MIL-DTL-17191E(SH)
4.4.2 Test methods for finished mounts.
4.4.2.1 Dynamic stiffness measurement.
4.4.2.1.1 Test system for measuring dynamic stiffness. The test system shall be capable of superimposing
cyclical vibration while statically loading the mount. Refer to the forced non-resonant section of ASTM D5992 for
discussion and a schematic of test systems capable of conducting low-frequency dynamic stiffness measurements.
A commonly used type of equipment for non-resonant testing is a closed-loop servo-hydraulic machine or a
vibration shaker with load frame (see figure 1). Transducers shall be arranged to measure force at the blocked
output or foundation side of the mount and displacement at the equipment or input side of the mount. The stiffness
of the test apparatus (machine, load-frame, force transducer, etc.) in series with the mount under test shall be
determined experimentally or via calculation. A test apparatus which is at least 100 times stiffer than the dynamic
stiffness of the mount being tested shall not require measurement adjustment to account for the flexibility of the test
apparatus. If the test apparatus fails to meet this relative stiffness requirement, the test apparatus compliance shall
be subtracted from the measurement to obtain the dynamic stiffness of the mount (Kmount). The dynamic stiffness of
the mount can be calculated using the classic equation governing the summation of springs in series:
(Kapparatus)(Kmeasured)
Kmount =
(1/Kmea sured) = (1/Kmount) + (1/Kapparatus)
yielding
(Kapparatus) - (Kmeasured)
To avoid over-correction, the stiffness of the test apparatus shall be greater than 30 times the stiffness of the
mount. Refer to ASTM D5992 for guidelines and requirements of the mechanical apparatus and instrumentation
used in the test system.
Dynamic calibration of the complete system shall be confirmed using steel springs of known spring constants,
having negligible damping, prior to the start of testing. Refer to the "mechanical and instrumentation factors
influencing dynamic measurement" section of ASTM D5992 for measurement discussion and selection of springs.
In addition, springs shall be selected on the basis of their linear force-deflection properties and ability to produce a
spring constant comparable to the stiffness of the mount to be tested. System confirmation shall be performed using
springs of known spring constants at the excitation amplitudes and frequencies at which the mount is to be tested.
System confirmation shall be performed at loads which produce spring constant measurements that bracket the
expected dynamic stiffness range of the mount to be tested. This shall be accomplished by adding or removing
springs within the stiffness range of intended mount measurements. Care shall be taken to assure measurements are
conducted within the linear load range of the springs. All spring dynamic stiffness measurements (corrected for test
apparatus stiffness) should be within 2 percent of the known spring constants. If this is not the case, the error shall
be investigated and corrected. This procedure is not intended to replace the periodic calibration of transducers,
signal conditioners, and other measuring instruments as required by ASTM D5992.
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