Page 242 - Advances In Productive, Safe, and Responsible Coal Mining

Engineered noise controls for miner safety and environmental responsibility 221

tasks, the shearer is provided with two rotating cutting drums—the headgate drum and
the tailgate drum. These drums are named in reference to their location as being either
nearest headgate or tailgate entries, but either can function as the “leading drum.”
When the shearer travels from headgate to tailgate, the tailgate drum is the leading
drum that performs most of the coal cutting, while the headgate drum is mostly in
charge of a cleaning operation by pushing coal left by the leading drum into the
AFC. When the shearer travels from tailgate to headgate, the drums switch functions,
with the headgate drum being the leading drum and the tailgate drum performing the
cleaning job.
Noise exposure samples collected from longwall system operators by MSHA
between 2002 and 2011 show that approximately 48% of these operators were
exposed to noise levels exceeding the permissible exposure level, or PEL [15].In
response to this finding, NIOSH conducted research to develop engineering noise con-
trols for longwall mining systems that would reduce the noise exposure of longwall
operators.
Noise assessments conducted by the US Bureau of Mines (USBM) indicated that
cutting drums are the dominant sound-radiating components on a longwall shearer
[16]. These drums are set into vibration by the excitation forces that arise from the
interaction of the cutting bits with the coal and transmitted to the drum through the
bit holders. Due to adverse conditions at the face while the drum is in operation, i.e.,
as the drum is sumped into the coal, vibration measurements are extremely difficult to
conduct on an operating drum. In addition, the presence of explosive gases at the face
of coal mines, as well as the limited number of instruments approved for underground
use in the US, further restricts the ability to perform any type of vibration and/or force
measurements. Therefore, in order to reduce the sound radiated by the cutting drums,
numerical models of the drums were used to explore the effect of various noise control
concepts on the surface vibration and on the acoustic radiation of the drum [17]. Inputs
to these models in the form of coal-cutting forces were obtained experimentally using
a self-contained, intrinsically safe instrumented bit developed during the course of the
project [18].
The longwall shearer-cutting drum examined in this study consists of a cylindrical
body with a 0.987-m outside diameter, a 1.067-m height, and a 0.05-m-thick wall.
Inside this cylindrical body, there is a circular mounting plate 0.10m thick having
a square opening at the center of the cylinder (refer to Fig. 12.2). The drum is made
entirely of steel and weighs 4707kg. Around the cylindrical body, four helical vanes
are welded, starting in the face ring and winding around the cylindrical body toward
the discharge side of the drum. The function of the helical vanes is to push the cut coal
into the AFC as the drum rotates. The vanes have a 1.91-m outside diameter. On the
outermost edge of the vanes, there are 28 bit holders that hold the cutting bits at var-
ious angles of attack. There are also 12 bit holders on the outermost edge of the face
ring and 4 bit holders in the flange of the face ring, making a total of 44 bit holders.
Water is carried through conduits inside the vanes to the bit holders, where the water is
sprayed through nozzles to reduce the risk of ignition of mine gases and for dust con-
trol purposes.