The design of the vibration test fixture is the most important step of a vibration test. It is the link between the shaker armature and the test item or unit under test (UUT).
Undesirable structural behaviour will hamper vibration testing. The structural behaviour of the vibration test fixture will impact the UUT behaviour. So, investing time into the design of the vibration fixture will always pay-off during testing.
Performing finite element analysis of the test fixture is common practice. This identifies resonances at the test unit mounting points.
But perform the analysis in situ. This then considers the stiffness of the armature and the vibration test fixture.
The orientation of the UUT is important, and should be representative of the installation.
If there are no fluids present in the UUT, test the three orthogonal axes vertically on the shaker. This approach is possible busing a book-end fixture.
The advantages of using this approach are:
less complex vibration test fixture geometry
more control of the structural dynamics of the fixture. Enabling better control of the shaker controller signal.
the test fixture requires less material. So reducing the payload of the shaker enables using a smaller shaker, that costs you less.
vibration testing with the shaker in the upright position, eliminates the slip table. Giving you more flexibility to where you can test.
Here we discuss the payload of the shaker and the fixture weight.
Based on Newton’s Second Law of motion, force equals the product of mass and acceleration (in the same direction as the force).
So, by reducing the payload, higher acceleration is possible with the same force. The opposite is also true.
The benefits of this approach is that one can use
a lower force rated shaker, which will be cheaper,
a bookend, if no fluids are present in the test unit.
you have more flexibility to where you can test, meaning you can negotiate the cost of testing.
The centre of gravity of the fixture and the test unit should be inline with the centre of the shaker armature. This allows pure vertical translation of the armature, and no tilting. And no moment loads are applied to the shaker.
This approach increases the shaker life expectancy . And controlling pure heave motion of the fixture is easier than with a pitch motion.
The higher the stiffness to weight ratio, the better. Using aluminium instead of steel gives you a better ratio.
Magnesium alloy is better than steel and aluminium. But the cost of the material and the complexity of manufacturing is somewhat higher. So this would come down to strategy, preference and financial return.
Preferably, machine the fixture as one piece. As bolting creates more flexibility (and damping) and makes the fixture less rigid. Welding the fixture is a less flexible approach, as long as no high stresses are present at the welds.
These are just a few key pointers on designing a vibration test fixture. If you need help, please get in touch.
Environmentally Sound Limited design products for vibration environments. Designing of vibration test fixtures is something in which we have many years experience.
So, to get a vibration test fixture designed for a successful vibration test, please call us on 01908 643 433.