The vibration shaker is a test equipment used to simulate real vibration environment effects in the laboratory. Vibration tests use different input signals to excite the specimen on the vibration shaker. Vibration tests are mainly divided into sine and random vibrations. Because of their different physical processes, there is no strict equivalent relationship between them. Therefore, when choosing the test mode, do not convert the severity level from sinusoidal to random.
The sinusoidal vibration test uses a sinusoidal signal of fixed or variable frequency and amplitude, and only one frequency is applied at each instant. The test conditions include frequency range or fixed frequency, amplitude, and test duration.
Sinusoidal vibration rarely appears as a single-frequency form in real environment. Even when measuring acceleration directly on rotating machinery. Such as gears and bearings, actual tolerances and clearances usually result in small variations in frequency. Some form of random vibration also occurs in the random characteristics of rotating machinery.
Sinusoidal vibration on the vibration shaker can be described as deterministic motion, which follows certain laws and can be completely determined from the past state at any specified time in the future. During the sinusoidal sweep test, this method is usually used to determine the time of failure because this failure is likely to be closely related to a specific frequency, and the correlation effect is not obvious with the random vibration test method. Of course, the sinusoidal test method usually takes longer to trigger the failure the random test method, because each frequency sweep only acts on each resonance point for a short time during each frequency sweep process. Although only one frequency is applied at any one time, it is indeed possible to maximize a particular resonant peak of the specimen if the sweep rate is slow enough. It can also be used to find potentially destructive resonance points, especially during design and development tests.
Motivation of random vibration tests of vibration shaker uses an irregular random input signal, which includes all frequency components within the specified frequency range (wideband) at all times. Its instantaneous value follows a normal (Gaussian) distribution. The distribution within the frequency range is represented by the acceleration spectral density (ASD) curve.
Random vibration is the most common type of trigger that occurs in the real environment. Its instantaneous value in the future cannot be predicted from the past state, and can only be predicted based on probability. In fact, these properties are suitable for most calculations related to random vibration, such as fatigue and alternating stress.
Unlike sinusoidal tests, random vibrations of the vibration shaker trigger the resonance throughout the duration, although it does not reach the maximum value. Most random signals in the laboratory have 3 times RMS value, which means that the instantaneous value of the trigger can extend from zero to 3 times the total RMS value in the test frequency range . A greater consideration for random trigger is that there will be a large amount of stress alternating between zero crossings, both positive and negative . This property will affect fatigue damage and thus the life expectancy to failure.
Whether it is sinusoidal or random vibration, only a understanding of production, transportation, use environment, the properties and characteristics of the test equipment, can choose your own suitable vibration mode more appropriately, so as to screen high-quality products and improve performances of defective products .