ManuelGG
Industrial
- Jun 12, 2008
- 6
Hello,
We have an equipment mounting over a damping base which resonance frequency is 21Hz. If must futfill the following shock requirement:
MIL-STD-810G Terminal Peak Sawtooth 40Gs 11ms, after a 30Gs test we have peak in the equipment over 70Gs. we are using a vibration exciter. In the standard there is a paragrah:
d. Classical shock pulses (vibration exciter). If a vibration exciter is to be employed in the conduct of a classical shock pulse, it will be necessary to optimize the reference pulse such that the net velocity and displacements are zero. Unfortunately, the need to compensate the reference pulse distorts the temporal and spectral characteristics, resulting in two specific problems that will be illustrated through example using a terminal peak sawtooth (the same argument is relevant for any classical pulse test to be conducted on a vibration exciter). First, a typical pre-pulse compensation of around 20% of the reference pulse peak will result in a time history that is outside of the 5% pre-pulse amplitude tolerances given in Figures 516.6-10 and 11. Second, as illustrated by the pseudo-velocity SRS in Figure 516.6-12, the velocities in the low frequency portion of the SRS will be significantly reduced in amplitude. Also, there is generally an area of increased amplitude associated with the duration of the pre- and post-test compensation. Observe that the low frequency drop-off in SRS levels between the compensated and uncompensated pulse is readily identifiable and labeled f1. Likewise, the frequency at which the compensated and uncompensated pulses converge is readily identifiable and labeled f2. The drop-off at f1 is considered to be acceptable if and only if the lowest resonant frequency of the item being tested, fN, is greater than f1 by a factor of two or more (fN ≥ 2*f1). The amount of gain in the region f1≤f≤f2 is directly related to the duration and magnitude of the compensation pulse. The potential for over-test in this spectral band must also be carefully considered prior to proceeding
Does anyone know if we are overexciting the equipment due to the use of a shaker? Could we avoid it using per example an impact machine or modifying the shaker control?
Regards Manuel
We have an equipment mounting over a damping base which resonance frequency is 21Hz. If must futfill the following shock requirement:
MIL-STD-810G Terminal Peak Sawtooth 40Gs 11ms, after a 30Gs test we have peak in the equipment over 70Gs. we are using a vibration exciter. In the standard there is a paragrah:
d. Classical shock pulses (vibration exciter). If a vibration exciter is to be employed in the conduct of a classical shock pulse, it will be necessary to optimize the reference pulse such that the net velocity and displacements are zero. Unfortunately, the need to compensate the reference pulse distorts the temporal and spectral characteristics, resulting in two specific problems that will be illustrated through example using a terminal peak sawtooth (the same argument is relevant for any classical pulse test to be conducted on a vibration exciter). First, a typical pre-pulse compensation of around 20% of the reference pulse peak will result in a time history that is outside of the 5% pre-pulse amplitude tolerances given in Figures 516.6-10 and 11. Second, as illustrated by the pseudo-velocity SRS in Figure 516.6-12, the velocities in the low frequency portion of the SRS will be significantly reduced in amplitude. Also, there is generally an area of increased amplitude associated with the duration of the pre- and post-test compensation. Observe that the low frequency drop-off in SRS levels between the compensated and uncompensated pulse is readily identifiable and labeled f1. Likewise, the frequency at which the compensated and uncompensated pulses converge is readily identifiable and labeled f2. The drop-off at f1 is considered to be acceptable if and only if the lowest resonant frequency of the item being tested, fN, is greater than f1 by a factor of two or more (fN ≥ 2*f1). The amount of gain in the region f1≤f≤f2 is directly related to the duration and magnitude of the compensation pulse. The potential for over-test in this spectral band must also be carefully considered prior to proceeding
Does anyone know if we are overexciting the equipment due to the use of a shaker? Could we avoid it using per example an impact machine or modifying the shaker control?
Regards Manuel
