High performance couplings are plain sailing

Marine power transmission systems make stringent demands on shafts and clamping components. Not only must they deliver high torque capacity, but they must also offer high levels of reliability combined with ease of maintenance. Marine power transmission systems can require frequent disassembly for maintenance, so quick mounting of dismounting is an essential feature. Then there is the challenging marine environment and the fact that there is often very little space in which to work. As yachts increase in size, outboard propulsion systems become inappropriate, whilst on the largest yachts and highest powered craft exotic propulsion systems such as water jets are becoming fashionable. But for yachts between 40 and 100 feet in length by far the most common propulsion arrangement where speeds above 40 knots are not required is the in-board drive with conventional submerged propellers. All propulsion systems have their advantages and disadvantages, but in-board drive lines benefit from an inherent simplicity and, when properly maintained, a near unlimited life. The emphasis on maintenance is crucial. The underwater appendages are subject to shock and there will always be inherent vibration. Excessive vibration would likely indicate serious wear due to misalignment, but there is inherent misalignment in all in-board systems: the engine is installed on rubber mounts and the shaft mounted in rubber bearings, so both engine and shaft can move. In addition, the boat hull itself is not completely rigid so there is more potential for movement. While it is vital that shaft coupling alignment is extremely accurate, it can be seen that it will be virtually impossible to eliminate misalignment between the shaft itself and the engine. When the engine is running, the shaft will tend to self align to a degree, but there will always be wear. Because of this, there is a need for reasonably regular maintenance to check and, if necessary, replace the rubber mounts and other key components. This makes ease of disassembly and dismounting of the drive line critical for a cost-effective propulsion system. Maintenance is further complicated by the fact that space is at a premium on yachts even of the very largest sizes, and in-board drives are often installed in areas where there is very little working space. This also impacts on power, with the torque handling capacity of a conventional coupling directly related to its physical size. So in a traditional in-board marine engine, the coupling between the engine and the shaft is one of the primary limiting factors in the maximum torque capability of the drive train. Traditional shaft clamping components fail to address all of these issues. Most mechanical shaft connections require numerous bolts; in the cramped engine and shaft tunnel, this makes the shaft a challenge to install and certainly does not facilitate easy maintenance. ETP-HYCON hydraulic shaft clamping connection couplings from Abssac offer a completely different kind of shaft connection, offering robust and compact high power transmission. Designed to deliver reliability, but also quick mounting and dismantling, the ETPHYCON couplings are ideal for a host of applications in difficult environments and heavy operations, including marine engine drives. Used to connect shafts within a driven system, the ETP-HYCON couplings use hydraulics to create the friction seal between the shaft and the coupling. The coupling consists of a double-walled hardened steel sleeve, filled with a pressure medium. The outer sleeve has two hydraulic connection points and the inner sleeve is coated with unique dry frictional substance called ETPHFC on the inside wall. When mounting, the two hydraulic connections are pressurised simultaneously. The captivated pressured oil lubricates the contact surfaces in the axial space between the seals. This pressure initiates the movement of the outer sleeve, which is forced to linearly move a small distance because of the axial force created by the oil pressure. The linear movement of the outer sleeve against the inner sleeve mechanically compresses the inner sleeve to an even surface pressure on the shaft. At a set pressure calculated to both holding force and required torque transmission capability, the hydraulic hoses are disconnected and the ETPHYCON is completely installed. A rigid and strong shaft connection has been created. Dismantling of the ETP-HYCON is done in the reverse way as in assembly, which also ensures an even dismantling pressure and no damage to the shaft. The ETP-HFC coating more than doubles the coefficient of friction from 0.15 (about the standard for conventional mechanical couplings) to better than 0.3. This means designers can specify a smaller shaft connection device with lower mass for any given torque requirement, making the whole assembly dynamically more efficient. sales@abssac.co.uk www.abssac.co.uk