DuPont—Leading the metals-to-plastics revolution in transmissions and driveline

 

General Motors' 6-speed transmission is a successful example of global scale transition to plastics in transmission components. Parts include transmission valve body cover and transmission control module in glass-reinforced Zytel® with gasket of Vamac®, ATF (automatic transmission fluid) level valve, ATF filter housing and transmission range sensor of Zytel®, and two baffles of Hytrel®.
Vespel® parts excel in seal rings and thrust washers in driveline applications such as automatic and continuously variable (CVT) transmissions, transfer cases and torque converters.
One-piece baffle of DuPont™ Hytrel® thermoplastic polyester elastomer has replaced a two-piece stamped metal baffle and rubber seal in 6-speed automatic transmissions for GM and Ford, saving weight, assembly time and cost.
Vespel® SP-21 thrust washers, located between the aluminum die cast case and second clutch of Jatco's 7-speed belt driven CVT for the Nissan March and Cube, have replaced larger all-metal needle bearings to save weight and space, and enhance fuel efficiency.
Highly abrasion-resistant low friction Vespel® SP sealing rings in ZF's stepless automatic transmission for the Mini seal oil at up to 55 bar and driving shaft speeds up to 12,000 rpm, while helping save cost, space and weight.
Transmission control module of glass-reinforced Zytel® is the 'brain' of the transmission, ensuring quick reaction time of key functions.
  

 

Talk to almost any automotive engineer involved in transmission and driveline development and they'll be looking for answers to these questions:

• How do I save weight without compromising part performance?
• How do I improve performance while reducing fuel consumption?
• How do I minimize friction losses between moving parts?
• How do I reduce cost?

 

For many applications, the short answer is to replace metals with high performance plastics and composites. But, while many in the auto industry have already made that transition, particularly in powertrain, some driveline engineers may need more evidence to prove that metals replacement really is the way to go to achieve those objectives.

 

Proving the solutions

 

AutoFocus interviewed Dino Tres, Global Transmissions Business Manager for DuPont Performance Polymers, and Dave Ritchey, DuPont™ Kalrez® and Vespel® Global Transportation Marketing Director, to find out how the company is collaborating with auto engineers and designers to provide answers to these questions.

 

Dino Tres has been working in the transmission segment for 20 years, specializing in helping customers reduce weight and integrate functionality with plastics composites like DuPont™ Hytrel® thermoplastic polyester, Zytel® GRZ, Zytel® HTN, and Rynite®.

 

Dave Ritchey has over 25 years polymer and coatings experience in the automotive industry and currently leads the 'Sealing, Wear and Friction' efforts within the DuPont™ Vespel® parts business. He is supported by a global team helping the transportation industry understand how materials, design and application knowledge can ultimately improve the efficiency of automotive and commercial vehicle drivelines.

 

AutoFocus: What mega-trends do you see in automotive drivelines?

 

Dino Tres: There is still a lot of metal in transmissions and drivelines, but there is also a tremendous push in the industry to reduce vehicle weight. OEMs are equating dollars to every kilogram of weight shaved off the vehicle, and they talk in terms of taking 10% off the chassis, 15% off the engine, and a very significant 20% off transmission and driveline. So if you are an auto engineer working on a big weight reduction program, the best way to achieve that is to use a lot of composites.

 

Much of the transition to plastics has already taken place in engines via intake manifolds, cam covers and other significant parts. But when we look between the engine and the wheels we still see a lot of metal and few composites. That has been the drive for DuPont to work with OEMs to develop components that replace metals and provide cost—and most importantly—weight savings. We are turning the corner in this area. With the use of computer simulation and commercial successes we have demonstrated how well polymers work in many applications where metal was heavily involved.

 

Dave Ritchey: A key mega-trend we are seeing is the shift away from purely manual transmissions to automated transmissions, particularly in emerging countries such as Brazil, China and India. This includes affordable, low cost, lightweight continuously variable transmissions (CVTs) as well as multi-speed dual clutch designs.

 

In addition, regardless of transmission design, engineers want to improve efficiency by reducing friction—that's another mega-trend. Globally we're seeing significant effort and funding for projects where reducing friction and frictional losses are now a key objective.

 

As transmissions get smaller, lighter and spin faster, engineers are pushing the limits of existing materials, particularly in their wear, friction and temperature capabilities. Through expertise in sealing, wear and friction we understand what leads to low friction, and through combinations of materials and design we can help lower friction and improve efficiency.

 

Vespel® works where most other resins don't. It is at the top of the polymer pyramid with its high load and high-speed capabilities (high PV limits). We enable new design solutions such as higher shaft speeds and loads. Vespel® can replace metal bearings to reduce overall size and weight, or enable the use of lighter weight metals. For example, due to its excellent wear properties against aluminum, it may eliminate the need for heavier steel inserts or sleeves.

 

AutoFocus: Can you give examples of applications where plastics can provide advantages over metals?

 

Dino Tres: The drive shaft is a dramatic example of the advantages provided by composites. In metal, it is just a heavy pipe. When CAFE standards and the need for greater fuel efficiency came into focus, the industry started talking about weight, so aluminum drive shafts were developed. We have run computer simulations that have shown it can be done in composites at half the weight and half the price.

 

Another example is a one-piece baffle of DuPont™ Hytrel® thermoplastic polyester elastomer that replaces a two-piece stamped metal baffle with a big rubber seal in 6-speed automatic transmissions. This has been so successful in cost and weight savings and in performance, that six different transmissions equipped with these baffles are in production globally at GM and Ford, and more are in development.

