Power unit that can be telescopically lengthened/shortened and performs linear movement which power unit comprises cylindrical parts (1-3) moving telescopically within each other in which case the mentioned parts are equipped with at least external or internal threads in such a way that the mentioned parts form a power unit that can be lengthened/shortened when the parts are being rotated in relation to each other and that at least one part (3) of the mentioned parts (1-3) is locked to be non-rotating or locked to rotate in a different way than the part (1) of the mentioned parts (1-3) to which part a rotating producing movement to the power unit is arranged. In order to reduce the construction weight of the power unit the mentioned parts (1-3) are made of plastic and of the mentioned parts the inner diameter (d) of the part (3) having the smallest diameter is over 25% of the useful height (H) produced by the mentioned smallest part.
