## 1 Introduction
In [6], the authors investigate the effects of infill volume while maintaining infill type, shell thickness and manufacturing speed. Also, in this paper, the service life of gears was tested, and failure types were characterized. Another paper that explores the infill percentage effects on part strength is [7]. The authors also present and test three distinct types of infill patterns. It is to be noted that in the tests conducted in this paper, the standard 15% infill on the printed gear was used, as will be shown in the following sections. Papers addressing the behavior of polymer gears are also [8] on durability, ref. [9] on loading conditions, ref. [10] on thermal behavior, ref. [11] on elastic tooth deflection and [12] on wear. The idea of using additive manufacturing for bearings is not new, and experiments with 3D-printed ball bearings were conducted by [PERSON] et al. in [13], in which the friction performance of the ball bearing was evaluated to verify its feasibility as an engineering part. Compared to our paper, in which we use FDM, they used multi-jet printing (MJP) as a method to prepare the ball bearing specimen, as it provides the advantage of high  
Figure 1: Gear bearing with double helical (herringbone) gears: (**a**) photo, (**b**) CAD model, (**c**) two herringbone gears meshing. surface quality. They designed a test rig and constructed experiments to evaluate the friction performance under various rotating speeds (500 rpm and 700 rpm) and radial load conditions (50 N, 70 N, 100 N), and their conclusion was that 3D-printed bearings can be used under a constant load and rotational speed. Another type of bearing is the herringbone grooved journal bearing. These are well known for their reliability and high rotor dynamic stability thresholds. In [14], research is conducted to determine whether current technology limits plastic 3D-printed parts' lubricant flow due to layer thickness, or whether 3D-printed parts can be used as an alternative in manufacturing the journal bearing. It is to note that this type of bearing uses hydrodynamic effects, and in comparison, the gear bearing does not use any lubrication, or at least this is the case in our experiments. In the field of polymer printed bearings, the Igus company provides custom 3D-printed plain bearing solutions for rotational motion, Figure 2. They use their proprietarily developed high-performance polymers that, according to their tests, last up to 50 times longer than standard 3D printing materials inside moving applications. Another important mention is that these plain bearings are dry running and maintenance-free [15]. In the literature, there are also studies on tribological aspects of 3D-printed parts. The study in [16] shows the impact of layer thickness, infill angle, infill pattern and orientation of the deposition and indicates the correlation between printing parameters and wear. Our experiments were designed to allow for the direct performance evaluation of the bearing mounted on the shaft, subject to different rotating speed and loads. The material used for the gear bearing manufacture is PLA, but an interesting and promising expansion of this paper could be the use of composite materials. An interesting study in this di-rection is presented in [17], in which a bronze-filled PLA-based composite filament is analyzed for use in FDM and shows improved mechanical characteristics, which in turn could increase the gear bearing's life.