The design, manufacture, and use of springs can be traced back in time to The Bronze Age. Spring design is a science based on complex arithmetic calculations combined with material science. Springs are used in everyday consumer devices including cellular phones and computers, in industrial applications including automotive and aerospace, and in precision medical devices where a spring with a diameter of .0036” (about equal to the size of a human hair) is used in catheters and endoscopic instruments.

While the wheel is often considered one of the most important inventions ever, the spring is arguably as equally important. Spring design is constantly evolving and requires more advanced testing and tolerancing. Determining a spring's characteristics and validating a spring's performance is critical to ensuring that the spring will perform to its specifications over its intended lifecycle for the application for which it was designed.

This article will introduce the latest measurement technology designed to ensure accurate, precise, repeatable, and reliable testing of helical compression and extension springs.

#### Hooke's Law

One of the basic principles of a spring is to withstand a force while having the ability to compress or extend and then return to its original position or shape. Robert Hooke, a 17th century British physicist, determined that the extension of a spring is in direct proportion with the load applied to it. Hooke's law is often used in spring design.

The most commonly encountered form of Hooke's law is probably the spring equation, which relates the force exerted by a spring to the distance it is stretched or compressed by a spring constant, k, measured in force per length.

F = -kx

where x is the displacement of the spring's end from its equilibrium position (a distance, in SI units: meters), F is the restoring force exerted by the spring on that end (in SI units: N or kg m/s2), and k is a constant called the rate or spring constant (in SI units: N/m or kg/s2).

Hooke's law only holds for some materials under certain loading conditions. Helical springs are examples of a product/material that in most cases correspond and perform according to Hooke's law.

#### Spring Testing

Spring test methods, such as load and free length testing for a compression spring, are useful to analyze and improve the spring-making processes. Selecting the appropriate test method is dependent largely on the spring's intended application, the test purpose, the ultimate spring design, and the instrumentation used for testing. Spring performance testing commonly uses load/rate testing where the spring's load and length are measured at 20% and 80% of either the spring's rated load or length. It is generally agreed that the measuring system used to determine the load/rate analysis be at least accurate to ±0.5% of full scale. The precision should be less than 0.1 times the load tolerance for the spring being measured. If the system is used to determine the length at various load limits, the measurements should be compensated for deflection of the load application and the measurement system as well as the spring when under applied loads. The precision of the height measurement must be less than 0.1 times the deflection tolerance or the load tolerance divided by the spring rate — whichever is less.

Extension springs also use a load/ rate method similar to compression springs but in the extension direction. Another attribute for an extension spring is to measure a property called initial tension. Initial tension is the load required to cause coil separation among all active coils.

#### Spring Force Testing Innovation

New spring force testing systems are available, designed for high-volume testing environments, the quality laboratory, and spring engineering and design applications. These systems are optimized to ensure accurate, precise, and repeatable measurements. The systems feature innovative measurement software that simplifies the testing process and allows operators with minimal spring measurement experience to perform testing in seconds to obtain reliable results.

For best results, high-volume force measurement solutions for compression or extension springs will:

• Offer a range of load capacities.

• Be supplied with strokes up to 40” (1016 mm) with adjustable speeds from 0.001” inch to 50” (0.02 to 1270 mm).

• Offer high-accuracy digital encoders with an outstanding resolution of better than 0.0001” standard.

• Ensure rigidity via a stable granite base and cast aluminum columns combined with software-controlled deflection compensation and linear error correction.

• Use interchangeable, low-profile load sensors with a measurement accuracy of better than 0.05% full scale.

• Have optimized sensors to ensure correct axial alignment and to compensate for off-center loading due to buckling, non-parallel surface conditions, squareness conditions, etc.

• Include software specifically designed for spring testing and measurement.

#### Force Software

Having unique templates for performing testing and measurement on compression springs and another for extension springs is useful. Each template allows the operator to perform sophisticated spring measurements using fill-in-the-blank forms complete with radio buttons that help the operator select the measurement functions necessary for their intended application. Test setup can be performed in seconds. Each template consists of four sections that the operator can use to set up their individual test: Pre-Test, Test, Data, and Post-Test sections. The Pre-Test section provides the operator with options that occur prior to the testing operation, including global settings, prompting, preconditioning and exceptions.

Test Section: Each template is unique to the spring type. The compression template's Test Section lets the user measure the spring's height and perform either a single- or two-point limit test. When the single-point test is used, the default result is the Spring Constant. When a two-point test is used, the default result is the Spring Rate. During setup, the user can specify the single point based on a load limit or length limit. When the two-point method is used, the user can specify two limits, which may be either a load or a length. The test speed is also specified. The extension template is similar to the compression template; however, instead of the option to measure height, the extension template has the option to measure initial tension.

Data Section: This is where the user selects and formats results. Additionally, the Data Section is used to apply a tolerance specification for the spring so that “pass” and “fail” status can be measured and reported.

Post-Test Section: This section provides the operator with options that occur once the test iscompleted. Post-Test options include the ability to return the crosshead to the home position once the test is completed and export raw data points via a USB port to an external device or network location. It will also export results (data) for a run and specify a runs limit for later recall.

#### Test Builder Software

Some spring testing solutions have advanced application options; for example, an advanced Test Builder application lets the user create a test setup without the use of a template. Instead, the user creates a test using stage movements and other stage types including holds, cycles, etc. The Test Builder provides flexibility to construct multiple stage test setups with the ability to report a significant number of more advanced test results. A spring test can be created using either the compression or extension template and then converting the test setup to the Test Builder application, allowing more flexibility and advanced testing features to be used.

Compression springs can also be tested using platens. Springs that may have non-parallel surfaces may benefit from self-adjusting platens. Specialized test fixtures can be used that secure the spring during testing. These fixtures are generally customized based on the spring's inside diameter. Hooks are used for extension springs.

Displaying measurement and test results is another consideration for force testing systems. Display views can include a data view showing numerical and textual results for a specific run. A batch/summary view will display the results for all runs within a batch in a tabular/spreadsheet style. A graph view displays graph lines based on the sampling rate and a statistics view calculates statistical results for user-selected results.

#### Conclusion

Today's spring force testing systems are highly accurate instruments for determining quality and performance characteristics for compression and extension springs. These systems are easy to use and can be applied on the production floor or in the laboratory.

This article was written by James M. Clinton, Product Manager for Force and Material Test Products at L.S. Starrett Co., Athol, MA. For more information, visit here .