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Compact drive line couplings for high performance and low inertiaAn innovative design of shaft clamp is delivering key benefits in a range of power transmission applications, not least in the highly challenging marine environment. We look at their use on the latest super yachts, and how they are delivering a new paradigm in torque transmission capability, reliability and ease of maintenance.
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 quickly become inappropriate, whilst on the very 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, from luxury yachts right up to the so-called super yachts, 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 propulsion systems: the engine is installed on rubber mounts and the shaft is 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. As well as the time and effort involved in working with so many bolts, there is also the potential to mark the shaft during assembly, which can be detrimental to the friction joint - and so impact on torque capacity - and increase the likelihood of premature corrosion.
Hydraulic shaft clamp
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 ETP-HYCON couplings are ideal for a host of applications in difficult environments and heavy operations, including challenging 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 ETP-HFC on the inside wall. In the inner sleeve, assembled cylindrical rings and seals prevent any hydraulic oil loss.
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 ETP-HYCON 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. Concentricity of the shaft is also maintained to a very high lateral run out tolerance.
Reduced mounting time
In contrast to other mechanical connections such as keyways or taper lock alternatives, ETP-HYCON dramatically reduces mounting times as the whole clamping process is hydraulically actuated. Mounting and dismantling can be achieved in seconds, without any risk of damage to the driven shaft.
ETP-HYCON couplings are available in both shaft-to-shaft (HYCON S) and shaft-flange (HYCON F) versions, and are approved by DNV for marine applications, as well as being approved by other common certification authorities including Lloyds, BAS and RINA. They are available for shaft sizes from 50mm to 220mm. Because of their decisive advantages, these innovative couplings are becoming an industry standard in luxury and super yacht applications.
One customer in Italy building an 80m luxury yacht with two 1640kW engines needed couplings with a torque capacity of 121kNm, easily accomplished with the ETP-HYCON range and within the available size envelope. A specific requirement was approval to LR-EMEA. And couplings with a design torque of 140kN meeting Germanischer Lloyd certification were supplied for a sailing super yacht with two 1740kW engines operating at 348rpm. Further, couplings with a design torque of 131Nm have been supplied for a German manufactured 85m luxury yacht with two 2000kW engines.
Of course the couplings are also suitable for a host of other marine drive tasks, and have been specified around the world on naval patrol boats and training vessels, fast stand-by and rescue boats, multi-purpose vessels and even an air cushion catamaran fast attack craft.
Abssac is the sole UK distributor for the full range of ETP hydraulic hub shaft connections. This outstanding array of products provides an excellent choice, addressing different customer needs for quick and easy clamping of shafts, quick release capability, strong and reliable clamping forces, precision tolerance bores with excellent runout, exact positioning and adjustment, frequent mounting, and the removal of the need for keyways. The ETP products are available in both steel and stainless steel models.
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