A mechanical tensioner for elongating and relaxing a stud having an axis and arranged in an object, comprises at least two parts including a first part connectable with a stud to pull the stud in an axial direction so as to elongate the stud and thereby to tension it in an object or to relax the stud, a second part connected with the first part movably in a transverse direction relative to the first part, and a friction element arranged between the first part and the second part and connected with the first part immovably in a transverse direction relative to the first part. The second part has a first engaging element engageable by a tool, while the friction element has a second engaging element engageable by a tool so that when an active force is applied to the second part through the first engaging element and an opposite holding force is applied to the friction element through the second engaging element the second part moves in a transverse direction and the first part moves only in an axial direction so as to elongate or to relax the stud.