5.3.2 Dominant FrequenciesEven in normal drilling there is some drilling induced vibration at levels which are acceptable, both from an operator comfort standpoint and structural integrity standpoint. In order to find relationships with the vibration signal, the drill was run at different speeds in small steps between 30 and 117 RPM (120 RPM attempted but halted due to safety concerns). These frequency domains were plotted and a relationship was found between the vibrational frequencies and RPM. The results showed a distinct peak in the vibration signal at 6‐times the rotary speed. At lower speeds, the dominant vibration peaks lost correlation with the rotary speed as the overall amplitude of vibration was low and the peaks blended into the ambient background noise. A plot of the data can be seen in Figure 5‐3.
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