Power transmission device

A power transmission device, in which a first gear and a second gear composed of helical gears are disposed coaxially to be movable in at least opposing directions and are accommodated in a case,

• the first gear and the second gear are respectively engaged with gears connected to a torque generating source, and
• angles of each helical tooth of the first gear and the second gear are respectively set such that thrust forces in opposing directions are respectively generated at the first gear and the second gear during transmitting, by the first gear and the second gear, a torque from the torque generating source, wherein
• the first gear and the second gear are fitted coaxially to generate a specific maximum static friction, and
• the first gear and the second gear respectively move in opposing directions in a case where an absolute value of a total value of the thrust forces in opposing directions of the first gear and the second gear exceeds an absolute value of the maximum static friction.