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Vibration Test Fixtures
 

Vibration test fixtures are required to allow mounting of the test specimen to the vibration table as well as to allow for testing in the three orthogonal directions.

The design of test fixtures is critical to avoid errors in equipment test response due to any resonances of the laboratory's shaker, table (movable part of the shaker) and the fixture itself. Ideally, the laboratory mounting should replicate the physical conditions seen in service, such as stiffness, mass and the consequent resonant responses of the actual service installation.

Theory of Fixture Design

The key issues in the design of a vibration test fixture are as follows:

1. Allow for ease of mounting of the test item to the vibration machine
2. Allow for vibration testing in each of the three orthogonal directions with minimum cross talk, i.e., motion in the two orthogonal directions not being tested
3. Ensure the absence of fixture resonances within the specified test frequency range by tailoring the dynamic response of the fixture and table
4. Weight and force limitations of the vibration machine
5. Distribution of vibration energy uniformly throughout the test item

Mounting

The fixture is the interface between the test unit and the table of the vibration machine. It must allow for proper mounting to the table. The number and size of attachment points must be sufficient to transmit the vibration energy equally to the fixture without losses.

Directional Characteristics

Vibration testing is usually performed in each of the three orthogonal axes, one axis at a time. The real service environment is seldom uni-directional. However for testing purposes, to allow for control and standardization of test specifications and methods, uni-directional motion is desired.

To avoid or minimize orthogonal motion, commonly known as cross-talk, various steps are taken;
Maintain the center of gravity of the test unit and fixture at or close to that of the vibration machine table
Increase fixture stiffness and mass where required to minimize the effects of test item resonance

Dynamic Response

Ideally, the vibration response characteristics of a test fixture should replicate those of the actual service installation. Often, however, these characteristics are not specified by the customer or are not known. The designer of a test fixture must therefore use engineering judgment based on experience to achieve an optimum design. Generally the approach is to make the test fixture as rigid as possible, within the allowable weight limits of the shaker. The test fixture should therefore have no resonances within the frequency range as specified by the customer. That is, the first resonant frequency should be above the maximum test frequency.

Vibration Machine Limits

The weight (test item, fixture plus table) and force limits of the machine must be observed to avoid overloading and possible damage.

Energy Distribution

The test fixture shall be designed to transmit vibration energy from the machine table to the test item in an approximately uniform manner over its mass. This goal may be achieved by designing the fixture to be as rigid as possible within the weight limitations of the machine.

 

* EXCERPTED AND REPRINTED WITH PERMISSION OF THE PUBLISHER FROM TEST ENGINEERING AND MANAGEMENT MAGAZINE, JUNE/JULY 2004, PAGES 10-11. COPYRIGHT 2004 THE MATTINGLEY PUBLISHING COMPANY, INC., OAKLAND, CALIFORNIA. WWW.TESTMAGAZINE.BIZ

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