Multi DOF spec question

[Baird]

I have a requirement that states "...the vibration will be applied as a tri-axis vectored exposure for 600 sec...". Has anyone seen this type of wording before? Does this mean I must have a multi-Degree-of-Freedom system to run this vibration test? Any help appreciated in advance.

 

[dampit]

In the aerospace world, we would run a qual-test 600 sec per axis. This usually is adequate unless the failure mode can't be excited in a single axis. Most large shaker tables can be turned over on their sides to excite the x or y direction as well as vertical z-dir. But, there are such things as multi-axis shaker tables which can excite in more than one direction at a time. But, usually the equipment is mounted at an angle (e.g. 45 deg assumes equal loading in two directions) to the direction of excitation. This gives a component of vibration in two axis of vibration using a single axis of excitation. Add another angle to the fixture and you have tri-axis excitation.

 

[TAOS]

Thanks, I've heard about those mult-axis fixtures. Wayne Tustin told me they don't excite the same modes as single axis testing does, so they're really quite conservative. I wanted to buy one to use but customer didn't like changes that weren't going to give "worst-case" single axis. I think this "multi-axis vectored" term is for a multple degree of freedom system.

 

[viblab]

I can't believe this is still going on. We've shot down angle mounted fixtures as multi-axis fixtures at least twice that I remember. All you are doing is exciting a different axis. Unless you know this the fragile axis, so what? I think you're misinterpreting Wayne. Why would a diffferent axis be necessarily conservative?