Rotational speed reduction device for truck tailgate

A rotational speed reduction device for a truck tailgate comprising:

• a housing having a cylindrical inner peripheral surface and an inward protrusion formed integrally on the inner peripheral surface in such a manner as to protrude radially inwardly from the inner peripheral surface;
• a rotor which has a cylindrical outer peripheral surface opposing the inner peripheral surface of said housing and has an outward protrusion formed integrally on the outer peripheral surface in such a manner as to protrude radially outwardly from the outer peripheral surface, and which is rotatably disposed inside said housing;
• a viscous fluid accommodated in an inner space which is defined by the inner peripheral surface of said housing, the outer peripheral surface of said rotor, and side surfaces of the inward protrusion opposing each other in a rotational direction, and which is partitioned into two, a first and a second, chambers, by the outward protrusion of said rotor;
• a first and a second passage provided in said rotor so as to allow the two, the first and the second, chambers to communicate with each other;
• a first one-way valve disposed in said first passage such that, in the rotation of said rotor with respect to said housing in one direction corresponding to the rotation in an opening direction of a truck tailgate, the flow of said viscous fluid from one of the two chambers to another one thereof through said first passage is allowed by an increase by more than a fixed level of the internal pressure of said viscous fluid in the one of the two chambers owing to the rotation in the one direction, whereas, in the rotation of said rotor with respect to said housing in another direction corresponding to the rotation in a closing direction of the truck tailgate, the flow of said viscous fluid from the other chamber to the one chamber through said first passage is inhibited;
• and
• a second one-way valve disposed in said second passage such that, in the rotation of said rotor with respect to said housing in the one direction, the flow of said viscous fluid from the one chamber to the other chamber through said second passage is inhibited, whereas, in the rotation of said rotor with respect to said housing in the other direction, the flow of said viscous fluid from the other chamber to the one chamber through said second passage is allowed,
• wherein said first one-way valve has a valve seat formed in said rotor, a spherical body which is seated on the valve seat and is movably disposed in said rotor, and a resilient member which is, at one end thereof, brought into contact with the spherical body so as to resiliently press the spherical body against the valve seat,
• said second passage has at least one notch formed in a protruding end of the outward protrusion, and
• said second one-way valve has a valve element provided at the protruding end of the outward protrusion movably in the rotational direction in such a manner as to straddle the protruding end of the outward protrusion,
• the valve element including a completely closing piece portion disposed on one surface of the outward protrusion which receives the internal pressure of said viscous fluid in the one chamber, so as to inhibit the flow of said viscous fluid from the one chamber to the other chamber through said second passage by closing the notch in the rotation of said rotor with respect to said housing in the one direction, an incompletely closing piece portion disposed on another surface of the outward protrusion which receives the internal pressure of said viscous fluid in the other chamber, so as to allow the flow of said viscous fluid from the other chamber to the one chamber through said second passage by partially closing the notch in the rotation of said rotor with respect to said housing in the other direction, and a connecting portion which is disposed at the protruding end of the outward protrusion and which is in contact with the inner peripheral surface of said housing and the protruding end of the outward protrusion slidably in the rotational direction, so as to connect together the completely closing piece portion and the incompletely closing piece portion.