Page 246 - Advances In Productive, Safe, and Responsible Coal Mining

Engineered noise controls for miner safety and environmental responsibility 225

practical to treat the whole cutting drum. Performing damping treatments on the outer
vane segments, which contribute the most to the total noise radiation, might be much
easier and more practical. However, by applying damping treatments to only the outer
vane segments, there is a theoretical maximum sound power level reduction, which
occurs when the treated components do not radiate any noise. That being the case,
the largest noise reduction that could be achieved is approximately 3dB(A), because
the noise generated by the vibration of the outer vane segments accounts for approx-
imately 50% of the total noise radiated by the whole drum, as shown in Fig. 12.4.

Structural modification
The predicted sound power level spectrum has two dominant characteristics that relate
to the vibration of the structure, and provide an excellent basis for developing struc-
tural modifications for suppressing noise radiation. The first characteristic is that the
sound power level has a large amplitude when the direction of the dynamic deforma-
tion of the cutting bit either aligns with, or has a large component along, the direction
of the excitation force. A straightforward solution to this condition is to minimize the
amplitude of the cutting-bit dynamic deformation in the frequency range of interest.
This is done by increasing the stiffness of the cutting-bit assemblies. The second char-
acteristic is that the outer vane segment vibration contributes the most to the total
noise radiated by the longwall-cutting drum. As a result, reducing the number of outer
vane segment modes in the frequency range of interest also reduces the radiated sound.
Helical plates (1 2-in. or 2.5 5-cm cross section) were added to the model to
connect bit holders to outer vane segments as shown in Fig. 12.6A. These plates served
to stiffen both cutting-bit assemblies and outer vane segments. Stiffeners provide
additional support for cutting-bit assemblies, and they also provide T-shaped supports
for outer vane segments. Further, these stiffeners connect all bit holder assemblies
located on the same vane, which significantly suppresses cutting-bit assembly out-
of-phase modes that occur along the vane.
Modal analysis results of the modified cutting drum with stiffeners show that the
number of modes in the frequency range of interest (below 2kHz) was reduced by
around 70 from the original 250 modes. For cutting-bit assemblies located on the face
ring, there is no vane segment for the bit holder to be connected to. Therefore, an
L-shape stiffener, highlighted in Fig. 12.6B, was added for each bit located on the face
ring. This approach was taken instead of using continuous plate stiffeners as were used
for cutting-bit assemblies located on vanes.
In order to assess the performance of the structural modifications, three different
cases with excitations applied at different cutting bits were analyzed. The excitation
locations for this analysis are highlighted with yellow circles in Fig. 12.6C–E. For all
three cases, the applied excitation consisted of the measured coal-cutting forces [18].
The predicted overall sound power level below 2kHz was compared with the baseline
prediction, and the reduction achieved for each case is given in Fig. 12.7. From the
simulation results, it can be seen that a promising sound power level reduction of
approximately 3dB(A) can be achieved by implementing these structural modifica-
tions on the longwall-cutting drum.