Frictional coupling device of vehicular power transmitting system

A frictional coupling device of a vehicular power transmitting system, comprising:

• a first rotary member disposed rotatably about an axis;
• a second rotary member disposed rotatably about the axis;
• a frictional coupling portion disposed between the first and second rotary members, and including a plurality of friction discs held in spline engagement with spline teeth of a radially outer splined portion formed on an outer circumferential surface of the first rotary member, and a plurality of friction plates held in spline engagement with spline teeth of a radially inner splined portion formed on an inner circumferential surface of the second rotary member, wherein the friction discs and the friction plates are alternately disposed adjacent to each other;
• and
• a piston disposed movably in a direction of the axis toward the frictional coupling portion, to press the frictional coupling portion,
• wherein a spacing distance between the adjacent spline teeth of the radially outer splined portion in a rotating direction thereof decreases along the direction of the axis in which the friction discs are brought into the spline engagement with the radially outer splined portion during assembling of the friction discs with respect to the first rotary member, and/or a spacing distance between the adjacent spline teeth of the radially inner splined portion in a rotating direction thereof decreases along the direction of the axis in which the friction plates are brought into the spline engagement with the radially inner splined portion during assembling of the friction plates with respect to the second rotary member.