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Engineered noise controls for miner safety and environmental responsibility 229

12.4.2 Noise controls for roof-bolting machines

Roof-bolting machines are extensively used in underground coal mines to drill and
install bolts for roof support. The majority of these machines involve a manual cycle
where the operator inserts a drill steel into the chuck and drills the hole, then inserts an
epoxy cartridge into the hole, removes the drill steel from the chuck, and replaces it
with a roof bolt tool to install the bolt. These many interactions between operator and
machine drilling head require that controls, and thus the operator, be in close proxim-
ity to the drill steel.

12.4.2.1 Noise source identification

To identify the location and the frequency content of dominant noise sources during
the roof-bolting cycle, a 1.92-m-diameter, 42-channel microphone phased array was
used. Testing was conducted in NIOSH’s hemi-anechoic chamber with a Fletcher
Model HDDR, dual head roof bolter, as shown in Fig. 12.10A. Interior dimensions
of this chamber are approximately 17.7m long by 10.4m wide by 7.0m high or
approximately 1300 cubic meters. This facility meets ISO 3744 requirements [23]
down to approximately 100Hz. Sound pressure level measurements were also taken
at the operator location, as shown in Fig. 12.10B, to determine the overall A-weighted
sound level at the operator’s ear while drilling.
A large steel support stand comprising rectangular tubes was fabricated by NIOSH
to hold the drilling media, shown in Fig. 12.10. To prevent the support stand from
radiating significant amounts of sound, sand was used to fill the hollow tubes except
for the diagonal tubes and the horizontal tubes along the short direction at the top of
the structure. This was done for convenience and to create a vibration impedance mis-
match in the structure to reduce vibration transmission.
During the formulation of the test plan, it was decided to use drill bits and drill
steels that were representative of industry usage. Therefore, round and hexagonal drill
steels were used along with a 34.9-mm drill bit. Granite was chosen as the drilling

Chain
Test
fixture

Urethane
sheet
Rock

Operator ear
microphone

(A) (B)

Fig. 12.10 Experimental setup used for drilling tests: (A) roof bolter in the hemi-anechoic
chamber, and (B) location of the operator ear microphone.