Vibrational sources

Time-domain plots must be used for all linear and reciprocating motion machinery. They are useful in the overall analysis of machine-trains to study changes in operating conditions. However, time-domain data are difficult to use. Because all the vibration data in this type of plot are added together to represent the total displacement at any given time, it is difficult to directly see the contribution of any particular vibration source. 

A frequency-domain vibration signature is generally used for the analysis because it is comprised of discrete peaks, each representing a specific vibration source. 

If there is no resonant condition to modify the resultant vibration phase, then the phase for both vertical and horizontal readings are essentially the same even though the vertical and horizontal amplitudes do not necessarily correspond. In actual practice, this may be slightly off due to other vibration sources such as misalignment. In performing the analysis, what counts is that when the source of the vibration is primarily from imbalance, then the vertical reading phase differences between one end of the rotor and the other will be very similar to the phase differences when measured horizontally. For example, vibrations 60° out of phase vertically would show 60° out of phase horizontally within 20 per cent. 

Possible vibration sources outside the building, such as nearby railroad tracks, should be considered also. In a new testing laboratory, heavy truck traffic immediately outside caused periodic vibration problems, even though the building sat on a solid concrete slab. Had this laboratory been on an upper floor, the vibration would have been even more severe. 

External vibration sources might include other equipment on bench tops, such as shakers, centrifuges, or sonicators. Local construction in the area or within the building is a common, though often overlooked, source of vibration. The testers should not be near hoods or significant airflow sources. Additionally, heavy foot traffic and door slamming should be avoided. 

In lieu of a perfect room, the specific problems your room presents should be known to protect the validity of your weighing as much as possible. For example, if you are having problems limiting your vibration sources, make your weighings at odd hours when elevators are not in use or when people are unlikely to enter the... 

Exposure monitoring is another element of HCP. The sound level and exposure time should be measured. It is very important that sound levels measured are typical of those encountered by the worker. Proper survey techniques include sound-pressure-level (SPL) meters that should be vigorously apphed at monitored workplaces. They measure the smallest pressure changes initiated by the vibrating source and transmitted through the air. 

Vibration increases noise and decreases lateral and vertical resolution. The most effective vibration isolation takes place in the instrument design. A goal is to keep vibrations resonances out of the band of frequencies used for data collection and feedback control, i.e., about 1-1000 FIz. Minimization of size and maximization of rigidity shifts vibration resonances to high frequencies (14,20). In addition, locating the SECM away from vibration sources such as air vents, pumps, or heavy machinery is advised. Note that bypassers are a source of vibrational and electrical interference and locations with foot traffic are inadvisable. 

Damping systems do not isolate the structure from the vibration source, but instead reduce the magnitude of the vibration within the structure. There is damping technologies that are passive and active. In passive systems, vibrations are diverted into special materials or structural components that dissipate the vibration energy as heat. In active systems, an electronic system monitors the response of the structure to vibration, and actuators then move the structure in opposition to the vibration, effectively canceling it out. Active systems can achieve greater performance and control than passive systems, but are much more expensive, take up more space, and require power. 

Figure 8 shows vibration of a machining center between its spindle and table caused by pressure fluctuation of coolant oil supplied to the spindle motor. Other major vibration sources were switched off during the measurement. The figures a and b show the vibration before and after the oil pressure was stabilized by adjusting the hydraulic accumulator, respectively. It demonstrates that vibration can be reduced by reducing the oil pressure fluctuation. 

Transmission of vibration to other structures can be a problem. Even on relatively simple applications, such as centrifugal pump bases, it is necessary to minimize the transmission of vibrations (usually by providing separate bases), otherwise standby machinery may be subject to bearing damage. In the case of control rooms, any noticeable transmitted vibrations are likely to be cause for complaint. Even when environmental standards concerning permissible control room vibration levels exist, and may seem attainable without special construction, it is advisable where possible to be conservative. Positively isolate the foundations and employ brick or concrete building methods if there is any vibration source in the vicinity, especially for low-frequency vibration exciters such as crushers and mills. 

Safety No vibration source, such as pumps, in the reactor vessel Primary sodium is unlikely to leak out because it is contained in the reactor vessel... 

The acoustic pressure can be measured through the effects produced in the propagation liquid, namely (1) heating, measured calorimetrically and (2) the action of radiation pressure, determined by measuring the force exerted on a surface (the radiometric method). While the former allows the measurement of the mechanical power of the vibrating source, the latter is associated with the acoustic power transmitted by the acoustic radiation. The acoustic pressure can also be determined using calibrated hydrophones (the acoustic method). 

What we perceive as sound is a series of compressions and rarefactions transmitted by some vibrating source and propagated in waves through the air. The compressions and rarefactions impinge on the ear drum (tympanic membrane) causing it to vibrate and transfer the movements through three small bones in the middle ear to the fluid of the inner ear. There they are received by rows of hairs (in the organ of corti), which vary in their response to different frequencies of sound, and are then transmitted to the brain and interpreted as sound. 

B) A competent person shall determine whether rock bolts meet the necessary torque, and shall determine the testing frequency in light of the bolt system, ground conditions and the distance from vibration sources. 

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