A mounting assembly is provided for mounting a sheave roller to a ladder-type cable tray. The cable cables can be laid on the cable tray for routing through a building. The mounting assembly includes first and second housing parts which are moveable relative to each other. The housing parts are attached to respective rungs of the cable tray. A lock prevents the relative movement of the housing parts relative to each other once the mounting assembly is engaged with the cable tray. The sheave roller is rotatably attached to the one housing part. This quickly and easily installs the sheave roller on the cable tray.