Linear actuator

A linear actuator comprising:

• a motor portion that includes a cylindrical rotary shaft having a first end and a second end and including an opening portion at least at the first end in an axial direction of the rotary shaft, a rotor fixed to the rotary shaft, and a stator disposed outside the rotor;
• a first bearing portion that rotatably supports the first end of the rotary shaft;
• a second bearing portion that rotatably supports the second end of the rotary shaft;
• a locknut threadably engaged on the first end of the rotary shaft and contacting an inner race of the first bearing portion to block movement of the inner race of the first bearing portion in the axial direction;
• and
• a ball screw mechanism including a ball screw nut and a ball screw, the ball screw nut including a nut body fitted in the opening portion of the rotary shaft and a flange portion integrally formed with the nut body and located outside the opening portion, the ball screw nut being screwed to the locknut using a plurality of screw members with the flange portion contacting the locknut, and the ball screw being threadably engaged with the ball screw nut with part of the ball screw extending in the rotary shaft, wherein:
• a male thread portion is formed on an outer peripheral portion of the first end of the rotary shaft;
• a female thread portion is formed on an inner peripheral portion of the locknut to be threadably engaged with the male thread portion of the rotary shaft;
• a first cylindrical outer peripheral surface portion and a second cylindrical outer peripheral surface portion that are unthreaded and face radially outwardly relative to an axis of said rotary shaft, said first and second cylindrical outer peripheral surface portions being formed, together with the male thread portion that is threadably engaged with the locknut, on the outer peripheral portion at the first end of the rotary shaft, the first cylindrical outer peripheral surface portion being located closer to the first bearing portion than the male thread portion is, and the second cylindrical outer peripheral surface portion being smaller in diameter than the first cylindrical outer peripheral surface portion and being located opposite the first cylindrical outer peripheral surface portion across the male thread portion;
• a first cylindrical inner peripheral surface portion and a second cylindrical inner peripheral surface portion that are unthreaded and face radially inwardly relative to an axis of the locknut, said first and second cylindrical inner peripheral surface portions being formed, together with the female thread portion threadably engaged with the male thread portion of the rotary shaft, on the inner peripheral portion of the locknut, the first cylindrical inner peripheral surface portion facing the first cylindrical outer peripheral surface portion with the male thread portion and the female thread portion threadably engaged with each other, and the second cylindrical inner peripheral surface portion being smaller in diameter than the first cylindrical inner peripheral surface portion and facing the second cylindrical outer peripheral surface portion with the male thread portion and the female thread portion threadably engaged with each other;
• a first gap is formed between the first cylindrical outer peripheral surface portion and the first cylindrical inner peripheral surface portion and a second gap is formed between the second cylindrical outer peripheral surface portion and the second cylindrical inner peripheral surface portion;
• and
• a first distance between the first cylindrical outer peripheral surface portion and the first cylindrical inner peripheral surface portion and a second distance between the second cylindrical outer peripheral surface portion and the second cylindrical inner peripheral surface portion are selected so as to prevent displacement between the center of rotation of the rotary shaft, the center of rotation of the locknut, and the center of rotation of the ball screw nut when the ball screw nut is fixed to the locknut using the plurality of screw members.