Fixtures for Shakers

❱ ❱ Test fixturing should accommodate vibration axis as well as product mounting considerations


he fixture for holding a test sample is the interface between the electrodynamic shaker and the equipment under test and is therefore a fundamental element

of a vibration and shock test programme. To ensure that the test programme delivers the best results it is therefore vital to consider fixture design from the very start.

ZERO MASS AND INFINITE RIGIDITY The theoretical ideal fixture has zero mass and infinite rigidity, as it would allow the shaker to never run out of thrust and for the fixture to always transmit the perfect motion. The fixture must also replicate, as closely as possible, the in-service mounting arrangements of the equipment under test and transmit the shaker forces to the equipment without significant attenuation or magnification. The fixture must also have minimal resonances within the

test frequency range, so the objective should be to design a resonance free structure. However, when this is not possible, attention should be focused on avoiding resonances in frequency ranges where a test sample resonance is known or expected. This may therefore require a modal test on the fixture design, using finite element analyses.

PERFORMANCE CONSIDERATIONS To ensure the physical integrity of the test fixture, the design engineer should consider the effect of vibration fatigue and peak stress on vibration fixtures. They should also accommodate suitable materials and features in the design where necessary, including fabrication techniques, such as casting, welding and fastening. It is also vital that the engineer understands how each of these techniques and designs will affect the performance of the fixture. As the test fixture may need to be rotated between axes during tests, it is imperative that it works satisfactorily in the three required orientations. The engineer must therefore add design features so that these axis changes can be made within

20 /// Climatic & Vibration Testing Vol 2 No. 1

❱ ❱ Chris Bowles of TÜV SÜD advises considering fixturing at an early stage of design

Chris Bowles, Environmental Test Supervisor at TÜV SÜD, explains the importance of fixturing for vibration tests.

the shortest time, without compromising the effectiveness or safety of the fixture. Something that is often overlooked is how

the test sample and all its functional leads and power leads can be accommodated, including the appropriate bend radii that are required. The test fixture must also allow for all of the visual inspection and functional test requirements required during test.

UNDERSTANDING THE SHAKER Prior to manufacture and commissioning of the fixture, the engineer should consider the force vectoring of the electrodynamic shaker. This will ensure that the shaker can drive the mass of the fixture and the equipment under test through the desired vibration spectrum. For example, at TÜV SÜD there are a number of vibration systems, ranging in force from 35 kN to 160 kN. This means that the relationship between weight; expected dynamic response of the test sample and fixture; required test specification; and available thrust, velocity and displacement of vibrators, must be both understood and considered. If this verification is incorrect, these factors may prevent the vibration test from running, or even cause damage to the shaker. Of course, nowadays, re-use and recycle must also be taken into account in order to minimise wastage. If possible, the test fixture should therefore be designed so that it can be reused. For example, you could consider incorporating interfaces into your product which match with those of the test house’s own vibrators and shock machines. After manufacture, TÜV SÜD will often perform a vibration fixture survey, in order to validate any theoretical modal analyses in the real-world test environment. This process involves fastening the finished fixture, minus the test sample, onto the shaker, which will then be run for a swept sinusoidal resonance search. Carefully positioned control and response accelerometers will identify any resonance or attenuation exhibited by the bare fixture, and the fixture would then be ready for test.

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