A good coupling system needs to reliably transmit torque between the two connected shafts. But when maintaining the propulsion system, it is important that the couplings can be quickly and easily dismounted and remounted. The longer this process takes, the longer the downtime and the higher the dry docking costs.
Every minute matters
In ships and vessels, couplings can either be installed externally (outside the hull) or internally to connect the shaft to the gearbox or to connect two shafts. Sterntube seals ensure that no water comes into the vessel. However, these seals need to be inspected at least every five years; to do this, the coupling often needs to be dismounted and the shaft pulled out.
“Many flange coupling solutions require heavy bolts,” explains Peter Edin, Area Sales Manager at SKF. “Reamer bolts are particularly prone to getting stuck, often requiring a large sledgehammer to remove them. When this happens, it can take days to remove the coupling for maintenance.” The result is high costs and lengthy downtime, and the ship owner cannot be sure exactly how long the process will take.
Another common method of torque transmission is to use a keyway in the shaft. However, this is not suitable for all applications since it requires a larger, heavier coupling. “Keyways can also increase stress concentration,” explains Edin. “And this can eventually lead to stress cracks.”
A tried-and-tested technique
In the early 1940s, SKF’s chief designer, Erland Bratt, invented a method that does not require a keyway. By injecting oil at high pressure between the tapered inner and outer sleeves of the coupling, friction is reduced. Then, a hydraulic jack creates a strong interference fit, ensuring high torque transmission. “Oil injection facilitates mounting and dismounting,” says Edin. “Ship owners know that they will only need one hour to remove the coupling.” So how does it work?
4 steps to mounting and dismounting
- The coupling is positioned and oil injectors are connected at positions A and B. (fig. 1)
- Oil is injected into A at high pressure, creating a film of oil between the inner and outer sleeves to reduce friction. (fig. 2)
- Excess oil flows out at the thicker end of the inner sleeve. As oil is pumped into B, the outer sleeve moves up the taper until the coupling reaches its final position. The oil pump is then stopped but pressure in B is sustained. (fig. 3)
- When the oil has drained from the contact surfaces of the sleeves and friction has been restored, the pressure in B is released. All oil connections are plugged. The coupling is now mounted, and will transmit torque between the shafts. (fig. 4)
Using this method, ship owners know how long the work will take and can schedule the job in a more efficient way. It can also help to extend the service life of the component. With the help of hydraulic couplings, ships spend less time in the dry docks and more time at sea.