Page 261 - Advances In Productive, Safe, and Responsible Coal Mining

240 Advances in Productive, Safe, and Responsible Coal Mining

12.4.3.3 Constraints

The most important requirement regarding noise controls in this particular application
was durability, especially since viscoelastic materials such as urethane were being
used. Another constraint, common to all underground mining equipment, is that noise
controls should not risk the structural integrity of the component being addressed,
which was the conveyor chain in this particular case.

12.4.3.4 Selected noise control
After laboratory and field testing of multiple noise control prototypes, the conveyor
chain with urethane-coated flight bars was selected as the most practical noise control.
Underground tests showed that this noise control provides an estimated noise reduc-
tion of 3dB in an 8-h time-weighted average using MSHA criteria [34]. Coated flight
bars proved to be significantly more durable than the treated tail roller, with in-mine
testing suggesting that the life of a coated chain equals or exceeds the expected life of a
typical metal-flight chain. This noise control is commercially available and has been
implemented in various mines to reduce the noise exposure of CMM operators.
Although the urethane-coated conveyor chain provides significant noise reduction,
the overall sound radiated by the CMM must still be further reduced. It is likely that a
CMM operator exposed to typical noise levels for an 8-h shift would still be exposed to
a time-weighted average sound level exceeding the MSHA PEL. To further reduce the
sound level at the operator location, and thus the noise exposure, sound radiated by the
dust scrubber system of the machine needs to be addressed. Preliminary work in this
area has identified various flow obstructions along the ducting and demister compo-
nents that result in an off-axis nonuniform flow upstream of the fan. These obstruc-
tions create air turbulence thereby decreasing fan efficiencies and increasing noise
emissions.

12.5 Conclusions

This chapter has presented an overview of research conducted by NIOSH aimed at
reducing occupational noise-induced hearing loss among different mining machine
operators. The approach used to achieve this goal was that of reducing sound radiated
by various pieces of mining equipment such as longwall mining systems, continuous
mining machines, and roof-bolting machines. For each machine, the process involved
identifying dominant noise sources and developing engineered noise controls that
would attenuate the sound radiated by these sources. Through this process, noise con-
trols were developed and implemented, and in most cases retrofitted, onto machines in
operation.
Interestingly, after initial dominant noise sources were attenuated by engineered
controls, other noise sources became dominant. For example, drill steel vibration
was identified as the dominant noise source for roof-bolting machines. Two noise con-
trols were developed: (1) the drill bit isolator to attenuate vibrations transmitted from
bit to drill steel, and (2) the collapsible drill steel enclosure, which acts as a barrier