What is vibration?
Vibration describes actual motion associated with a source. The motion can be in one or more directions – up and down, side to side, front to rear, or rotation. For each of these directions there is a corresponding vibration intensity (acceleration), which describes “how much” vibration exists in each direction. How quickly the motion occurs in each direction is the frequency of vibration. Vibration can be continuous or impulsive; each made up of a wide range of vibration frequencies. Degree of vibration is determined by simultaneously measuring three vibration acceleration measurements (up and down, side to side, front to rear) where the person’s body comes into physical contact with the vibration. These measurements are then all electronically recorded, stored, and analyzed to determine how long during a daily work shift a worker can safely operate a given power tool or drive a vehicle such as a fork-lift.
In its simplest form, vibration can be considered to be the oscillation or repetitive motion of an object around an equilibrium position. The equilibrium position is the position the object will attain when the force acting on it is zero. This type of vibration is called “whole body motion”, meaning that all parts of the body are moving together in the same direction at any point in time. The vibratory motion of a whole body can be completely described as a combination of individual motions of six different types. These are translation in the three orthogonal directions x, y, and z, and rotation around the x, y, and z-axes. Any complex motion the body may have can be broken down into a combination of these six motions. Such a body is therefore said to possess six degrees of freedom. For instance, a ship can move in the fore and aft direction (surge), up and down direction (heave), and port and starboard direction (sway), and it can rotate lengthwise (roll), rotate around the vertical a
(part 3) DLI Engineering Posted 11-17-03 Logarithmic Frequency Scaling So far, the only type of frequency analysis discussed has been on a linear frequency scale, i.e., the frequency axis is set out in a linear fashion. This is suitable for frequency analysis with a frequency resolution that is constant throughout the frequency range, commonly called “narrow band” analysis. The FFT analyzer performs this type of analysis. There are several situations where frequency analysis is desired, but narrow band analysis does not present the data in its most useful form. An example of this is acoustic noise analysis where the annoyance value of the noise to a human observer is being studied. The human hearing mechanism is responsive to frequency ratios rather than actual frequencies. The frequency of a sound determines its pitch as perceived by a listener, and a frequency ratio of two is a perceived pitch change of one octave, no matter what the actual frequencies are. For instance if a sound of
If we could watch a vibrating object in slow motion, you could see movements in different directions. Any vibration has two measurable quantities. How far (amplitude or intensity), and how fast (frequency) the object moves helps determine its vibrational characteristics. The terms used to describe this movement are frequency, amplitude and acceleration. Figure 1 – Representation of the Measures of Vibration Exposure Frequency A vibrating object moves back and forth from its normal stationary position. A complete cycle of vibration occurs when the object moves from one extreme position to the other extreme, and back again. The number of cycles that a vibrating object completes in one second is called frequency. The unit of frequency is hertz (Hz). One hertz equals one cycle per second. Amplitude A vibrating object moves to a certain maximum distance on either side of its stationary position. Amplitude is the distance from the stationary position to the extreme position on either side an