The evolution of drill bit and chuck isolators to reduce roof bolting machine drilling noise

Among underground coal miners, hearing loss remains one of the most common occupational illnesses. To address this problem, the U.S. National Institute for Occupational Safety and Health Office of Mine Safety and Health Research (NIOSH OMSHR) conducts research to reduce the noise emission of underground coal mining equipment, an example of which is the roof bolting machine. Field studies show that, on average, drilling noise is the loudest noise that roof bolting machine operators would be exposed to, and it contributes significantly to the operators’ noise exposure. OMSHR has determined that the drill steel and chuck radiate a significant amount of noise during drilling. OMSHR and the Corry Rubber Corporation (CRC) have developed a bit isolator that breaks the steel-to-steel link between the drill bit and drill steel and a chuck isolator that breaks the mechanical connection between the drill steel and the chuck. This effectively reduces the noise radiated by the drill steel and chuck and in turn reduces the noise exposure of roof bolter operators. This paper documents the evolution of the bit isolator and chuck isolator. Laboratory testing confirms that production bit and chuck isolators reduce the A-weighted sound level generated during drilling by 3.7 to 6.6 dB. Introduction Hearing loss prevention is one of 21 priority research areas listed in the the U.S. National Institute for Occupational Safety and Health (NIOSH) national occupational research agenda (NIOSH, 1996). Further, the U.S. Department of Labor’s Mine Safety and Health Administration (MSHA) collects noise sample data that assists OMSHR in selecting equipment whose operators are most likely to be overexposed to noise (MSHA, 2005). Sample data collected from 2000 to 2005 show that seven types of machines compose the bulk of the equipment whose operators exceed 100% noise dose per the MSHA permissible exposure level (PEL) (Table 1). Among operators of all equipment used in underground coal, the table shows that roof bolting machine (RBM) operators were the second most numerous group of miners to be overexposed to noise. To develop effective noise controls, it was important to determine the tasks and noise levels associated with significant noise exposure for roof bolting machine operators. The objective was to reduce operators’ noise exposure to a time-weighted average (TWA) of 90 dB(A) or less for an eight-hour shift. This would correspond to a noise dose of 100%—the maximum allowed per the MSHA PEL. OMSHR performed noise dosimetry and time-motion studies to determine the noise doses and sound levels associated with the typical tasks required of RBM operators and the time spent conducting these tasks. Post-processing of this data revealed the tasks that operators devoted the most time to, the noise dose accumulated during each task and, of course, the tasks which are the primary contributors to the operators’ noise exposure. This information helped OMSHR to determine where noise control research should be focused. Tables 2 and 3 summarize data collected during a timemotion study of a dual-boom J.H. Fletcher roof bolting machine (Peterson and Alcorn, 2007). The tasks shown are those for which the operators were engaged at least 1% of their shift. The ‘other’ category of Tables 2 and 3 includes time spent riding the elevator, when the operators were bolting alone, both drilling or both bolting, and other times where the activity could not be documented. Table 2 shows that the operators spent a significant amount of time drilling and bolting simultaneously—roughly 3.5 hours of the shift. As shown in Table 3, Operator 1 accumulated 52% PEL dose during this time, or two-thirds of his daily noise exposure, 78% PEL dose. Operator 2 accumulated 68% PEL dose during this time, or greater than half of his full-shift exposure of 127%. Further analysis showed that drilling sound levels far exceeded those associated with the bolting portion of the duty cycle (Table 4). Table 1 — Percentage of noise overexposures by machine.