## 2 Materials and Methods
Figure 5: Locations of strain gauge measuring points, shims/force transducers, and inductive linear displacement sensors on physical models of innovative screw friction joints (with M16 or M24 bolts). The connecting angle brackets allowed us to achieve complete contact with the working surfaces in each physical model of a given innovative bolted friction joint or bolted connection node (Figure 7). Figure 6: Locations of strain gauge measuring points, force pads/transducers, and inductive linear displacement sensors on physical models of bolted junctions containing innovative bolted friction joints (made at 1:1 and reducing scales). Figure 7: Views of the physical model of the screw connection node with four friction joints and a package of disc springs (centrally pressing the angles while “whitening” their surfaces) after installing four strain gauge force transducers (under the screw heads) and four inductive linear displacement sensors. The experimental stand had an adequately strong and rigid structure at the maximum values of force \(F\) for loading the physical models (up to 550 kN for destructive tests). The stand included upper and lower tables on single and multi-axis joints. This allowed us to map the boundary conditions of the support and the physical loads of the models as closely as possible to those factors occurring in the real bolted joints and connection nodes in the cranes' headframes with girders. The stand was designed to load physical models (with force F) via a pressure arm using a hydraulic press or load-blocking bolt. It included strain gauge force transducers in the form of strain gauge pins in an articulated connection between the lower table and the stand base. This allowed us to control the quasi-statically changed-loaded force F, the values of which were recorded with those of the other quantities determined using the measuring system, as schematically shown in Figure 8. The schematic shows the following:  
* Resistance strain gauges with a measuring base of 3 mm, in pairs on the physical angles of the models (at the locations indicated in Figures 5 and 6), in half-bridge arrangements. These were supplemented by full Wheatstone bridge arrangements with pairs of the same strain gauges on the outer plates to provide temperature compensation at each measuring point. * Resistance strain gauges (with a measurement base of 3 mm, a resistance of 120 Ohms, and a constant of k = 2.05) taped to the \"strain gauge pins\" of the stand (in the articulated connection of the lower table to the stand base) in full-bridge arrangements to measure the vertical and horizontal components of the lower table's response to the external force \(F\) loading a given physical model of a bolted friction joint or junction node. * KMR-type strain gauge washers/force transducers from HBM with a measurement range of 200 kN under preloaded M16 bolts in physical models of bolted friction joints and bolted junctions, accuracy class 1.5. * MR-type strain gauge pads/force transducers from Hottinger Baldwin Mestechnik with a measurement range of 40 kN under M8 bolts with preloads in scaled physical models of bolted friction joints, accuracy class 1.5. * MDK-a-type inductive linear displacement sensors from VIS with a measuring range of 3 mm and an accuracy of 0.1% F.S.O. * KMM20-type force sensors from EMSYST with a measuring range of 0-5 kN and an accuracy of 0.25% F.S.O. * QuantumX MX840B- and MX1615B-type measuring multi-input amplifiers from Hottinger Baldwin Mestechnik to measure signals from points and strain gauge transducers and inductive sensors, with accuracy classes of 0.05 and 0.03, respectively. The results of the first tests of this \"whitening\" sawing showed that primer paint on these surfaces, even in a thin layer, clogged the spaces between the teeth of the working surfaces of these rings (Figure 9).