As automatic transmissions become more advanced as a system, the components within must evolve as well. Performance drives the design and there are many factors that must mesh together to achieve the overall goal.
Efficiency drives system improvement
Improved efficiency is the leading factor when it comes to system improvement. The move to eight, nine, and ten-speed automotive transmissions are focused around improving fuel economy. Obviously, with the increased number of gear ratios, the number of clutches and clutch pistons has also increased. A reduction in the amount of energy needed to engage and retract a clutch piston directly reduces the parasitic loss of energy within the entire system. This has resulted in a significant focus on the impact of seal drag within each clutch piston.
D-Rings provide advantages for automotive transmission clutch pistons
For more than a decade, D-Rings have been the preferred sealing technology for automotive transmission clutch pistons as they provide 
several advantages to other seal designs including:
Bi-directional sealing capability,
Symmetrical design simplifies assembly,
Lower drag than other radial compression seal designs,
Reduced variation in drag response, and
- Eliminates potential spiral failure existent with O-rings.
In the past, there was always enough fluid pressure to overcome seal drag and activate the clutches. Because these same size pumps must supply so many more clutch circuits in the transmission, this energy supplied is in greater demand. Similarly, heavy springs in the past provided enough return force to ensure immediate disengagement of the clutch. Limited availability of space, as well as lower cost targets, have resulted in smaller, lower force return springs.
D-Ring cuts drag forces by 50%
Our engineers have developed a low drag D-ring which reduces the drag forces in the application by 50%. All other advantages of D-ring seals are maintained including the symmetrical profile which simplifies the assembly process. This improves the first time through capability and reduces warranty costs.
By creating volume space to which the deflected rubber can easily flow to, the reaction forces against the mating surface (normal force) is reduced while still maintaining sufficient sealing forces.
In addition, Parker’s unique manufacturing approach provides a significant cost reduction over compression and injection molded designs. The absence of mold parting lines in this new technology also eliminates molded rubber flash on the sealing surface which is inherent with injection or compression molded products.
Lower clutch seal drag provides several advantages including:
Smoother shifts,
Overall improvement in system efficiency, and
Smaller return springs save space, weight, and cost.
This technology is also available for applications beyond transmission clutches. Any reciprocating piston application can benefit from low drag D-rings. Examples would include active differentials and other torque management applications.
Please contact one of Parker’s application engineers to discuss how low drag seals can improve the performance and efficiency of your applications.
Article contributed by Scott A. Van Luvender, automotive applications engineering manager, Engineered Materials Group
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