R+W recently received a customer inquiry which turned into an exciting project for our development team, a flange mounted safety coupling (STF). Normally this is nothing out of the ordinary, but in this instance a special marine certification was required.

The requested STF safety coupling must be maintenance-free and will be installed in a ship’s propulsion system to protect the gearbox on the bow steel rudder. The propeller drive has an engine power of 2,560 kW and speed of 800 rpm. If the bow steel rudder jams, the safety coupling disconnects the shaft connection to prevent damage to the gearbox. The STF series has flange connections on both sides and an integrated bearing for high speeds.


Flange connections are friction-locked bolted connections that can be mounted quickly and easily. In addition to the bolt circle diameter (mounting bolts), a centering pilot is required in the flange to ensure concentricity. This centering feature ensures alignment of the flanges and mitigates the possibility of off-axis rotation. Additional radially mounted studs are used for speed measurement.
A RINA certification was required for this project. RINA is an acceptance organization like TÜV, but specifically for the marine industry. The design of the STF coupling had to be adapted to the following RINA requirements: All components in the power flow had to be forged steel in accordance with 3.2 RINA acceptance, a safety factor of 2.8 was required, all remaining components need a 3.1 material test certificate, and an acceptance of the coupling by a RINA inspector.
In the end, we received the RINA certification for our custom STF safety coupling and thus assuring customer satisfaction.