# Evaluation of the Variability of Vibration Measurement Results in Rolling Bearing Quality Control
## 4 Results and Discussion
### Influence of Measuring Point on the Result of Rolling Bearing Quality Control  
The end of each bearing manufacturing line features two devices that verify the bearing in terms of generated vibration: on the first device, the measurement is conducted on one of the sides, while on the other device, the same bearing is fitted inversely and tested again. This allows for obtaining general information on the manufactured bearings and verifies the entire technological process. From a metrological point of view, a single measurement for each side (which in essence must be treated as a different measurement object) is not sufficient to obtain important information about the tested bearing. The testing of the accuracy of rolling bearing vibration measurement systems requires an increase in the number of measurements and measurement points along the outer ring's circumference. When conducting a series of measurements at different points along the outer ring's circumference, it is possible to observe the result's dependency on the sensor's application point. In this case, testing was conducted on 12 bearings with different technical conditions: 4 bearings without substantial manufacturing defects or damage, 4 bearings with the inner race's manufacturing imperfections and 4 bearings with local operating damage on the outer race. Eight equally spaced measurement points (every 45\({}^{\circ}\)) were marked along the circumference of each bearing's outer ring. Three measurements were conducted at each point. Thanks to this methodology, it is possible to obtain plots in polar coordinates that demonstrate the result's variation depending on the outer ring's measurement point. Selected results are presented in Figures 6-9. The type and location of the defects assigned to the tested bearings were detected on three different test devices. The result of the vibration measurement at the bearing's given point depends mainly on the geometrical structure of its outer ring's race. This results from the fact that this point is stationary as a point of contact with the sensor. Other rolling bearing components (inner ring, cage and balls) perform periodic rotating motions. The impact of other factors, such as grease, contamination or random errors, should be very similar at all measurement points and be distributed evenly along the entire circumference. The scale of the result's variation will be best described in this case by a spread of mean results for each measurement point. The result's variation is illustrated well by the plots showing the RMS value of the vibration velocity expressed in Anderons, obtained depending on the sensor's application point. The results are presented in three frequency bands. Figure 6 presents the variation of the result for an efficient bearing. In this case, the vibration level is low (does not exceed 6 And in any band). The spread of result at various points along the outer ring's circumference fluctuates from 1 to 2.2 And depending on the frequency band. On the other hand, Figures 7 and 8 present the measurements of bearings with substantial outer race damage. In the first case, it is local damage, while in the second case, it is a local geometrical structure alteration. In Figure 7a, it is possible to note a drastic change in the vibration level in the low frequency band when applying the sensor to the point designated as 270\({}^{\circ}\). Changing the sensor's application point can result in a spread of over 15 And. However, the damage does not affect other bands. Figure 8 presents a situation in which the device type substantially affects medium and low bands. It is possible to observe a substantial result change near the local surface alteration on the outer ring (point marked as 90\({}^{\circ}\)), resulting in a spread of over 8 And in the medium band and over 10 And in the high band. This device type has less impact on the result's variation in the low band. Figure 9 presents the impact of the sensor's application point on the measurement's result for local inner race damage.