Thermoplastic materials for gears: status, future trends and solutions*
In this article an overview of different lubricated thermoplastic materials, their corresponding
performances and simulation methods will be given. In particular, we will be showing
wear behavior and fatigue data for number of thermoplastic matrixes and fillers. Moreover,
the results of new developments will be highlighted. These new technologically advanced
materials exhibit higher fatigue performance over a broad range of temperatures in comparison with widely used thermoplastic materials in gears.
In the past decades, internally lubricated thermoplastic materials have been increasingly used in gears. Starting from technologically easy applications, such has simple business-machines, thermoplastic gears are today able to withstand progressively more challenging environments where increasingly elevated loads, high temperatures and safety requirements are needed, like for example in actuators of high Performance cars’ motors. Moreover, thermoplastic gears can be reproducibly and quickly manufactured via injection molding offering economic advantages versus metal gears. This advantage combined with Elimination of external lubricants, weight reduction, improved corrosion resistance, low noise and high degree of freedom in gear-geometry have driven the selection of internally lubricated thermoplastics over metals by many gear designers. However, a number of gear applications are not accessible to thermoplastics gears due to their limitations in terms of fatigue and high temperature resistance in comparison with metals, therefore new thermoplastic solutions that can bridge these gaps, at competitive prices, might further increase the application space of thermoplastic gears [1–4].
SABIC has been pioneering this field, since the 1970s, with LNPTM LUBRICOMPTM internally lubricated thermoplastic compounds and composites [5–6]. Today SABIC product portfolio is based on a wide range of high performance thermoplastics in combination with different internal lubricants and fibre reinforcements. The development of this portfolio was driven mainly by five criterions: high working temperature (>80°C), high tooth root stress, high geometrical accuracy, low wear and low noise.
In this contribution an overview of different lubricated thermoplastic materials, corresponding performances and simulation methods will be given. In particular, we will be showing wear behavior, fatigue data for number of thermoplastic matrixes and fillers. Moreover, the results of new developments will be highlighted. These new technologically advanced materials exhibit higher fatigue performance over a broad range of temperatures in comparison with widely used thermoplastic materials in gears. These superior combination of fatigue and heat resistance performances offer possibilities to the gear designers to meet the future gear trends such as ability to withstand increasingly high loadings and temperatures.
Comparison of different thermoplastics
Polyamide 66 (PA6.6) is a relative low-cost thermoplastic with good mechanical and tribological performance. Kukureka et al. observed abrasive wear by introducing fiber reinforcement in PA6.6 (aramid, glass and carbon fiber). As a hard phase in polymer matrixes, glass fibers enhance the load carrying capability and the thermal conductivity. A positive effect is the lowering of the wear rate of the polymer although the steel Counter faces may be abraded. They observed that glass fiber reinforcement in PA66 decreased the coefficient of friction and allowed the material to be used for higher duties without exceeding the softening point of the matrix.
Figure 1: SABIC portfolio of materials for tribological applications.
This increase in duty is, however, at the expense of an increased wear rate and shorter component life. It was suggested that the wear debris is initially retained in the System and, because of the more rigid substrate, is incorporated into a highly deformed surface layer. Wear rates and coefficient of friction are then determined by the behavior of the layer and by the strength of its bond with the underlying material . In order to reduce the wear rate, polytetrafluoroethylene (PTFE) is typically used as an internal lubricant. These macro PTFE molecules slip easily along each other, similar to lamellar structures. After a service period, the PTFE particles compounded in the plastic will form a PTFE film over the mating surface (metal or plastic counter face) resulting in a reduction of friction and wear. SABIC developed PTFE lubricated reinforced PA6.6 grades with enhanced wear resistance - LUBRICOMPTM RFL36, RCP36 and RAL23SXC - to enable increased duty and lifetime of gear components by internal lubrication (Fig. 2).
