Power transmission device

A power transmission device that is mounted on a vehicle equipped with a motor and transmits power from the motor to a driving wheel side via a hydraulically driven friction engagement element, the power transmission device comprising:

• a mechanical pump that is driven by the power from the motor to produce a hydraulic pressure;
• a pressure regulating valve that regulates the hydraulic pressure from the mechanical pump;
• an electric pump that is driven by supplied electric power to produce a hydraulic pressure;
• a switching mechanism that is formed of one or more switching valves, that is connected to a pressure regulating valve oil passage through which oil output from the pressure regulating valve flows, an electric pump oil passage through which oil discharged from the electric pump flows, and a hydraulic servo oil passage communicated with a hydraulic servo of the friction engagement element, and that selectively switches between a first state in which the pressure regulating valve oil passage is communicated with the hydraulic servo oil passage while communication between the electric pump oil passage and the hydraulic servo oil passage is shut off and a second state in which communication between the pressure regulating valve oil passage and the hydraulic servo oil passage is shut off while the electric pump oil passage is communicated with the hydraulic servo oil passage, and
• a drain valve that opens to drain oil when a hydraulic pressure of a drain oil passage in fluid communication with the electric pump oil passage is a preset pressure or more, wherein
• the switching mechanism is further structured to be connected to a drain oil passage equipped with the drain valve, and to allow communication between the electric pump oil passage and the drain oil passage in the first state, and shut off communication between the electric pump oil passage and the drain oil passage in the second state.