Definitions
Vibration may be defined simply as any movement which a body makes about a fixed point. This movement can be regular, like the motion of a weight on the end of the spring, or it can be random. The vibration experienced from machinery is usually a very complex, but regular, motion. Using appropriate analysing techniques, any complex motion can be analysed in terms of a number of simple components. This type of analysis-Fourier Analysis-is outside the bounds of this book but can be found in the most texts dealing with vibration theory.
With vibration being defined in this simple way, it follows that a body can vibrate in a number of different directions, although the International Standards Organization (1974) suggests that any movement should be defined and measured in terms of three orthogonal components : ‘x’-front to back : ‘y’-side to side; and ‘z’-up/down. Using these coordinates, a person jumping up and down, for example, would be vibrating in the z direction. If the same person was rotating about an axis from shoulder to shoulder (for example, tumbling face-forwards from a roof) then the body would be vibrating in the x and z axes in the same time. Finally, a weightless astronout in space may rotate uncontrollably and might be vibrating in all three axes together. For most experimental purposes, however, the effects of vibration are normally considered in one axis only-and this has generally been in the vertical (z) direction.
Types of vibration :
- Sinusoidal : influenced by regular vibration.
- Random : influenced by irregular vibration and the vibration cannot be predicted, ussually it comes from the real world tools.
The types of vibrations based on affected location consist of:
- Whole body vibration : there are three kinds i.e., vertical vibration, horizontal, and lateral vibration.
- Vibration in certain location: on the shoulder part and fingers affected by hand tools.
The reactions caused by vibration are:
- Physiology : depending on frequency and intensity.
- Work performance reaction : depending on frequency, duration, body support, angle, age.
Once the direction of the motion has been determined, the amount of vibration is defined in terms of two parameters : its ‘speed’ and ‘intensity’.
The ‘speed’ of a vibrating body is expressed in terms of its frequency of movement-simply the number of times the body, the weight on the end of a spring for example, completes one cycle movement (i.e. from its fixed reference point to its higest point, to its lowest point and finally back to its reference point) within a spesific time priod (usually 1 second). The faster the body vibrates, therefore, the more cycles that occur per second. The normal unit of frequency in the Hertz (Hz) where 1 Hz = 1 cycle per second.
Vibration intensity is normally expressed in terms of the maximum amount the body moves from the fixed point-usually in distance for amplitude (in cm or in mm). More recently acceleration units have been used to define vibration intensity. These are conventionally expressed in ‘g’ unit (1 g being the amount of acceleration needed to lift a body off the earth’s surface); although the convention hase been changed slighty in recent years to use the metric units of meter per second2 (1 g = 9.81 ms-2).
Each of these parameters is related, thus ;
Acceleration in 'g' unit :
Where f is the vibration frequency and a is the vibration amplitude.
Conversion Table of displacement, velocity, and acceleration :
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