Q & A: Do I need a fixture for vibration testing?

April 13, 2016

Question: One of the Qualification test requirements for my product includes vibration testing. Do I need to provide a test fixture in order to perform the vibration test?

Answer: The primary purpose of a vibration test fixture is to adapt the service mount of the unit under test (UUT) to the vibration test equipment (shaker, slip plate, etc) and to transmit the intended vibration to the UUT with as much fidelity as possible.

In order to accomplish these goals, a fixture should include good engineering design, specific materials and precision fabrication. Of course, all of these factors combine to make vibration fixturing both relatively expensive and time consuming to produce. This is undoubtedly why many companies seek to forego the design and build of dedicated fixturing and to seek other means of attaching their test items to the vibration equipment or simply leave it up to the test lab.

When no fixturing is provided, the test facility must find a way to attach the test item to the vibration equipment however they can. This usually means strapping or clamping the articles using eye bolts and nylon straps or threaded rod and clamp bars placed across the test item. As you can imagine, there are compromises inherent in this approach.

First, most test specifications are written with the assumption that the vibration will be input at the service mount of the test article. Strapping or clamping items to the vibration equipment rarely allows for this.

The second challenge is that there is a limit to the force that can be applied to an item by strapping or through clamp bars. Therefore, coupling between the test item and vibration system is usually compromised. This typically results in a frequency selective transmission of the vibration with some frequency bands being under the desired amplitude and other frequencies being over the intended amplitudes. The net result can be an under-test or over-test of the test article.

Finally, control of the input vibration levels can be compromised due to the ambiguity of sensor placement for control and monitor accelerometers. These locations are usually provided as an aspect of good fixture design.

So when vibration testing of your product is required, always weigh the time and expense of fixturing against all of the possible compromises inherent in the undefined approaches of clamping/strapping or “best way” mounting as decided by the test facility.

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