*2.1. Experimental Setup*

The experiments were performed on a laboratory-built tribometer setup for bidirectional, translatory movements with high normal loads, and large sliding amplitudes (Figure 1a). In this setup, a self-lubricating journal bearing made of a bronze alloy with polymer lubricant macrodepots is horizontally mounted on the tribometer and held in a fixed position. The counterpart, a shaft made of hardened and polished Cr-steel, slides inside this bearing in a translatory oscillating movement driven by a pneumatic cylinder. Two adjustable electronic switches define the reversal points of the oscillating movement. The normal load is applied by a second pneumatic cylinder. The executed force is transmitted via a parallelogram structure, which ensures that the horizontal position of the bearing is always maintained even in the event of wear-induced lowering (Figure 1b). The pressure applied on the bearing was calculated as the normal force acting on the nominal cross-section, according to engineering standards for journal bearings.

**Figure 1.** (**a**) Translatory oscillating tribometer setup, (**b**) lateral force measurement, (**c**) position measurement, (**d**) bearing temperature measurement, and (**e**) normal force measurement.

The tribometer is equipped with several sensors to monitor and document the defined experimental parameters, the environmental situation, and the reactions of the tribometer to the different friction conditions. The instrumentation of the setup is described in detail below. In the current study, the focus lies on the data generated by the lateral force sensor.

A commercially available linear inductive position sensor (Turck Li300P0-Q17LM0- LiU5X2) measures the oscillating movement of the shaft. The applied normal load and the lateral force, i.e., the force in sliding direction, are recorded by two load sensors

(HBM Type U2B and HBM U9C, respectively). The wear of the journal bearing can be qualitatively monitored by the vertical movement of the cantilever, which is measured by a laser triangulation sensor (Keyence IL-030). The temperature of the bearing is measured by a thermocouple (type K, diameter 0.5 mm), which is mounted inside a drilled hole at the top point of the bearing's front face, where the highest contact pressure and therefore the highest temperature is to be expected. Figure 1 indicates the mounting positions of the force, position, and laser triangulation sensors as well as the thermocouple.

In addition, several sensing techniques are used to detect friction-induced vibrations. Two acoustic emission sensors (NF-Corporation AE-900M-WB), one mounted at the shaft and one mounted at the bearing holder, measure high-frequency structure-borne noise in the range between 100 kHz and 5 MHz. Three MEMS (micro-electro-mechanical system) acceleration sensors (Analog Devices ADXL1002) are mounted at the bearing holder and detect low-frequency vibrations up to 11 kHz in all spatial directions. The emitted airborne noise is measured in the frequency range between 20 Hz and 20 kHz using a high-precision microphone (Brüel & Kjaer 4189-A-021).

Furthermore, the ambient air temperature and humidity is monitored in the vicinity of the experiment by a TE Connectivity HTM 2500 LF sensing module and the supply air pressure of the pneumatic drive by a Telemecanique XMLP016BC71V pressure transducer.

The oscillation frequency of the shaft was set to a nominal value of 1 Hz and a stroke amplitude of 30 mm, which ensured that each contact point of the shaft was moved out of the contact completely in each stroke. It has to be noted that the oscillation frequency was not constant during the experiment but varied with the resistance the pneumatic cylinder had to overcome to move the shaft. During the first 1.5 h of the experiment, the normal load was gradually increased until a nominal bearing pressure of 8 N/mm2, corresponding to a normal load of 6 kN, was reached. The experiments were performed until at least one of two thresholds was exceeded. The first threshold was set for the bearing temperature at 150 ◦C and the second one was set for the uncorrected lateral force at ±3.5 kN. However, it should be noted that the temperature threshold was never exceeded, and all experiments were stopped after exceeding the lateral force threshold.

In total, data from 9 experiments performed under the described conditions were used for this study.
