Engineering polymer gears - advantages and challenges:
Today polymer spur gears permanently enter new fields of applications. In comparison to metal gears, they have significant advantages, e.g. weight efficiency and dry running capability. However, there are still challenges like their low operating temperature or their temperature dependent material properties. There are ways to uplift gear performance: Material modifications with fibers can increase mechanical part properties. Usage of additives or external (downstream-processing) modifications like radiation crosslinking can decrease friction and wear. The injection molding process also offers the possibility to increase the potential of polymer gears by changing the morphological structure of the polymer with different processing settings.
Radiation crosslinking as performance enhancement
Radiation crosslinking is an external downstream-processing technique. Manufactured parts are exposed to high-energy radiation, for example electron or gamma radiation. To prevent a high level of heat input during the radiation process, parts are radiated multiple times with smaller doses until they reach the defined dosage. This process starts a permanent chemical alteration within the polymer matrix that causes a change in material and therefore part properties. Polymer molecules become stimulated and ionized during the radiation process. The resulting separation of lateral groups or atoms and the following formation of free radicals in the chosen material is the basic principle, which leads to a development of a three-dimensional crosslinked network. Due to a high molecule mobility in the amorphous boundary regions of typical semicrystalline thermoplastics, these regions will be crosslinked predominantly. The crosslinking procedure also reduces the temperature-dependent mobility of the molecule chains. This leads to a change in glass-transition temperature to higher temperatures. It also modifies the softening behaviour to higher temperatures with an increased density in crosslinkings. Crystalline regions on the other hand are less mobile. Here free radicals cannot cause crosslinking to the same degree as in amorphous boundary regions. Therefore, a good initial state for crosslinking is a small degree of crystallinity of the thermoplastic material. Overall radiation crosslinking of polymer materials causes a material dependent superimposition of the reaction effects. Free radicals can recombine by forming new connections and this can lead to an improvement of the material and part properties.
Tribological performance of radiated spur gears
The effect of radiation crosslinking is investigated with regard to mechanical, thermal and tribological wear capability and operating temperature of thermoplastic polymer gears to assess the possibilities and several application scenarios. Gear tests were performed on a gear test bench with regard to VDI 2736. The performance increase in terms of thermal, mechanical and tribological material behavior was analyzed, particularly the change in wear behavior and the resulting operating temperatures at tooth flank and tooth root for radiation crosslinkable and non-crosslinkable Polyamide 66 spur gears against steel pinions at two ambient temperature settings (23 °C and 140 °C). Results show a performance uplift of up to 50 % for the radiation crosslinkable materials, especially in wear at high operating temperatures in comparison to non crosslinkable Polyamide 66, as can be seen in Figure 1 and Figure 2. The tooth width decrease after 1.2x106 load cycles at an effective tooth flank temperature of 165 °C has been halved. This can be derived to the higher heat deflection and mechanical stability as well as increased wear resistance of the crosslinked parts due to the radiation process and its resulting chemical alteration of the polymer matrix.
Figure 1: Unirradiated PA66 - gear example with visible tooth wear after 1.2x106 load cycles at a drive speed of n1 = 3000 min-1 and braking torque of M2 = 0.5 Nm, resulting in 165 °C tooth flank temperature; RzPinion = 2 µm. (Source: LKT)
Figure 2: Radiated PA66 - gear example with no visible tooth wear after 1.2x106 load cycles at a drive speed of n1 = 3000 min-1 and braking torque of M2 = 0.5 Nm, resulting in 165 °C tooth flank temperature; RzPinion = 2 µm. (Source: LKT)
The performance increase in comparison to other unirradiated materials and possible application scenarios for radiated spur gears as well as analytical data and other results will be presented and discussed at the International Conference on Gears 2019.
Dipl.-Ing. Bernhard Gierl, Institute of Polymer Technology (LKT), Friedrich-Alexander-Universität Erlangen-Nürnberg
Prof. Dr.-Ing. Dietmar Drummer, Institute of Polymer Technology (LKT), Friedrich-Alexander-Universität Erlangen-Nürnberg