An assembly for advancing a piston through a rotationally fixed cartridge, the cartridge having a 2. distal/front end, an open proximal/rear end and a substance filled therein between the piston and the front end, the assembly comprising: an axially fixed first member rotatable in either a first rotational direction or an opposing second rotational direction; an axially movable, rotatable second member connected to the first member via a threaded connection and connectable to the piston, wherein the first member comprises a first shaft and the second member comprises a second shaft; and a friction pad rotationally fixedly connected with a distal end of the second member, the friction pad comprising: a central disk; a first set of fingers extending radially outwardly from the central disk; and a second set of fingers extending radially outwardly from the central disk, wherein both the first set and the second set of fingers (34, 36) are configured to contact an interior side wall of the cartridge, the first set of fingers being angled toward the first rotational direction and configured to wedge the friction pad between the interior side wall and the second member upon rotation of the first member in the first rotational direction, thereby substantially preventing rotation of the friction pad and the second member in the first rotational direction relative to the cartridge, the second set of fingers being angled toward the second rotational direction and configured to wedge the friction pad between the interior side wall and the second member upon rotation of the first member in the second rotational direction, thereby substantially preventing rotation of the friction pad and the second member in the second rotational direction relative to the cartridge, characterised in that each finger of the first set of fingers is positioned opposing a finger of the second set of fingers, thereby forming a respective pair of fingers. an axially fixed first member rotatable in either a first rotational direction or an opposing second rotational direction; an axially movable, rotatable second member connected to the first member via a threaded connection and connectable to the piston, wherein the first member comprises a first shaft and the second member comprises a second shaft; and a friction pad rotationally fixedly connected with a distal end of the second member, the friction pad comprising: a central disk; a first set of fingers extending radially outwardly from the central disk; and a second set of fingers extending radially outwardly from the central disk, wherein both the first set and the second set of fingers (34, 36) are configured to contact an interior side wall of the cartridge, the first set of fingers being angled toward the first rotational direction and configured to wedge the friction pad between the interior side wall and the second member upon rotation of the first member in the first rotational direction, thereby substantially preventing rotation of the friction pad and the second member in the first rotational direction relative to the cartridge, the second set of fingers being angled toward the second rotational direction and configured to wedge the friction pad between the interior side wall and the second member upon rotation of the first member in the second rotational direction, thereby substantially preventing rotation of the friction pad and the second member in the second rotational direction relative to the cartridge, characterised in that each finger of the first set of fingers is positioned opposing a finger of the second set of fingers, thereby forming a respective pair of fingers. a central disk; a first set of fingers extending radially outwardly from the central disk; and a second set of fingers extending radially outwardly from the central disk, wherein both the first set and the second set of fingers (34, 36) are configured to contact an interior side wall of the cartridge, the first set of fingers being angled toward the first rotational direction and configured to wedge the friction pad between the interior side wall and the second member upon rotation of the first member in the first rotational direction, thereby substantially preventing rotation of the friction pad and the second member in the first rotational direction relative to the cartridge, the second set of fingers being angled toward the second rotational direction and configured to wedge the friction pad between the interior side wall and the second member upon rotation of the first member in the second rotational direction, thereby substantially preventing rotation of the friction pad and the second member in the second rotational direction relative to the cartridge, characterised in that each finger of the first set of fingers is positioned opposing a finger of the second set of fingers, thereby forming a respective pair of fingers.