The amount of time required to complete one cycle of the vibration is the period of vibration. If a period of one second is required to complete one cycle of vibration then during one minute the cycle there will be repeated 60 times or 60 cycle per minute. This measure of the number of cycle for a given interval of time (minute, second, hour etc.,) is the frequency of the vibration and is more useful than the period. For vibration works, frequency usually expressed in terms of cycles per minute, abbreviated as CPM.
The total distance travelled by the vibration part, from one extreme limit of travel to the other extreme limit of travel is referred to as “peak to peak displacement” and it is normally expressed in microns.
Since the vibrating weight in figure is moving, it must be moving at some speed. However the speed of the weight is constantly changing. At the top limit of the motion speed is zero. Since the weight must come to a stop before it can go it is constantly changing throughout the cycle, the highest or peak velocity is selected for measurement. Vibration velocity is normally expressed in terms of mm/sec. in the opposite direction. The speed or velocity is greatest as the weight passes through the neutral position. The velocity of the motion is definitely a characteristic of the vibration but since
Discussing vibration velocity, it is pointed out that the velocity of the part approaches zero at the extreme limit of travel of course each time the part comes to a stop at the limit of travel, it must “accelerate” to pick up the speed as it travels toward the other extreme limit of travel. Vibration acceleration is important characteristics of vibration. Technically acceleration is the rate of change of velocity.
Another important characteristic of vibration is phase. Phase is defined as the position of a vibration part at a given instant with reference to fixed point or another vibrating part”. Phase readings are normally expressed in degrees (0 degree to 360 degree) where one complete cycle of vibration equals 360 degree.
SIGNIFICANCE OF VIBRATION
The real significance of the characteristics of vibration lies in the fact that they are used to detect and describe the unwanted motion of a machine. Each of the characteristics of vibration tells something significant about the vibration. Therefore, the characteristics might be considered to be the symptoms used to diagnose inefficient operation or impending trouble in a machine.
An analysis of machinery vibration and noise should also be made at the start of a preventive maintenance program to determine whether the machine is in good operating condition or not. Analysis data taken with the machine in good operating condition is called “base line data” and provides the basis for comparison with failure and for periodical analysis.
Finding the part of the machine causing the vibration or noise is easy. The frequency of noise and vibration is normally some multiple of the RPM of the part in trouble. The table lists the vibration frequency and likely causes. The table lists the most common causes of vibration is likely encounter together with the amplitude frequency and strobe picture characteristics for each cause. The remarks column provides helpful information about any peculiar characteristics.
ACTIVE VIBRATION CONTROL
Active vibration control system uses actuators, sensors and control units to reduce vibration without necessarily adding damping or introducing vibration isolation. Vibration is attacked with a counter force that is exactly opposite to the systems response to other outputs.
PASSIVE VIBRATION CONTROL
The Passive Vibration contol system consists of a spring and a damper.The spring is intended to soften vibrations and the damper is to terminate the oscillation which is excited in the system.The active system uses accelerometers and electromagnetic drives which allows higher degree of vibration isolation to be achieved.