Vibration damping device for a vehicle body

A vibration damping device for a vehicle body, the vibration damping device comprising:

• a cylinder including a first end supported at a predetermined first mounting position of the vehicle body;
• a piston rod slidably extending through a second end of the cylinder, and including a first end which is inserted into the cylinder and a second end which is supported at a predetermined second mounting position of the vehicle body;
• a first piston through which the piston rod extends, the first piston being fixed to the first end of the piston rod and slidably fitted in the cylinder;
• a second piston through which the piston rod extends, the second piston being fixed on the piston rod between the first piston and the second end of the cylinder and slidably fitted in the cylinder;
• a free piston slidably fitted between the first piston and the first end of the cylinder;
• a gas chamber between the free piston and the first end of the cylinder and filled with a gas of a predetermined pressure;
• a first oil chamber between the free piston and the first piston and filled with a hydraulic oil;
• a second oil chamber between the first piston and the second piston and filled with the hydraulic oil;
• a third oil chamber between the second piston and the second end of the cylinder and filled with the hydraulic oil;
• a first hydraulic oil passage provided in the first piston that allows the first oil chamber and the second oil chamber to communicate with each other;
• a second hydraulic oil passage provided in the second piston that allows the second oil chamber and the third oil chamber to communicate with each other;
• a first restrictor provided in the first hydraulic oil passage;
• a second restrictor provided in the second hydraulic oil passage;
• and
• a damping force change mechanism provided in the piston rod that selectively allows the first oil chamber and the second oil chamber to communicate with each other;
• wherein
• the damping force change mechanism includes:
• a through hole opening in an end surface at the first end of the piston rod and extending toward the second end of the piston rod;
• a communicating hole that allows the through hole and the second oil chamber to communicate with each other;
• a third hydraulic oil passage extending from the through hole to the communicating hole, and that allows the first oil chamber and the second oil chamber to communicate with each other;
• an on-off valve movably fitted in the through hole, the on-off valve including a fully opened state by moving to a predetermined first direction so as to open the third hydraulic oil passage, and including a fully closed state by moving to a second direction reverse to the first direction so as to close the third hydraulic oil passage;
• and
• an operation rod inserted into the piston rod in a side of the second end of the piston rod with respect to the on-off valve in the through hole, the operation rod being movable in the first direction and the second direction so as to determine a moving direction of the on-off valve;
• the on-off valve includes an expanded diameter portion projecting from the through hole into the first oil chamber and extending outward in a radial direction of the piston rod;
• and
• the expanded diameter portion faces an entire opening edge of the through hole and contacts the end surface of the piston rod.