Figure 2: Wear study of fiber reinforced thermoplastic Composites versus steel at room temperature at velocity 50 fpm, pressure 40 psi. From left to right: 1) Neat PA6.6; 2) PA66, 30% Glass Fiber; 3) PA 66, 30% Carbon Fiber; 4) PA66 10% Aramid Fiber; 5) 30% Glass Fiber, lubricated with PTFE [LUBRICOMPTM RFL36]; 6) 30% Carbon Fiber, lubricated with PTFE [LUBRICOMPTM RCP36]; 7) PA66 Aramid Fiber, lubricated PTFE [LUBRICOMPTM RAL23SXC]. Wear is expressed in the units: 10-10 .in5 .min/ft.lb.hr.
Aramid-fibre filled polymer composites, such as RAL23SXC, are known to have an intermediate friction and an intermediate wear factor while the wear of their metallic countersurfaces is low. Therefore such aramid fiber filled composites are typically used against soft metals such as aluminum, brass and copper .
In order to assess the effect of temperature on wear resistance of different thermoplastics, various wear studies on Commercial glass fiber reinforced composites were conducted (using thrust-washer, counter surface stainless steel disc at speed of 50 fpm and pressure of 40 psi, at different temperatures). As shown in Fig. 3, lubricated glass fiber reinforced polyoxymethylene (POM) has poor wear performance at room temperature whereas PA6.6 (RFL36) and polyphthalamide (PPA) (grade name LUBRICOMPTM UFL36) based composites demonstrated excellent wear resistance. It has been found that lubricated GF PA6.6 and GF polyetherimide (PEI) composites has the optimal wear resistance performance at temperatures below 100
°C. Moreover, at 150 °C wear experiments of these series composites indicated that PTFE lubricated glass fiber reinforced PPA (UFL36) exhibit a low wear over a large temperature range, 23 up to 150 °C, suggesting this is good material for low wear and high heat/strength applications.
Figure 3: Wear study of different short glass fiber reinforced thermoplastics, lubricated with PTFE versus steel at various temperatures
From left to right data of LUBRICOMPTM KFL36, RFL36, UFL36AS, OFL36, LFL36, FL36).
Various polymer-polymer combinations were evaluated by thrust washer wear experiments. As depicted in Fig. 4, PTFE lubricated carbon fiber reinforced PPA (LUBRICOMPTM UCL36ASP) and polyphenylene sulfide (PPS) (LUBRICOMPTM OCL36) showed the best wear resistance against steel as well against itself, therefore UCL36ASP and OCL36 are preferred composites for high load and long lifetime gear applications at elevated temperatures up to 150 °C. The gear fatigue strength at elevated temperatures of some of these reinforced composites will be discussed in the next paragraph.
Figure 4: Plasticplastic thrust washer wear study of PTFE lubricated short fiber reinforced thermoplastic composites against each other at room temperature at 50 fpm 40psi
From left to right THERMOCOMPTM grades RFL36, RCL36, UFL36AS, UCL36ASP, LCL33, OCL36, EFL36).
Figure 5 shows gear fatigue Performance of three crystalline SABIC grades at 23 and 150 °C.
Figure 5: Gear Fatigue, at 23 and 150 °C, of LUBRICOMPTM RFL36, UCL36 and OCL36, based on PA6.6, PPA and PPS respectively, with same loading of short glass fiber and lubricant. Spur gear fatigue testing was performed using a computer-controlled gear wear testers LRI-2GL, GH (Lewis Research, Inc.).
It can be seen that LUBRICOMPTM RFL36 (which based on the P66) shows the best fatigue performance over UCL36 and OCL, which are based on the higher melting Point resins PPA and PPS respectively. This is one of the reason PA66 lubricated material is one of the most used resin in high fatigue applications.
SABIC have been developing a new experimental grade which has similar properties of PA66 with extremely higher fatigue resistance. In Fig. 6, the tensile fatigue of RFL36 is compared with the new experimental grade (both grades contain same loading and type of short glass fiber and lubricant). It can be clearly seen that the experimental grade shows a tensile fatigue of one million cycle without breakage, at both 25 °C and 150 °C. This experimental grade largely outperforms the grade RFL36 (based on PA66). What is more remarkable is that this experimental (more advanced) material exhibit higher fatigue performance over a broad range of temperatures in comparison to a widely used thermoplastic material in gears like RFL36. The superior combination of fatigue and heat resistance performances offer possibilities to the gear designers to meet the future gear trends such as ability to withstand increasingly temperatures and loadings.
