THE SIZING PROCESS: Random

SELECT A SYSTEM WHICH YOU THINK IS THE CORRECT ONE THEN, PROVE IT.

Ideally with 20% - 30% spare. Do not size a system to the limit!

For this example we will use the system ratings for a V830-335 SPA10K and a Test Load of 20kg with the following Test Specification.

RANDOM TEST

Is the upper frequency greater than 500Hz?(Note 1)
1000Hz is greater than 500Hz
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YES
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PSD at 20Hz = 0.0502377 g/Hz
PSD at 1000Hz = 0.0502377 g/Hz
Calculate the total grms
Total grms = 11.90023553 grms
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Is the total grms less than the rated grms?
11.9 grms is less than 60 grms
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YES
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Calculate the total moving mass
20kg + 12.85kg = 32.85kg (Note 2)
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Will the load support system support the load? (Note 3)
32.85kg is less than 160kg
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YES
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Calculate the rms force required
32.85kg x 11.9grms = 391kgf rms
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Is the required force less than the rated force?
391kgf rms is less than 1000kgf rms
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YES
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Calculate the travel required for this test
D (required): 3.102mm p-p Crest factor: 3 Lf roll off: 24dB/octave
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Is the vibrator rigidly mounted?
If the vibrator is not rigidly mounted, body motion will occur.
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YES
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If No, the maximum available travel must be calculated. In this case it would be 49.37mm p-p
Is the travel required for the test less than the travel available?
3.102mm is less than 49.37mm
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YES
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Would a smaller system do the test?
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NO
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If Yes, select a smaller system and repeat the process.
Perform checks for other loads and test specifications

Note 1

Where a test specifications highest frequency is below 500Hz this test can be considered as narrow band random test. Full system force may not be achievable under these conditions. Refer the test specification to the factory for analysis.

Note 3

Locating the payload in the mid position of the armature travel is normally done by using the internal load support system, but this may be supplemented by using an external load support such as bungies.

Introduction  General notes on Sine Ratings  General notes on Random Ratings  General notes on Shock Ratings  Sine Test Worked Example  V830-335 SPA10K Specification

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