## 2 Common Faults of Gearboxes and Causes Analysis  
The internal elements and structures of gearboxes are various to meet the transmission performance requirements of different applications. For example, planetary gear trains,widely adopted in wind generation transmissions, consist of either spur, single-helical, or herringbone gears to increase the rotational speed of the rotor to match that required by the generator [28; 29]. Despite the diversity of the internal structure and components, most gearboxes include various kinds of gears, gear shafts, bearings, enclosures, etc., and all these elements are prone to failure because of the following possible reasons [30]: (1) wearing or damage due to long-term operation with high alternating loads; (2) insufficient component lubrication under gearbox intensive operation; and (3) high instantaneous impacts at emergent loaded starting- or stopping-operation. According to [PERSON] et al. [31], the elements with high failure rates in gearboxes are bearings and gears, a total of up to more than 60% of failures occur on these two components. Therefore, the faults of bearings and gears were taken as research objects in this paper for the following diagnosis research, which is a general study of gearboxes without focusing on a particular gearbox type.  
According to [PERSON] et al. [32], the common faults and their causes of gears and bearings are summarized in Table 1.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Elements**} & **Common** & & **Causes** \\  & **Faults** & & & \\ \hline \multirow{8}{*}{Gears} & Broken & - & Strong shocks in operation from \\  & \multirow{2}{*}{Teeth} & & heavy load. \\  & \multirow{2}{*}{-} & \multirow{2}{*}{Periodically intense bending stress.} \\  & & & & Periodically intense bending stress. \\ \cline{3-4}  & \multirow{2}{*}{Pitting} & - & The shear stress on tooth flanks makes the \\  & & & metal on tooth surface deform and peel off \\  & & & to form uneven pits. \\ \cline{3-4}  & \multirow{2}{*}{Cracks} & - & Constant impacts during \\  & & & long-term operation. \\  & & & A defect of temperature control in the gear \\  & & & production process. \\ \hline \multirow{8}{*}{Inner Ring Faults} & Mainly include two types: wearing and pitting. \\  & \multirow{2}{*}{Outer Ring Faults} & Mainly include three types: wear, pitting \\  & & & and fracture. \\ \cline{3-4}  & \multirow{2}{*}{Outer Ring Faults} & - & Same causes as the wearing and pitting \\ \cline{3-4}  & & & faults of the inner ring. \\ \cline{3-4}  & & & Continuous impact due to overloading \\ \cline{3-4}  & & & induces fracture faults. \\ \hline \multirow{8}{*}{Bearings} & \multirow{8}{*}{Outer Ring Faults} & - & All inner and outer ring faults can cause \\  & & & roller faults. \\ \cline{3-4}  & \multirow{2}{*}{Faults} & - & Extra friction due to foreign particles or \\ \cline{3-4}  & & & poor manufacturing causes damage to \\ \cline{3-4}  & & & the rollers. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Common faults of gears and bearings [32].  
To identify the above common faults of gears and bearings, vibration signals of gearboxes are normally taken as featured signals [33]. Based on these vibration data, signal preprocessing, fault feature extraction, and classification methods are studied in the following sections.