Abstract: A device for test running of power nut runners comprises a main body (10;50) with a nut (11;54), a screw (12;52) with a nut runner engaging head (13;55), a clamping element (14;56) co-operating with the screw (12;52) and spring unit (17;59) to be clamped between the clamping element (14;56) and the main body (10;50) to generate a torque resistance on the screw (12;52), wherein a one-way clutch (24;64) is provided between the nut (11;54) and the main body (10;50) to prevent rotation of the nut (11;54) at rotation of the screw (12;52) in the tightening direction and to permit rotation of the nut (11) when rotating the screw (12;52) in the loosening direction, a friction clutch (30;72,73) between the nut (11;54) and the main body (10;50) is arranged to prevent rotation of the nut (11;54) during loosening of the screw (12;52), and a lock element (34;70) secured to the screw (12;52) is arranged to rotationally lock the screw (12:52) to the nut (11;54) at continued rotation of the screw (12;52) in th loosening dire