## 5 Results and Discussion  
Figure 12 is a graphical representation of the dependence of the emerging axial force in the bolt on the tightening torque for tests without lubricating grease and with different alternatives of the tested grease.  
Figure 11: Dependence of emerging force \(F_{OC}\) on time.  
Figure 12: Dependence of the loading tightening torque on the emerging axial force with and without lubricating grease.  
The dependence of the calculated actual friction coefficients on the tightening torque for the different test variants with or without lubricating grease is shown in Figure 13.  
The calculated coefficients of friction in the thread were slightly distorted. This could be due to many factors, including the running-in of the screw connection. This caused an increase in the axial force in the screw threads, given by law, which was the main cause of the scatter of the measured values. Therefore, after each measurement, we completely unscrewed the screw and screwed it in again so that the same threads did not meet again.  
Unlike linear theory, the actual dependence of the axial force on the tightening torque is progressive.  
Based on the results of the verification experiments, we recommend a linear formula for estimating the lower limit of the axial force \(F_{O}\) when applying the tightening torque \(M_{T}\), considering the following:  
* The actual \(F_{O}\) value can be up to 50% higher, especially at large \(M_{T}\) values, which are suitable for screws with a high yield strength, because the thread surfaces grind against one another. The actual course of \(F_{O}\) is progressive.
* In order to achieve reproducible values on a set of the same screws, attention must be paid to the same method of lubrication [21].  
We determined the coefficients of friction in the thread during dry and lubricated friction. By using better lubricants, the value of the coefficient can be minimized to values close to 0.2, which is the standard value of the steel-steel shear coefficient.  
In conclusion, it is advisable to use modern hydraulic tightening machines for demanding applications due to their better accuracy and capability for operation without harmful effects on the operator.  
For better-quality measurement, it would be competent to use modern hydraulic tightening machines to derive a more accurate value of the tightening torque. These machines have a small deviation value [22].  
Due to the dimensional diversity of the screw connection, it would also be appropriate to adapt the measuring device to a more universal format:  
* By making inserts for fastening screws of smaller dimensions. The design of the inserts is of great importance. By making them, the coefficient of friction could also be measured in other types of threads.
* By modifying the device so that it is possible to carry out a second method of measurement, in which the nut under the washer is tightened. With this measurement, another quantity would enter our calculations--the friction coefficient between the  
Figure 13: Dependence of the tightening torques on the actual calculated coefficients of friction with and without lubricating grease.  
nut and the washer. In this method, it is necessary to ensure that the screw head is guided to ensure that the screw does not rotate but only moves in the axial direction.
* By adjusting the device frame or its parts so that screws with smaller and larger lengths can be used.