Joos takes spring measurement accuracy to new heights with Zwick’s latest tester

The company „Technische Federn GmbH, Otto Joos“ produces the highest technology springs having wire diameters from 0.2 to 8.4 mm. The springs are mainly used in injection pumps for vehicles, locomotives and ships, and are individually produced based upon customers’ specifications.

To meet its customers’ demanding requirements all springs are monitored during their production by the Joos quality control system. The level of accuracy required is only possible with testing machines which fulfill a much higher grade of precision than the springs to be measured. Additionally the reliability of the testing machine is characterized by the measurement capability index, and this is evaluated in practical tests using a reference spring.

During production control, different spring heights and spring forces are measured on random samples. Important is the ability to compare the results of the tested specimens to the results of the entire batch. This can be done with a histogram, and these charts enable the distribution to be calculated and applied to the production batch ensuring that the percentage of parts manufactured outside specification is minimized.

For production control, the Joos Quality lab uses two zwickis, (0.5 kN and 2.5 kN capacity), manufactured by Zwick. These are equipped with precision compression spring testing devices, which are necessary for testing these high level springs. Absolute co-planarity and linear guidance virtually eliminate any measuring errors and this is independent of the spring’s actual position on the compression platens.

The inherent deformation of the testing equipment (machine compliance) is measured and eliminated in real-time by the testing machine controller. This makes sure that measuring errors are extremely small even without direct spring height measurement. The real-time optimization of the machine’s operating parameters to each individual test condition guarantees its excellent measuring capability.

The test is performed without stopping at each measuring point. Important results are spring forces and spring heights at given height or force values. Additionally to the live display of the force-height curve, Zwick software, testXpert^® II features a screen layout showing the histogram and the frequency distribution of any test result. Histogram parameters, the number of classes, the backlog classes and the class limits can be preselected. As graphs, the Gaussian distribution curve, the average value and the confidence limits can be displayed.

The laboratory personnel are united in their praise for the machines. “Our Zwick spring testers are reliable, high precision instruments, and are very easy to use. The histogram which can be integrated in the test report provides us with a helpful feature which shows us the result distribution of our springs”.