An actuator converting a rotary movement into a linear movement is disclosed. The actuator includes a drive means (11), a primary low friction screw mechanism (3, 6, 14) driven by the drive means, a secondary traditional screw mechanism (7, 13) which is connected to the primary screw mechanism and moved linearly by this. The secondary screw mechanism transfers the linear movement to a driven means, such as a clutch. The primary screw mechanism is performing the working stroke proper, while the secondary screw mechanism is arranged to compensate for changes in the engagement point of the driven means. One of the screw mechanisms is provided with a spring (4) making the actuator self-regulating.