## 2 Materials and Methods
### Gear Test Rig and Experimental Testing  
#### 2.3.1.  
The testing of non-lubricated gears was carried out on an in-house-developed non-mechanically closed-loop test rig at an ambient temperature of 23.5 \(\pm\) 1 \({}^{\circ}\)C and a relative humidity of 50% \(\pm\) 10% (Testo 175H1, Titisee-Neustadt, Germany). The test rig enables the testing of gear pairs with a torque of up to 20 Nm, and thus offers the possibility of testing many gears of various materials at different loads and speeds. In accordance with VDI 2736 [62], non-mechanically and mechanically closed-loop test rig can be employed to investigate polymer gears. The non-mechanically closed-loop test rig was adopted due to the greater flexibility, and a configuration that allows for earlier failure detection and efficiency determination. Figure 5a shows a schematic representation of the test rig with indicated important parts, while the in-house-developed test rig is presented in Figure 5b.  
The driving three-phase asynchronous electric motor (EM), with an output power of 3 kW and a nominal speed of 1430 rpm (Seipee JM 100 LB 4 B3, Campogalliano, Italy), and an identical braking electric motor (generator) are controlled and regulated with a frequency converter (Schneider ATV 930, Rueil-Malmaison, France). The braking electric motor is connected to the regenerative unit (Altivar ATVRD15N4, Rueil-Malmaison, France) to increase the energy efficiency of the system and to avoid the overheating of the braking electric motor. In order to maintain the desired torque and speed during the test, two dynamic torque sensors (KTR Dataflex 16/50, Rheine, Germany) with corresponding steel laminae couplings (KTR Radex NC25, Rheine, Germany) were installed. The resolution of the torque sensors is \(\pm\)0.2%. Torque sensors were used to transmit the measuring signals, input/output torque and rotational speed to the Siemens Logo! Basic Module. The Siemens Logo! Basic Module device with a data logger was programmed to maintain the  
Figure 4: Hybrid polymer gear designs: (**a**) first, (**b**) second, (**c**) third and (**d**) fourth.  
desired torque level at each testing stage using the proportional-integral-derivative (PID) regulation. The input and output torque, as well as the rotational speed, were recorded during the entire gear operation. All of the data were recorded using the Siemens LOGO! Soft Comfort V8.2 software, which enables the results to be previewed and exported.  
To speed up the investigation process, an accelerated life testing strategy for gear testing was performed in this study. The employed step load testing procedure provided a balance between the reliability of the results, the number of tests and the time required [27]. The graph of the accelerated life test strategy, with the indicated four 15-min load stages at a speed of 1000 rpm, is presented in Figure 6. To avoid a sudden impact load, the gears were gradually accelerated to 1000 rpm for 20 s, and then the load was applied. The 60-min accelerated step load test began as soon as the gears were loaded with a first stage load level. The testing time multiplied by the rotation speed defined the number of load cycles. The bulk temperature was measured 3 times in each stage, at every 5 min of the test duration. Gear testing was performed through four stages of the step load testing procedure or until gear failure.  
Figure 5: (**a**) Schematic representation of an (**b**) in-house-developed test rig.  
Figure 6: Accelerated service life testing graph.