GM’s next-generation lightweight six-speed manual transmissions for Europe use gears with wide, asymmetrically-cut dog teeth, and triple-cone synchronizers for first/second gear, with double cones for third/fourth. Reverse gear is also synchronized.
In recent years, many of the headlines and much of the news coverage generated in the transmission market has involved automatic transmissions. Stepped automatic transmissions (ATs) continue to add more gears, and the new technology variants such as dual clutch transmissions (DCTs) and continuously variable transmissions (CVTs) are growing in popularity. However, the manual transmission (MT) continues to be the leader, accounting for almost half of all global production and maintaining a healthy majority share in Europe and in many large developing markets such as China.
The pursuit of improved fuel efficiency and driver comfort has sparked ongoing technology and materials improvements in manual transmissions. New synchronizer materials such as carbon, sinter and molybdenum now offer a range of durability and shift quality enhancements, improving reliability and shift feel. As always, these changes have put increased pressure on the lubricant operating system, as they result in increased torque-to-weight ratios and higher operating temperatures.
Dedicated Transmission Lubricants Ensure Highest Performance
What is sometimes overlooked by oil marketers and end users is that this modern manual transmission technology requires the development of dedicated lubricants capable of extracting the highest possible efficiency and driveability without compromising durability. Throughout the global market, Lubrizol has found that many passenger car and commercial vehicle owners frequently service their manual transmissions with a non-dedicated fluid, or even an ATF. Results of field trials have demonstrated that this can result in poor performance and shortened equipment life expectancy.
Manual transmission lubricants must be compatible with the wide variety of new materials being used in the assembly, and also must have good thermal stability, shear stability, and load carrying and extreme pressure performance. Increasingly, the lubricant is tailored to match the particular requirements of the target transmission, including improved corrosion resistance and the use of viscosity modifiers to maintain protection in extreme operating conditions. Developing fluids that deliver these improvements is critical to enable smooth and repeatable gear shifts, while maximizing transmission component life.
Delivering these vital requirements is dependent upon the friction performance in the synchronizer system. The fluid must provide the proper friction performance to deliver a consistent shift for an extended drain period (up to 800,000 km in the case of some commercial vehicles), with a response that is optimized for the synchronizer materials used. As the demands on the synchronizer grow, and the range of available materials becomes more complex, optimization of shift performance becomes a complicated balance.
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Highly sophisticated dual-clutch transmissions such as the Audi S-tronic system still use the same basic principles as the manual gearbox but fluid choice is more critical still as lubrication of wet clutches must also be assured.[/caption]
Examining Friction Performance and Synchronizer Wear
To help optimize shift performance, it is essential to understand the interaction between fluids and synchronizer friction materials. Historically, research has centered on dynamic friction performance and the control of synchronizer wear. However, further focus has been placed on the examination of the friction profile of individual synchronizer engagements to optimize shift performance and provide wear protection. Mastering the relationship between speed and friction allows Lubrizol researchers to optimize the shift performance, which is the first step to develop an appropriate transmission fluid.
In the development of new fluids, we also analyze how the chemical structure of friction modifiers can affect friction and wear performance. This is carried out on a wide range of synchronizer materials such as brass, carbon, molybdenum and phenolics.
When formulating a lubricant to perform with new materials, the selection of friction modifier chemistry can provide the difference between an optimized fluid and one that will cause rapid wear and poor shifting. In comparison testing, otherwise identical fluids with different types of friction modifier added showed enormous differences in their properties. The goal is to develop a friction modifier that provides both optimized shift quality and overall low wear on synchronizer parts across the widest possible range of materials. Once the optimized candidate materials have been determined through extensive testing on dedicated synchronizer durability rigs, Lubrizol’s technicians scrutinize components in microscopic detail to fully understand how and why these systems work, aiding in future development.
Surface Analysis Yields New Discoveries and Understanding
Today, we know surface analysis is the best way to understand synchronizer friction modifiers. Auger electron spectroscopy of a synchronizer can establish the chemical composition of the synchronizer surface, and by etching away surface layers, we can measure the depth of any lubricant-derived film that may be present. This analysis reveals the depth and chemical nature of any interaction between the lubricant and the material, which though likely to be only nanometers in thickness, performs a very important role in protecting the components and subsystems.
The performance of different friction modifier systems can vary dramatically. As such, certain types of friction modifiers are more effective at tribofilm formation. The tribofilm found within Lubrizol friction modifiers provides a strong and deep interaction with the surface of the material, resulting in a protective film with the depth and chemical composition to help balance the friction and wear properties. While it is important for the tribofilm to have a deep interaction with the friction modifier, the chemical nature of the film must also provide a balance of wear protection and shift quality.
Synchronizer Tests Show Strong Results with Film Protection
To understand the kinetics of tribofilm formation, experiments were conducted to test new synchronizers from break-in to 10,000 cycles. The results were surprising. When using a next-generation fluid, a substantial film was created very quickly (in the equivalent of just 200 gear shifts of a test program). This film protection showed rapid increase and then equilibration, to maintain strong protection and low wear of the synchronizer parts for the duration of the test procedure. When parts were analyzed after 100,000 cycle durability tests, this protection was shown to have been maintained.
Using advanced surface analysis techniques to cut and extract sections of the tribofilm enables researchers to visually examine these chemical layers and their structures using a transmission electron microscope. These advanced surface analysis techniques help researchers interpret the mechanism by which the friction modifiers added to the lubricant turn into a protective tribolayer. Researchers can then determine what can be altered in the structure of the friction modifiers to improve their overall performance. The results of these fundamental studies are already being implemented in the development of new dedicated manual transmission fluids with optimized shift quality for new and future manual transmission designs.
The manual transmission world is dynamic. The performance and durability of a manual transmission fluid relies on careful and thoughtful formulation, using an advanced additive package that has undergone rigorous testing and research. Lubrizol leads the way in understanding how modern manual transmission additive and viscosity modifier technologies need to be developed to ensure maximum performance and provide end users with a comfortable driving experience. Lubrizol also knows that OEMs need a fluid tailored to suit their specifications and hardware. For oil marketers and end users, the use of a dedicated manual transmission fluid provides the required durability and performance to provide years of reliable and comfortable vehicle performance.
