# Evaluation of the Variability of Vibration Measurement Results in Rolling Bearing Quality Control
## 3 Materials and Methods  
The manufacturing of rolling bearings involves several stages. These stages include the plastic processing, machining and heat treatment of particular bearing elements. The bearing's assembly is the last stage of manufacturing [11,12]. The bearing manufacturing process is presented in more or less detail in specification and videos published on bearing manufacturers' websites and is available in scientific publications [13,14]. Quality control takes place at many stages of production [15,16]. However, the most significant test for a manufactured bearing is the vibration control of the finished product, to which 100% of manufactured bearings are subjected. Vibration measurement is therefore the ultimate test for a manufactured bearing. This is because bearing vibration is proportional to the other parameters which determine bearing quality (thrust torque, noise, life, positioning accuracy, etc.) [17]. It is therefore reasonable to assume: low vibration high quality, high vibration poor quality. Excessive vibration levels in rolling bearings have a negative impact on the operation of electric cars in particular [18] and multi-bolted systems [19]. The manufactured bearing components are delivered to the automatic production line, first being washed. Then, the inner/outer ring pairs are accurately measured on the line and, based on these results, balls are selected from the appropriate dimension group. This ensures that the finished bearings have the required internal clearance. The balls are placed inside the bearing with the outer ring deformed smoothly. After fitting the cage, the bearing is still washed in a liquid which also serves as a conservation lubricant. The assembled and washed bearing then goes on automatic vibration acceptance measurement. The rolling bearing vibration measurement methodology is based on company procedures, which must comply with the standards of the ISO 15242 group [20, 21, 22]. According to the measuring principle presented in the first part of the standard, the rolling bearing to be tested is placed on a measuring shaft matched to the size of the inner ring. This shaft is seated in a spindle which has a universal slot for shafts adapted to different bearing types. According to the standard, the rotational speed of the shaft, and consequently of the bearing inner ring, should be 1800 rpm. This is a reference speed closely related to the three analysed frequency bands: low 50-300 Hz, medium 300-1800 Hz and high 1800-10,000 Hz. By agreement between the manufacturer and the customer, the speed can be changed and, where appropriate, be 3600 rpm, 900 rpm or 700 rpm. In such cases, the bands' limits will also change. Proper vibration measurement requires axial loading of the bearing under test. This is achieved by a mechanical or pneumatic pusher (usually three-pronged) in contact with the bearing outer ring. The clamping action keeps the outer ring from rotating. The force of the pusher depends on the type and size of the bearing and is also regulated by standards. Usually, the pushers are suitably adjustable, interchangeable or, within certain limits, universal to match the size of the bearing under test. The radial vibration of the operating and loaded bearing is recorded through an electrodynamic vibration velocity sensor in direct contact with the fixed outer ring. The sensor is mounted in a holder above the bearing, which has the ability to move it in two axes. The exact position of the sensor stem is also governed by the aforementioned standards. The measurement principle described is based on the procedures used in practice in bearing manufacturing plants, which are in accordance with the applicable standards [20; 21; 22]. Figure 1a shows a section of the production line where the quality assessment of the manufactured bearings is carried out. Figure 1b shows the test head of the automatic production line. Visible is the trough inside which the bearings move and the pneumatic actuator that places the bearings on the test shaft.