 

One of our customers, looking for a material with excellent wear resistance for a transmission fork application, was convinced that only a move from aluminum to bronze would give the wear and friction characteristics needed. But bronze is very heavy. We provided the solution using Zytel® HTN high performance polyamide with two Vespel® wear pads which have proved to be significantly more resistant to wear than bronze.

 

Dave Ritchey: Almost any new transmission design project can be an opportunity to replace metal. For example, Jatco replaced metal needle bearings in a new CVT transmission for the Nissan March and Cube with Vespel® SP-21 thrust washers. These are significantly thinner than the metal needle bearings and one-third the weight, allowing Jatco to reduce the overall size and weight of the CVT unit.

 

ZF Friedrichshafen AG also gained weight and space-saving and maintenance-free advantages by designing with Vespel® parts. ZF installed highly abrasion-resistant low friction Vespel® SP sealing rings in its CVT drive for the Mini.

 

AutoFocus: Where do you see the greatest contribution of plastics?

 

Dino Tres: There is enormous potential for polymers in automatic transmissions and drivelines. Wherever engineers need to take out weight and cost and reduce component size while sealing fluids, resisting wear, friction and loads, and withstanding chemical attack at low and high temperatures—we have the products to do that.

 

The specific gravity of composites is about 1.2 to 1.5 compared with steel at about 8.0, making weight reductions of up to 80%, and cost savings of 30-50% possible. As a result, we already have many commercial and developmental applications across the driveline sector.

 

Dave Ritchey: As loads, fluid pressures and shaft speeds increase, high performance polymers can help provide new levels of vehicle performance—more power from smaller, lighter components. Vespel® parts excel as seal rings, thrust washers and wear pads in automatic and continuously variable transmissions, transfer cases and torque converters because they can enable new designs and concepts to be successful.

 

AutoFocus: How do you convince engineers of the long-term reliability of these materials?

 

Dino Tres: DuPont™ Hytrel® thermoplastic polyester elastomer has been used for CVJ boots for more than 20 years and has demonstrated outstanding performance in this very demanding application with high rpm, temperatures up to 150°C, aggressive grease and chemicals inside the boot, and impact from dirt and pebbles outside. When GM switched their CVJ boots from standard elastomers to Hytrel®, it extended part warranties from 25,000 miles to life of car.

 

Transmission sensors of DuPont™ Rynite® PET thermoplastic polyester resin and Zytel® high performance polyamide have been in production since the early 1990s. As operating conditions have become tougher in terms of heat, chemical attack and mechanical stresses, we have kept pace by developing super-structural materials such as the Zytel® HTN family of thermoplastics.

 

Dave Ritchey: For our Kalrez® and Vespel® business, giving our customers 'peace of mind' is really where the 'Science of Sealing, Wear and Friction' provides its value. First, we have the historical material science capabilities of DuPont backing our resin development and selection process. Second, since we are the supplier of Kalrez® and Vespel® parts, we have part testing capability in all three regions—Europe, Asia and the Americas. It's very typical for us to confirm part performance in DuPont laboratories under actual end use conditions as specified by the customer.

 

These test facilities also allow us to optimize design and material selection before the parts are installed in the actual components. Finally we have production capability in all three regions as well. All facilities operate to global quality standards as required by our customers. It's not by accident that I often hear from customers how our parts continue to perform flawlessly years after commercial introduction.

 

AutoFocus: Can you comment on some product capability highlights?

 

Dino Tres: We have many structural offerings in our portfolio of polymer composites, engineering polymers and elastomers that provide one-stop solutions for customers. Products range from rigid plastics that can be molded or machined like metal, to elastomers with plastics-to-rubber flexibility. They offer combinations of excellent heat and chemical resistance, sealing ability, mechanical strength, flex fatigue, wear and aging resistance, and low friction.

 

The automotive industry already uses these products extensively. They include Crastin® PBT thermoplastic polyester resin, Hytrel® thermoplastic polyester elastomer, Kalrez® perfluoroelastomer parts, Rynite® PET thermoplastic polyester resin, Vamac® ethylene acrylic elastomer, Vespel® parts and shapes, Viton® fluoroelastomer and Zytel® high performance polyamide.

 

Dave Ritchey: We continue to develop new polymers and new designs in Vespel® parts and shapes. For example, we expanded the portfolio by launching Vespel® SP-2515 polyimide parts to offer a coefficient of thermal expansion close to aluminum. By having an expansion rate similar to the mating metal, our Vespel®
SP-2515 parts allow designers to maintain the intended dimensions and clearances over a broader temperature range. Vespel® SP-2515 is already commercialized in transmission seal rings offering very low friction, tolerance control over a broad temperature range and long life.

 

It is one of several new product and design innovations which enable reduced component weight and friction, boost efficiency, and drive out cost in hot and dirty operating environments.

 

AutoFocus: What are the advantages of working with DuPont?

 

Dino Tres: We work closely with OEMs on computer simulation involving almost the entire driveline including axles, wheels, springs, shock absorbers, etc. DuPont offers the advantage of using high-speed computers to do that sort of modeling. We also have in-house capability to run material behavior projections that allow modeling of dynamic crash events in milliseconds, to meet government crash worthiness requirements, for example. This advanced material behavior characterization is accomplished by stretching a material fast enough in milliseconds to show stress-strain curves at very high strain rates. Those are some of the excellent capabilities we offer our customers.

 

Dave Ritchey: I believe it really centers on 'peace of mind'. As mentioned earlier, we apply our science heritage all the way from material selection to end-use testing. And because we are a global company, we can rely on our global network to support our customers anywhere they need us. If we prototype a part in Detroit or Frankfurt, we can confidently commercialize it in China or Brazil with the same expectations and quality our global customers demand.

 

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