The present application relates to a shaft locking device. The device has an locking element configured to be mounted on a shaft in a first position so that the locking element indicates the angular position of the shaft. The locking element is also configured to be mounted on a shaft in a second position so that the locking element is locatable against a stop to prevent rotation of the shaft. The present application also relates to a device for determining the angular position of a shaft, a valve assembly and an actuating unit for a valve.