## 1 Introduction  
For machine tools, moving components for material processing often come into contact with each other. Fatal and permanent wear occurs on the contacting surface by disintegration damage. We use lubricants, such as grease, to reduce wear by friction and provide the components with a smooth movement. Due to unexpected external influences, such as humidity and temperature variations, lubricants may not work properly, which causes severe frictional contact between metals. This frictional contact generates vibration and noise as well as heat, causing overload throughout the production process and eventually producing defective outputs or failures. The friction and wear of various materials on the profiles of surfaces have been conducted after considering the friction coefficient [1; 2; 3; 4]. [PERSON] et al. [4] studied friction behavior between the wafer and transfer film attached to each other. For a constant normal load, the friction coefficient gradually increased with the increasing sliding speed. For the ball-shaped chemical components, the friction coefficient was maintained in spite of high sliding speeds. [PERSON] and [PERSON] [1] studied the variation of friction coefficients depending on the material coated on one or both sliding metal surfaces. The surface coated with magnetron tin film showed a smooth operation without much constraint. To investigate the friction of materials, the wear caused by friction always accompanies the vibrational excitation [5; 6; 7]. [PERSON] and [PERSON] [5] studied the change of friction coefficients for various normal vibration amplitudes for materials of varying hardness. The friction coefficient decreased with the increasing vibration amplitude and converged to a specific value with the increasing rub cycle. To study the transient phenomenon, [PERSON] et al. [8] studied the difference of some kinds of vibration by friction. [PERSON] et al. [9] also investigated the frictional vibration and noise and influence of the transient friction characteristic. Many recent manufacturing machines require increased precision operations for applications to high-tech productions. In the manufacturing process, it is difficult to identify whether friction and wear occurred and whether maintenance operations are required. Acoustical signature is a useful tool of application to continuously monitor production lines. The noise generation by friction and wear was a research subject for decades [10; 11; 12]. [PERSON] et al. [10] concentrated on the mechanism of sliding phenomenon on rough surfaces in terms of three steps (tribology, dynamics, and acoustics). The sound pressure level (SPL) was linear with surface roughness and sliding speed. [PERSON] et al. [11] studied the characteristics of sounds from abrasive and adhesive wears in frequency domain. In terms of amplitude and frequency, the adhesive wear generated sounds in various frequency bands. For the abrasive wear, it was single but had a wide frequency range. Regardless of the type of materials used in the experiments, it maintained consistency in its generation frequency spectrum range. There was a transition period between adhesion and abrasive wears. The frequency bands of sound radiation from contacting surfaces are different. Therefore, through a series of signal processing, it was necessary to classify the sliding mechanism and derive the factors for the occurrence of wear. It was possible to identify whether the current state is influenced by friction or not. Friction noise based on frequency domain was analyzed using fast Fourier transform (FFT) with statistical calculations [12; 13; 14; 15; 16]. [PERSON] [13] studied the power spectral density (PSD) of sounds for various roughness of surfaces and relative motion speed of materials. [PERSON] et al. [14] evaluated surface roughness from friction noise, and a statistical index was used for artificial neural network (ANN). From the index feature, different roughnesses of surfaces were well discriminated with the ANN algorithm.