Power transmission apparatus of four wheel drive apparatus

A power transmission apparatus of a four wheel drive vehicle including a front wheel (Wf) to which a drive force of an engine (E) is directly transmitted, and a rear wheel (Wr) to which a portion of the driving force of the engine (E) is indirectly transmitted via a multiple disc clutch (C), wherein the multiple disc clutch (C) is engaged by a hydraulic pressure generated by hydraulic pumps (Pf, Pr) in accordance with a difference between rotational speeds of the front wheel (Wf) and the rear wheel (Wr);

• the power transmission apparatus comprising:
• a torque cam mechanism including
• a first cam member (62) moved in cooperation with rotation of the front wheel (Wf), and
• a second cam member (63) moved in cooperation with rotation of the rear wheel (Wr),
• the multiple disc clutch (C) being engaged by a thrust force in an axial direction generated in accordance with a difference between rotational speeds of the first cam member (62) and the second cam member (63), wherein
• the torque cam mechanism (61) generates the thrust force when the rotational speed of the front wheel (Wf) exceeds the rotational speed of the rear wheel (Wr) in running the vehicle forward, and does not generate the thrust force when the rotational speed of the rear wheel (Wr) exceeds the rotational speed of the front wheel (Wf) in running the vehicle forward, and
• a gear ratio (β) of a second gear mechanism (5) for transmitting rotation of the rear wheel (Wr) to the second cam member (63) via a second differential apparatus (6) is set to be larger than a gear ratio (α) of a first gear mechanism (4) for transmitting rotation of the front wheel (Wf) to the first cam member (62) via a first differential apparatus (2).