This next generation material in set to complete the SABIC broad portfolio of grades for tribological applications, which can enable plastic gear to reach high loads, increased service life and high heat resistance (Fig. 1).
Gear damage mechanism and its
The coupling of wear and fatigue and its link to the gear geometry, environmental condition and material property make the gear failure prediction challenging. Currently, two standards are helping plastic gear designers, the VDI 2736  and JIS B 1759 .
Analytical equations to evaluate the gear temperature, tooth root stress, contact pressure, tooth bending and wear behavior are proposed.
Nevertheless, the accuracy of the norm prediction is not only depending on the accuracy of the analytical equations but also strongly dependent on the material data quality that the designer is using. To do a full gear sizing based on the VDI 2736 Part 2, the material data Tab. 1 are needed.
Gear designer are often facing lack of material data which prevent them to use the material to their full capacity. One reason is the complexity of data generation. For example, in the case of S-N flank fatigue, gears Need to be tested at different load levels, temperature levels, with different lubrication types and material pairings. As the choice of gear material and lubrication is broad, an infinity of material combination would need to be tested. Moreover, the flank fatigue results might be influenced by the tested gear geometry. For this reason, SABIC propose another approach based on a wear process simulation rather than the S-N flank fatigue.
The wear simulation is based on the VDI 2736 Equation (1). With Fn, the local the normal force on the tooth at the contact, b the face width, NL the number of load cycle, ζ the local specific sliding and kW the wear coefficient.
This wear equation is then implemented in a contact analysis simulation in order to take into account the exact gear geometry and contact pattern. SABIC approach is sum up on the Fig. 7.
Figure 7: SABIC service life Evaluation strategy.
Gear life estimation strategy on Fig. 7 is made of three steps. First, the tooth root temperature is computed. If the temperature is acceptable for the chosen material pair, the tooth root fatigue safety and bending safety are evaluated. If all results are sufficient, a wear simulation is done. Finally, considering the maximum allowable wear profile a new tooth root stress evaluation based on the worn profile can be done to double check if no important stress increase has occured. This workflow can easily be done with the KISSsoft software where SABIC material portfolio for tribological applications (shown in Figure 1) is available.
Wear simulation example
The most sensible material data in a wear simulation study is the material wear coefficient. The wear coefficient is directly related to the amount of material removal. Nevertheless, in the literature an important variability is observed due to the measurement techniques used and the possible anisotropy of the material in case of for fiber filled plastics. The wear coefficient is strongly influenced by the material temperature and surface roughness in the case of plastic metal contact . In the following section, an insight in these differences is given to promote the use of more accurate material data.
Figure 8: KISSsoft wear simulation results
On Fig. 8 a wear simulation in case of POMsteel and POM-POM contact is presented. The gear geometry used is based on the LKT reference geometry from VDI 2736 Part 4 .
The results in Fig. 8 show an important Service life different due to a high wear rate of the POM-POM pairing. This result underline the need of using appropriate wear coefficient depending on the material pairing in order to use its full capacity.
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 VDI 2736, Blatt 4, VDI Richtlinien, Thermoplastische Zahnräder Ermittlung von Tragfähigkeitskennwerten an Zahnrädern, June 2014.
International VDI Conference on „High Performance Plastic Gears 2015“: Dr. Domenico La Camera, Material Science, Technology and Innovation, SABIC, Bergen op Zoon, The Netherlands, Dr. Bart Vandormael, Industrial Europe, Technology and Innovation, SABIC, Bergen op Zoon, The Netherlands, Dr. Ing. Julien Cathelin, Reinforced Plastics Center, Technology and Innovation, SABIC, Elsloo, The Netherlands