Linear actuator with drop prevention mechanism

A linear actuator comprising:

• a housing;
• an outer cylinder firmly secured to said housing;
• an inner cylinder fitted in said outer cylinder such that said inner cylinder can project from and contract within said outer cylinder while it is prevented from rotating relative to said outer cylinder;
• a drive screw shaft rotatably supported by said housing and driven by a rotating drive source for rotation at a fixed position;
• a driven nut secured to said inner cylinder and threaded with said drive screw shaft for causing said inner cylinder to project from or retract within said outer cylinder in response to rotation of said drive screw shaft;
• a drop prevention nut threaded with said drive screw shaft in proximity to said driven nut;
• a shear pin secured at one end to, and projecting in a radial direction from, one of said drop prevention nut and said inner cylinder or said driven nut;
• and
• the other of said drop prevention nut and said inner cylinder or said driven nut having an axial shear pin guide groove in which a projecting free end portion of said shear pin is slidably received,
• wherein said shear pin is designed to be sheared by engagement with an end wall of said shear pin guide groove when said inner cylinder and said driven nut are displaced relative to said drop prevention nut in a direction of a load acting on said inner cylinder, and
• wherein upon shearing of said shear pin, said drop prevention nut sup ports thereon said inner cylinder or said driven nut to thereby keep said inner cylinder in position against drop.