## 1 Introduction  
Combine harvesters operate in varying crop and field conditions, such as under conditions of high moisture, environment dustiness, and exposure to straw material. The harvesting season is typically short. During harvesting time, crops might be wet, laid down, and vary in straw content. In addition, due to field conditions, the driving speed of the combine varies as well. These conditions could lead to a variable amount of feeding crop to the threshing mechanism of the harvesting combine, resulting in an excessive load on the combine drive mechanisms and their bearings. Such an excessive load not only aggravates the threshing process, but also harms gears and decreases their longevity. Very small soil particles rise into the air during the movement of combine harvesters, tractors, and other agricultural machinery. This leads to the accumulation of soil dust on bearing housings, and eventually penetrates the bearings. In addition, the bearing units experience significant dynamic load in the form of shocks and vibrations [1]. The threshing mechanism is the key part of the combine harvester on which the grain separation and quantitative cleaning work indicators depend. Thus, it is important to ensure that all the components of the threshing mechanism, including the threshing drum bearings and all other bearings of the combine, are properly functioning. Proper technical maintenance ensures smooth and continuous performance that fulfills the agrotechnical requirements with the least amount of crop losses and expense. The rolling element of the bearing is the basic component of almost all mechanisms that transmit power in agriculture equipment, aircraft, ships, vehicles, various industrial installations, etc. [2]. Unexpected damage to the bearings can lead to disruption in the operation of equipment and financial expense. Damaged bearings often destabilize the rotation of the shaft, and can result in disruption to the operation of the entire mechanism. To avoid such incidents, it is important to identify any bearing defects as early as possible. The diagnostics of bearing defects have been investigated frequently in the past few years [3]. Bearing defect diagnostics include the time domain, frequency domain, combined time-frequency analysis (as presented in [4; 5]), vibration-acoustic measurements, temperature measurements, and wear debris analysis. Vibration measurement diagnostics seem to be the most widely used [6]. Vibration signature diagnostics allow the condition of operating machinery to be checked. They can identify potentially approaching failures at a particular location, avoiding significant damage to the machinery that could lead to expensive repairs. Periodic scheduled vibration measurements could ensure the reliable operation of machinery [7]. Vibration analysis provides pertinent information regarding the progressive deterioration of bearings, and gives a baseline signature for future monitoring. Frequency domain spectrum analysis could identify defect frequencies, and predict the presence of defects in roller bearings [8]. In addition, multiple faults can be detected using vibration analysis in the frequency domain mode [9]. Vibration analysis techniques for rolling bearing fault diagnostics are extensively used by researchers. When the bearing rollers are operating in a damaged cage, vibration shock impulses are generated. However, similar vibration signatures can also be produced by various other mechanical units and mechanisms, such as worn-out gears [6; 10; 11; 12; 13]. Bearing defects are recognized not only by measuring the velocity; if the periodic vibration measurements indicate that the bearing velocities are increasing, it could mean that bearing defects are serious, and it could lead to a serious mechanical failure. However, a measurement of the bearing rotational speed does not provide the required information of bearing status. The bearing vibration velocities need to be measured to assess the bearing condition.