Multi-disc frictional engagement mechanism

1. A multi-disc frictional engagement mechanism for coupling a first member and a second member which are coaxially arranged, comprising:

a plurality of first friction plates mounted on the first member slidably in an axial direction;

a plurality of second friction plates mounted on the second member slidably in the axial direction, the plurality of first friction plates and the plurality of second friction plates being alternately arranged;

a hydraulic piston arranged displaceably in a direction perpendicular to the pluralities of the first and second friction plates and including a radially extending pressure receiving portion;

a piston oil chamber configured to cause a hydraulic pressure to act on the pressure receiving portion;

an elastic member configured to come into contact with the pressure receiving portion from a side opposite to the piston oil chamber so as to bias the hydraulic piston toward the pluralities of the first and second friction plates; and

a lock mechanism comprising a holding member and an engaging member, the lock mechanism being configured to restrict a movement of the hydraulic piston in a coupling direction toward the pluralities of the first and second friction plates in a state in which the hydraulic pressure is supplied to the piston oil chamber, the elastic member is compressed by the pressure receiving portion, and the pluralities of the first and second friction plates are not coupled,

wherein the lock mechanism is locked when the hydraulic pressure of the piston oil chamber acts on the lock mechanism, and

wherein the hydraulic piston is configured such that the restriction of the movement in the coupling direction by the lock mechanism is removed in a state in which the hydraulic pressure acts on a piston portion from the side opposite to the piston oil chamber and the holding member is separated from the engaging member.

2. The multi-disc frictional engagement mechanism according to claim 1 , wherein:

the engaging member is movable toward the pressure receiving portion;

the holding member is configured to come into contact with the engaging member moved toward the pressure receiving portion to hold the engaging member;

the pressure receiving portion includes a receiving portion with which the engaging member is configured to be engaged, at an end part of the pressure receiving portion; and

the engaging member is moved by the hydraulic pressure supplied to the piston oil chamber so as to be engaged with the receiving portion and to come into contact with the holding member such that the movement of the hydraulic piston in the coupling direction is restricted.

3. The multi-disc frictional engagement mechanism according to claim 2 , wherein:

the holding member includes the piston portion, which is configured to receive the hydraulic pressure of the piston oil chamber, and is configured to move to a position where the holding member is held in contact with the engaging member by the hydraulic pressure supplied to the piston oil chamber.

4. The multi-disc frictional engagement mechanism according to claim 2 , further comprising:

a tubular member interposed between the pressure receiving portion and the holding member and including a plurality of radially extending holes,

wherein the engaging member is a plurality of balls held in the plurality of holes.

5. A multi-disc frictional engagement mechanism for coupling a first member and a second member which are coaxially arranged, comprising:

a plurality of first friction plates mounted on the first member slidably in an axial direction;

a plurality of second friction plates mounted on the second member slidably in the axial direction, the plurality of first friction plates and the plurality of second friction plates being alternately arranged;

a hydraulic piston arranged displaceably in a direction perpendicular to the pluralities of the first and second friction plates and including a radially extending pressure receiving portion;

a piston oil chamber configured to cause a hydraulic pressure to act on the pressure receiving portion;

an elastic member configured to come into contact with the pressure receiving portion from a side opposite to the piston oil chamber so as to bias the hydraulic piston toward the pluralities of the first and second friction plates; and

a lock mechanism configured to restrict a movement of the hydraulic piston in a coupling direction toward the pluralities of the first and second friction plates in a state in which the hydraulic pressure is supplied to the piston oil chamber, the elastic member is compressed by the pressure receiving portion, and the pluralities of the first and second friction plates are not coupled, wherein:

the lock mechanism is locked when the hydraulic pressure of the piston oil chamber acts on the lock mechanism,

the lock mechanism includes an engaging member and a second elastic member configured to bias the engaging member toward the pressure receiving portion;

the pressure receiving portion includes a receiving portion with which the engaging member is configured to be engaged at an end part of the pressure receiving portion;

the engaging member is configured so as to move and engage with the receiving portion by a biasing force of the second elastic member prior to the hydraulic piston being pushed back by the elastic member such that the movement of the hydraulic piston in the coupling direction is restricted when the hydraulic pressure supplied to the piston oil chamber is reduced from a state in which the hydraulic pressure is supplied to the piston oil chamber and the elastic member is compressed by the pressure receiving portion;

the engaging member is partially exposed to the piston oil chamber; and

the restriction of the movement of the hydraulic piston in the coupling direction by the lock mechanism is removed by temporarily supplying the hydraulic pressure to the piston oil chamber and retracting the engaging member against the biasing force of the second elastic member.