PAI replaces metal in cam sprocket

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  • Advantages over phenolic materials
  • PAI sprocket offers high strength, stiffness and fatigue resistance up to 275°C
  • Polimotor 2: an all-plastic, four-cylinder engine project

The Polimotor 2 project, led by legendary automotive innovator Matti Holtzberg, selected polyamide-imide (PAI) from Solvay Specialty Polymers to replace conventional metal in the fabrication of an innovative cam sprocket design. The project aims to design and manufacture a next-generation, all-plastic engine for competitive racing in 2016.

The making of the PAI cam sprocket

Allegheny Performance Plastics, LLC, a leading processor of high-performance thermoplastics, injection molded the net shape. Gates Corp., a manufacturer of power transmission belts and a global maker of fluid power products, performed final machining to incorporate a spur tooth design that reduces wear and optimizes transfer of transmission torque between the sprocket and the belt. Ultimately, the Polimotor 2 engine will incorporate two 102-mm (4-in) diameter sprockets, and one 51-mm (2-in) diameter sprocket in its valve train drive system. Cam sprockets are attached to one end of the cam shaft in an automotive combustion engine and, along with the timing belt, help maintain timing between the cam shaft and crankshaft.

PAI cam sprocket

The Polimotor 2 project selected Solvay Specialty Polymer’s Torlon polyamide-imide (PAI) to replace conventional metal in the fabrication of an innovative cam sprocket design (source: Solvay Specialty Polymers)

PAI sprocket offers high strength, stiffness and fatigue resistance up to 275°C

Despite constant exposure to high torque, extreme temperatures and vibration, as well as dirt, automotive fluids and road salt, cam sprockets must reliably deliver precise timing control to maintain optimal engine performance. If these sprockets overheat, chip, lose their shape or fail to perform reliably under load, everything from the crank to the pistons can quickly cease to work properly.

Cam sprockets are typically made from sintered steel, aluminum or occasionally thermoset phenolic polymers. However, Polimotor 2 opted to mold its engine’s spur tooth cam sprockets using Solvay’s 30 percent carbon fiber-reinforced Torlon 7130 PAI. The material offers high strength, stiffness and fatigue resistance up to 275°C (525°F). The material features a specific strength of 1.4 105 J/kg (5.4 x 105 in-lbf/lb) and specific stiffness 15 106 J/kg (6 x 107 in-lbf/lb). Stainless steel, in contrast, delivers specific strength and stiffness of 0.8 106 J/kg (3.1 x 107 in-lbf/lb), and 24 106 J/kg (9.7 x 107 in-lbf/lb) respectively.

In practical terms, this allows the Polimotor 2 cam sprocket fabricated from the PAI material to deliver comparable mechanical properties with a 75 percent weight reduction over a similarly sized stainless steel cam sprocket that weighs 1.1 kg (2.4-lb).

Advantages over phenolic materials

Unlike metals, Torlon 7130 PAI does not conduct heat, helping to promote longer belt life. It also eliminates potential chipping of the sprocket, which can be a concern when using phenolic materials because they are more brittle. In addition, the material offers very good fatigue resistance and very good wear performance at elevated pressures and velocities, thereby decreasing noise and vibration, and offers broad chemical resistance to automotive fluids.

Polimotor 2: an all-plastic, four-cylinder engine project

The Polimotor 2 project aims to develop an all-plastic, four-cylinder, double-overhead CAM engine that weighs between 63-67 kg (138 to 148 lbs), or about 41 kg (90 lbs) less than today’s standard production engine. In addition to the current cam sprocket application, the program will leverage other materials of Solvay to develop up to ten engine parts. These include a water pump, oil pump, water inlet/outlet, throttle body, fuel rail and other high-performance components.

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