A tension rod mechanism for a tension rod having a first outer shaft with a second inner shaft slideable therein has a unidirectional slip-lock configured to be received in the first outer shaft for rotational movement therewith and unidirectional slidable axial movement relative thereto. A threaded shaft is fixedly connected to the unidirectional slip-lock. A collar is threadingly engaged with the threaded shaft and configured to be fixedly received in the second inner shaft for movement therewith. When the unidirectional slip-lock is in the first outer shaft and the collar is fixedly received in the second inner shaft, slidable axial movement of the unidirectional slip-lock relative to the first outer shaft may occur only in an expansion direction A rotation of the first outer shaft relative to the second inner shaft increases the axial distance between the unidirectional slip-lock and the collar.