Study on Novel High Efficiency Dog Clutch for Stepped Automatic Transmissions

Abstract

A new dog clutch for automatic transmission (AT) was developed to improve transmission efficiency. With this dog clutch, it is possible to replace a conventional 1st gear clutch (combination of a multi-plate wet-type clutch and a one-way sprag clutch) in AT. The dog rotor and rotary shaft are constantly engaged through helix splines. If the dog rotor and stator are engaged, albeit a small amount, the rotary force of the dog rotor is converted to a translating force of the engaging direction. Therefore, the engaging force of the dog clutch is assisted by this translating force when it is turned on. In contrast, the direction of the translating force is inverted when the 2nd gear clutch is engaged. The dog rotor receives the disengaging force, and it is passively disengaged as in a one-way sprag clutch when shifting upward from 1st to 2nd gear. Therefore, the transmission torque of the 1st and 2nd gears does not overlap and AT torque-gap does not occur when shifting gears with the proposed dog clutch. We confirmed that the longitudinal G-gap during the gear shift was the same as that of the conventional AT in the vehicle running test. Moreover, the drag losses of the dog clutch were 50-60 % lower than those of the multi-plate wet-type clutch. When AT downshifts during power-on, it is necessary to maintain a low driving torque until the relative rotational speed between the dog rotor and stator is reduced and the dog rotor engages completely. However, the proposed dog clutch can transmit the driving torque in the middle of engaging, thus reducing the waiting time. We demonstrated that the waiting time in the simulation was reduced because of the new dog clutch. The simulation model included the edge contact of the dog tooth and the frictional resistance forces. The friction coefficients measured in the clutch unit test were used for the model. The proposed dog clutch performs better than conventional clutch; therefore, it will be convinced to apply AT as well as drivetrain of the EV and PHEV.