The Salvaged Arming Crank is a part of a damaged Trebuchet that you have to deliver to Siegemaster Lormar as part of the Fort Ranik mission. It can be found in one of the nearby Wrecked Catapults.
Note that the Salvaged Arming Crank is a "Bundle" (i.e. an object that does not go into the inventory, but has to be held by hands).