Page 245 - Advances In Productive, Safe, and Responsible Coal Mining

224 Advances in Productive, Safe, and Responsible Coal Mining

Fig. 12.5D, and the main part of the drum were still given the material properties of
steel. For the baseline case, the whole drum was defined as steel.
For the force isolation case, in the frequency range of interest (below 2kHz), the bit
and bit holder vibrate almost as a rigid body with relatively low natural frequencies,
due to the flexibility provided by the rubber layer. Meanwhile, the main part of the
drum has many flexible modes with relatively high natural frequencies, some of which
are significant contributors to the total noise radiation. For frequencies above the
highest natural frequency for which the bit and bit holder behave as a rigid body,
the force transmitted to the main part of the drum can be significantly reduced, due
to the 20dB/decade slope of the transfer function. However, at frequencies where
the bit and bit holder behave as a rigid body, larger forces can be transmitted to
the main drum structure due to the resonance.
To reduce the force transmission for all the frequencies, the drum design should be
modified so that the highest natural frequency of the bit and bit holder assembly rigid
modes is lower than the first flexible mode of the main drum structure. In practical
terms, natural frequencies of the bit and bit holder system can be adjusted by using
different rubber materials.
In this study, the properties of actual industrial rubber materials were used to eval-
uate the effect of the bit isolation concept on sound radiation, and significant sound
power reduction of up to 25.9dB(A) was achieved. However, after the authors dis-
cussed this concept with cutting-drum design engineers, it was concluded that this
concept is not suitable for the cutting drum due to adverse cutting performance and
durability issues that the viscoelastic material would pose.

Damping treatment
Experimental modal analysis tests conducted on a newly manufactured drum indi-
cated that the longwall-cutting drum is very lightly damped [17]. A uniform 0.01 loss
factor was used for the structure in the structural-acoustic simulation as an approxi-
mation of the damping ratio obtained experimentally [20]. Due to the small damping
ratio, there are many sharp peaks in the predicted sound power level spectra. Those
peaks can be suppressed by increasing the damping ratio of the drum. Therefore, the
effect of increasing the damping on the predicted overall sound power level of the
noise radiated by the longwall-cutting drum was evaluated using numerical models.
The overall sound power level below 2kHz, predicted using a uniform 0.01 loss
factor, was taken as the baseline. The overall sound power levels for two additional
cases—one with a uniform 0.02 loss factor and another with a uniform 0.03 loss
factor—were calculated and compared with the baseline prediction. For the 0.02 loss
factor and for the 0.03 loss factor, the overall sound power level reductions were
3.3dB(A) and 5.2dB(A), respectively.
Despite these reductions, this noise control concept does not constitute a very
attractive strategy for the longwall-cutting drum. On the one hand, increasing the
damping of the drum would require some type of damping treatments (e.g., attaching
a layer of viscoelastic material to the surface of the drum), which, due to the adverse
environment, would have durability issues. On the other hand, it would not be