From the Super Mario Wiki
The Maintenance Hub is a room inside of the Old Clockworks in Luigi's Mansion: Dark Moon. The Maintenance Hub is accessed by taking the door on the left on Floor B1 of the Service Elevator or by taking the door on the right inside of the Synchro Gantry. The Maintenance Hub is a small room. In the middle of the room is a large gear that only turns once the hidden gear in the Crank Room has been revealed. A zipline is used to allow Luigi to travel between the entrances to the Synchro Gantry and the Service Elevator. On the ground below, there is nothing but sand, mice, and a hidden Treasure Chest. The door on the left takes Luigi back to the Synchro Gantry while the door on the right back to the Service Elevator. There is an invisible door to the north that takes Luigi to the Crank Room when shone with the Dark-Light Device.
During C-5: Piece at Last, Luigi must go to the Maintenance Hub to reach the Synchronization Room where the Toad assistant is. However, the way to the room is locked, so Luigi must go north into the Crank Room to find a key. After finding the key, Luigi must head back to the Maintenance Hub and into the Synchro Gantry. After rescuing Toad, when Luigi returns to the Maintenance Hub, he is attacked by three Greenies wielding mallets and a piece of metal as a shield.
Only during C-5: Piece At Last, Luigi should drop down to the lower floor of the Maintenance Hub. In the middle of the room, there is an Treasure Chest hidden away by Spirit Ball. Using the Dark-Light Device, Luigi can reveal the Treasure Chest that releases a ruby once it has been investigated.