scope:

Purpose.

This handbook establishes a methodology for conducting force limited vibration tests for all NASA flight projects. The purpose is to provide an approach which may be consistently followed by those desiring to use force limiting, without having to conduct an extensive literature search or research and development effort before conducting the test. A monograph on Force Limited Vibration Testing is available for reference and is recommended for those needing more detailed technical information (NASA-RP-1403).

Applicability.

This handbook recommends engineering practices for NASA programs and projects. It may be cited in contracts and program documents as a technical requirement or as a reference for guidance. Determining the suitability of this handbook and its provisions and providing for correct implementation is the responsibility of program/project management and the performing organization. Individual provisions of this handbook may be tailored (i.e., modified or deleted) by contract or program specifications to meet specific program/project needs and constraints.

For the purpose of this handbook, a force limited vibration test is any vibration test in which the force between the test item and shaker is measured and controlled. The recommended means of measuring the force is with piezoelectric force gages, but other means, e.g., shaker armature current or strain gages, may be useful in special situations. Similarly, the control of the force is preferably accomplished in real time, but iterative, off-line control may be employed as a stepping stone. If the force is not measured, the test is not considered a force limited vibration test, and this handbook does not apply. This distinction is important because in the past some have found it convenient to simulate a force limited test and then to use the analytical results to notch the acceleration input in the test. The simulation approach is not recommended because measurement of the force is considered to be the essential element of the force limiting approach.

This handbook is applicable to all force limited vibration tests of NASA flight hardware including aircraft and launch vehicle equipment, spacecraft and space experiment hardware.

Since the purpose of force limiting is to mitigate the effect of test item resonances in the vibration test, the technique is most useful for structure-like equipment and for fragile equipment such as optics and complex instruments. References 2, 5, 7, 10, and 13 provide examples of the use of force limiting in vibration tests of space flight hardware.