Rope tensioner. Rope was cheaper than leather belting and didn't have to be re-laced because of stretching. The tensioner could accommodate a lot of stretch. Of course, you needed someone who could make a 10 foot long splice in the rope. A Navy Chief Boatswain's Mate could. One rope drive that I have seen drove a large sawmill in Mississippi. Close to the rope was hanging a wire connected to a cow bell. If the rope started to fray, the frayed piece would hit the wire and ring the bell. Then the Engineer would know there was a problem.