Measurement_of_Vibration_and_Acceleration.pptx

Measurement_of_Vibration

1. Measurement of Vibration and Acceleration Measurement of motion and vibration parameters are important in many applications. The experimental quantity that is desired may be velocity, acceleration, or vibration amplitude. These quantities may be useful in predicting the fatigue failure of a particular part or machine or may play an important role in analyses which are used to reduce structure vibrations or noise level.

2. Two Simple Vibration Instruments WEDGE METHOD: Consider the simple wedge shown in Fig. 1 which is attached to a vibrating wall. When the wall is at rest, the wedge appears as in Fig. 1 a ; when the wall is in motion, the wedge appears as in Fig.1 b . An observation is made of the distance x . At this distance the thickness of the wedge is equal to the double amplitude of the motion .

3. In terms of x, the amplitude is given by a = x tan (Ɵ/2) where θ is the total included angle of the wedge. The wedge-measurement device is necessarily limited to rather large-amplitude motions, usually for a > 0.8 mm.

4. REED VIBROMETER A simple vibration-frequency-measurement device is shown in Fig. The small cantilever beam(Reed) mounted on the block is placed against the vibrating surface, and some appropriate method is provided for varying the beam length. When the beam length is properly adjusted so that its natural frequency is equal to the frequency of the vibrating surface, the resonance condition will result. The natural frequency of such a beam is given by

5. ω n = Cn .√[E.I/mL⁴] where ω n = natural frequency, Hz E = Young’s modulus for the beam material, N/m² | = moment of inertia of the beam about the centroidal axis in the direction of deflection, m4 m = mass per unit length, kg/m L = beam length, m C n = 0.550 for SI units

6. Under the resonance condition the reed vibrates with maximum amplitude. The reed length is calibrated directly in frequency units. Typical frequency range is 5Hz to 10000Hz