. In simplest terms, a fatigue test is performed by placing the spring within a specialized machine equipped to load and unload the spring at high frequencies to a predetermined length, force, and number of cycles. Springs can be fatigue tested in either extension or compression.
Warning Signs of Bad Coil Springs
The following are general guidelines for red flags or warning signs that a spring is not performing properly and may experience reduced performance or even premature failure. The warning signs are listed in order of severity and ease of catching.
Fractures or Cracks in Spring: After removing the load, inspect the spring again (See Step 3). If you find cracks, small fractures, or other surface damage that weren’t present during the initial load test, the spring is unfit for use in the application.
Permanent Set or Uneven Coil Spacing: A “permanent set” is the term given to springs when they do not return to their original length after unloading. If the spring does return to its length but has uneven coil spacing, this is another type of permanent set. The spring has experienced plastic deformation in both cases and stretched beyond the material's elastic limit. Such a spring is unsuitable for application use and will not perform as intended.
Decrease in Spring Rate: If the calculated spring rate during load testing is below the specification value, the spring does not meet the stiffness specification. This is a big sign that your spring will not function as intended in the application. Such a spring may even fracture in application because it is at risk of being deformed beyond its maximum load-rated capability.
In addition to these warning signs that may manifest after load testing, you can evaluate the following in your spring’s operational history to determine if you should replace a spring.
Fatigue Life: It’s helpful to evaluate the number of cycles your spring has already undergone and ensure you do not exceed the spring’s fatigue life specification value. You should never exceed a spring's rated fatigue life.
Environmental Effects: If your spring is used in a highly corrosive environment or with extreme temperature fluctuations, you should consider replacing it at more frequent intervals.
When to Replace Your Coil Springs
Simply put, if you observe any warning signs outlined above during testing, you should replace your spring.
Furthermore, suppose you notice any signs of damage on your spring, such as dents, cracks, or surface corrosion. In that case, you should remove them from the application and perform rigorous testing before considering placing them back into your mechanism. If your spring operates hidden from inspectable view in your application, consider creating a regular maintenance and testing interval that involves disassembly of the mechanism, removal, and careful inspection of the spring according to the general principles outlined here.
Before purchasing your spring, at the spring design and selection stages, you should always inform the spring supplier of the design life of your spring, representing the number of cycles you expect the spring to withstand in your application. This information allows the supplier’s experts to tailor their spring sizing calculations and evaluate whether additional design life testing is required to verify a spring will meet your design life criterion.
If you follow these recommendations, you will avoid an unexpected, premature failure of your coil spring.
Century Spring: Your Trusted Partner for High-Quality Coil Springs and Testing Solutions
Century Spring is a quality-first manufacturer with decades of experience designing and manufacturing stock and custom springs for the most demanding industries. As the most trusted name in spring and wire form products, our dedicated customer support teams can answer your questions about spring performance, maintenance intervals, and replacement schedules.
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