Linear actuator and a medical delivery device comprising such linear actuator

A medical delivery device comprising a telescopic piston rod that comprises a linear actuator that comprises

• a first member provided with threads cut in a first direction;
• a second member provided to be axially displaceable in relation to the first member and being provided with threads cut in a second direction opposite said first direction;
• an intermediary member being engaged with and axially displaceable in relation to the first and second members, respectively, via first and second threads that are complementary to the threads provided on the first and second members, respectively;
• means for preventing rotation of the first member in relation to the second member;
• and
• drive means for rotationally driving the intermediary member,
• wherein
• the threads provided on the first and second members are provided as internal threads;
• the threads provided on the intermediary member are provided as external threads;
• the means for preventing rotation of the first member in relation to the second member comprises a sheath stretching across the intermediary member, said sheath being inrotatably, but longitudinally slidably arranged in relation to the first and second members;
• and
• the drive means comprises a driving rod that is inrotatably, but longitudinally slidably arranged within the intermediary member.