THE IMPORTANCE OF DYNAMIC FATIGUE TESTS ON SPRINGS
THE REAL CHALLENGE
Starting from the assumption that from a physical point of view springs are very particular and complex components, we have carried out several studies focused on the influence of external forces that can affect them, both in static and dynamic mode.
It is intrinsic in the behaviour of springs, and especially in their use, the ability to be able to withstand continuous elastic deformations, which at the time of their application momentarily change their construction size;
Springs must also be able to absorb large amounts of elastic energy, which must then be released when the external force ceases to be applied.
It is mainly for this reason, that the study of their behaviour when subjected to one or more external forces becomes crucial.
The questions we have asked ourselves are many. They are the result of a combination of technical reasons, which are, however, linked to the commercial needs that the market imposes on spring manufacturers.
HOW WE FACE THE CHALLENGE
At the moment the real challenge is: “To produce the best spring at the best price“. But we are also sure that the following issues are discussed every day by every spring manufacturer:
- How is it possible to make sure that a spring will work correctly, before it actually begins to work?
- How can we be sure that a particular spring is really capable of sustaining the working life cycle for which it was designed?
The fields of use of springs are extremely wide and varied. One of the typical examples of very severe use, is that of springs used inside internal combustion engine valves, where in addition to being subjected to millions and millions of cycles, the springs must also be able to withstand high temperatures, which could cause the so-called “tempering” phenomenon.
In order to obtain these very high mechanical performance levels consisting of correct deflection and continuous endurance of cycles with very high unit loads, the spring manufacturer must use special steel alloys containing vanadium, silicon, and other alloying elements, to ensure high yield strength. All this without forgetting the importance of correct heat treatment, aimed at increasing both hardness and yield strength.
In spite of the high-quality levels required by the customers, due to the highly critical mechanical aspects, the springs must be produced quickly and at low cost. All this makes “the spring component” an extremely delicate element.
It follows that the quality control of springs is crucial to ensure that the customer receives a product that fully meets their needs, trying to eliminate, or at least minimize, faults in the final product, which will be sold and used in everyday life.
THE BENEFITS OF PERFORMING DYNAMIC FATIGUE TESTS
For more than 35 years, Easydur has specialized in the construction of measuring equipment. It therefore has a wealth of experience in the mechanical testing of springs of all kinds. Based on this know-how, we strongly believe that, today more than ever, it is imperative to carry out long duration cyclical tests (Fatigue) designed to simulate the behaviour of the spring during its “life” cycle.
Dynamic fatigue tests on springs are often underestimated, and basically conceived as destructive tests, since most of the testing machines used for this purpose are mechanical instruments with very poor technological content, and without load cells and control software (the so-called “crank-handle system”).
These machines have the objective of applying stress to several springs of the same type, until one or more of them break, identifying only the number of “blows” that led to the breaking of a spring. It is not possible to set loads and, more importantly, it is not possible to detect the load drop moment during the cycles.
Identifying the relaxation and consequent loss of elastic characteristics of the springs is an important time saver, and actually allows the springs to be characterized in a correct manner.
It begs the question: Why wait to reach breaking point, when the object could be achieved much sooner by detecting the relaxation?
During the last year, we have sold several machines specifically designed to allow our customers to perform dynamic fatigue tests, controlled by our specific “EasyDynamics” software, which allows to quickly, correctly and effectively simulate a life cycle of the tested springs, making the research and development of new production solutions much easier.
It is important to be able to produce charts as a function of different physical measuring parameters (Force, Position, Time, Speed, Temperature, etc.)
FORCE/POSITION CHART
FORCE/TIME CHART
It is possible to interrogate the sine wave chart, inspecting its points of interest, in addition to setting test parameters such as: Stroke, Speed, Strength, and Frequency.
These technical features are now the dominant factor for an accurate and thorough analysis of the simulation of the life cycle of the spring before its failure.
Example of Electromechanical machine for dynamic fatigue tests and static tests, with multi-cell system, with total bottom of scale: 50 kN
This type of machine is normally equipped with multi-channel capability, and can be fitted with several load cells and additional channels, so that signals can be acquired from external sensors.
MULTI-CHANNEL FORCE/POSITION CHART
MULTI-PARAMETER CHART AXES
Considering that normally a “cyclical duration” test requires a substantial amount of time, it is also necessary to have the possibility of selecting the data acquisition times and frequencies.
Last but not least, it is absolutely possible to fit the machine with climatic chambers for “hot tests” or “cryogenic tests”, or saline mist chambers, for tests with aggressive agents.
We would like to underline that in machines for these types of tests, attention to energy saving cannot be ignored, using systems for the recovery of kinetic energy and elastic potential energy.
Listening to the needs of many spring manufacturers, we have understood that such a tool would first of all help to establish new experiments and projects. It is particularly true during the manufacturing of a batch of new springs, allowing to assess new materials and more cost-effective heat treatment. This would enable the spring manufacturer to quickly understand how to build a reliable spring at a cheaper cost, therefore ensuring commercial success. But why at the same time not give the possibility to also carry out static tests using the same machine, with the same precision and the same objectives of a standard “Load test”?
This type of dynamic characterization is not just recommended for compression springs, but also for traction and torsion springs.
Our goal is to study innovative solutions, aimed not just at traditional quality controls, but also at responding to real issues that spring manufacturers are facing, helping them to make the most of their time, preserving the main goal of “ZERO DEFECTS” production.
