## 9 Experimental Verification Tests of Bearing Temperatures  
As already mentioned, experimental verification of temperatures is necessary, as the results of simulation tests differed, among others, in for modeling contact areas and distribution of heat sources. Experimental research should allow for the identification of the best, i.e., the closest to reality, method of FEM simulation modeling.  
In the Department of Mechanical Engineering of the SUT there is a research stand for testing resistance to motion and temperatures in bearings [17]. Figure 13 shows the view of this station. The stand is in the form of a headstock in which the spindle is mounted on two angular contact ball bearings. The headstock rests on the Kistler 7292 torque sensor, which measures the resistance to motion. The preloading of the bearings is performed by the nut and the Kistler 9102A axial force sensor, while the spindle rotation is performed by the milling spindle on the table where the entire station rests.  
The temperature measurement was carried out with the use of Pt-100 contact sensors, glued on the outer rings of the bearings (Figure 13b), with the OptisCtLaser pyrometer (temperature measurement on the inner ring) not shown in Figure 13b and with the VigoCam V50 thermal imaging camera, not shown in Figure 13b.  
Figure 12: Exemplary radial, thermal deformation (**a**), radial, thermal and mechanical deformation along the axial axis (**b**), thermal and mechanical contact loads (**c**); \(\delta_{(0)T}\), \(\delta_{(0)T}\)—radial outer and inner thermal deformation, \(\delta_{(b)}\), \(\delta_{(0)}\)—radial outer and inner mechanical deformation, 231N—contact load due to thermal deformation, 61N—contact load due to the preload and centrifugal forces.  
The tests consisted in recording the temperatures on the outer and inner ring and the entire headstock (using a camera) until a steady state was reached.  
Figure 14a shows an example of the temperature course on the outer and inner rings, and Figure 14b shows an example of a thermal image from a thermal imaging camera for similar conditions in which the simulation tests were carried out [17].  
Comparing the temperature courses from the simulation tests with the course from the experimental tests in Figure 14a, one can formulate an opinion about their compliance, first of all qualitative. The temperatures on the outer and inner rings differ in the steady state by approx. 10 \({}^{\circ}\)C, and the time to reach the steady state is approx. 1 h. On the other hand, quantitative differences are noticeable, i.e., the temperatures from the experimental tests in the steady state are slightly lower than those from the simulation tests. In contrast, in the transition state for the outer ring, it is significantly different in the first 1000 s.  
Figure 14: Examples of experimental and simulation curves of the temperature of the outer and inner rings: (**a**) time course, (**b**) image from a thermal camera, and (**c**) the effect of bearing speed; TirExp, TorExp—experimental results, Tir, Tor—simulation results.  
Figure 13: View of the stand for testing resistance to motion in the bearing (**a**) and temperature sensors (**b**).  
Figure 14c also shows the simulation and experimental test results in the form of the influence of the bearing rotational speed on the temperatures of the inner and outer rings. Comparing these test results, in this case we can also speak of a qualitative agreement, i.e., the increase in speed is accompanied by an increase in temperatures and the temperature of the outer ring is higher than the inner ring. The quantitative comparison of these results indicates larger differences (TirExp), up to 10 \({}^{\circ}\)C. This is mainly due to technical problems in measuring the temperature of the inner ring, which was difficult to access, and due to calibrating the pyrometer, i.e., choosing the correct emissivity factor.