Engineering Controls Database
Nail-Making Machine – Noise Control Study
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Overview: The case history presented here is one of sixty-one case histories that were published by the National Institute for Occupational Safety and Health (NIOSH) in 1978 as part of an industrial noise control manual [NIOSH 1979]. The case histories are examples of engineering tasks that have been completed not only by professional noise control engineers but also by non-acoustical specialists who used common sense to solve their noise problems. The case histories were chosen primarily because the amount of noise reduction actually achieved was measured. Such engineering results, even if not directly applicable to a specific situation, illustrate general principles that may point the way to a successful result. They are intended to be useful to production and safety engineers, health personnel, and other factory personnel who are not specialists in noise control. Case study: A nail-making machine was operating under conditions causing severe impacts. The vibration was solidly transmitted to a weak concrete floor, which radiated considerable noise. There were 10 machines, operating at 300 strokes/min. Operator sound level was 103.5 dBA. |
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A sound level of 103.5 dBA only allows for 7 minutes and 30 seconds of exposure time. The goal is to lower the level to a point where the operator can work an entire shift at a safe exposure level. Hearing loss is one of the most common occupational diseases in America today and the second most self-reported occupational illness or injury. Approximately 30 million workers are exposed to hazardous noise on the job and there are approximately 16 million Americans with noise-induced hearing loss. |
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It was decided to use vibration isolating mounts to reduce floor radiated noise. Because of the repeated shock situation, selec¬tion of the isolator followed these rules: • The natural period of isolator plus machine should be much greater than the shock pulse duration (10 msec). • The natural period of isolator plus machine should be less than the time between pulses (200 msec). Elastomer-type isolators were used, which had a static deflection of 0.1 in. under machine load. This corresponds to a natural period of 100 msec, thus fulfilling the design conditions. |
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| NIOSH [1979]. Industrial noise control manual – revised edition. Cincinnati, OH: U.S. Department of Health Education and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 79-117 | |
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nail manufacture nail manufacture nail-making machine nail-making machine noise noise noise control noise control |
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Figure 1 shows octave-band spectra at the operator's position after all machines had been vibration-isolated. The sound levels have been reduced about 8.5 dB to 95 dBA, a level still in excess of permitted levels. Additional noise control is needed. Comments: To maintain the isolation, maintenance people should be warned not to short-circuit the isolators by any solid connection from machine to floor. This short-circuiting can also occur when dirt and grease are allowed to build up around the pods. As a reduction to a sound level of 95 dBA is not considered satis¬factory for full-day operator exposure, additional noise reduction could be obtained by the design of a barrier between the major noise source in the machine and the operator. Depending on the needs for vision through the barrier, plywood, lead-loaded vinyl curtain, or Plexiglas could be used. Such a barrier should yield a reduction of 5 to 8 dB at the operator position. This noise reduction should result in lowering the sound level to 87—90 dBA. Where there is a series of machines, additional reduction of several decibels could be obtained by added room absorption, either in the form of spray-on acoustic absorbent on ceilings and walls or in the form of hanging absorbent baffles from the ceilings. [img=1] |