Designing Vibration Fixtures

[Cryo1]

We are designing fixtures for ESS and qualification testing. We keep hitting a wall with management over the design. The engineers say "build a cube, and bolt directly to the test shaker." How common is a cube structure in vibration testing? Why not bolt directly to the shaker table?

 

[BobM3]

What do you mean by "cube"? Is it meant to be a universal structure that your specimens would be bolted to?

For high performance, mass and inertia are your enemies. You'll have to look at the tradeoffs between fixturing ease and performance. Every system I worked on bolted as direct as possible right to the table.

 

[TPL]

Very common - a 'cube' gives 5 faces to which a component can be attached for testing. On each face, depending on how the bolt holes are arranged, the component being tested can be mounted in several differecnt orientations, allowing the vibration response in different axes to be determined.

 

[Cryo1]

The simplest thing we thought of is a block or cube to mount to the table and placing the UUT on each exposed face. What I was told to use in the in attachement below. Each block represents a aluminum block and everything is held together with bolts.

 

[monkeydog]

If that is to some sort of scale, I don't see anything good about that set-up.

As BobM3 said "mass and interia". Think of how the UUT will respond on the set-up.

 

[IRstuff]

A fixture is generally needed for:
> as an adaptor to the shaker head, since it's usually undesirable for everyone to drill their own holes into the head; you wind up with Swiss cheese that no longer can hold anything.

> as a means of altering orientation of the UUT. As mentioned above, vibration tests often require running with different orientations, so it's usually easier to have a single fixture that can orient the UUT as required.

What the fixture looks like depends on the vibration profiles, etc., but the requirement is that the fixture not introduce anomalies into the test. Obviously, if it needs to support different UUT orientations, it needs to be awfully stiff, so a cube might be a plausible solution.

One caveat is that slip tables don't like top-heavy configurations, as that potentially can cause the slip table to rock on its oil film, resulting in unintended resonances.

TTFN

FAQ731-376

 

[MAGNAFEAMAN]

Fixture can be made for different testing configurations. What I normally do is create a fixture for multiple parts on test provided that the parts function will not interfere with each other. The shaker table has bolt pattern that you can easily adapt to. Ensure that the pattern is symmetrical such that you can rotate fixture for different directions. Another important consideration is the frequency of your fixture with and without the part on it. You should notch the frequency if you can not develop a fixture that has no resonance at test frequencies otherwise your test is meaningless.