Rotary actuator and linear actuator

A linear actuator comprising:

• one hollow motor;
• two hollow reduction gears having a same reduction ratio;
• and
• a rotation/linear-motion conversion unit having one screw shaft and two nuts,
• wherein the hollow reduction gears are coaxially arranged on both sides in a direction of a center axis line to sandwich the hollow motor therebetween;
• the hollow motor has:
• a hollow motor shaft;
• and a motor hollow part that extends to penetrate the hollow motor in the direction of the center axis line and is defined by a hollow part of the hollow motor shaft;
• each of the hollow reduction gears has:
• a hollow input shaft;
• a hollow output shaft;
• and a reduction gear hollow part that extends to penetrate the hollow reduction gear in the direction of the center axis line and is defined by a hollow part of the hollow input shaft;
• the screw shaft coaxially extends to penetrate the motor hollow part;
• the hollow input shafts of the hollow reduction gears are coaxially connected to shaft end parts on both sides of the hollow motor shaft;
• the nuts are screwed on shaft portions on both sides of the screw shaft, the shaft portions protruding from both ends of the motor hollow part;
• the hollow output shafts of the hollow reduction gears are coaxially connected to the nuts via connection mechanisms;
• and
• at least one of the connection mechanisms has an adjustment part to adjust a connection position of the hollow output shaft to the nut by a predetermined angle in a rotational direction of the hollow output shaft.