Abstract: A device for providing relative movement between three telescopically co-ordinated parts (1, 2, 3) which comprises a power device (7) of a type variable in length and acting between a point (8) on a first (1) and a point (9) on a second (2) of the parts, and a flexible traction element (12), which is arranged to cause relative movement between the third part (3) and the first and said second parts (1, 2) by transmitting forces between the first and the third parts when relative movement between the first and second parts (1, 2) is effected by the power device. The power device located externally of the parts is designed with a longitudinal internal passage (13), in which the traction element extends in its extension between a point (14) on the third part and a point (15) fixed in relation to the first part (1), whereby the power device will operate as a casing variable in length for the traction element.