The requirement for system stability discussed in the
previous paragraphs imposes restrictions on the type of vibration system that
the taped technique should be applied to. This technique is ideally suited to
single product acceptance test programs where the entire vibration system
(with the exception of the test article) remains unchanged from test run to
test run. The effect of minor differences between test articles can be
minimized by testing with fixtures that are significantly more massive than
the test article. Rigidity of the fixture should be maximized to limit the
number of resonance that must be equalized.
Because these conditions cannot always be realized in practice, the
application of the technique stresses the details of the test setup to assure
the highest degree of uniformity from setup to setup. Test setups with high
degrees of flexibility, non-linearities, or high mechanical noise levels
should be avoided in the application of this technique.
The verification of tolerances applied to the taped
random vibration test presents problems since it is assumed that many of the
test laboratories utilizing this random technique will not have spectrum
analysis equipment. A technique was therefore developed to use a low level
sine frequency sweep to determine if the random test will be within tolerance.
These tolerances are discussed in detail in Chapters 5 and 6