The Fundamentals of Compression Spring Testing
Why is compression spring testing important? Get to know all the details by reading further on.
The design, manufacture, as well as use of springs dates back to the Bronze Age. Spring design is a unique science based primarily on material science combined with complex arithmetic calculations. Springs are used in a wide range of applications in day-to-day consumer devices such as computers, cell phones, pens, staples, tire gages, mattresses, electrical contacts, etc.
Springs are also used extensively in several industrial applications, including aerospace and automotive. Every precision medical device you know or don’t know uses a spring with a diameter not exceeding 0.0036 inches, i.e., approximately equal to the size of a single human hair. You will find such springs in endoscopic instruments and catheters.
Since springs are used in nearly every industry and product, testing springs for their unique design attributes and performance is now more critical than ever as advances in manufacturing processes and spring technology continue to evolve.
Determining a spring’s characteristics as well as validating its overall performance is essential to ensuring that the spring will perform according to its specification over its planned life-cycle for its proposed application.
Springs and Hooke’s Law
One of the fundamental principles of a spring is its ability to withstand a force while retaining the capacity to extend or compress and return to its original shape or position. A 17th-century British physicist named Robert Hooke took it upon himself to determine that the extension of a spring is directly proportional to the load applied to the spring. Thus, Hooke’s Law was born.
Hooke’s Law is often employed in spring design. The spring equation is perhaps one of the most encountered forms of Hooke’s Law. It relates the force or load exerted on a spring to the distance it is compressed or stretched by ‘k,’ a spring constant, often measured in force per length.
e.g. F = – kx.
where ‘x‘ = the displacement of the spring’s end from its equilibrium position, i.e., the distance, often measured in meters (SI Units);
‘F’ = is the restoring force or load exerted on the spring on that specific end, measured in kg m/s2 or N;
‘k’ = this is a constant known as the rate or spring constant, often measured in kg/s2 or N/m.
Hooke’s Law is only applicable for some materials under specific loading conditions. For instance, helical springs correspond to and perform ideally according to Hooke’s Law.
What You Need to Know About Compression Testing
Compression refers to the application of crushing or pushing forces that act on a material or an object, allowing it to be squashed or squeezed. It is the opposite of the application of pulling forces or tension.
In this light, compression testing is used in determining how a material or product – like springs, etc. – reacts when crushed, compressed, or flattened by measuring the fundamental parameters that determine the behavior of the specimen under compressive loads.
The fundamental parameters in this regard include the elastic limit – which is approximately equal to the yield point/yield strength or proportional limit, Young’s Modulus (often linked to tensile testing), and compressive strength.
Compression testing can sometimes be undertaken as one aspect of the design process, in the quality control laboratory, or within the production environment.
For example, it can be used to assess the overall performance of a spring or product, check out the strength of components such as compression springs, PET containers, etc., or characterize the compressive attributes of materials like metal, foam, etc.
Typical applications of compression testing feature in these sections:
- Automotive and Aerospace Industry
- Packaging Industry
- Fitness, Health, Leisure, and Safety Industry
- Medical Device Industry
- Cosmetics Industry
- Construction Industry, etc.
Spring test methods – such as free length and load testing for a compression spring – are generally helpful for analyzing and improving spring-making processes.
However, selecting the most appropriate test method depends primarily or largely on the test purpose, the spring’s intended application, the instrumentation employed for testing, as well as the ultimate spring design.
Spring performance testing typically utilizes rate/load testing in which the spring’s length and load are measured at 80 percent and 20 percent, respectively, of either the spring’s rated length or load.
Experts agree that the measuring system employed in determining the load/rate analysis must be accurate to at least +/-0.5 percent of full scale. In addition, the precision should be less than 0.1 times the load tolerance for the spring under measurement.
If this system is utilized in determining the length of the spring at several load limits, these measurements should be compensated for deflection of the load application, the spring when subjected to applied loads, and the measurement system.
The precision of the height measurement must also be less than 0.1 times the load tolerance or deflection tolerance divided by the spring rate, no matter which is less.
A typical spring tester usually comprises the following:
- A motorized or manual force test stand
- Force gauge or indicator alongside a load cell
- Data collection software, and
- Compression plates
Why Compression Test is Important
A compression test is helpful as it helps determine how a particular product or material reacts when subjected to crushing loads. The displacement and load are recorded when the sample is compressed.
Additional calculations are also made, including compressive strain and stress, which may be plotted on a stress-strain diagram or graph in order to determine the proportional limit, elastic limit, yield strength, and yield point.
Compression tests help ensure the safety of products as they must be manufactured to the highest quality standards. The data obtained during the tests can be utilized in several ways, such as:
- Facilitating or optimizing design processes
- Determining consistencies in the manufacture of products and boosting productivity
- Evaluating batch quality and maximizing growth profit and turnover
- Minimizing the costs of manufacturing products as well as achieving lean manufacturing objectives
- Establishing strict compliance with industry or regulatory as well as international standards.
These are some of the benefits of compression spring testing.
Compression spring testing is implemented in a wide range of industries in order to ensure safety in product design, quality, and reliability. Force measurement is crucial in order to ensure compliance with reliability and performance standards.
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