Solvay Specialty Polymers for High-Performance Plastic Gears

Thermoplastic injection-molded parts in automotive applications is growing and proving significant reductions in NVH issues

Solvay recently helped Allegheny Performance Plastics and Getriebe Technik GmbH (GETEC) explore the possibility of using their Torlon PAI plastic to replace a traditionally metal helical pump gear, which powers an oil pump that lubricates and cools the transmission. When the car is idling and during low engine speed, with RPMs from 800–2000, this gear is known to be the single largest contributor to system noise. The use of high-temperature, high-performance thermoplastic injection-molded parts in automotive applications is growing and proving significant reductions in NVH issues.

Finite element analysis is a challenging task when dealing with gear applications, and this is especially true since different software is providing different sets of answers. Solvay continues to perform tests on actual plastic-gear prototypes using different material combinations for both machined and molded plastics. The bending and contact stress data for different materials are being updated to simulate gears in KISSSoft and Abaqus. Most notably, relevant data is available for Torlon 4203, Torlon 7130, and KetaSpire KT-880 CF30.

KetaSpire PEEK and Torlon PAI are excellent candidates for gear use not just for their continuous-use temperature properties but for their friction and wear performance in challenging gear mesh conditions. KetaSpire is semicrystalline and Torlon is amorphous. These metal alternatives offer a significant reduction of weight, NVH, and moment of inertia.

In testing, they show excellent performance but in different loading regimes. KetaSpire has improved life over conventional materials at higher load cases, whereas Torlon offers potential for extremely high cycle counts and continuous use, but at moderate loads.

Amodel PPA, Xencor LFT, and Ryton PPS each have very different properties from one another. The partially aromatic, semi-crystalline grades of Amodel PPA resins have low moisture absorption, high chemical resistance, excellent mechanical properties, outstanding dimensional stability, exceptional elevated thermal performance, and good processing characteristics. Xencor LFT contains long glass or carbon fiber reinforcement inside, which offers high fatigue resistance along with friction and wear properties, particularly suitable for gear applications. Ryton PPS has high chemical resistance, high continuous use temperature, and exceptional dimensional stability for high precision tolerances for motor parts.

In testing, Amodel PPA and Ryton PPS both have good performance in bending fatigue, but Amodel gives a better low-cycle count fatigue, and it’s less susceptible to damage during shock loading. At the same time, Amodel is slightly more susceptible to wear than Ryton making Amodel better for intermittent operations with medium to low cycle counts but are exposed to demanding impact targets. Ryton can have a place in higher stress applications, but it is more susceptible to shock loading, however, it has greater wear resistance than Amodel, making it well suited to applications that do not require high shock-load resistance but need to meet higher wear and cycle requirements at lower loads.

Tests include single-tooth bending tests to evaluate tooth root stress using a hydraulic pulsator; a 4-Square test rig to evaluate the number of cycles to fail and whether its cause was wear, tooth root breakage, pitting, micropitting, scuffing, tooth flank fracture, etc.; and noise testing to detect gear noise levels during different stages of rotation.

Tests have been performed both on machined and molded gears, which each have advantages and disadvantages. Machined gears are cost-effective and easy to process, but the fiber orientation and tooth flank surface quality are not as uniform as they would be with a molded gear. The challenges for molding are that tooth shape, dimensions, and concentricity need to be maintained after molding, which can cause shrinkage.

The benefits of Torlon PAI include the economies of injection molding and lower noise, vibration, and harshness (NVH) will continue to drive the replacement of many traditional metal parts with thermoplastic components. Traditionally, thermoplastic parts have only been considered for metal replacement on noncritical components. However, that assumption is shifting. During the last decade components manufactured from high-performance thermoplastics have replaced bearings and other components in demanding automotive transmission applications. The Torlon PAI oil pump gear represents a thermoplastic replacement of yet another crucial component and places thermoplastic parts within the critical function of the transmission.

At this early stage, the Torlon PAI gear was designed to be a drop-in replacement for the metal gear sharing the exact same dimensions. GETEC completed a full NVH test at their testing facility, and the plastic part performed 3 decibels lower than the metal gear at the critical idling and low engine RPMs. This represents a significant reduction in noise. Sven Steinwascher, GETEC managing director and CTO said, “The metal gear that we replaced with this first prototype had undergone six years of refinement to reach the level of NVH performance it has today. It’s encouraging that we surpassed that on our very first attempt with this thermoplastic part.”

Torlon PAI Helical Pump Gear

 

Solvay’s head of marketing-automotive Brian Baleno said, “Reducing NVH and identifying components to save space are two significant challenges for electrified vehicle powertrains. Torlon PAI has a long history of replacing metal in thrust washers and bearings, so we see metal replacement in oil pump gears as the next evolution and are excited to be partnered with Allegheny Performance Plastics and GETEC to help to refine the part and perhaps even decrease the NVH further.”

Solvay, observing key trends and factors affecting the transportation sector, has developed, tested, and applied materials for a wide variety of automotive uses. Central to those objectives are efficiency and regulatory targets, engine size reduction, increased electrification of the powertrain, low NVH, and higher efficiency through lightweighting. It’s no longer a question of whether high-performance plastics are meeting NVH and other challenges in e-mobility environments, but which polymers are good for high-performance gears?

RESOURCES

Solvay Specialty Polymers

Solvay Webinar on Plastic Gear Applications