The invention relates to a device (1) and to a method associated with an assembly mechanism for a rotor system of an axial turbo engine having a central tension anchor, as is particularly known from engine technology. To this end, the invention provides a tensioning tool that pre-tensions or biases the tension anchor prior to and during the insertion of the anchor into the compound rotor assembly. After inserting the biased tension anchor in the compound rotor assembly, the tension anchor is relaxed, whereby the anchor exercises a compressive force on the compound rotor assembly. According to a preferred embodiment, the compound rotor assembly is also biased (compressed) and relaxed after assembling the tension anchor, so that a desired nominal bias Sigmanominal can be particularly easily achieved. The use of a shaft locking nut is no longer necessary in the device according to the invention, thus avoiding the difficulties that are associated with the use of this nut, such as complexity and safety problems. The invention also relates to a method for assembling a tension anchor (2) for a rotor system (6) employing the assembly mechanism according to the invention.