Abstract: A locking device for securing a tube to a threaded nipple has a threaded nut for mating with the threaded nipple. The nut has a through opening formed therein and a camming surface formed in the opening. A locking member has a first body section adopted to be positioned over the tube and received in the opening of the nut. The locking member also has a second body section extending away from the first body section and presenting an inclined surface positioned for engagement by the camming surface. A gripper component extends from an inner surface of the second body section for engagement with the tube. When the nut is threadably received on the nipple, movement of the tube away from the nipple results in the inclined surface engaging the camming surface to cause the gripper component to tightly engage the outer surface of the tube.