Shaft couplings, propeller nuts and shaft flange bolts designed for easy handling and trouble free operation Wrtsil hydraulic equipment includes varieties of shaft couplings, propeller nuts and shaft flange bolts. Hydraulic couplings provide a simple and quick way to connect shafts, eliminating the need for keyways, tapers or thrust rings. The hydraulic shaft coupling range consists of four design types: OHSM, OHSM-B and OHSM-V are flange couplings and connect a shaft to a gearbox, engine or another shaft. OHSN couplings are designed to connect two standard shafts.