Production machinery and equipment that generate intense noise include machines that operate with impacts such as forging hammers [100, 101], cold headers, stamping presses, riveters, jolting tables, some machine tools, and impact‐generating assembly stations. With the exception of forging plants, in which forging hammers are the dominant source of noise, noise sources in manufacturing plants can be typically classified in order of intensity/annoyance as: (i) compressed air (leakages, air exhaust, air blowing nozzles), (ii) in‐plant material handling systems, and (iii) production and auxiliary machinery and equipment. Reference [102] discusses noise in production areas of manufacturing plants.
The two basic noise reduction techniques are (i) use of acoustical enclosures, which are expensive to build and maintain, may reduce efficiency of the enclosed equipment, and are not always feasible, and (ii) noise reduction at the source, which is effective but often requires a research and development effort. Reference [103] contains a large number of case histories of successful noise reduction projects carried out in industry.
11.5.1 Machine Tool Noise, Vibration, and Chatter
A common problem in the manufacturing industry today is the vibrations or chatter induced in machine tools during machining. Such problems occur, for example, in turning, milling, boring, and grinding. Chatter is very undesirable. Not only does it create noise but the vibration that it produces also results in an uneven cut and undesirable cut quality. The vibration of machine tools may be divided into three different classes: (i) free or transient vibrations of machine tools excited by other machines or engagement of the cutting tool and the like, (ii) forced vibrations usually associated with periodic forces within the machine tool, for example, unbalanced rotating masses, and (iii) self‐excited chatter that may be explained by a number of mechanisms. These mechanisms include, among others, the regenerative effect, the mode coupling effect, the random excitation of the natural frequencies of the machine tool caused by the plastic deformation of the workpiece material, and/or friction between the tool and the cut material. Vibration in machine tools affects the quality of machining, particularly the surface finish. Furthermore, machine tool life can be correlated with the vibration and noise levels produced. Machine tool chatter may be reduced by selective passive or active modification of the dynamic stiffness of the tooling structure and/or by the control of cutting data and its use to maintain stable cutting. Forced unbalance vibration in rotating tooling structures may be reduced by passive balancing or active online balancing.
Free or transient vibrations of machine tools may be excited by other machines in the environment via the machine tool base or/and by rapid movements of machine tables, engagement of the cutting tool, and the like. The forced vibrations are usually associated with periodic forces within the machine tool, for example, unbalanced rotating masses, or the intermittent tooth pass frequency excitation in milling. This type of vibration may also be excited by other machines in the environment of the machine tool via its base.
Machine tool vibrations during machining operations are usually denoted self‐excited chatter or tool vibration. Depending on the driving force of the tool vibration, the vibration is generally divided into one of two categories: regenerative chatter (secondary chatter) and nonregenerative chatter (primary chatter). See Refs. [104–106] for examples. Extensive research has been carried out on the mechanisms that control the induction of vibrations in the cutting process. The majority of this research has involved the dynamic modeling of cutting dynamics focusing on analytical or numerical models. Usually, the purpose of this work is to produce dynamic models for the prediction of cutting data that ensure stable cutting and maximize the material removal rate. Active control has also been investigated for turning and boring operations [107, 108]. Reference [109] discusses the problem in the manufacturing industry of the chatter‐induced vibration in machine tools during machining.
11.5.2 Sound Power Level for Industrial Machinery
Although the present chapter has discussed a number of procedures for calculating the sound power and sound pressure levels of industrial machinery, more information can be found in Ref. [110]. The measured or calculated sound power levels can be used for predicting the sound pressure levels in a space or developing purchase specifications for new equipment. With any project, acoustical data measured and calculated in accordance with recognized standards should be obtained. Many manufacturers provide sound power levels or measured sound pressure levels at 1 m from their equipment, and some offer special low‐noise options. In the European Community (EEC) it is required to determine the sound power level of some items of machinery and to provide a label on the machine giving this information. If manufacturers’ data are unavailable, efforts should be made to measure a similar unit in operation. If this is not practical, then theoretical estimations can be used.
It has to be noted that most of the equations presented in this chapter are based on measured data and tend to be conservative, usually predicting somewhat higher sound pressure levels than are measured in the field. Due to recent efforts at reducing equipment noise, sound pressure levels for some equipment may be significantly (10 dB) quieter than the levels calculated in this chapter. Some equipment consists of several different sound‐producing components such as motors, pumps, blowers, and the like. The sound power levels for each component should be determined and then combined (using correct decibel addition) to get the total sound power levels.
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