A device (10) for shifting one structure with respect to another, is disclosed. The device comprises a body (12) defining a cylinder, a core (14) within the cylinder and rotatable in the cylinder with respect to the body (12). A coil spring is connected between the body (12) and the core (14) and is wound or unwound as the core (14) is rotated in the cylinder. A compressible ring (48) is carried by the core (14) and is in frictional contact with the body. The ring (48) can be compressed by a nut (66) screwed onto a shaft (50) forming part of the core (14) so that the frictional resistance offered by the ring (48) to turning of the core (14) in the cylinder is increased.