Reverse rotation prevention mechanism and motor with reducer

1. A reverse rotation prevention mechanism provided on a torque transmission path between an output shaft and a driving shaft of a motor, comprising:

a first frictional force generation unit that resists rotation of a braking rotational member provided on the torque transmission path relative to a non-rotating fixed member; and

a second frictional force generation unit that generates, when an external force is exerted on the output shaft, a braking force that prevents reverse rotation, wherein

the second frictional force generation unit generates the braking force as a portion of the braking rotational member is pressed against the second frictional force generation unit as the first frictional force generation unit resists rotation of the braking rotational member.

2. The reverse rotation prevention mechanism according to claim 1 , further comprising:

a braking and pressing member that moves axially away from the braking rotational member due to a reactive force responsive to a force that presses the braking rotational member against the second frictional force generation unit when an external force is exerted on the output shaft; and

a third frictional force generation unit that generates, when an external force is exerted on the output shaft, a braking force that prevents reverse rotation, by causing a portion of the braking and pressing member to be pressed.

3. The reverse rotation prevention mechanism according to claim 1 , wherein

the braking rotational member is pressed against the second frictional force generation unit by a driving shaft side rotational member provided on the torque transmission path more toward the driving shaft of the motor than the braking rotational member,

the driving shaft side rotational member is engaged with and rotated, relative to the non-rotating fixed member, along with the braking rotational member when the motor is driven, and

the first frictional force generation unit is provided between the braking rotational member and the driving shaft side rotational member.

4. The reverse rotation prevention mechanism according to claim 1 , further comprising:

a spacing mechanism that causes a portion of the braking rotational member to be spaced apart from the second frictional force generation unit when the motor is driven.

5. The reverse rotation prevention mechanism according to claim 1 , wherein

a portion of the braking rotational member is a tapered surface tapered with respect to a central axis of the braking rotational member.

6. The reverse rotation prevention mechanism according to claim 5 , wherein

a taper angle of the tapered surface is not less than 1° and less than 30°.

7. A motor with a reducer comprising:

a motor;

a worm to which a rotational force of a driving shaft of the motor is transmitted;

a worm wheel in mesh with the worm;

an output shaft to which the rotational force exerted on the worm wheel is transmitted; and

the reverse rotation prevention mechanism according to claim 1 .

8. The motor with a reducer according to claim 7 , wherein

the reverse rotation prevention mechanism is provided on a torque transmission path between the driving shaft of the motor and the worm.

9. The motor with a reducer according to claim 7 , wherein

the reverse rotation prevention mechanism is provided on a torque transmission path between the worm wheel and the output shaft.

10. A reverse rotation prevention mechanism provided on a torque transmission path between an output shaft and a driving shaft of a motor, comprising:

a worm, to which a rotational force of the driving shaft of the motor is transmitted via the torque transmission path;

a worm wheel in mesh with the worm, the worm wheel coupled to the output shaft such that the rotational force exerted on the worm wheel is transmitted to the output shaft;

a first frictional force generation unit that resists rotation of a braking rotational member provided on the torque transmission path relative to a non-rotating fixed member;

a second frictional force generation unit that generates, when an external force is exerted on the output shaft, a first braking force that prevents reverse rotation;

a braking and pressing member that moves axially away from the braking rotational member due to a reactive force responsive to a force that presses the braking rotational member against the second frictional force generation unit when an external force is exerted on the output shaft; and

a third frictional force generation unit that generates, when the external force is exerted on the output shaft, a second braking force that prevents reverse rotation, by causing a portion of the braking and pressing member to be pressed, wherein

the second frictional force generation unit generates the braking force as a portion of the braking rotational member is pressed against the second frictional force generation unit as the first frictional force generation unit resists rotation of the braking rotational member.

11. A reverse rotation prevention mechanism provided on a torque transmission path between an output shaft and a driving shaft of a motor, comprising:

an output shaft side rotational member coupled to the output shaft;

a braking rotational member provided adjacent to the output shaft side rotational member;

a driving shaft side rotational member engaged with and rotated along with the output shaft side rotational member and the braking rotational member when the motor is driven, the driving shaft side rotational member being coupled toward the driving shaft of the motor; and

a frictional force generation unit which generates, when an external force is exerted on the output shaft, a braking force that prevents reverse rotation, by causing the output shaft side rotational member to move axially away from the braking rotational member and cause a portion of the braking rotational member to be pressed.

12. The reverse rotation prevention mechanism according to claim 11 , further comprising:

a spacing mechanism that causes a portion of the braking rotational member to be spaced apart from the frictional force generation unit when the motor is driven.