Brake element for transmission and control system therefor

A control system for a brake element for a transmission, the brake element comprising a first piston configured to press friction plates, and a second piston configured to move the first piston toward a side close to the friction plates, wherein the first piston and the second piston are arranged in series along a stroke direction thereof, in this order from the side of the friction plates;

• and wherein the first piston is fitted in the second piston in such a manner as to be movable together with the second piston and relatively movable with respect to the second piston, and wherein the brake element further comprises a first oil pressure chamber configured to be supplied with an oil pressure for moving the first piston by a stroke toward the side close to the friction plates, a second oil pressure chamber configured to be supplied with an oil pressure for moving the second piston by a stroke toward the side close to the friction plates, a biasing member biasing the second piston toward a side away from the friction plates, and a restriction member configured to prevent a stroke of the second piston toward the side close to the friction plates from becoming greater than a given value, the control system being characterized in that it comprises an oil pressure control device configured to control supply and discharge of an oil pressure with respect to the first oil pressure chamber and the second oil pressure chamber,
• wherein the oil pressure control device being operable,
• when it is necessary to set the brake element to a disengaged state,
• to (1) supply no oil pressure to the first oil pressure chamber and the second oil pressure chamber to thereby move the second piston and the first piston toward the side away from the friction plates by a biasing force of the biasing member, to set the second piston and the first piston at their respective initial positions;
• when it is necessary to engage the brake element being in the disengaged state,
• to (2) supply an oil pressure to the second oil chamber to thereby move each of the second piston and the first piston by a stroke toward the side close to the friction plates, to set the second piston and the first piston at their respective standby positions determined by the restriction member, and then,
• to (3) supply an oil pressure to the first oil chamber to thereby move only the first piston by a stroke toward the side close to the friction plates, to set the first piston at a pressing established position where effective pressing of the friction plates is established;
• and when it is necessary to disengage the brake element being in the engaged state,
• to (4) discharge the oil pressure in the first oil pressure chamber to thereby move only the first piston toward the side away from the friction plates, to set the first piston at a zero-clearance position where a pressed state of the friction plates is released, and then,
• to (5) discharge the oil pressure in the second oil pressure chamber to thereby move the second piston and the first piston toward the side away from the friction plate by the biasing force of the biasing member, while maintaining a relative positional relationship between the first piston and the second piston, to set the second piston and the first piston at their respective initial positions.