A study of U.S. western hard-rock miners noise exposure revealed that 96% of mining machine operators are exposed to noise levels exceeding the Permissible Exposure Level (PEL), with jackleg drill operators having the most rapid noise dose accumulation rate¹. Jackleg drills, simply referred to as drills throughout this article, are used to drill blast holes that are filled with explosives. They are also used to drill bolt holes, through which bolts are driven for roof support, especially in narrow situations. Traditionally, these drills have been driven by pneumatic power, i.e. compressed air. However, there are drills currently available that are powered by electricity. These electric drills are less noisy and have lower vibration levels than pneumatic drills, and constitute an attractive alternative to pneumatic drills. However, electric drills also have lower penetration rates than pneumatic drills². As a consequence, the reduced noise of the electric drills tends to be overshadowed by the slower penetration rates. One of the outstanding issues to be addressed is to quantify the noise exposure reduction benefits of the electric drills, so that their value can be assessed by mine operators.

In this context, the National Institute for Occupational Safety and Health (NIOSH) conducted a study to assess the acoustic performance of pneumatic drills and electric drills used in the mining industry. The objective of the study was to determine, from an occupational noise exposure perspective, whether electric drills constitute a viable alternative to pneumatic drills. This article presents the most relevant results from the study. A more comprehensive report can be found in Reference 3.

 

(a)    Pneumatic drill. (b) Pneumatic drill with muffler. (c) Electric drill.

Figure 1. Drills under test.

 

 

Figure 2. Experimental setup for noise source identification measurements.

 

 

3 RESULTS

 

3.1 Noise Source Identification

 

Noise source identification was conducted to determine the physical location and the frequency content of the dominant drill noise sources. To accomplish these tasks, a microphone array was used to measure the acoustic field. The data were then processed using a beamforming algorithm. This algorithm focuses the array to a particular point in space where it is suspected a noise source is located. The results from this processing are acoustic maps in one-third octave frequency bands.

 

Figure 3 shows typical beamforming acoustic maps. From these maps it can be observed that for the pneumatic drill without muffler, the dominant noise source is located at the drill body. When a muffler is installed, the sound is reduced by approximately 5 dB and now two noise sources are present; one at the drill body and one at the drill-steel-rock interaction place. Therefore, the muffler reduces the source at the drill to a level that is comparable to the noise source at the drill bit-rock interaction place. When the electric drill is used, it can be seen that the sound is further reduced by approximately 9 dB, and now the dominant source is located at the drill-steel-rock interaction place.

 

 

(a) Pneumatic drill. (b) Pneumatic drill with muffler. (c) Electric drill.

Figure 3. Typical acoustic maps for the tested drills (2000 Hz).

 

3.2 Noise Dose Accumulation

 

The cumulative dose was measured using a dosimeter placed at the operators shoulder, as shown in Figure 2. Figure 4 shows the cumulative dose as a function of time while drilling into concrete. In this figure, the three segments corresponding to the dose accumulated while operating each tested drill are clearly identified. From this figure, it can be seen that the operator accumulates dose more rapidly while operating the pneumatic drill. These results were used to estimate average cumulative dose rates per minute which are summarized in Table 1.

 

 

Figure 4. Cumulative dose for the three tested drills while drilling in Concrete.

 

 

Table 1. Operator cumulative noise dose.

 

 

3.3 Penetration Rate

 

The penetration rate was determined by dividing the hole depth by the time required to drill a particular hole. The penetration rate of the electric drill, as shown in Figure 5, is approximately 60% the penetration rate of the pneumatic drill. Using these penetration rates, and the cumulative dose, presented in Table 1, allows estimating the time required to drill a reference 1.2 meter (48 inch) hole. It would take 1.98 minutes to drill a reference hole in concrete using the S83 with muffler as opposed to 3.24 minutes using the TE MD20. Similarly, it would take 3.22 minutes to complete a 1.2 meter (48-inch) reference hole in granite using the S83 with muffler, while it would require 4.89 minutes to drill the same hole using the TE MD20 drill. During this time, the operator of the pneumatic drill with muffler would accumulate approximately 8.1% dose, while the operator of the electric drill would accumulate 4.2% of the maximum allowable dose.

 

 

Figure 5. Penetration rate.

 

 

3.4 Determination of Sound Power Levels

 

The sound power level is the actual sound energy emission of a device. Thus, it is a valuable tool for comparing the noise generated by the drills under test. The sound power level radiated by the tested drills while drilling into granite is presented in Figure 6a. Similarly, Figure 6b shows the sound power level radiated by the drills while drilling into concrete. Figure 6a reveals that the difference in sound power level between the pneumatic drill without muffler and the electric drill is 10.5 dB(A). When the muffler is installed, the difference in sound power level between the pneumatic drill (with muffler) and the electric drill is reduced to 7.2 dB(A). Figure 6b shows a similar trend when drilling into concrete. The difference in sound power level between the pneumatic drill without muffler and the electric drill is 8.1 dB(A). When the muffler is installed, the difference between the pneumatic drill (with muffler) and the electric drill is 4.8 dB(A).

 

 

(a)    Granite. (b) Concrete.

Figure 6. Sound power level radiated by the drills.

  4 CONCLUSIONS

 

A comparison between a pneumatic drill and an electric drill was conducted by NIOSH. The results from the various tests conducted in the study indicate that from an occupational noise exposure perspective, the acoustic performance of the electric drill, despite its slower penetration rates, overcomes the acoustic performance of traditional pneumatic drills. These findings provide quantitative information to assist the mine operator in balancing potential miner hearing loss and productivity with a viable means to meet regulations.

 

 

5 ACKNOWLEDGMENTS

 

The authors would like to thank Gary Roberts, Ron Key, and Rusty Lynn Howard for providing the drills, and to Patrick McElhinney for his support for setting up the test.

 

 

6 REFERENCES

 

1. Spencer E.R., Assessment of Equipment Operators' Noise Exposure in Western Underground Gold and Silver Mines, 2009 SME Annual Meeting and Exhibit, February 22-25, Denver, Colorado, preprint 09-073. Littleton, CO: Society for Mining, Metallurgy, and Exploration, Inc., 2009.

2. Phillips, J.I., Heyns, P.S., and Nelson, G., Rock Drills used in South African Mines: a Comparative Study of Noise and Vibration Levels, The Annals of Occupational Hygiene, Vol. 51, No.3, pp. 305-310, 2007.

3. Camargo, H.E., Peterson, J.S., Kovalchik, P.G., and Alcorn, L.A., Acoustic Assessment of Pneumatic and Electric Jackleg Drills Used in the Mining Industry, Proceedings of NoiseCon 2010, Baltimore, MD, April19-21, 2010.