Injection device for delivering a defined number of equal doses of a fluid substance

An injection device for delivering of a defined number of equal doses of a fluid substance contained in a reservoir, the device having a housing within which an arming mechanism and a dose delivery mechanism are arranged along a longitudinal axis of the housing, the housing being coupled to an enclosure adapted for receiving the reservoir with the fluid substance, wherein the arming mechanism comprises rotatable setting sleeve, the setting sleeve being coupled to a spring,

• the dose delivery mechanism comprises a screw ring having an internal thread, a piston rod having an external thread and a non-circular cross-section, and a blocking sleeve engaged with the piston rod to prevent rotation of the piston rod,
• wherein the device further comprises a rotatable knob which is coupled to the setting sleeve in a way allowing to transfer rotational movement and an axially displaceable trigger,
• wherein the device is being armed for delivery of doses of the substance by rotation of the knob,
• wherein during arming of the device, the screw ring and the axial displaceable trigger are in an engaged position so that the screw ring is immobilized,
• during delivery of each dose, the axial displaceable trigger is released allowing rotation of the screw ring and displacement of the piston rod along the housing by a defined distance due to unwinding of the spring, the displacement of the piston rod causing the fluid substance to be discharged from the reservoir,
• wherein a certain play L exists between the axial displaceable trigger and the screw ring in the engaged position so that after arming of the device and before releasing of the axial displaceable trigger, both the setting sleeve and the screw ring undergo a priming rotation with the limits of said play L.