R&D Portfolio - Research Goal 3.2 : Reduce noise generated by roof bolting machines using wet and mist drilling
Roof bolting machines are employed in underground coal mines to drill vertical holes in the roof of the mine and then insert 3 to 4 foot bolts into these holes, which help to secure the stability of the mine roof and prevent roof falls. Drilling into hard rock in a confined space generally results in sound pressure levels over 90 dB(A) at the drill operator position. MSHA coal noise data collected from 2000 through 2003 indicated that roof bolting machine operators were the second most common type of underground coal equipment operator whose average occupational noise exposures exceeded 100% of the PEL.15 Adding water to a drilling process on a coal mine roof bolting machine can reduce the drilling noise, but can result in other problems such as wet and slippery mine floors and an uncomfortable work environment for the drillers who are exposed to the water coming out of the drill holes. Mist drilling methods require much less water than conventional wet drilling methods, but can provide the same noise reduction benefits.
The HLR program conducted tests of dry, wet, and mist drilling configurations in its acoustic test chamber to evaluate and optimize drilling technologies for noise reduction. HLR program staff used sound power level testing methods to do this study. The study found that wet and mist configurations emit less noise than dry drilling configurations. Depending on the compressive strength of the rock media, field testing shows that sound pressure level reductions of
2 to 7 dB(A) occur at the bolter operator’s position when employing wet drilling configurations. This result is most apparent in the one-third octave band center frequencies of 1,000 Hz or greater. The water attenuates noise in these frequency bands, reducing the sound power level emission. This research also showed that wet drilling results in better performance, i.e. rock penetration rate, and if properly optimized, can reduce respiratory dust production. After the technologies were optimized for different mining conditions, they were tested at field sites where the lab findings were confirmed. The mist drilling system has the same noise-reduction benefit as wet drilling, but uses much less water, creating a better work environment for the operator and discharging less water on the mine floor. In mines where wet drilling creates a problem, mist drilling is an effective alternative.
Mist system on a roof bolting machine
 Peterson JS, Kovalchik PG, Matetic RJ . A sound power level study of a roof bolter. SME Preprint No. 05-72, 2005 SME Conference, Salt Lake City, Utah, 8 pp