Browse Publications Technical Papers 2010-01-0857
2010-04-12

General Motors Small Front Wheel Drive Six speed Automatic Transmission Family 2010-01-0857

General Motors introduced a family of small front wheel drive six speed automatic transmissions for the 2008 model year. The family currently has two variants: 6T40 and 6T45, which cover a range of vehicles from small & compact cars to small SUVs and handle engines torque capacities up to 240 Nm Gas(280 Nm Diesel) & 315 Nm Gas (380 Nm Diesel) respectively.
The 6T40/45 transmissions replace GM traditional four speed automatic wrap around transmissions 4T40/45. The wrap around transmissions have Torque Converter, Pump & Controls on the engine axis and the rest of the transmission content on the output axis. The 6T40/45 have an on-axis architecture with majority of the transmission content on the engine axis and final drive & differential on the output axis. The 4T40/45 have input chain transfer whereas the 6T40/45 have an output chain transfer. The objectives behind the creation of the 6T40/45 transmissions were improved fuel economy, performance, mass and NVH as compared to the transmissions they replaced. Packaging in the existing vehicle architectures and high mileage dependability were givens. This architecture was required to offer low cost for a front wheel six speed transmission while meeting the givens and objectives.
A six speed powerflow, common with GM's 6T70/75 transmission, was selected by General Motors Propulsion Systems Research Laboratories, for the 6T40/45. The powerflow includes three simple planetary gear sets, three brake clutches and two rotating clutches. The resultant on-axis transmission architecture utilizes a hyper-elliptical (squashed) torque converter, an on-axis pump and three close coupled gear sets. Two rotating clutches have been radially stacked for minimized axial length of the transmission. The architecture also includes a chain axis transfer and a planetary final drive. This architecture yields a 6.1 overall ratio spread, enabling up to 7% improvement in 0-60 mph performance because of a deeper first gear ratio and up to 4% better fuel economy due to lower N/V for the top gear when compared to the 4T40/45 transmissions that it replaces. A wide range of axle ratios are available by utilizing the flexibility of sprocket sets and final drive ratios.
The architecture is cost effective by virtue of the chain axis transfer, on axis pump, one piece case and significant part commonality among the variants of the family. The transmission controller and control system are similar to the General Motors 6T70/75 transmission family. There was also special attention focused on flexible manufacturing design features that enables a global manufacturing footprint.
The architecture utilizes a center clutch housing that enables the required shape of the transmission case, making it optimal for packaging in a variety of applications. The 6T40/45 transmissions package in the existing vehicle architectures with 197 mm center distance between engine & output axes and an output axis vertical drop of 75 mm from engine axis. On axis design makes the architecture package similar to manual transmissions, and hence, opens it up to more global vehicle architectures. The 6T40/45 transmissions, depending on applications, offer 1-3 kg mass savings when compared to the 4T40/45 while offering two extra forward speeds and an internal transmission controller.
The 6T40 and 6T45 have components with identical radii but component lengths have been flexed for torque capacity differences. Additional family variants based on input & output torque, packaging, and other geometry/hardware parameters are also possible. The overall result is a robust, compact and cost effective transmission family which offers significant fuel economy and performance benefit over its four speed counterpart transmissions and should provide an attractive balance of overall metrics in the automatic transmission market.

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